DIY tunnel drive

[CDK]

To fully appreciate this project, you should look at http://forums.boatdesign.net/forumdisplay.php?f=34 "Why are all prop shafts water lubricated?".

For my 26 ft. Draco Twincab, now being equipped with Mercruiser gasoline engines and Berkeley jets, I designed an experimental tunnel drive system that will be powered by two small turbocharged diesels.
The boat was originally equipped with sterndrives, so the engine bay location and lengthare unsuitable for a conventional prop shaft construction. Also, the height is insufficient to use V-drives.

At the present I am preparing two GRP tunnels, 17" diameter, about 2 ft long that will be laminated to the transom once the jet drives are removed and the engine area is cleaned and degreased. The stainless steel construction is already finished.

The prop shafts are enclosed in stainless steel tubes with welded on flanges that will initially be bolted to the transom. Then the inner tubes will be fully laminated in the 2" transom wall to obtain a solid construction.
The outer (ball)bearing is a sealed, grease filled type, mounted behind two marine oil seals that are neoprene coated. The gap between the two seals is filled with a mixture of lithium grease and silicone oil. I added silicone oil because it has unique properties as a water repellant. The gap between the 2nd seal and the ball bearing receives oil through a small opening at the top of the bore where the bearing is pressed in. The bearing carrier is only held in place by an O-ring between the carrier and the cover, located in a V-shaped space. The ring was pressed outward against the tube when the 4 bolts were tightened. When loaded with a 500 lbs force it did not visibly move.
The short inner tube has two back to back mounted tapered roller bearings, thrust rings and a neoprene oil seal, held in place by a steel cover plate and 4 bolts. With the latter the bearing preload can be adjusted.
A small stud with a threaded hole will be connected to a transparent oil reservoir mounted well above the waterline. The whole tube is filled with SAE 15W40 engine oil, making water intrusion virtually impossible.

Here are some photo's and drawings of this part of the project.

[Frosty]

Are you using surface props?

[CDK]

The props are still a big question mark.
To test the drives for stability and steering at low speed I will start with 3-blade 14x17" normal props. I hope to be able to determine the speed at which the tunnels start to draw air. If that is near my 20 kn. target, I will try to improve the bottom section in front of the tunnels. Does it occur much sooner, I will have to use sp props, in which case I shall also feed the exhaust gases to the tunnels.

[c-cat]

Be sure to vent the tunnels via. tubing.Otherwise probably won't acheive planning speed.Thought about doing this to some merc.sportjets but reverse would be an issue. You also may think about halo rudders behind the props, traditional rudders tend to chatter in turbulence, and stop working all together at @45-50mph.

[TerryKing]

CDK, nice mechanical work there....

Can you help us visualize what the bottom profile and tunnels look like, from the side?? I'm not sure how this all goes together...

[CDK]

Progress is a bit slow due to the weather. The tunnels are attached to the (double) transom and the prop tubes are in place. Also the ugly bottom holes from the jets are closed with over 1/2" of GRP but work outside is hampered by rain, salt deposits after NE wind and low temp. As soon as the weather permits I will clean up the mess outside and make pictures.

-"You also may think about halo rudders behind the props, traditional rudders tend to chatter in turbulence, and stop working all together at @45-50mph."-
If I will be able to reach half that speed with my two small diesels I will be more than satisfied....

[PetterM]

Interesting project and very nice mechanical work.
How much power do you have?
What is expectation of speed?

[CDK]

Interesting project and very nice mechanical work.
How much power do you have?
What is expectation of speed?

Twin VW 1,9 ltr. turbodiesels, 75 HP in a VW t-4 transporter, perhaps a bit more after marinising because there is no EGR anymore and the injection pumps can easily be adjusted to deliver approx 100 HP for a short time.
Speed target is 20+ knots.

[Pericles]

CDK,

Have you see this?

http://www.dbdmarine.com/CD-300-Marine-outdrive.htm

Pericles

[Frosty]

Personally I am well aware of dbd. Passed from some place or another my enquiries were thought of as being not serious when I actually wanted to buy 2.

I was also told that they were'nt actually in construction.

Have things changed?

[CDK]

CDK,

Have you see this?

http://www.dbdmarine.com/CD-300-Marine-outdrive.htm

Pericles

No, I have not.
Looks good though, clearly inspired by Arneson and/or Q-SPD.
The ball shaped CVJ under water is an expensive construction that requires regular maintenance and also the submerged tiller arm seal is something to worry about.
Anyhow, my DIY tunnel drives are almost ready....

[CDK]

Here are some pictures of the work in progress.
The camera of course is very unforgiving and shows every irregularity, but I comfort myself with the thought that once ready, the whole structure will be submerged and after a few weeks will look like a giant broccoli anyhow.

The tunnels are made from 16 layers of glass and resin, a little over 1/2" thick. Inside they are smooth, the outside is covered with resin mixed with a liberal amount of talcum powder. The local hardware stores here offer only one type of resin and large bags of talcum powder to obtain any viscosity between syrup and butter. I watched local fishermen make new boats by covering the old ones with GRP and a powder filled top coat. The commercial boatbuilders flatten and fill the whole surface, the private ones only polish the dry area, the hasty ones just paint it and go fishing.

The boxlike transom I added 3 years ago to obtain enough engine room length for Berkeley jets and Mercruiser engines.
Although it lost its function now, I decided to leave it in place because the original transom had so many holes it would have needed a complete renewal. Now it provides addition strength to the tunnels.

There are 4 small rudders (3 shown) with a wing profile, made from solid GRP with a stainless steel insert to attach them to the steering shafts. Levers and a tie-bar still have to be made, and so is the contraption to connect it to the Morse steering cable.

The far too shiny red paint is a first layer of copperbased antifouling, applied minutes before the pictures were taken. This type of paint is banned in some countries and it does not completely avoid marine growth, but I also tried several modern anti-foulings that were even less effective. The broth we call the Adriatic sea contains life forms that attach even to teflon. I experimented with silicone oil but gave up because it slowly washes off and people kept staring and commenting at the oil film around the boat.

[CDK]

This concludes the DIY tunnel drive project. There is still some paintwork to be done to the dry part of the transom, but that is not essential.
The 4 rudders pivot on ss shafts with ball bearings behind neoprene seals, greased for life. Their angle is limited to 40 degrees, otherwise the propeller tips come too close. One of the middle rudders is connected to the long shaft with a steering arm that is pushed in and out by a bar that has ball joints at both ends. Because the geometry dictates approx. 3/4" sideways movement and 1/4" vertical for the bar, the transom hole had to be fairly large and elliptical. It is covered with a ss plate and an angled tube with a neoprene bellows. Not an easy part to make, but it's there.

Because the prop shaft ends inside the boat have no provisions like splines or keyways - I have no tools for such a job - and the Spicer 1350 universal joints need 1-1/4" 10 spline stubs, adapters had to be made.
It proved impossible to buy such splined shafts without a gearbox or large electric motor attached to the other end, so I called in the help of an old friend far away who has access to machines I can only dream of. He cut the SAE-B splines in piece of high tensile strenght shaft, I drilled holes that exactly fit the prop shaft ends. The half disks that are held together with 4 very strong Allan bolts compress the stubs around the prop shafts so they can take of the torque of the turbocharged diesel engines that are already waiting.

[CDK]

At least 2 months behind schedule, today we made the first test run with diy tunnel drives and diy turbocharged diesels. Not yet at full speed of course, there were enough troubles at less than half throttle already.

The good things are that the engines make no more noise or vibration when compared with the old Merc's. In fact if they didn't smoke like a woodfire after startup, nobody would suspect there were oil burners under the floor.
Another good thing is that the boat at speeds under 12 kn. tends to go straight ahead, so I don't have to do much steering to keep her on a heading, just compensate a bit for crosswind.
And an engineering experiment I had bad feelings about proved to be an excellent idea: The raw water, after passing through the heat exchanger and the gearbox, is injected IN the exhaust end of the turbocharger that now forms a swirl chamber. An enormous difference in temperature between the turbine and the end where the exhaust hose is clamped, but the drop forged steel housing can take it.

But there are bad things as well. The steering with 4 small rudders at the end of the tunnels is so ineffective that running on one engine is out of the question unless the intention is to drive around in circles.
With 2 engines at the same rpm small corrections are possible with the steering wheel, but for anything else increasing one engine's rpm is the only solution. As the purpose of dual engines is extra safety in case of an engine failure, the rudders must be able to keep the boat at least on course with only one engine running.
On the way back to the harbor the port engine suddenly became very noisy but that was caused by the coupler disk that touched an insufficiently tightened nut and can easily be corrected. The steering problem is much more serious.

[FAST FRED]

The whole tube is filled with SAE 15W40 engine oil, making water intrusion virtually impossible.

Seals leak and in time the Sheen Police may be after you.

There are commercial stern bearing lubricants that are not petroleum based , and are "green"at least in terms of being fined.

FF

[CDK]

The same would apply to all the outboard engines, sterndrives and jets on this planet. I'm sorry, but I have more urgent problems to take care of.
Btw, between the TWO outer seals there is a chamber filled with silicone oil: water repellent, non-toxic, not (quite) petroleum based.

[PetterM]

Very impressive project. :)
Your rudders look a bit small for the speeds you are doing.

[CDK]

Thank you for your admiration.
That the rudders are too small became manifest in the first turn, adequate steering is only possible with engine rpm.
I already ordered some ss plate and shaft material and will make new rudders with 40% more surface. These I can enlarge by welding: the current ones are GRP with an ss insert to attach them to the steering pins.

[CDK]

The new stainless steel rudders are in place. They really make a difference: running on one engine it is now possible to stay on course and even make a port turn with only the port engine engaged. With both engines at the same rpm, steering is sufficient for all situations except cramped spaces; with one engine in reverse the boat can be turned on the spot in any direction.
So far we've made only short tests covering 26 miles and there is still some work to be done on the engines and instrumentation, but it is already clear that the efficiency has vastly improved when compared to the stern drives the boat was originally equipped with. Not 10 or 20% better, but probably over 50% more mpg!

[PetterM]

any pics? :)

[CDK]

I'm sorry, no. I was so focussed on improving the performance that I didn't even pull the boat out but installed the rudders while submerged. A hell of a job because every small piece you drop is lost.
They are just stainless steel plates, corners rounded and a piece of shaft welded at 1/3 of the 12 inch length.

[mydauphin]

The new stainless steel rudders are in place. They really make a difference: running on one engine it is now possible to stay on course and even make a port turn with only the port engine engaged. With both engines at the same rpm, steering is sufficient for all situations except cramped spaces; with one engine in reverse the boat can be turned on the spot in any direction.
So far we've made only short tests covering 26 miles and there is still some work to be done on the engines and instrumentation, but it is already clear that the efficiency has vastly improved when compared to the stern drives the boat was originally equipped with. Not 10 or 20% better, but probably over 50% more mpg!

Did the flanking rudders ever work out? I am build something similar but only one rudder per prop, thinking of putting more traditional in middle. How did reverse work out. Would love to put rudders on side to reduce vibration etc. But I am afraid of low speed handling, your input is appreciate since you have done it.

[tom kane]

Props in tunnels make a boat feel that it is on rail lines and they do not want to go right or left.Any type of rudder gives very little turning effect especially at speed.

[Frosty]

Sony drive owners are now cutting off one side of the tunnel to have one leg like the flexitab drive. I have done this with amazing results with 3 knots on the cruising speed and incredible improvement on cornering.

Apparantly according to my friend Rudy at levi drive this is a well know modification on twin drive vessels.

I can not emphasize just how much of an improvement it is.

[PetterM]

3 knots from the reduced rudder drag? what's your boat speed?

[mydauphin]

Sony drive owners are now cutting off one side of the tunnel to have one leg like the flexitab drive. I have done this with amazing results with 3 knots on the cruising speed and incredible improvement on cornering.

Apparantly according to my friend Rudy at levi drive this is a well know modification on twin drive vessels.

I can not emphasize just how much of an improvement it is.


Do you cut the inner or outer rudder and what is rotation of blades?

[Frosty]

I have a cat with a hydraulic tie bar I had no choice but the cut off the inner side or I would need to change all the hydraulics.

This is opposite to what a mono hull would do to keep rudder in the water when turning.

Rotation is left left--right right

I used to cruise at 17KNT at 3400RPM 4200max.

I can now cruise at 20.2 at 3400.

With the old tunnels I would turn into a corner and would need plenty of rudder and it would pull it down to 13 -14. With the cut offs it bites into the corners and looses little speed.

There is no difference to the feel of the steering or is it heavy, the forces on the one legged rudders are ballanced out.

[FAST FRED]

There are stuffing boxes that use shaft packing as the bearing too.

These usually will have 15 to 20 packing rings , and do require lubrication , usually a turn of a grease cup at the end of a run to stop drips .

With the new graphite style packing no lube is ever required as they run very very cool.

Perhaps an even simpler solution to solve a problem?

FF

[CDK]

Sorry for my slow response time.
We've bought a motorhome and made a 10 day trip trough Europe to check it out.

I've added two pictures of the latest modifications, with a better shape for the tunnel entrance.
Unfortunately one of the rudders has been severely damaged before I could make the first test run to measure the increase in top speed. A violent storm combined with high air pressure pulled more water from the bay than ever before, so the rudder hit a rock that has always been there, but until then with enough water over it. The climate change.....

The rudders I attached last year and made several day trips with them. You cannot compare their performance with stern drives or outboards because the boat is much more reluctant to change its course. With the original Mercruisers, slow cruising meant steering continuously to stay - more or less - on course. Now you can walk around or drink a beer and hardly see a change at the compass when you come back.

Running on one engine, it is possible to make a 360 degrees turn to both port and starboard, but a starboard turn with only the starboard engine has quite a radius. Also, with one engine you can maintain a straight line, so you can get home.
With both engines running you don't need any rudders, just push the throttle of one engine a bit to increase the rpm and pull it back if the desired course is reached. Much easier and more effective than turning the steering wheel.

Reversing with two engines goes perfect, again without touching the steering wheel. Turning the boat around without moving forward and positoning the bow in any direction with one engine fwd and the other in reverse is a piece of cake. Reversing on one engine is not impossible, but the steering response is very slow so you need a lot of space.

I hope this answers your question.

[Frosty]

Convetional stuffing boxes dont work with surface drives ,--not enough submersion in water, How do I know this?

You have to have an oil bath system. Simple to make--a bronze fitting with oil seal in both ends and oil feed from a static resoviour and a cooling outlet from the engine into the stern tube.
Been on 4 years now.

Also stuffing boxes allow very very little movement to accomodate the movement of the engine. Exessive shaft wear will be the result.

PS I also use a special cutlass bearing of 10 inches to accomodate side thrust from the prop. Its all easy stuff if you start right.

Ive never seen a stuffing box with 15 -20 rings,--usually 4-5

[CDK]

Much, much behind schedule because of damage, weather and other matters needing my attention, I finally made some test runs with the improved tunnel entrances.

The good news is that steering response has improved dramatically. Except when mooring in a cramped space - which still goes best by playing with the engine rpm and fwd-reverse- it is not necessary to touch the throttle controls. The 4 small rudders prove very capable to change direction, both on one engine and both.
Also the violently agitated white wake has gone, so the flow in and behind the tunnels is much better now.

But the bad news is, that the speed has gone down from 14 knots during the try out last year to no more than 10 knots now.
The engine rpm at full throttle barely reaches 2800, so the engines cannot produce more than 60 pct of the horses I expected.
This is certainly caused by the better prop efficiency now that there is no more air in the tunnels, but the result is more than a little bit disappointing.

Imho there are 3 ways to improve the situation, and I'll have to make the right decision:

1. Increase the engine torque by modification of the turbo chargers or the waste gate valve. Not good for the engines, but I probably just need it to reach planing speed.

2. Invest in 2 new props with less pitch or a smaller diameter.

3. Somehow introduce a controlled amount of air in the tunnels again.

Any bright ideas are very very welcome!

[CDK]

Baeckmo wrote in post #43 of http://www.boatdesign.net/forums/inboards/changing-pitch-28009.html :

""CDK, you are correct about my writing dia instead of radius when discussing blade adjustment, thanks for the note! As for the wake factor, there are also different engineering cultures; in "my world" an 8% wake means that the propeller is working in a mean velocity field 8% slower than the main flow (=vessel speed). Consequently, when I guess that you have a 30% wake factor, it means that the inflow to the prop is 7 knots when the boat is doing 10 kn.

If we check your prop with a humble 20% wake (my definition....), it would draw about 53 hp and need a blade area ratio of 0,69 in order to keep cavitation within 10 % of the blade surface. In this condition the incoming flow one dia in front of the prop needs a throughflow area equal to over 18" in diameter. A correct propeller tunnel must be adapted to the flow field of the propeller!!

This operating point means an engine torque of 132 Nm (incl. transmission), which I believe is more than you get from the VW engine. The prop is certainly cavitating, otherwise your engine would have topped at a lower rpm.

If you could produce a basic power curve for this engine we could use the balance (propeller power)/(engine power) to find a likely wake factor for your tunnel, but at this stage, it is only of academic interest! (Btw, is this the six cyl engine? What exhaust manifold is used? Std watercooled ones are inferior to car manifolds in terms of power!)

We have built a couple of workboats with "external" tunnels here; I will check a few old files and see if there are any photos for you. If so, I propose we continue the discussion where it belongs; in the "DIY tunnel" thread. OK with you?""

Thanks for explaining the wake factor in detail.

Of course I cannot contradict the fact that the tunnels or in fact shrouds (the bottom is open) have a negative effect on the flow towards the props, but I have no means to do reliable measurements. But they are very functional: with props that close to the surface, tunnels are a necessity. When reversing at idle rpm, the props already create a vortex at 5-8 inches behind the boat, from the surface to the hubs.


The 1th thing I'll do is calibrate the electronic tachometers, they may be a bit optimistic.
I will also record the boat speed at various rpm settings, the way it is now is too much guesswork. With the currently low fuel consumption it is impossible that the engines together generate over 100 hp at half throttle position.

I used VW 1,9TD engines, type ABL. According to the manual for a VW transporter, they produce 55 KW at 3700 rpm and have a torque of 127 Nm between 1700 and 2500 rpm. The same engine in a VW Passat has the same output but 150 Nm torque. Because of the lower exhaust temperature due to the water jacketed manifold, the turbo chargers are probably less efficient at lower rpm.

[baeckmo]

CDK,
Now, in this project you have a very small margin between success and disaster. Please take no offence here, but your good craftsmanship is ruined by lack of basic understanding when it comes to hydrodynamics. Along the design route, you have taken the wrong decisions on what you think are trivial issues, but in fact are crucial. To turn this job reasonably right we have to start from the beginning again. In the process I (and others....?) may occur as bl--dy besserwisser; sorry for that, but it is not intended to hurt anybody! Instead, by showing the logical design progress, we may inspire other fiddlers to avoid the most common mistakes. There are similar problems in a couple of threads at the moment!

Reading through previous notes, I find that "Ad hoc" comes very close to my preliminary estimates regarding performance and what information is needed, so I hope he will fill in where I have wite spots myself!

First we have to get reliable data on engines (=possible thrust) and hull (=possible resistance).

Starting with power source, we have dubious info on the engine; power has been said to be 75 hp @ 3600 rpm, 85 hp @ 4600, there is one VW notation of 75 hp @ 4200 rpm aso.

Here the immediate task for an "Experimental engineer":

Find out, through engine nr´s what is the likely original configuration plus relevant running data (power curve, turbo pressure, exh temp, specific fuel consuption). Then, to find the real operating condition, take readings of inlet (=turbo outlet) pressure, inlet gas temperature and exhaust temp (after or before turbine, depending on how VW present their data) at whatever full load you reach when running free. In addition, run a bollard pull test, using a dynamometer or suitable scale to find static thrust as functon of rpm. Calibrate tacho´s and check the real gear ratio (including decimals). During testing, preferably use a digital light tacho.

With this info at hand, we may continue with the hull.

[CDK]

Baeckmo, I detect some German-like Gründlichkeit in your reply. Not bad, but I'm afraid you overestimate my possibilities a bit. This is a beautiful island with camping places, restaurants and tourist shops. I could get you a T-shirt with any popstar or football-player, but no optical tachometer, exhaust gas thermometer and certainly no dynamometer. The latter exists though, but it is built in a vehicle test stand (like Tüv or MOT) and I don't think they'd lend it to me.
What I could do is try to break a rope tied to the bollard or find a number of overweight tourists and tell'em to pull the boat to the pier while I move off. My mooring lines are 10mm nylon: I can tear them easily with my 4WD, but the boat almost certainly doesn't have that pull.

Because the engines started their automotive life in 1998 and 1999 VW transporters, the output is 55 kW @3700 rpm, so 75 HP. They are equipped with Garrett T3 turbochargers with 0,7-0,8 bar max. boost pressure.

The identical engine, with different injection pump setting was also used in VW passenger cars where it delivered 85 HP@4600 rpm. There is also a well documented tune-up story on the VW Transporter forum from someone who increased the output to 112 HP. For me that is not feasible because it includes an air cooler and another spring in the turbo charger's waste gate to obtain 0,9 bar.

Engine torque is not exactly specified in my workshop manual because they only give data for the non-turbocharged version, 127 Nm between 1700-2500 rpm. Similar data I find in the spec sheets for VW marine and industrial engines of the self-aspiring type; these engines were never turbocharged until the 1.9TDI came on the market, but that is not what I have installed. All diagrams I've seen for the 1.9D engine are very flat between 1000-3300 rpm with just a very slight hump near 1700 rpm.

For the rpm vs boat speed data I need a few more days.

[baeckmo]

Hehe, CDK, no German genes here as far as I am informed......., but yes, when it comes to my professional issues, I may be irritatingly meticulous. Hope you can bear with me.....! And beeing an islander myself, I guess I have to stick to the local socker team and its T-shirts; anything else is regarded illoyal... but thanks for the offer!

Anyway, "dynamometer" also means a scale used in tension. Any fishing vessels around? They often use such a device in a winch hook to weigh their catch. The idea with this is to generate a first reference point on the possible thrust curve. Compared with the calculated thrust at the achieved rpm´s it also gives an idea about the drag caused by your tunnels.

From the original engine setup (with "undented" planing bottom) we can get a reasonable thrust figure (equals hull resistance plus Merc Alfa resistance) for the maximum full power speed. This figure is used to "homing in" some constants in a hull resistance program, so we can get an idea on the actual hull resistance, in particular around te hump speed, with your tunnels.

So for the engine performance you feel confident about it now? Ten year old engines, without a history? There went 10% of your power!! What fuel do you use? "Ecofuel, Envirodiesel, Citydiesel" etc.? If yes, there went another 8%! If reasonably shure, however, (btw a reading of charging pressure would be within your capacity, I guess?) it is then possible to draw a power curve based on generic engines, doublechecking with the info from prop calculations. Now we have the thrust required at various speeds. Remains to reshape your tunnels and match resistance with possible prop/rpm/gearing combinations.

"The head should do the job and the fingers assist; not the other way around....."

[apex1]

Baeckmo
Reading through previous notes, I find that "Ad hoc" comes very close to my preliminary estimates regarding performance and what information is needed, so I hope he will fill in where I have wite spots myself!

I doubt that will happen soon, he is in Singapore at present.

And whats wrong with German Gründlichkeit ? All the world loves our products, but most do´nt accept the way we achieve the quality of them!?;)

[CDK]

Anyway, "dynamometer" also means a scale used in tension. Any fishing vessels around? They often use such a device in a winch hook to weigh their catch. The idea with this is to generate a first reference point on the possible thrust curve. Compared with the calculated thrust at the achieved rpm´s it also gives an idea about the drag caused by your tunnels.

you use? "Ecofuel, Envirodiesel, Citydiesel" etc.? If yes, there went another 8%!



Whenever we buy from a local fisher he asks me if I have a scale....

Picture taken in December early in the morning, warm sea, cold air.

[baeckmo]

Baeckmo


And whats wrong with German Gründlichkeit ? All the world loves our products, but most do´nt accept the way we achieve the quality of them!?;)

Well, Richard, we are sliding a bit offline here, but let me add that I believe that when it comes to engineering, we share much of the culture. I studied math in Mannheim for a short period in late 60-ies, doubling as a prentice in a Konstruktionsbüro. Admittedly, some water has passed under the bridges since, but that period somehow set the standard for me.

[apex1]

Quite obvious Baeckmo, but lets not hijack the thread, I was joking, as Cornelis did.
I hope Ad hoc will chime in again after the weekend.

Regards
Richard

[CDK]

Calibrating the tachometers meant a bit more than just sticking a screwdriver in the hole and turn it. The tacho-signal from the alternator is almost 17 times engine rpm, so modifications were necessary. I posted the circuit on 'on-board electronics & controls' last week.

We made a 40 mile trip in several directions, compared the gps with the Navman log and decided to use the log data data because of the sea currents that were several knots at the time.
With both throttle levers to the stop, the engines reach only 2100 rpm and the speed just touches 10 knots. So the power generated is 55x3700:2100x0.736= 42 hp @ engine.
At 6 knots and 1200 rpm that it 13 hp, which explains the extremely low fuel consumption at that speed.

The speed vs. rpm curve is attached, and so is a drawing of the wake pattern. Photographing is no good because the directions of the flow cannot be seen. There is a superficial flow from the flat bottom parts at port and stbd sides to the center, where it mixes with the outflowing water from the tunnels, creating an uneven surface for several meters behind the transom. This looks a bit funny but I guess it is because the props are near the surface. You see the same with stern drives in a tilted position.

[baeckmo]

Well, confusion is here again..... . Hope you appreciate my Gründlichkeit regarding consistence and accuracy in measuring now. 2100 rpm is quite a bit from 3000! First, you can't calculate power as linearly varying with rpm's. According to a power curve for a dynotest on this engine, you have the following performance for a unit in mint condition:

n=1500, P=25 hp
2000, 39.5
2500, 52
3000, 62.8
3500, 71.5
3700, 73.5

To be realistic regarding engine conditions, with real world installation, fuel temp, exh back pressure and transmission these figures ought to be reduced by some 13 %. This leaves us with 64.2 shaft hp's at 3700 rpm and 36 at 2100 rpm full throttle.

Next, when backing off on throttle, the propeller power curve follows an exponential path; P~(n2/n1)^2.65. The exponent may vary depending on hull resistance and propeller type (for waterjets it is exactly 3.0). This means that your power at 6 knots/1200 rpm is instead about 8.2 hp.

Even so, comparing with the existing data from your boat with Merc's, the achieved efficiency is a disaster due to the turmoil in your tunnels. I'll be back on this issue later.

[CDK]

Well, confusion is here again..... . Hope you appreciate my Gründlichkeit regarding consistence and accuracy in measuring now. 2100 rpm is quite a bit from 3000!

Yes, I already felt embarrassed when I wrote it, no need to rub it in. Of course I should have heard it, but the engines are under deck, lots of foam under the floor and their feet are resting on 6 silent blocks. I've told people numerous times not to trust instruments, particularly digital ones, but I fell in the same trap.

I also know that linear calculation of engine hp is not exact because the torque changes, but in the absence of data it does serve it's purpose. Your 36 hp and my 42 at 2100 rpm are at least in the same order of magnitude.

Also with a rule of thumb, I calculated the slip to be approx. 30% at 600 rpm (prop). Is that really bad?

[PetterM]

Interesting project.
At what speed does the transom cavity develop?


Calibrating the tachometers meant a bit more than just sticking a screwdriver in the hole and turn it. The tacho-signal from the alternator is almost 17 times engine rpm, so modifications were necessary. I posted the circuit on 'on-board electronics & controls' last week.

We made a 40 mile trip in several directions, compared the gps with the Navman log and decided to use the log data data because of the sea currents that were several knots at the time.
With both throttle levers to the stop, the engines reach only 2100 rpm and the speed just touches 10 knots. So the power generated is 55x3700:2100x0.736= 42 hp @ engine.
At 6 knots and 1200 rpm that it 13 hp, which explains the extremely low fuel consumption at that speed.

The speed vs. rpm curve is attached, and so is a drawing of the wake pattern. Photographing is no good because the directions of the flow cannot be seen. There is a superficial flow from the flat bottom parts at port and stbd sides to the center, where it mixes with the outflowing water from the tunnels, creating an uneven surface for several meters behind the transom. This looks a bit funny but I guess it is because the props are near the surface. You see the same with stern drives in a tilted position.

[CDK]

Interesting project.
At what speed does the transom cavity develop?

I guess you meant cavitation.
Frankly, I don't see any but that doesn't mean there is none. I have experienced the phenomenon many times with badly trimmed stern drives, damaged or grown-over props and of course plastic bags. In all of these cases engine rpm rises, but the boat speed doesn't change of even drops.
In this particular case that does not happen, the engine rpm doesn't increase above 2100 at half throttle or any position beyond it.
It feels like climbing a hill with an overweight car without a gear shift lever.

[Ad Hoc]

baeckmo

you're doing very well..no need for me to chime in, except to reiterate what i have said before about the angle and tunnel losses. If that isn't changed first all you'll keep doing is enjoying your own private jacuzzi.

Except to say the shaft should be strain gauged for a more accurate figure.

[baeckmo]

CDK,
Did you use the acoustic method for calibrating your tacho? Still feel a bit concerned about those figures, so until verified by other means, discussion on propeller details is not meaningful. Depending on how the Fourier transform is used, there may be a filter bandwidth that is too wide for correct analysis. Remember that propeller power at a fixed operating point is proportional to rpm^3. A distortion +/- 5 % of frequency thus corresponds to a bandwidth of (1.05/0.95)^3 = 1.35 in power.

Meanwhile, let’s see what we can learn from your data from the Mercs. Generally, for a preliminary idea on IO thrust, you may use the producer’s engine power with a total overall efficiency of abt 0.55 (hydrodynamic losses from lower housing included) to find a rough figure at full throttle (provided engine and propeller matching is ok). With the setup you started from, you had 28 knots (14.4 m/s) from 2 * 135 hp at 4600 rpm. This is reasonable with a weight to power ratio of ~14 kg/hp and single prop outdrives. Each side is then producing a net thrust of 3795 N, (135*736*0.55/14.4). So we have one point on the resistance-versus-speed line. We also have a propeller load factor (Bp = n*P^0.5/Va^2.5) of ~6.6, which is comfortably low to give good efficiency.

In addition, you gave a figure of 22 knots @ 3500 rpm. With the rpm exponent of 2.65 this would give a power (each side) of roughly 65 hp (about what you have in your engines right now), resulting in a thrust of 2340 N each side, giving a second point on the R/V line. Now, these two poins may be used as reference points when the available hull data are analyzed in a hull resistance program; for instance fiddling with weight and longitudinal C of mass until a reasonable fit is found. A quick check on your hull reveals a marked hump resistance, a real ”camel” in the R/V diagram. This is fairly easy to overcome with the trim facility and the fair thrust of the 15” props that are recommended for this speed range. If nothing in the hull shape, weight or weight distribution were changed, those 20 to 22 knots would have been possible with the output of your VW engines.

But with the added drag from your steep tunnel inlets, the hump resistance becomes even higher and the propeller performance is seriously impaired. Using smaller props does not make it easier; the load factor is now into ”tug territory”! The working condition at surface level also means that you need more blade surface to avoid cavitation, even if you were successful in avoiding ventilation (which you are not….).

You can go two ways to find remedy; either you go Frosty’s way with surface piercers or you rebuild the tunnel arrangement (I assume refitting the Alpha’s is not an option). Even if I personally very much like the spp’s, and have good experience from them, I have to advice against them in this application because of the technical risk with the hump resistance of your hull. I would be happy to have Rudi’s opinion on this though, as I know he has Levi drives operating in lowspeed hump range! As a contrary, the steadily rising resistance (no hump) of a slender catamaran makes it ideal for spp’s, but even so, I think Frosty reported a slight sluggishness in midrange (pls correct me if my memory is wrong Frosty…..).

So, if I were in your shoes, I would redo the tunnels, including propshaft outer bearing. Attached as thumbnails (if the Ghost of zeroes and net bugs is friendly to me today) are sketches showing the basic proportions of a tunnel compromise that allows you a retrofit inside the existing ones. The inlet part is extremely important, the ramp (”rooftop”) must connect to the bottom via a soft radius (R>= 2* Dprop) and its inclination to be not more than 15 degrees. (Yes Ad Hoc, waterjet inlets often see 22 to 25 degrees, but that is with a lower surface to guide the flow into the inlet, and they still generate serious velocity gradients). Note the flat ”roof” which effectively increases the aspect ratio of the bend, making for lower losses! The tunnel sides reach down to bottom level, with an outside ”wing portion” that stops aerated water to be sucked around the tunnel side into the propeller inflow. The transverse radius between this surface and tunnel allows smooth transverse flow into the tunnel when turning and during high load operation, when increased flow is called for.

The shroud aft of propeller disc section must be long enough and at the correct angle to prevent air from backflowing into the propeller. I have seen numbers of tunnels with this part parallell to keel and too short; it must be adapted to the outflow streamlines. It’s probably wise to design for a prop dia of 15 in; this will be better for the thrust required. Even so, the Bp will be pretty high, but tolerable. You should cut away the part of your shaft tube reaching into the tunnel; it is a major disturbance and causes an early boundary-layer detachment. Let the shaft run free and use a rubber cutlass bearing, carried in a single strut. It is easy to overlook the rubber bearing, taking it for a trivial chunk of gum, but it is a cleverly designed piece that performs three main tasks. First, wet rubber is about as slippery as it gets, so it’s a good bearing material. Second, rubber has excellent spring/damping qualities; it is very effective in reducing shaft vibrations. Third, in metal hulls it is acting as a barrier to galvanic currents, improving corrosion resistance.

A single ”one legged” strut will work inside the wake from the shaft, in fact often straightening it to some extent. From a fluid point of view this is to be preferred instead of the (mechanically stiffer) V-strut arrangement you have used. In that arrangement you get three wakes in front of your prop. It takes really advanced flow studies to get the V-type in line with the 3D flow, and yours are certainly not adjusted to the local flow!! Just make sure the strut is stiff enough (DNV rules refer to the load carrying capacity of the cutlass bearing for the bending strength) to avoid contact propeller-shroud!


Good Luck!!

[Ad Hoc]

beackmo

nicely put.
Those sketches look "familiar"...you don't happen to work for "that" company do you?? ;)

[Frosty]

Too many things wrong here with this design. You cant work this out with a pencil.

How can you work out or even suggest thrust when you don't know if you have cavitation or part. Hes already suggested 30% slip--

Oh well,-- im a doer not a talk abouter. Next year when your still pontification and coming up with theories the poor guy will be still boatless.

Enjoy the theory!

[baeckmo]

beackmo

nicely put.
Those sketches look "familiar"...you don't happen to work for "that" company do you?? ;)

Hah.. Ad Hoc, guess what you're aiming at, but nope! Been designing AND building (I still hold an EU license for Aluwelding) special work boats, fifi/resque boats, maritime surveyors et c. for over 35 years now. Also "doubling" as a non-affiliated/third party consultant in trouble shooting and technical disputes on pumping and turbine systems, including w-jets.

And Frosty, maybe I dug too deep into CDK's problem, but I felt sorry for the guy having spent so much effort along a dead-end alley. As I have a long experience, both practical and theoretical, from the designing and building of hulls with propeller tunnels I imagined that my input could lead him forwards. For some reason that combination of skills seems to have pissed you but I can live with that. I asked you a polite question and get your pukes back, why?

[PetterM]

...who would that company be then?

[apex1]

Petter, is it really so important for us to know if Baeckmo got his skills with KaMeWa, Ulstein, ABB or any of the many others in the field of high tech propulsion???
His advice is sound and valid imho, and thats what CDK needs.

[baeckmo]

Ouch Petter, didn't know I had to submit CV and customer references to participate here.....

[CDK]

Too many things wrong here with this design. You cant work this out with a pencil.

How can you work out or even suggest thrust when you don't know if you have cavitation or part. Hes already suggested 30% slip--

Oh well,-- im a doer not a talk abouter. Next year when your still pontification and coming up with theories the poor guy will be still boatless.

Enjoy the theory!

That is quite a lot of negative waves, Frosty.
But it isn't all that bad. First of all the boat is in the water, moored 25 meters from my garden gate. Lots of people would kill for that.
We make regular trips to nearby bays and the lighthouse island 6 miles out. The boat has a kitchen, stainless steel fridge, toilet and an enormous (albeit triangular) bed. It takes us anywhere at 6 knots and digests so little fuel that I cannot remember when I last filled the tank. The fuel gauge says 50%, which means approx. 150 liters of red diesel, enough till November when the season ends.
My wife's happy, the dog is happy, and I... Well, I could be happier if the boat reached my 22 knots target, but for now I can live with it. I've had boats for 45 years, from modest 15 ft. sports boats to a 60 knots Glastron Carlson, even the smallest one needed more fuel than this one.

And now about the 30% slip. I asked if that was bad, but nobody said Yes, that is very bad!

This is my calculation; a bit clumsy perhaps because only metric values have meaning for me.

A 14x17" prop advances 43,18 cm for one revolution.
1200 engine rpm is 600 prop rpm is 36000 rph, times 43,18 cm makes 15,55 km/h or 8,4 knots. If the boat were to be propelled by rack and pinion, that would be its speed. But it does only 6 knots, which is 71% of 8,4 so the slip is 29%. Is my line of reasoning correct?

Should the engines have a bit more zest and could reach 3600 rpm, the slip should be near 13% to obtain 22 knots. I may have been a bit optimistic when I started this project.

[Frosty]

propeller slip while still on the hump is irrelevant. Prop slip becomes an issue when at cruising speed or when the boat is up and running.

However at this point prop slip is an issue or clue as the boat isnt working properly.

My post was aimed at the mathematicians that were enjoying the mathematics of the problems pontificating with each other as you were in need of some answers.

But if you are happy with it and can use it, then I am less concerned

[Ad Hoc]

beackmo

"...Ouch Petter, didn't know I had to submit CV and customer references to participate here....."

Sadly those that feel inferior and those that have no formal qualifications with prerequisite training and those that think "we" must be part of their "lets sit around a camp fire and pretend", do!...all very sad.

I get hit with all sorts of criticism because I'm not part of the "smiley happy people" group...jesssssss....very sad and childish. And they wonder why they're not real professional engineers!

I'm sure CDK appreciates the reply, which is all that matters, even if others do not, as it is way above their pay grade to comprehend....

If one doesn't understand the mechanisms (ie theory and scientific mathematics which many seem to loath and hate, for obvious reasons really), one cannot arrive at a proper solution, it just becomes pure guess work and/or iterative trail and error. Wastes time, time = money!

[CDK]

Baeckmo, thank you for spending so much time on my project. I will need some time to digest your various recommendations.

The rpm issue bothers me a bit also.
I downloaded the software bit could not find a mike with a plug that fits my wife's laptop. Instead I used an oscilloscope with the mike and looked for a recurring waveform. From the length in mS (75) I calculated the idle rpm to be a little over 800. Then I looked at the alternator signal, which was approx. 10 mS and modified the tacho circuit for this much shorter than expected length, connected it to the alternator and set the meter to 0.8 on the dial.
To make sure I will try to glue a magnet on the crankshaft pulley and use a pickup coil I didn't know I had. The pulley is the only rotating part I have access to and even that is not as simple as it sounds.

The V-struts I made because with the original blunt tunnel entrance there was a lot of vibration. More rpm and more speed, but I was afraid the welds between stern tube and flange would not hold.
I left the struts in place just because I made them, but with the current tunnel shape they are not really necessary.

Between the alpha-ones and the tunnels I installed Berkeley jets, using the Mercuiser engines. Very poor steering, immense fuel consumption and devastating corrosion made me decide to remove these and sell them on Ebay. But I noted that the few US companies who sell this junk have all kinds of intake scoops for sale to guide more water to the pump. Do you think it is worth the experiment to install something similar under the tunnel entrances?

[Frosty]

beackmo

"...Ouch Petter, didn't know I had to submit CV and customer references to participate here....."

Sadly those that feel inferior and those that have no formal qualifications with prerequisite training and those that think "we" must be part of their "lets sit around a camp fire and pretend", do!...all very sad.

I get hit with all sorts of criticism because I'm not part of the "smiley happy people" group...jesssssss....very sad and childish. And they wonder why they're not real professional engineers!

I'm sure CDK appreciates the reply, which is all that matters, even if others do not, as it is way above their pay grade to comprehend....

If one doesn't understand the mechanisms (ie theory and scientific mathematics which many seem to loath and hate, for obvious reasons really), one cannot arrive at a proper solution, it just becomes pure guess work and/or iterative trail and error. Wastes time, time = money!

Hmm not quite sure who you are talking to but now you've got that off your chest can you tell us all whats wrong with CDK's boat and what he should do or do you have any other things to say to mere mortals that dare talk with you.

Mathematics can tell you how to build and close on the results of a new build, even where the water line will be, but you cant work out a mistake.


You can work out what he should have and how it should be and what the result should be but,--you cant tell him what to do from here with a pencil.

Or can you?

[PetterM]

Ouch Petter, didn't know I had to submit CV and customer references to participate here.....

Hehe, sorry, that was not what I meant, I was only curious who the company was, kind of recognize the tunnel design. Just wondering if I was right.
Good that you have taken the time to offer well thought through advise.

[baeckmo]

Hmm

You can work out what he should have and how it should be and what the result should be but,--you cant tell him what to do from here with a pencil.

Or can you?

Watch me, and you will be surprized what may be achieved with the help of daddy's ol' pencil, provided one knows which end to point to the paper....

[baeckmo]

And CDK, your project is a stimulating challenge; it contains ingredients from various disciplines. I take it that you have some useful know how about electronic devices to share, that will come handy sooner or later so don't worry about my time so far.

I am sketching on a possible working schedule for the alterations (they are not very complicated, compared to what you have done so far), so please have some patience before you reach for the big cutter!

Additional scoops would only add to the already high drag, so nono! The medicine for you is still a correct inlet "rooftop"! As for the bearing struts, I agree that the double strut is preferred in terms of mechanical strength, but it should be oriented with one leg hiding in the shaft wake and number two more or less at right angle to the first. In a cavitating environment, some positive prerotation (=same as prop) of the incoming flow will improve the cavitation performance of the propeller. Thus it is not trivial which side the second leg is mounted. It must be fitted to the side where the blade is moving downwards, and also be given some camber. I'll show you how.

[tom kane]

If you are going to run a propeller close to the water surface you should expect that you are going to run your prop in surface piercing mode if you want a good speed and will have to design your drive accordingly.Tunnel drive and jets are surface drives.Hickman got good speed from his design which would have been better with modern surface-piercing props.The tunnel in the picture was used in race boats (60 MPH +) and did not like turning.You can not expect water to change direction abruptly what ever the design is unless it is in a cylinder which would increase drag.Hickmans drive would have been effected by interference from the side rudders when turning the same as a tunnel does when turning.The tunnel in the picture has half a venturi built into the top half which did improve the drive.

[Ad Hoc]

baeckmo

"....And CDK, your project is a stimulating challenge; it contains ingredients from various disciplines..."

Indeed, fully concur and and "interesting" problem to solve, especially with more than one liners!

I agree a single leg bracket/strut is best, for all said reasons. Establishing some basic loads from an out of balance prop, by losing one blade, this is a relative simple load case to ascertain the strength requirements; I've used this load case many times on single P-bracket arrangements with success all to Class, and/or as using a Class based type as you've noted.

I agree, the 15 degree would be better - I was just noting (for ref) that w/j's use the 20~22.5 angle to minimise space requirements and hence didn't really want to add too much potential costs to the solution, poor bloke is probably spending heaps of money.

"...but I was afraid the welds between stern tube and flange would not hold..." Yes, this detail is critical and requires more than just a cursory glance. The whole structural arrangement in terms of load paths and localised stiffness is far more important. Not to mention the detailing of the joint and alloy grade and temper. You need to ensure you can get a full pen weld.

Frosty
Look at the previous posts by me and beackmo, ie one liners wont solve this, it is like any design issue, it is iterative.

[tom kane]

Enclosed picture is a S/P prop on an (American) displacement hull. Surface drive replaced two Volva inboard-outboard drives which gave low speed. One S/P drive gave big improvement in speed and economy.

[Frosty]

Thats not a surface prop Tom. If it was the blades would be concave looking from the back, cupped.

Thats more of an equipoise.

[tom kane]

Acording to PAUL KAMEN N.A. What is a surface-piercing propeller? Simply stated,a surface-piercing propeller (or surface propeller) is a propeller that is positioned so that when the vessel is underway the waterline passes right through the propeller hub.This is usually accomplished by extending the shaft out through the transom of the vessel,and locating the propeller some distance aft of the transom in the relatively flat water surface that flows out from the transom bottom edge.etc.....My comment.The design of a surface-piercing propeller is difficult to define.Any propeller can work in surface-piercing mode.You could say that Hickmans propellers are not surface-piercing propellers.

[Frosty]

Thats a description of its duty not of what the prop is.

Read further down paragraph starts with "What distinguishes a surface propeller from an underwater design".

I would have thought you knew that Tom.

Here it is,--- http://www.well.com/~pk/SPAprofboat.html

[tom kane]

Nearly all succesful designs have moderate to heavy trailing edge cupping..what would you call Hickmans props as in the photo.They seemed to be successful props for his purpose.

[Frosty]

Its an equipoise propeller,---Very successful and popular 40 years ago. Cupping is something you can do after and generally tames a torquey engine or its what the prop man would do if you took it back to him and said your RPM was high.

[tom kane]

Thanks Frosty,I will have to asume that the American surface propulsion company who built this drive did not know what they were doing when they discribed the prop as being suitable for their surface drive propulsion unit.It is strange that the magazine editor did not know.I suppose at a quick glance it has been missed.

[Frosty]

The propeller in the posted picture does not cover the criteria of your surface drive guru Paul Kamen does it?

Now if you have the right picture or not I don't know or do I know what that is.

You described the picture as a surface prop on a displacement boat.????

I have no idea where you are going or what you are talking about

[baeckmo]

What you see in Tom's photo is a transcavitating propeller, probably Newton-Rader series, which is the section to select when there is a risk for air ingress into the flow. With varying degrees of cup it also serves fine in surface piercing operation at moderate speeds, where pure cleavers are overkill. Arneson drives will be found with similar props from Rolla. Vosper Thornycroft have used very similar ones on their smaller SES ships (HM series). Correct cavity size was ensured by introducing ventilating air through the bearing struts.

For those who might wish to go beyond speculation, test data under sp conditions will be found in "Fast-91, Methodical Series Test Results" reported by Rose and Kruppa. Further info on the VT versions of Newton-Rader props at various working depths is found in "Schiff und Hafen", 5/1973: "Modellversuche mit Schiffspropellern an der Wasseroberfläche" by H Brandt.

Ventilated propeller operation is not restricted to the situation with the hub at surface level, but can be applied even with fully submerged propellers, depending on the specific design goal.

[apex1]

There´s a additional paper by Brandt / Maksoud reffering to the farfield effect. 3D test were done at Ad Hocs main playground.
see:
http://www.fsopt.com/downloads/1995_5-1bericht.pdf
in German, but I guess Cornelis will understand that.

Regards
Richard

[CDK]

If you are going to run a propeller close to the water surface you should expect that you are going to run your prop in surface piercing mode if you want a good speed and will have to design your drive accordingly.Tunnel drive and jets are surface drives.Hickman got good speed from his design which would have been better with modern surface-piercing props.The tunnel in the picture was used in race boats (60 MPH +) and did not like turning.You can not expect water to change direction abruptly what ever the design is unless it is in a cylinder which would increase drag.Hickmans drive would have been effected by interference from the side rudders when turning the same as a tunnel does when turning.The tunnel in the picture has half a venturi built into the top half which did improve the drive.

That tunnel resembles mine, but I used a longer tube so the prop is located near the end. When I first presented my design with an oil filled stern tube I met a lot of friction on the forum. Now I see that I am not the only one on this planet, it feels good!

[CDK]

There´s a additional paper by Brandt / Maksoud reffering to the farfield effect. 3D test were done at Ad Hocs main playground.
see:
http://www.fsopt.com/downloads/1995_5-1bericht.pdf
in German, but I guess Cornelis will understand that.

Regards
Richard

Yes Apex1, I do understand the language, but I need a bit of time staring at the math. Haven't seen diff's and integrals for quite some time, but I'll catch up.

[apex1]

Yes Apex1, I do understand the language, but I need a bit of time staring at the math. Haven't seen diff's and integrals for quite some time, but I'll catch up.

It´s not that important Cornelis, Being focussed on Bäckmo´s calc.s is leading to a solution (at least we all hope that). Though sometimes it´s nice to know the theory behind the mystic machines.;)

Richard

[CDK]

Baeckmo, double checked and triple checked the rpm gauge.
2100 rpm is 2100-2150, dependent on the viewing angle, sunglasses and the number of drinks. At sea, both engines have the same rpm at the same throttle setting, in neutral the port engine seems a bit lazy, i.e. needs the throttle a bit more advanced to obtain the same rpm. It also runs more uneven at idle, so I'm gonna take that one out in the autumn. Both reach 4000 rpm without load.

[baeckmo]

.....50 rpm variation, seems a rather constant alco concentration!! Fine, then we can proceed without worrying about that. And high idle 4000 seems ok for a 3700 rpm powermax. Btw, what is the inner radius of the existing tunnels?

Attached please find an example on tunnel arrangement in a crew trsp vessel I designed and built some years ago. That specific tunnel was built to replace a waterjet that didn't meet thrust specifications due to restricted inlet area (classic!!!!). Power is 240 hp VM diesel @ 3600rpm. Max operating speed 31 knots. We had to keep existing 1:1 gear ratio, necessitating a supercavitating propeller within a nozzle to control cavity stability at lower speeds.

This tunnel is deeper than yours, and rooftop inclination higher due to design restrictions. These drawbacks could be handled because we had exact data on thrust over the operating envelope. A sister vessel had been equipped with a Volvo single prop drive leg for testing purposes. With a precision manometer in the pressure line to the trim cylinder, we recorded thrust at all relevant operating conditions (manometer calibrated against dynamometer at bollard pull). Finally, "real world" data were compared to our calc:s.

Anyway, you can see the looong soft inlet curvature, the side wings (horizontal in this case), and the radius between wings and tunnel inlet, that I suggest you adopt as well.

[CDK]

.....50 rpm variation, seems a rather constant alco concentration!! Fine, then we can proceed without worrying about that. And high idle 4000 seems ok for a 3700 rpm powermax. Btw, what is the inner radius of the existing tunnels?



The attached drawing I made for myself last year when I decided to change the tunnel entrance. It is not according to DIN, but it shows the dimensions and the warped ceiling I made.

[baeckmo]

Mmmm, it occurs to me that you, Apex1, Ad Hoc and others, may contitute an acceptance committee, checking out the competence of newcomers! That might make sense to the figures you come up with...... because you are saying that a 14 in dia propeller, rotating 1050 rpm would absorb abt 36 hp! NO WAY!!!! Not even with the stem to the quayside. Possibly if working in quicksilver.....or dragged backwards! If your gearing was something like 1.2 to 1 it might happen, but with the 2.0 to 1 that you said, nope!

So please, I asked you before: what transmission are you using, can you verify the gearing (note: can be different FW/Reverse)? Is it hydraulic, if so is the servopressure correct, ie no clutch slip? Is the manouvering valve coming all way to full position? Tape magnet (good ones found as closing strips on refrigerator doors; pay a visit to your neighbour)to the shaft. With electric coil as pickup (guess you have better fixes though), compare shaft frequency to net freq. via your oscilloscope if necessary.

And: can you check exhaust back pressure after the turbine (use transparent plastic tubing to make a waterfilled U-tube manometer. Make connection with more heat resistant material). What does your exhaust system look like (Wet/dry, sound dampers, dimensions etc)?

[CDK]

So please, I asked you before: what transmission are you using, can you verify the gearing (note: can be different FW/Reverse)? Is it hydraulic, if so is the servopressure correct, ie no clutch slip? Is the manouvering valve coming all way to full position? Tape magnet (good ones found as closing strips on refrigerator doors; pay a visit to your neighbour)to the shaft. With electric coil as pickup (guess you have better fixes though), compare shaft frequency to net freq. via your oscilloscope if necessary.

And: can you check exhaust back pressure after the turbine (use transparent plastic tubing to make a waterfilled U-tube manometer. Make connection with more heat resistant material). What does your exhaust system look like (Wet/dry, sound dampers, dimensions etc)?

Gearboxes: Twindisc Technodrive, 2.00:1.00, mechanically operated double cone clutches.

Exhaust: tangential water injection with 8 mm nozzle in the lower end of the turbo-charger body, 150 mm rubber hose, dia 85 mm internal,to a stainless steel jar, axial input 81 mm at the top, radial output 65 mm dia. at the bottom, approx. 70 cm rubber hose (internal 68 mm) to the 65mm transom feed-through, approx. 120 mm higher than the bottom of the jar. No valves, knees or other obstructions.

[baeckmo]

Check that the wastegates are not stuck in open position; pretty common trouble! Connect manometer (preferrably ~0-2.5 bar) via T-branch to the pressure-control tube and run full load. Any pressure from compressor?

Is that a watercooled intercooler to be seen on top of inlet manifold (if yes, seawater or engine coolant inside?) or is it a water/water heat exchanger?

The jar you mention, is there one each engine, or is it one common? Anyway, the 65 mm outlet seems rather small to me, remember there is a considerable volume of steam competing with the exhausts for flow area! The exhaust turbine is extremely sensitive to back pressure; you should use the 85 mm inner dia all the way through!!! Many turboblown marine engines in the 80-100 hp size use 100 mm here. My suggestion here: bypass the jars and go directly to the transom outlet with 85 mm hose for testing; work on one engine only untill we find the snag.

Max acceptable exhaust back pressure at full load and rpm is ~300 mm H2O, ie 1/3 of what the same engine tolerates when naturally aspirated. With your 2100 rpms you should have just a few cm:s of backpressure.

[Ad Hoc]

baeckmo

"...Mmmm, it occurs to me that you, Apex1, Ad Hoc and others, may contitute an acceptance committee,.."

Hmmm, I'm not sure where you get that impression, but nothing could be further from the truth. If anything, I'm always on the receiving end of the "i have a computer program, so you can't tell me anything, even though I've never designed a boat before" and the "I've been on this website longer than you, gotta be part of the club type etc etc"..jesssss, get real. I'm sorry you feel that way from me at least, non intended at all. I think you're making an excellent summary, far better than i could. Especially since it does appear you do this type of investigation far more frequently than I do. I only did this type of investagtion on sea trails, about 2~3 times a year. So your diagnostic skills on this are more current.

Trouble is, as you can tell from this thread, small amounts of info are given, bit by bit, which makes it very difficult to ascertain the true problem, other than proved "guesstimates", which are all subject to large errors. And as already noted, the problems do not rest in one discipline too.

The fact you are asking, in some ways, the same questions you did at the beginning, even now, highlights this. Without the answers to the questions, it is difficult to give anywhere near an exact answer.

[apex1]

baeckmo

"...Mmmm, it occurs to me that you, Apex1, Ad Hoc and others, may contitute an acceptance committee,.."

Hmmm, I'm not sure where you get that impression, but nothing could be further from the truth. If anything, I'm always on the receiving end of the "i have a computer program, so you can't tell me anything, even though I've never designed a boat before" and the "I've been on this website longer than you, gotta be part of the club type etc etc"..jesssss, get real. I'm sorry you feel that way from me at least, non intended at all. em, other than proved "guesstimates", which are all subject to large errors. And as already noted, the problems do not rest in one discipline too.



May I please just sign that! to make it short...........
Richard

[CDK]

There is a small plugged tube at the turbo charger's output. When I remove that, there is quite a lot of air escaping, but my manometers are 0-10 bar and higher. I will order a boost pressure gauge 0-1,2 bar. The waste gates are OK, I disassembled and cleaned them at construction time. Since then, the engines made only 20 hours.

On top of the manifold there are two rubber hoses with an adapter for the difference in diameter. The other stainless steel item is the air intake: just a hollow cylinder with mesh wire at the end to keep the mice out, and a 20mm rubber hose near the end for the crank-case ventilation. There is no intercooler.

Each engine has the jar mounted under the turbine. Because the water is tangentially injected at the lower part of the turbine, it swirls around the inside of this jar cooling it very effectively. I've used certified exhaust hose with 80 mm outside diameter because that seems to be very common here. With 85 mm inner dia I would need also larger transom exhausts. I'll see what is available.

[baeckmo]

Don't worry Ad Hoc, I was joking; I do feel at home here with you guys! Question is if CDK will stand the "Breakdown investigators manifesto" much longer;

1) Never trust anybody's witness; they all cover their asses and mistakes.
2) Never trust anybody's calculations; they are all wrong since they are all based on trillion color printouts from buggy computers.
3) Never believe in anybody's measurements; instruments were always uncalibrated and applied at the wrong place.

CDK, the Bosch VE rotary injection pumps normally used with the VW engines have a manifold pressure compensation device that adjusts fuel flow according to the prevailing manifold pressure. It is a diaphragm thing on top of the pump housing. Check that the connection to the manifold is intact. To check if it operates, connect to a bicycle pump and pressurize slightly.

While you are at the pump side, is the surplus fuel return to the tank operating without back pressure (ev. valves open?). The injecton timing is working on pump housing pressure; any back pressure will upset timing.

These are car engines right? Are they complete with fuel feed pumps, or were they depending on electric "tictac pumps" that were forgotten when you took delivery? The VE pumps must have a feed pump!

Those turbine housings will radiate an awful lot of heat into your engine compartment. Do you know the breathing air temperature? Are fuel lines at risk, generating vapour locks into injection pumps?

[Ad Hoc]

baeckmo

Your 1~3, sounds like the advice given to me by my old chief designer when i was a newbbie. When i presented my "findings", he would always ask have YOU witnessed it or have YOU calculated it, really emphasing the YOU and not ANYONE ESLE!

I think CDKs patience is grand....but the alternative is to pay a professional to go there and sort it out...so in comparison, I cannot image he can complain too much at the endless questions by "us" for free advice/suggestions.

Ive no need to chip in since you're making all the right noises :)

Long road ahead though CDK...hope you're in it for the long haul..

[daiquiri]

CDK, the Bosch VE rotary injection pumps normally used with the VW engines have a manifold pressure compensation device that adjusts fuel flow according to the prevailing manifold pressure. It is a diaphragm thing on top of the pump housing. Check that the connection to the manifold is intact. To check if it operates, connect to a bicycle pump and pressurize slightly.
While you are at the pump side, is the surplus fuel return to the tank operating without back pressure (ev. valves open?). The injecton timing is working on pump housing pressure; any back pressure will upset timing.
These are car engines right? Are they complete with fuel feed pumps, or were they depending on electric "tictac pumps" that were forgotten when you took delivery? The VE pumps must have a feed pump!

I just love to read this kind of posts. No frills here.
I feel that I could stand for hours behind your back, watching what you're doing, learning... :)

[apex1]

I just love to read this kind of posts. No frills here.
I feel that I could stand for hours behind your back, watching what you're doing, learning... :)

Jepp, it´s a joy. No bantering and no "show me your pictures to proof you´re alive" crap around. And although the topic is too much above my head (was that the right phrase?), to contribute seriously, I enjoy being at least a passive part of it.

[Ad Hoc]

"...I feel that I could stand for hours behind your back, watching what you're doing, learning... :)

Ditto..

[CDK]

CDK, the Bosch VE rotary injection pumps normally used with the VW engines have a manifold pressure compensation device that adjusts fuel flow according to the prevailing manifold pressure. It is a diaphragm thing on top of the pump housing. Check that the connection to the manifold is intact. To check if it operates, connect to a bicycle pump and pressurize slightly.

While you are at the pump side, is the surplus fuel return to the tank operating without back pressure (ev. valves open?). The injecton timing is working on pump housing pressure; any back pressure will upset timing.

These are car engines right? Are they complete with fuel feed pumps, or were they depending on electric "tictac pumps" that were forgotten when you took delivery? The VE pumps must have a feed pump!

Those turbine housings will radiate an awful lot of heat into your engine compartment. Do you know the breathing air temperature? Are fuel lines at risk, generating vapour locks into injection pumps?

No there is no diaphragm thing on the pump, nor is there a feed pump.
On a turbine charged diesel with crude mechanical injection, the intake pressure is known so there is no need to adjust the injection pump for variations. Besides, VW offered their transporter Van with the same engine without turbine and only 10 hp less, so the engine is not pushed to its limit. The turbo version was there for buyers who wanted to pay a bit more. VW used the opportunity to install a very simplistic intake plenum where the air has to conquer 5 square angles instead of the much more streamlined one for the naturally aspiring engine.

The feed pump is part of the injection pump. In the car they used a small thermostatic valve on top of the filter housing. At low temperatures the return fuel didn't go all the way back to the tank but was allowed in the filter again. I made return connections on the top of the fuel tank, which is very nearby and used a short piece of 4 mm copper tube to create a restriction that prevents the fuel from siphoning back to the tank. VW uses something similar, made from plastic.

With the water jacketed manifold and the peculiar water injection in the turbine housings they do not radiate much heat, several 100 degrees less than the turbine on my Kia Sorento. I will try to measure it for you.

Daiquiri: >>I just love to read this kind of posts. No frills here.
I feel that I could stand for hours behind your back, watching what you're doing, learning... >>

Thank you for admiring us so openly. Some rep. points perhaps?

[baeckmo]

Sorry, CDK, a quick correction on the pump theme. In a naturally aspirated diesel the induced massflow of air is varying practically linearly with rpm,s as long as the atmospheric pressure is constant. But with the turbocharged engine, the air massflow is varying with rpms AND load (=fuel flow).

Imagine rolling at constant speed on the highway, level and flat. Engine power is constant, so is fuel flow and turbo-rpms (=inlet pressure). Enter an uphill. To keep speed constant you increase fuel flow.

The air flow in the NA engine is the same as before, due to rpm dependance, but the fuel/air ratio is increased.

In the turbo-engine, the increased fuel flow generates increased exhaust flow, the turbine is speeding up and the air flow is increasing until a balance is reached. Fuel/air ratio is kept more or less constant all the way!

If there were no sensor for inlet pressure, a defect turbounit would send massive smoke as soon as a load was applied. This really makes me wonder what engines you are stuck with!

Also, the internal feed pump is there to maintain a controlled housing pressure. It is not designed to feed the engine in a boat.

Please describe your fuel system a bit more in detail. Are there any common parts (filters, rubber hoses, whatever) or are the systems completely separated all the way? Have you checked the tank outlet for dirt/bacterial gel or other debris?

[baeckmo]

Anyway, the pressure compensator (also called "smoke limiter") is not there, and that´s it. It is not as crucial in a marine engine as in a road vehicle, since the propeller load has about the same power characteristics as the charger unit, so we can get the same power when everything else is ok.

One problem with the system without a separate feed pump is air leaks. Since it is ten times as difficult to make pipe connections tight against air leaking into a sub-atmospheric system pressure, than stopping fuel to leak out at overpressure, you are asking for trouble in the tiny internal priming pump.

So, next exercise for you: Disconnect the fuel delivery tube at the pump. Install 20 cm transparent, fuel resistent tubing in between, with a clean, white paper behind. Run (both) engines under full load and watch carefully for any trace of gas bubbles in the incoming fuel.

And I just have to ask the obvious: is your throttle control really pulling the control lever to full flow position, and if there is a mechanical stop; does it really go all the way back in the "run" position? If there is just a solenoid; does it really get its full 12 Volts to open fully?

[CDK]

Anyway, the pressure compensator (also called "smoke limiter") is not there, and that´s it. It is not as crucial in a marine engine as in a road vehicle, since the propeller load has about the same power characteristics as the charger unit, so we can get the same power when everything else is ok.

One problem with the system without a separate feed pump is air leaks. Since it is ten times as difficult to make pipe connections tight against air leaking into a sub-atmospheric system pressure, than stopping fuel to leak out at overpressure, you are asking for trouble in the tiny internal priming pump.

So, next exercise for you: Disconnect the fuel delivery tube at the pump. Install 20 cm transparent, fuel resistent tubing in between, with a clean, white paper behind. Run (both) engines under full load and watch carefully for any trace of gas bubbles in the incoming fuel.

And I just have to ask the obvious: is your throttle control really pulling the control lever to full flow position, and if there is a mechanical stop; does it really go all the way back in the "run" position? If there is just a solenoid; does it really get its full 12 Volts to open fully?

At or below 50 hp @ liter, the amount of fuel injected is substantially lower than the available airmass can handle, so there is no need for an aneroid. The VE pump is a marvelous piece of mechanical engineering that - with just a handful of springs, rollers and a plunger - does the right thing at the right moment. The same engine in a VW Golf-3 or Passat is labeled AAZ. It generates more power, has a stronger spring in the waste gate actuator for more pressure at higher rpm; there they installed an aneroid to prevent a fuel overdose.
And yes, a faulty turbo charger manifests itself clearly in the rear mirror.

Internal pressure is of little concern in the VE pump. The submerged plunger that meters the amount to be injected takes always the same volume, both at idle and at full power. A sleeve around it has a waste port and can slide back and forth whenever the governor requires that.
In the VW transporter, the pump draws fuel from the tank near the rear wheels and under all driving conditions keeps the pump body filled.

In my setup the only common part is the 300 ltr fuel tank, located directly behind the bulkhead. There are two fuel lines, filters and water separators screwed against the bulkhead and very short rubber hoses connect them to the engines. The fuel returns are also separate.

The tank is clean, it was used for gasoline in the past.
The fuel lines are transparent, so show any air that passes. None does!
The actuators that control the pump setting are (of course) not simple solenoids but electronically controlled motors with spindles, limit switches and potmeter feedback. They release all tension at idle and can reach more than 95% of "full throttle". I left a few mm of travel unused to prevent the motors from drawing current continuously.

[baeckmo]

Well, now is the time to make a summing up of round one. So far we seem to have produced a decent amusement factor for our friends (which is not bad per se) but no solution to your problem. The tacho calibration issue may be temporarily settled. The high idle of 4000 rpm (~8 % over max power rpm’s) is what convinced me that your readings are in the right region, if not with the best of accuracies.

So, we have a propeller working at 1050 rpm. With available info on the operating point and propeller data, the absorbed power might be up to abt 19 hp (make it 20 with transmission losses) in the worst of cases. At this speed, your engines should produce at least 36 hp according to available information, which leads to conclusion no one: both engines are at least 45 % low on power.

If it had been only on one side, it could have been the occational sh-t, but it is on both, which makes it more likely that it is caused by ”the last hand on the bolt”. Then, what makes a diesel run; enough fuel to produce the reqired power, enough air to burn that fuel and big enough holes to get rid of combustion gases. Everything else is a question of life span. First round survey of the installation revealed ”too small holes for the residues”, but no complaints on air inlet conditions (”mice are absent…”). If air inlet flow had been low, you would most certainly have reported black smoke, but you haven’t! Now, with the reduced power, the outlet should not be too critical and the chargers are not yet really alive, which leaves us with our noses pointing to the injection pumps and fuel system (”not enough fuel to produce………”).

On your pic’s the details of the pump and its connections are not quite visible, but I suggest we concentrate our efforts there. I take it that any immobilization paraphernalia (if originally present) has been removed without destruction of anything essential. As previously noted, the housing pressure is used for injection timing control (please don’t argue on that!). The return connection (center bolt in banjo conn.) has means for controlling this pressure (varying from calibrated nozzle to spring loaded non-return valve). Are all the original connection parts in place?

Also, I have a vague idea of having seen some VW diesels with Lucas/CAV rotary injection pump, could this be the case here?? If so, I think Lucas sometimes use additional electrical inputs for cold starting and timing control. There may also be a fuel flow limiter, reducing injection quantity until engine has reached normal operating temp. Questions: are there any unused (as for now) electrical connections on the pumps, looking like the fuel sol. connections? Can you verify the pump supplier and part number? Is there anything else that might indicate something sensing engine coolant temp? Any extra, unused temp sensors in the engine water jacket that might be there to ”report” to the injection pump?

A couple of years ago we had a similar problem with a brand new VM diesel pulling a waterjet. With fairly exact data on the jet, we knew the engine was some 35 % low on power. It turned out that these engines, although ok for the purpose, are in fact ”overcooled” for our climate (big, mediterranian duty oil coolers). The pump was electronically controlled rotary Bosch, and the temp sensor reported some two degrees C too low engine temp, which caused control circuits to limit fuel flow. Any associations of thoughts?

[CDK]

As previously noted, the housing pressure is used for injection timing control (please don’t argue on that!).


Don't you think that is a bit condescending? If you don't want me to argue, I'll leave it for what it is, but please try to remember I studied mechanical engineering and they even gave me a very official document to confirm that.

There was an immobilizer circuit on the pump which communicated with a coil around the key lock in a very complicated fashion. All it did was keep the fuel valve from opening if the proper key wasn't present. I dissected the black box and re-installed just the solenoid that operates a needle valve.

If there was a Lucas pump installed, don't you think I would have told you?
Both pumps have their original banjo bolts, the one with the restrictor on the return side. The fuel lines are short and of adequate dimensions and so are the Volvo Penta filters/ water separators.

At the beginning of this discussion, I mentioned that the turbo charger behaviour may have changed because I use water jacketed exhausts.
Although I wasn't able to retrieve any factory data, I remember that forums like the dieselschrauber-community mention full turbine pressure at 1600 or 1800 rpm, which means that the waste gate should start to open at that point.
My theory is, that with 90 C. water circulating a light alloy manifold, the exhaust temperature will be considerably lower than with a cast iron one under a heat shield. At higher gas velocities the difference will decrease, but around 2000 rpm there certainly is less gas volume to turn the turbine and there will be less intake air. Not to the point of black smoke forming, but still noticeable as a reduced performance. The manometer will provide the answer.

[CDK]

Thinking about my own last post I must admit it is not as strong a case as I thought when writing it....

[baeckmo]

Ouch, I tried to avoid rubbing your nose with the housing pressure issue, but you really want it don't you? Thought it was waste of my time, because the info is available netwise, but I can scan the information from the "Bosch VE information page" if you really insist......

And, in fact I cannot find that you have mentioned the pump type before at all; it was my own automatic first assumption that there were only Bosch pumps in VW engines. Anyway, the VE pumps also have a variety of temperature compensating bits and pieces. For instance there are pumps with a small watercirquit for engine coolant, that acts on injection settings, depending on engine temperature. So my questions about surplus temp sensors et c are still relevant.

Of course you have a point on the reduced xh temperature, but the turbine power is a product of mass flow times enthalpy difference, and the massflow is still there. The temperature of the bulk of gas is not changed dramatically during its short residence time in the manifold. Your turbine housing (the "spiral" housing) is NOT watercooled on your engines and you might add some heat insulation around it to keep gas temperature up, but the effect is marginal. Instead, I still believe that the xh back pressure is too high; it remains an open question until measured. But again: it should not be critical at the low rpm end.

Having full turbopressure at 1800 rpms is b------t! Possible in a dualturbo installation but not here.

[CDK]

The pump type was a matter of prior knowledge. I wrote all about it in another thread:http://www.boatdesign.net/forums/diy-marinizing/marinizing-vw-turbodiesel-engines-19118.html , but of course you couldn't know that.

Do not spend time searching for the housing pressure issue. I've read lots of texts about these pumps, the most detailed ones are in German. I also looked inside such a pump and decided to leave mine untouched as long as possible because at re-assembly too much can go wrong. And in the gray past I briefly worked at Robert Bosch KG in Amsterdam investigating diesel fuel system malfunctions. But back then they only made inline pumps.

There is a slight overpressure in the pump housing to prevent vacuum forming around the fast moving and rotating plunger. Early models suffered from decreased output volume near max rpm, so they resolved that with an insert in the banjo bolt or a washer if there were axial connectors. If I remember correctly there should be a minimum of 100 mm H2O, but that may not apply to all pumps.
You are correct, there are many different types with or without appendages and even the optically identical ones have different springs.

But the pumps on ABL diesels are very basic. There was this immobilizer with two wires, one for 12V the other for serial data from the key lock sensor. No further electrical connections, no vacuum, just the "throttle lever" and a bowden cable for the starting device that advances the injection and increases the minimal fuel amount so idle is slightly over 1000 rpm.
I ignored that last one because I thought it would be required for low temps only, but that was a mistake: starting without the cable pulled results in poor idling with lots of smoke from unburnt fuel leaving the exhaust. That stops as soon as the engines are loaded, but when idling you can make a blue smoke curtain with the sharp smell of unburnt fuel, so there probably is still injection after the exhaust valves open. I solved that with a linear motor and a switch on the instrument panel.

In the above thread I also gave some details about how I inject cooling water in the turbine housing and waited with bated breath for the cracks to appear, but they didn't.

"Having full turbopressure at 1800 rpms is b------t! Possible in a dualturbo installation but not here."

Take a turbine designed for 0,9 bar at 3000 rpm and replace the waste gate spring with one that opens at 0,5 bar. Remember that the engine output difference between SAP and turbocharged is only 5 kW (1996/97) or 10 kW (1998/99).
The manometer hasn't arrived yet from Germany.

[baeckmo]

For your info on the VE pumps, I dug down into documentation sediments since I worked (long ago) as a r&d engineer with the green diesels, arranging fuel mapping and timing optimizing among other tasks.

In the attached document you find the function of the injection timing device and how it works, using housing pressure as a rpm signal. You also find the recommended supply pressure range.

I also compiled a diagram, showing typical air inlet pressures (expressed as a percentage of the respective max pressure) for two diesels, operating in the same rpm range as the VW. The Volvo TMD31 is a 4 cyl. without wastegate and intercooler, with total volume of 2.4 l. Power is 100 hp (ie 42 hp/l.). The VM is a 6 cyl 4.2 l. engine with intercooler and wastegate (60 hp/l). It has a 4 cyl brother with exactly the same mean brake effective pressure. There is no such thing as full turbo-pressure at less than half rated rpm's in a standard application! Since I am not allowed to disclose the VM figures in absolute terms, I made the comparison nondimensional regarding pressure levels.

Your VW will have a turbocharacteristics similar to, but slightly lower than the VM. It would probably have a relative pressure of 0.4 to 0.45 of pmax (ie ~0.35 to 0.4 b.) at 2100 rpm. The "waste-gate engines" show a very marked pressure kick when their "undersized" turbo's start working. When the w-g opens, the pressure increase flattens to a linear rise up to max. pressure. Within a very limited rpm range, the power increase is very steep, and these engines are often notorious for bad behaviour under propeller load conditions. The Volvo TAMD 63L is a typical example, and your VW belongs to this cathegory as well.

It also has the consequense, that even a relatively small deviation from ideal conditions will result in a dramatic change of performance. So after checking your installation, there are two areas that deviate from established marine standards; the exhaust diameters are small and creating a high back pressure to the turbine, and the supply pressure to the injection pumps are insufficient.

It is of no use to discuss propellers and tunnels until the engine installation is corrected, but from your photos there seems to be some details that have to be changed as well, but let's take this when your engines are up and running!

PS If you use paper cartridges for your fuel filters: Take care that they are stowed dry, with undamaged moisture-tigh wrapping until installed. They tend to absorb H2O moisture when lying around unprotected. Some types swell from this, blocking fuel flow after installation!

[CDK]

Baeckmo, the 1999 VE pumps Bosch brochure has slightly different pictures of the automatic timing device and detailed description of the operation.
The vane pump pressure is used to determine rpm and advance timing up to 24 crankshaft degrees, starting at 300 rpm. In their paper there is a spring-loaded valve at the exit side of the chamber, so there is an evenly distribution of pressure throughout the pump and they use the pressure from the chamber the high pressure plunger is in.

My knowledge is a bit dated I'm afraid. There have been - probably still are - pumps where the vane pump connects to the chamber with a small bore and the timing advance pressure came from the vane pump directly. That timing plunger bore also has a channel to the main chamber, partly closed by the plunger until the spring was compressed. It resembles the drawing in your document: the vane pump pressure determined the advance, the chamber pressure didn't matter.

So I stand corrected, even more so since I saw the new drawing.

The first thing I will do is unscrew the banjo bolts to see what is inside/underneath. Then I will read through some papers from the "Dieselschrauber Community" to see if it is safe to remove the advance mechanism with the pump in situ. I expect to find sediment in at least one engine blocking the plunger's travel.

[baeckmo]

The channel from vane pump to the spring side of the plunger (lower left in pic) is the suction side. The advancing pressure enters from pump housing to the right side of the advancing plunger through the small nozzle seen just to the right of the pin in the plunger center.

I think the banjo with the ball (non-return) valve primarily was used in applications where the return fuel was led to the filter body, to prevent pump draining during standstill. I don't think it is worth your time to bother with that.

The spring side of the advance mechanism is available under the low cover at the side of the pump. It can be opened with the pump in situ, but beware the "jumping spring". You are correct that in older engines, maybe having been unused for some time, "slime" is collected here, as it is the lowest part of the wet inners. But, if my memory is correct, you should not try to withdraw the plunger; it is bl--dy hell to get it back in position!

[baeckmo]

CDK, came to think of your return lines. You omitted the original fuel filters with their thermostatically controlled shunt, having quite narrow bores. Could it be that your "new" return has too low flow resistance, compared to the original combination: banjo ball valve>thermo shunt>filter, so that the filling of the injection pump cylinder is not complete? That would certainly slow you down!

After a marinizing project of an Iveco ohc engine long ago (similar power and VE-pump as your VW's), I saved the following data for the pump housing pressure:

Engine rpm 1000, pressure 2.4 to 3.0 b.
1500, 3.5 to 4.1
2000, 4.6 to 5.2

I expect your engines to run with the same levels of housing pressure. Check the old filter heads for possible nozzles or equal! Unfortunately, there is no pressure connection in to the housing. I was allowed to borrow a longer center bolt and a manometer adaptor from a local diesel pump shop, but I guess that is not an option for you.

[pistnbroke]

I give up CDK do you want to check out these engines to be sure you have no stupid problem or not ....yesterday baeckmo did not know his waste gate from his power valve now he is a diesel god ..before you do any of this dismantling you are taking about you need to check fuel and air ..its a diesel for gods sake !!!
simple basic things first .....

I have seen no post of boost pressure and of course it will have maxed at 1800 rpm
Nor have I seen any details of checks on the power valve settings ,,,

you need a pressue gauge 0-15 psi or 1 bar usually a fuel pressure gauge is the cheapest or a vac gauge with a reverse scale ....whats all this manometer crap .... bye till you call

[baeckmo]

CDK, Just spoke to a friend who is servicing a fleet of MAN vehicles in Norway. He confirms that there were two systems for the fuel return in light commercial vehicles. On the one you have there is an additional constriction in the original fuel filter head, together with this thermovalve. This version has never reached the marine environment (except through DIY marinizers).

Easiest way to correct your system might be either to reinstall the old filter head and connect both return paths to tank, or to check the orifice size (would be something in the 0.5 mm dia range) and make two plugs to fit into your tank connection.

As said before, low housing pressure will upset injection pump performance totally, so imho this is the first thing to adress.

[CDK]

CDK, came to think of your return lines. You omitted the original fuel filters with their thermostatically controlled shunt, having quite narrow bores. Could it be that your "new" return has too low flow resistance, compared to the original combination: banjo ball valve>thermo shunt>filter, so that the filling of the injection pump cylinder is not complete? That would certainly slow you down!

After a marinizing project of an Iveco ohc engine long ago (similar power and VE-pump as your VW's), I saved the following data for the pump housing pressure:

Engine rpm 1000, pressure 2.4 to 3.0 b.
1500, 3.5 to 4.1
2000, 4.6 to 5.2

I expect your engines to run with the same levels of housing pressure. Check the old filter heads for possible nozzles or equal! Unfortunately, there is no pressure connection in to the housing. I was allowed to borrow a longer center bolt and a manometer adaptor from a local diesel pump shop, but I guess that is not an option for you.

Baeckmo, I think the riddle is solved.
I have a Brooklands Workshop manual for the VW transporter models 1996-1999. It says that the fuel line connectors are marked and must not be mixed up because the return one has a restrictor. It also has a description and several pictures of the control valve on top of the fuel filter, that should open one way at 12 C. and the other way at 31 C.
That is all crap. Maybe they employed a bible-translator who understood no technical German, some text got lost or they are just ignorant fools, but that doesn't alter the fact that it is crap.

When the sun went down yesterday evening and you were probably composing your post I lifted the floor boards on my boat and unscrewed the banjo bolts on one engine and guess what, they are identical. In
the rapidly falling light I think I saw some markings like E and A, but certainly nothing that remotely looked like an orifice to reduce fuel flow.

The Bosch VE brochure says that the mechanism advances the injection by 24 degrees, starting already above 300 rpm. As you may remember when asked for black smoke I responded there is only blue smoke and the smell of unburnt fuel when the engines are started without the "Starthilfe" lever being pulled.
That figures! If the commence of injection at idle rpm occurs that late in the power stroke, there is still some fuel injected after the exhaust valves start opening, hence the smell and smoke. And if there still is 24 degrees missing at higher rpm, more that a quarter of the power stroke passes before the injection starts.

Unfortunately I do not have the VW filters and valves because the engines came from a recycler where they used bolt cutters to free the engine from its environment. But I have a lathe and a variety of materials, so I can make something with an adjustment screw, a narrow passage or even a spring loaded valve if I must. Some guidance would be helpful because I have no example at hand.

I take the liberty to add that leaving engine timing to a remotely mounted plastic device is very poor engineering!

[liki]

Unfortunately I do not have the VW filters and valves because the engines came from a recycler where they used bolt cutters to free the engine from its environment.

Hello again, your missing part is:

(41) 068130157 banto bolt with return flow restrictor / ABL - (41) is not part of the spare part code.

You might want to first ask your local VW shop for the price or ask other recycle yards for that bolt. Likely you'll be surprised by the price, either way. Vagcat lists it at 9,2 EUR in Germany but it is not at all reliable with prices.

[CDK]

Hello again, your missing part is:

(41) 068130157 banto bolt with return flow restrictor / ABL - (41) is not part of the spare part code.

You might want to first ask your local VW shop for the price or ask other recycle yards for that bolt. Likely you'll be surprised by the price, either way. Vagcat lists it at 9,2 EUR in Germany but it is not at all reliable with prices.

Thank you Liki. No local VW shop within 80 miles, but I'll search the internet. Is the ABL code part of the component description? And why do my engines not have it? I never unscrewed them before and I doubt very much that the vehicle owner(s) exchanged them for the wrong ones.

[baeckmo]

CDK, not only is the injection timing upset, the short time for each injection stroke makes the housing pressure a critical factor for the filling of the pump cylinder. The plunger of the 4-cylinder distributor pump is working with a 4 times higher stroke frequency than each pump element in a 4-cylinder line pump, hence the high housing pressure necessary for filling during the inlet period.

Just spoke to a friend who is in charge of servicing a fleet of MAN-vehicles. He confirmed that there had been a fuel system with the VE-pumps, where part of the pressure restrictor was "displaced" to the shunt valve in the filter housing.

Normally, a pump for ~75 hp would pump about 23 liters/hour at full load. Of this the engine consumes about 15.5 l/h. The rest, 7.5 l/h is recirculated mainly in order to cool the injection pump. All the friction and turbulence within the pump itself generates heat that must be removed. If you plan to make two orifices, the nozzle bore would probably be something in the 0.5 to 0.7 mm range.

PS: Your banjo bolt does not have the same small restriction as the pumps with "normal" fuel return to tank. In your system, part of the restriction lies in the filter housing! DS

[pistnbroke]

back leak off from the injectors usually goes stright back to the tank without restriction ....3-4 mm bore

[liki]

Thank you Liki. No local VW shop within 80 miles, but I'll search the internet. Is the ABL code part of the component description? And why do my engines not have it? I never unscrewed them before and I doubt very much that the vehicle owner(s) exchanged them for the wrong ones.

Those bolts were located atop the fuel tank on the transporter, wasn't it so that you didn't get the other end of the fuel lines from the recycling yard?

The exact description is:
Spare part code: "068130157"
Description: "banto bolt with return flow restrictor / ABL"

[baeckmo]

CDK, Went down to the yard and unscrewed the return banjo from an Iveco 8041 MO8, that has the same pump as you, and about the same fuel flow. There are two holes, diameter 0.35 mm (NOTE: NOT 3 to 4 mm as suggested by the p-and-b man) in the cylindrical part of the bolt. This adds up to almost exactly the same effective orifice area as the single 0.5 mm I proposed earlier.

Of course it is by far better if you can get hold of the original parts, but with your tooling and machining skill you may come up with a useful compromize if the VW parts are unavailable.

[CDK]

CDK, Went down to the yard and unscrewed the return banjo from an Iveco 8041 MO8, that has the same pump as you, and about the same fuel flow. There are two holes, diameter 0.35 mm (NOTE: NOT 3 to 4 mm as suggested by the p-and-b man) in the cylindrical part of the bolt. This adds up to almost exactly the same effective orifice area as the single 0.5 mm I proposed earlier.

Of course it is by far better if you can get hold of the original parts, but with your tooling and machining skill you may come up with a useful compromize if the VW parts are unavailable.

Rest assured, I'll make something. A 0.35 drill isn't in my box, but 0.4 is there.
With just one fixed restriction, the flow will change with temperature and viscosity, but it is a good starting point.

[pistnbroke]

Being a smelly diesel person ie thick ..I thought we were of the opinion that engine output was low /wanted checking ....I dont quite see how restrictions in the back leak off line are going to affect performance.... this is just the fuel not required by the injectors and as I said on cars the pipe goes straight to the tank or back to the filter to keep the fuel warm in winter .

did the boost pressure ever get checked ? diesel ...got enough air ?..yes tick that box ..so still low on power ....you aint got enough fuel .....KISS

[CDK]

Being a smelly diesel person ie thick ..I thought we were of the opinion that engine output was low /wanted checking ....I dont quite see how restrictions in the back leak off line are going to affect performance.... this is just the fuel not required by the injectors and as I said on cars the pipe goes straight to the tank or back to the filter to keep the fuel warm in winter .

did the boost pressure ever get checked ? diesel ...got enough air ?..yes tick that box ..so still low on power ....you aint got enough fuel .....KISS

No need to get grumpy.
You missed a crucial point in this conversation. We are not talking about the injector leakage but the fuel return from the pump. The internal pressure is used in the pump for the injection advance mechanism; without the proper restriction there is zero pressure, the injection start may be up to 24 crankshaft degrees retarded, so ignition starts way after TDC.

[CDK]

Today we did a full throttle test with the starter levers still pulled to the max. I didn't do that until now because idle rpm increases and the gearboxes don't like switching above 1000 rpm. The levers advance injection by 4.8 degrees (crankshaft).
The boat reached nearly 11 knots at 2600 rpm, see attached graph. We could make just one test because of the upcoming bora )* and had to return to the bay, but the result proves that the lack of power is caused by injection timing.

)* Bora is a local wind caused by air masses falling down the mountains at the coast. Gusts well over 100 km/h. are normal, in extreme cases up to 240 km/h. There are no reliable telltale signs.

[Ad Hoc]

CDK

ah, so, slowly honing in and ticking off the boxes now, Good...progress :)

Nasty wind you have there...not referring to eating too many beans either ;)

[baeckmo]

....The levers advance injection by 4.8 degrees (crankshaft). .....AND increases max injected fuel!!!!

Feels fine to have a confirmation of our thoughts; interesting to see coming results from return restrictor tests. Now I will be gone for a week (youngest daughter getting married....), so I won't upset any of you with comments for a while! Good luck Cornelis!!

[Frosty]

[QUOTE=pistnbroke..I dont quite see how restrictions in the back leak off line are going to affect performance.... this is just the fuel not required by the injectors and as I said on cars the pipe goes straight to the tank or back to the filter to keep the fuel warm in winter .

You are still in the stone age, things have moved on, this is not a 108 with leak offs.

My brand new Yanmar 6 LP's would not run with the return back to the filter. Well they would fast idle only. Simply fitting the returns back to the tank as per the book cured it.

I figure that the fuel supply from the tank offered more resistance than the return so fuel was being sucked from the return and was starving itself of fuel.

[pistnbroke]

good I like the stone age ..no technology....I think I said that returns to the tank are normal ...

[Rick Willoughby]

Today we did a full throttle test with the starter levers still pulled to the max. I didn't do that until now because idle rpm increases and the gearboxes don't like switching above 1000 rpm. The levers advance injection by 4.8 degrees (crankshaft).
The boat reached nearly 11 knots at 2600 rpm, see attached graph. We could make just one test because of the upcoming bora )* and had to return to the bay, but the result proves that the lack of power is caused by injection timing.

)* Bora is a local wind caused by air masses falling down the mountains at the coast. Gusts well over 100 km/h. are normal, in extreme cases up to 240 km/h. There are no reliable telltale signs.

CDK
Taking the data points from your speed v rpm curve I have produced the corresponding estimates for the power and drag.

The good news is that each engine is now producing 22kW providing the props are not aerating. A substantial improvement over the situation at 2200rpm - almost double the power. It also gets you into the maximum torque region for the engines once you get the fuel sorted.

The bad news is that the drag, calculated assuming no aeration, is now higher than my prediction. This could be due to the sinkage being greater than I am allowing.

It would be useful to know the actual angle of inclination. It would also be useful to know the water level on the side of the hull just before the transom. Not at the transom but slightly in front so sinkage can be estimated.

It may be worthwhile looking for the aeration as well. Is there any noticeable change in the flow out of the tunnels - bubbles noticeable.

Rick W

[baeckmo]

Looking at the "lines drag" curve; are you really trying to make us believe that this Draco would be completely without a resistance hump in the Fd range around 1.5?!! Please note my sincere doubts!

Referring to my comments on Savitsky algorithms in another thread, that is another strong cause for questioning the validity of these calculations. Have you made any references to full scale towing tests in this velocity range? As I mentioned before, I have been struggling myself with resistance prediction methods. Thus it would be interesting to know if you are using any corrections for wetted bow, aspect ratio or friction factor et c. in this speed range. And, of course what verification tests you have performed.

In addition to the clean hull drag, we must not forget the drag, caused by flow detachment in the tunnels. Here a rough estimate:

The flow that enters the tunnel sections at 11 knots is about 1.38 m3/s in total. With a 30% loss in mean velocity, the resulting pressure loss is nearly 8 kPa. This equals an extra fluid power of 11 kW, to be supplied by the props. With the propefficiencies we have here (~60%), the tunnel losses cost 25 hp in addition to the reduced propeller efficiency due to the variations in inlet flow, both directionally and velocity-wise.

So let us take the opportunity here, and see if we can improve our predictions in this speed range, I feel we have a black hole to fill there!

[Rick Willoughby]

Looking at the "lines drag" curve; are you really trying to make us believe that this Draco would be completely without a resistance hump in the Fd range around 1.5?!! Please note my sincere doubts!

Referring to my comments on Savitsky algorithms in another thread, that is another strong cause for questioning the validity of these calculations. Have you made any references to full scale towing tests in this velocity range? As I mentioned before, I have been struggling myself with resistance prediction methods. Thus it would be interesting to know if you are using any corrections for wetted bow, aspect ratio or friction factor et c. in this speed range. And, of course what verification tests you have performed.

In addition to the clean hull drag, we must not forget the drag, caused by flow detachment in the tunnels. Here a rough estimate:

The flow that enters the tunnel sections at 11 knots is about 1.38 m3/s in total. With a 30% loss in mean velocity, the resulting pressure loss is nearly 8 kPa. This equals an extra fluid power of 11 kW, to be supplied by the props. With the propefficiencies we have here (~60%), the tunnel losses cost 25 hp in addition to the reduced propeller efficiency due to the variations in inlet flow, both directionally and velocity-wise.

So let us take the opportunity here, and see if we can improve our predictions in this speed range, I feel we have a black hole to fill there!

I am not using an empirical method for calculating the drag. However I have to make manual corrections for the trim. The Savitsky calculator actually gives a drag hump as you would expect for such a heavy boat relative to planing surface.

The prop can absorb a maximum of 29kW at 1300rpm. This occurs when the flow presented to it is at 5kts. If the flow is any slower than this, the blades stall out and the power demand actually drops off.

The issue for the tunnels is not if they slow the water flow onto the prop compared with the open water case but if they allow air into the props. Any air will change their operation dramatically. The slowing flow simply introduces a wake factor.

Hopefully the trim is much worse than I am using and there is a hump in the drag curve as a result and it will reduce when the boat gets on the plane.

Main point is that the motors were not delivering full torque before and they are now doing a little better. Many had written off the tunnels as a complete mess up. A few calculations, even with incomplete data, can lead to the correct conclusions and right course to correct.

Rick W

[Ad Hoc]

That's a no then!

[baeckmo]

.....well actually a whole load of no's....!

And, you are "....not using an empirical method.....". What method are you using to get results like that, then?

[M-Sasha]

.....well actually a whole load of no's....!

And, you are "....not using an empirical method.....". What method are you using to get results like that, then?

PLAYSTATION as apex mentioned several times! And not to forget "assumptions" as Rick mentioned several times! Alltogether "muppet show" results, nice to impress some non educated poor members looking for a supporting hand!

Sasha

[rambo!]

This is one of the most interesting threads to follow, a lot of knowledge and ways to break down a problem into possible causes.
So why do some waste their time to misconduct or be sarcastis over some of the partcipants...who is right or wrong...CDK will hopfully have the answer in a near future.
So let the creative thoughts flow, read and learn abot different ways to approach problems.

And written comments are not always what they seems to be....sometimes written with a smile...sometimes in anger.....you don´t see that in the text....so bear that in mind.
rgds
Olle

[Frosty]

Watch me, and you will be surprized what may be achieved with the help of daddy's ol' pencil, provided one knows which end to point to the paper....


Im still waiting!!

[CDK]

Your VW will have a turbocharacteristics similar to, but slightly lower than the VM. It would probably have a relative pressure of 0.4 to 0.45 of pmax (ie ~0.35 to 0.4 b.) at 2100 rpm. The "waste-gate engines" show a very marked pressure kick when their "undersized" turbo's start working. When the w-g opens, the pressure increase flattens to a linear rise up to max. pressure. Within a very limited rpm range, the power increase is very steep, and these engines are often notorious for bad behaviour under propeller load conditions. The Volvo TAMD 63L is a typical example, and your VW belongs to this cathegory as well.



Some measurements made today. The KTY81-110 temp sensor was pressed to the marked area of the Garrett turbine. Pressure was measured with a fancy boost pressure indicator of which the dial can only been seen with the illumination on (totally useless design feature).

[pistnbroke]

I assume you put a tee in the pipe that feeds the wate gate actuator ....then rev up ... ..pressure rises then peaks and drops back when the waste gate opens ...... 6 psi seems a bit low ..you carnt have too much turbo pressure ( well you can like 20 psi) as the fuel burns in excess air ...I would have thought you would have had at least 12 psi..

[CDK]

I assume you put a tee in the pipe that feeds the wate gate actuator ....then rev up ... ..pressure rises then peaks and drops back when the waste gate opens ...... 6 psi seems a bit low ..you carnt have too much turbo pressure ( well you can like 20 psi) as the fuel burns in excess air ...I would have thought you would have had at least 12 psi..

No I didn't, there is a small brass stud on the manifold with a rubber cap over it. I guess it's there for service or the vans had permanent pressure gauges.
The measurements were done at high idle only, so with much less fuel injected than under normal operating conditions. That also explains the low turbine temperatures. I assume that under load there will be much more pressure, otherwise VW would not have spent money on a waste gate.

At the present I cannot freely do what I want to: there are tourists everywhere, police, government inspectors etc. In this season all kinds of laws are in force, like no noise, no speed over 5 mph near the coast etc.
And the daily temp. near 40 C. also doesn't help.

[Frosty]

Why dont you try dis connecting the waste gate see what happens. Its not going to run away because you have the throttles in your hand.

It finishes the argument of poor boost immediately. Racers don't use waste gates. 6 psi is nothing

[CDK]

....The levers advance injection by 4.8 degrees (crankshaft). .....AND increases max injected fuel!!!!

Feels fine to have a confirmation of our thoughts; interesting to see coming results from return restrictor tests. Now I will be gone for a week (youngest daughter getting married....), so I won't upset any of you with comments for a while! Good luck Cornelis!!

I pressed thin brass sleeves in the banjo bolts and drilled one 0.4 mm hole in them. The results are disappointing. I now need not pull the start levers anymore to obtain 2600 rpm, and the engine respond better to a change in throttle setting, but the maximum rpm did not increase and there was still no black smoke.
When the floor boards were away, I also checked the functioning of the throttle actuators and found that the small electric motors are developing axial play. The power levers cannot reach the end of their stroke anymore, so "full throttle" in reality is somewhere between 90 and 95%. Changing that is a major operation that I will postpone until the autumn.
The port engine is developing an oil pressure problem: after prolonged running at full power the oil light goes on below 1200 rpm.

Below are a few pictures of the wake at different rpm settings. I temporarily clamped an inclinometer to a side window, leveled it with the boat at rest and saw that the hull angle reaches 6 degrees at 2600 rpm.

[apex1]

Cornelis, how can you be so focused on that minor problem? Your bench upholstery needs a refurbish, THAT is important.

Richard

[CDK]

Cornelis, how can you be so focused on that minor problem? Your bench upholstery needs a refurbish, THAT is important.

Richard

You are quite right Richard. It is not the upholstery but a cover that was marine blue when new, now bleached out to ash gray. The dark patch is where the solar panel lies.
I'll tell my wife!

[pistnbroke]

KISS .....its a diesel ..enough fuel to produce the power and more than enough air will burn it with out smoke ....so its the fuel then ...KISS

[Frosty]

The Upholstery needs replacement? well it a thought" its as good as what other professionals are coming up with ---may work.

[baeckmo]

Hello everybody; back again after 7 days of pouring rain in Ireland..... now I understand why there are so many pubs on that island....!

CDK: nice job you have done; it tells us that the engines are now running with fuel restricted by rpm-limits (i.e. centrifugal rpm map, NOT max fuel map)! The 0.4 mm bores are working fine; don't change anything there yet! Note; NO disappointment there due to the following:

If engines had been running on max fuel curve, the xh gas temp would have been 450 to 500+ centigrades in pipe (~120 deg. lower on housing surface), and pressure 0.6<>0.8 bar at max rpm. Note: A turbine housing without water-jacket is normally glowing in a dark cerise colour when at max load! Don't bother with wastegate! And if excessive xh back pressure had occurred with engine running on fuel max mapping line, the temp would have been equally high, but with low boost pressure and black smoke as a result.

The pic's from the boat show absolutely excessive aeration of props, which supports the conclusions above. (And Richard, sharp as ever, with his focus on details, noted the real problem before the rest of us. Shame I missed that.... How 'bout "Argus1" instead, Richard?).

So, performance is now restricted due to prop ventilation, with centrifugal regulator limiting fuel flow (and thus rpm). Are the locking devices intact on the throttle lever stops, or have these settings been tampered with?

And.... aah, yes I am still waiting for an independent calibration of tacho's!. If you still have the instruments installed, you may now provoke the engines to show a different behaviour if you run a "bollard push test", because it seems that the whole tunnel arrangement is submerged at zero speed, hence less aeration in that case, and probably higher values for xh temp and boost pressure.

[Ad Hoc]

baeckmo

aah yes, bollard test, good call. The most obvious does tend to elude us at times!

[apex1]

Jaja Baeckmo.
(And Richard, sharp as ever, with his focus on details, noted the real problem before the rest of us. Shame I missed that.... How 'bout "Argus1" instead, Richard?).
If only I could, I would change my nickname. This apex1 name has it´s roots in the early 80ies when I held shares in a computer and software developing company. We brought the first branch specific, integrated hardware/software system worldwide into the market, named apex1. Well, and sold ONE. Was too early for such stuff.

Sorry for the aside.

Richard

[CDK]

Hello everybody; back again after 7 days of pouring rain in Ireland..... now I understand why there are so many pubs on that island....!

Yet somehow they manage to make TV documentaries with only sunshine. My wife wants us to go there with our motorhome but I've always refused.

CDK: nice job you have done; it tells us that the engines are now running with fuel restricted by rpm-limits (i.e. centrifugal rpm map, NOT max fuel map)! The 0.4 mm bores are working fine; don't change anything there yet! Note; NO disappointment there due to the following:

If engines had been running on max fuel curve, the xh gas temp would have been 450 to 500+ centigrades in pipe (~120 deg. lower on housing surface), and pressure 0.6<>0.8 bar at max rpm. Note: A turbine housing without water-jacket is normally glowing in a dark cerise colour when at max load! Don't bother with wastegate! And if excessive xh back pressure had occurred with engine running on fuel max mapping line, the temp would have been equally high, but with low boost pressure and black smoke as a result.

I measured the back pressure. It dances around 100 mm H2O @2000 rpm, at lower speeds measuring is hampered by the exhaust coughing up cooling water irregularly.


The pic's from the boat show absolutely excessive aeration of props, which supports the conclusions above. (And Richard, sharp as ever, with his focus on details, noted the real problem before the rest of us. Shame I missed that.... How 'bout "Argus1" instead, Richard?).

So, performance is now restricted due to prop ventilation, with centrifugal regulator limiting fuel flow (and thus rpm). Are the locking devices intact on the throttle lever stops, or have these settings been tampered with?

I didn't tamper with them (yet) and there is no need to because the throttle lever is at the end of its stroke approx. 2 mm before the stop is reached.
There is a minor change in behaviour I didn't mention before: the whole throttle stroke is active now until the end. If I also had the last few mm. available I think there might be another 100 rpm more.

And.... aah, yes I am still waiting for an independent calibration of tacho's!. If you still have the instruments installed, you may now provoke the engines to show a different behaviour if you run a "bollard push test", because it seems that the whole tunnel arrangement is submerged at zero speed, hence less aeration in that case, and probably higher values for xh temp and boost pressure.

The rev counter program confirms the tacho readings. I was a bit hasty the first time when I used the program as it was installed. But there are a few parameters that need to be set first, like a min/max setting and a conversion factor. Once that is done you get proper readings.

A bollard test has to wait until the tourists have gone. A fierce NE wind (bora) is also making life difficult for all boat owners in the area.

I found an interesting topic about increasing engine output here:
http://www.t4forum.de/wbb2/thread.php?threadid=90661

These guys want to drive dangerously fast with their old VW vans, so they install an intercooler, oil cooler and a waste gate bypass. But the extra power comes only from turning the "Mengenschraube" at the back of the VE pump.
This might be a solution for my boat engines to deliver the extra horses to get the boat over the hump and pull back the throttles before the whole engines start to glow in the dark.

Not visible in the wake pictures is the changed position of the hull at speed. I wrote several times that the bow lifts 6 degrees, but I also noticed that the stern digs in. At rest there is approx. 80 mm of anti-fouling visible at the flanks, at speed that is completely gone. Not a very objective observation I admit, because the bow wave raises the level along the hull.
There is also an increased level directly behind the stern, but I read in another post where Ad Hoc answered, that it is typical for a sub-critical wave pattern.

Funny that in 50 years of boating I always looked forward and now find myself staring at the stern most of the time. Must be old age.....

[CDK]

baeckmo

aah yes, bollard test, good call. The most obvious does tend to elude us at times!

BOLLARD PULL TEST.

The only thing I could think of to indicate the pulling force was the use of nylon fishing line.
We used 20 strands of 0,5 mm, breaking strength 18.7 kg, loosely connecting two spliced lines, each 15 m. long. One end attached to the center of the stern, the other to a large bollard on a pier. There was also a 6 mm safety (650 kg breaking strength) line of 50 cm long to hold the ends together if the fishing line should snap.

That happened at approx. 1500 rpm, using only the port engine. The safety line snapped less than 1 second later because of the boat's momentum, while the rpm increased to 2000.

Not a very scientific setup I admit, but it tells us that one engine pulls with at least 375 kgf, so both engines generate a force of more than 750 kgf at zero velocity.

[Ad Hoc]

as a rough guide most boats/engines will get 750kg per 100hp

[CDK]

as a rough guide most boats/engines will get 750kg per 100hp

Does that mean that the average 100 hp engine generates 750 kg at a bollard pull test or that an engine when actually delivering 100 hp during such a test will generate 750 kg pull? At the breaking point I saw only 1500 rpm, so the engine was unable to deliver full power.

[Ad Hoc]

"..Does that mean that the average 100 hp engine generates 750 kg at a bollard pull test.."

Correct

[baeckmo]

One of my issues with the BP test was to check wether the propellers, when fully submerged (as in static condition), would allow the engines to come closer to full load condition. As we still have a lowload condition when "steaming", due to aeration, this would be one way to ascertain that the engines now (after correction of return restriction et c.) are getting enough fuel and that thermostats, cooling and everything else is ok.

So, just put her stem to the quayside and try to move the island; full throttle! Don't worry about thrust, but check exh temp and boost pressure if possible. If engine data now indicate that this department is ok, we may proceed to seek a solution to the propeller problem, that fits your ambitions!

[powerabout]

take an accurate measure of fuel consumption and do the bsfc calculation.
Peak torque will be the lowest figure.

[pistnbroke]

sorry your bollard pull is unlikley to prove anything ..your assumption is that each strand took its share of the load .....if one strand was only 1mm shorter than the others it would take all the load and snap followed by the next shortest etc ....sorry I would love you to get this boat A1

[CDK]

One of my issues with the BP test was to check wether the propellers, when fully submerged (as in static condition), would allow the engines to come closer to full load condition. As we still have a lowload condition when "steaming", due to aeration, this would be one way to ascertain that the engines now (after correction of return restriction et c.) are getting enough fuel and that thermostats, cooling and everything else is ok.

So, just put her stem to the quayside and try to move the island; full throttle! Don't worry about thrust, but check exh temp and boost pressure if possible. If engine data now indicate that this department is ok, we may proceed to seek a solution to the propeller problem, that fits your ambitions!

What bothers me about the test is that I didn't design the propulsion system to move the island. I regularly help tourists pulling their boats out at the slip and am witness of the smoke coming from the clutch and the rear wheels. For a boat the situation is comparable to a car with an automatic gearbox: no smoke from the clutch, but the strain is the same.

Anyway, the throttle setting at the 'moment supreme' was for 2000 rpm, while actually is was only 1500, so full torque must have already been there. I am willing to repeat the test and measure the boost pressure, but I do not expect any spectacular figures because the rpm will be insufficient for a noticeable boost pressure.

During the bollard test, there was again no black smoke, so I need to adjust the general fuel setting on the injection pump first.
I wrote about this on the German VW-T4 forum in a very detailed thread about tuning these specific (ABL) engines. As installed in a van, these engines produce no more than 50 kW; the identical engine in a passenger car delivers 55 kW because it has an aneroid on the injection pump and a small oil cooler under the radiator.

These guys squeeze up to 104 kW out of the same engine by changing the pump settings, installing an intercooler, an oil cooler and a bypass valve before the waste gate aneroid to raise the pressure to 0,9 bar. They also replace some cylinder head bolts with stronger ones (12 instead of 8.5).
I suggested that in my case the modifications could be limited because I do not need to double the output and will throttle back as soon as the boat reaches planing speed. But the general opinion is, that the engine is thermally already at its limits and will need a fresh water or blower assisted oil cooler before any other modifications are made.

I found a guy with a V8 powered new Searay who was willing to participate in an experiment where I would run my boat at full throttle and he would supply the extra power to pull my boat over the hump using a long line. That way I might have found out if the boat can sustain planing speed, but the next day the Searay suffered prop damage when the guy drove backwards and hit a rock, so we had to abandon the plan.

[CDK]

sorry your bollard pull is unlikley to prove anything ..your assumption is that each strand took its share of the load .....if one strand was only 1mm shorter than the others it would take all the load and snap followed by the next shortest etc ....sorry I would love you to get this boat A1

All strands were of equal length. I wrapped the fishing line around 10 times and only knotted the ends so each strand could slide and distribute the load evenly.

[powerabout]

I would have thought that peak torque is also max boost. If not you would probably end up with 2 torque peaks.

Without an anerometer on the pump I would have also thought you would get black smoke as the pump will assume you have boost.

What is peak torque rpm for your engine?

Cheers

[apex1]

What is peak torque rpm for your engine?Cheers

It is all, but really all said here in the thread. It is good practice to read a thread before contributing.;)

[baeckmo]

What bothers me about the test is that I didn't design the propulsion system to move the island. I regularly help tourists pulling their boats out at the slip and am witness of the smoke coming from the clutch and the rear wheels. For a boat the situation is comparable to a car with an automatic gearbox: no smoke from the clutch, but the strain is the same..

Come on now CDK, your propeller is just a torque converter working in- and cooled by huge amounts of water. The questions are: What rpm's does your engine reach with full throttle at zero speed ahead, and with fully submerged propeller? And what is the accompanying boost pressure and xh temperature? There is nothing strange or risky about this test, as long as you can steer 90 degrees to the jetty, it is common practice, just get it done! And don't worry about geographic dislocation.......!

To "powerabout": boost pressure is never max at torque max, see earlier notes on this subject!

[Rick Willoughby]

All strands were of equal length. I wrapped the fishing line around 10 times and only knotted the ends so each strand could slide and distribute the load evenly.

Did the line break at the knot? Do the bollards have any surface defects? Very difficult to get 100% strength conversion efficiency from single strand to multi-strand.

I calculate 2000N thrust at 1500rpm on the engine. To give an idea of the engine speed sensitivity, if it was 1600rpm then the strain would be no more than 3200N.

Rick W

[CDK]

Come on now CDK, your propeller is just a torque converter working in- and cooled by huge amounts of water. The questions are: What rpm's does your engine reach with full throttle at zero speed ahead, and with fully submerged propeller? And what is the accompanying boost pressure and xh temperature? There is nothing strange or risky about this test, as long as you can steer 90 degrees to the jetty, it is common practice, just get it done! And don't worry about geographic dislocation.......!

To "powerabout": boost pressure is never max at torque max, see earlier notes on this subject!

Baeckmo, it isn't the part in the water I'm concerned about, nor is it the thrust bearing that can take 3,5 tons in its oil filled chamber. But it leans against a cover that is held in place by 4 long M8 bolts and nuts.
I know, M8 stainless survives over 800 kg static load, and there are 4 of them, but still.....Now I wish I'd have used M10.

But I'll humour you and measure rpm and boost pressure. Xh temp I cannot promise because I don't think it is feasible to press a sensor against the turbine housing on a rocking boat without getting your fingers burned. But I'll look for something I can temporarily attach.

[CDK]

Did the line it break at the knot? Do the bollards have any surface defects? Very difficult to get 100% strength conversion efficiency from single strand to multi-strand.

I calculate 2000N thrust at 1500rpm on the engine. To give an idea of the engine speed sensitivity, if it was 1600rpm then the strain would be no more than 3200N.

Rick W

The fishing line broke somewhere and in more than one place. It connected 2 spliced rope slings, not the bollard. Contrary to my expectation it did not break at the knot, but I used a fisherman's knot where the line is wrapped several times around itself.

[Rick Willoughby]

The fishing line broke somewhere and in more than one place. It connected 2 spliced rope slings, not the bollard. Contrary to my expectation it did not break at the knot, but I used a fisherman's knot where the line is wrapped several times around itself.

If it did not break at the knot then it means there must have been a weak point elsewhere. If it broke in more than one place then this also means the strands were not sharing the load. The friction over the rope slings must have been enough to cause significant variation in individual strand tension.

A fisherman's knot will achieve about 80% of the filament strength at very best. So the failure was less than 80% of the rating. Was the line dry when the test was being carried out? Was the line moist when the knot was preloaded.

Rick W

[apex1]

You do´nt know the Dutch have a tradition in seafarers usances? Do you? They must not learn from computer players how to distribute loads on ropes! They breathe Ocean when they have their first glimpse at the cruel world, and they fought the sea more and better than any!

Ähh, just my two cent of course (some may not know)

[CDK]

Come on now CDK, your propeller is just a torque converter working in- and cooled by huge amounts of water. The questions are: What rpm's does your engine reach with full throttle at zero speed ahead, and with fully submerged propeller? And what is the accompanying boost pressure and xh temperature? There is nothing strange or risky about this test, as long as you can steer 90 degrees to the jetty, it is common practice, just get it done! And don't worry about geographic dislocation.......!



Damn it Baeckmo, look what happened!

There are between 2000 and 2100 rpm, the boost pressure gauge said +0.6 bar.
Air temp. 38,5 degrees, the exhaust I couldn't measure.

[baeckmo]

........, M8 stainless survives over 800 kg static load, and there are 4 of them, but still.....Now I wish I'd have used M10.


With your setup so far (and what is known about engine power characteristics), the engines should be able to reach something like 2600 rpm at Va=0, producing a bollard pull of <~4 kN each. So if you made the threads ok there should be nothing to worry about!

I feel pretty comfortable that the engines will be performing ok now, maybe some final adjustment of fuel flow and throttle linkage has to be done, but nothing "hidden" anymore.

Now to the wet jobs! your boat weighs in at 3600 kg you said. That is nearly one tonne more than it was advertized (and designed for) in mid 80:ies..... Each time you go onboard, there is a bolt or a nut or an ?? dropping from your pocket; take an hour and clean her from surplus weight!!

That done, she still needs all the planing (NOTE: NOT PLANNING) surface she can get. To this end, continue the original bottom (no twist, no hook) between inner tunnel sides as far back as to the very leading edge of your rudders. NOTE: Inner tunnel sides to be extended vertically down until they meet bottom extension! Use longitudinal stiffening in line with "jet box" sides and centerline. No flexing allowed!

This way you get 1) more planing area, 2) virtual LCG when planing is increased, 3) air leak path from inside into tunnel is broken, 4) the resistance at 16 knots is reduced ~20 % and trim reduced ~2.5 degrees!

The next step depends on how far your ambition can stretch..... , but one thing is certain; the propellers are far too close (axially) to the tunnel ends, they are ventilated from behind! Even if you don't want to put more midnight oil into a radical revision of the tunnels, at least add 150-200 mm to the tunnel length, with the top sector inclined ~half the shaft inclination!

[Rick Willoughby]

Damn it Baeckmo, look what happened!

There are between 2000 and 2100 rpm, the boost pressure gauge said +0.6 bar.
Air temp. 38,5 degrees, the exhaust I couldn't measure.

CDK
Prop absorbed power under these conditions is 16kW. Gives engine torque of 73Nm.

Rick W

[Ad Hoc]

In addition to baeckmo's excellent precise, don't forget to reduce the angle of the 'inlet' from its 45 degrees to the hull bottom, or so, to around 20~25. This also helps with the 150~200mm and shaft inclination as he noted too.

[CDK]

Now to the wet jobs! your boat weighs in at 3600 kg you said. That is nearly one tonne more than it was advertized (and designed for) in mid 80:ies..... Each time you go onboard, there is a bolt or a nut or an ?? dropping from your pocket; take an hour and clean her from surplus weight!!



This particular boat was built in 1979 and delivered March 1980. The Draco company was a bit vague about the weight because the boat was available with different engines. If I remember correctly, they stated 2800-3000 kg depending on motorization.
There is a 300 ltr fuel tank and a 100 ltr water tank.
I added a large GRP roll bar, radar, stainless anchor, 30 m. chain, electric winch, davits and carry an inflatable in the aft cabin. The propulsion is lighter than the stern drives, but the box at the stern is an extra 70 kg.

Maybe I exaggerated by 100 kg or so, but I remember than once in an Italian marina the mobile crane operator stopped lifting the boat and positioned the crane closer because an alarm sounded, telling him that the load exceeded the 3 tons limit for that beam angle.

[baeckmo]

Damn it Baeckmo, look what happened!

There are between 2000 and 2100 rpm, the boost pressure gauge said +0.6 bar.
Air temp. 38,5 degrees, the exhaust I couldn't measure.

Wow, did you cross any national borders while pushing around??? Cruising the archipelago has certainly got a wider meaning....

Now, for the propulsion; the boost pressure indicates that your engine is not too far from its nominal power characteristics at this operating point. Propeller data indicate same; most computer programs rely on algorithms that fail at low speeds of advance. So back to daddys pencil, propeller diagram and a calculator... (see that, Frosty?).

With an average figure of kq=0.12 for this kind of prop, water density 1000 kg/m3, 1050 shaft rpm and a transmission efficiency of 94 %, we have a power consumption of 33 hp. This is for a prop in an unlimited volume of water; the flow restriction of the tunnel will increase power further, say at least 10 %. The resulting total developed power is then ~36 hp (~27 kW), which is very close to the preliminary engine data collected earlier!

As in all engineering processes, be it electronics, mechanical or else, trouble shooting in marine engineering most of all demands a logical testing sequence, based on knowledge of loading characteristics of the machinery involved (and calibrated instruments......). Add some sceptic attitude to flashy (fishy??) computer wizards, and you are well off!

So, lean back now, have a cold beer with me, and try to figure out how to proceed. I suggest that for now, you let the engines be as they are, and focus on the tunnel issues as outlined in my previous note. With the davit and dinghy in mind, stretching the whole bottom, also between tunnel and chines, should be contemplated. And, as a final step of course, a total reshaping of tunnels, according to Ad Hoc's and my previous suggestions.

[Ad Hoc]

Consistently good advice again baeckmo....nice to see someone else with their 'sharp' pencil being used too :D...mine is always by my side at the ready..

I'm not a beer person, but i'd certainly enjoy leaning back..admiring the view and chatting about...well, plenty really, with the two of you :)

[CDK]

So, lean back now, have a cold beer with me, and try to figure out how to proceed. I suggest that for now, you let the engines be as they are, and focus on the tunnel issues as outlined in my previous note. With the davit and dinghy in mind, stretching the whole bottom, also between tunnel and chines, should be contemplated. And, as a final step of course, a total reshaping of tunnels, according to Ad Hoc's and my previous suggestions.

While you were composing your reply, I converted one of the 1th drawings to represent the approximate current situation.
Would it be advantageous to extend the side skirts all the way to the edge of the transom, following the bottom's incline?

[Ad Hoc]

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
http://www.boatdesign.net/forums/boat-design/hull-design-small-displacement-boat-28618-6.html

[CDK]

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
http://www.boatdesign.net/forums/boat-design/hull-design-small-displacement-boat-28618-6.html

It seems that I didn't express my thoughts correctly.
On both sides of the tunnels there are horizontal surfaces to reduce air entrance. I could extend these in the shape of triangles or squares until the edge of the transom, but not quite horizontal because the profile of the hull dictates the shape there.

[Frosty]

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
http://www.boatdesign.net/forums/boat-design/hull-design-small-displacement-boat-28618-6.html


12 pages of this now. Are we any where nearer than we were at page 2?

Ad Hoc said he could work out this problem with his dads pencil ( dont make me go look for it).

Your tunnels are too short and your under powered.

CDK you know that don't you, just read Ad hocs post he has'nt a clue.

[Ad Hoc]

CDK
I'd need to see a picture/sketch to see more, (plan and profile) before i comment, as not 100% clear, sorry.

[CDK]

CDK
I'd need to see a picture/sketch to see more, (plan and profile) before i comment, as not 100% clear, sorry.

I used the word skirt and you thought of something hanging down. To me there is always an association with 'lady in red'.

Baeckmo says it is imperative to obtain maximum planing surface: two options are shown in yellow below.

[Ad Hoc]

CDK

A planing surface has length and breadth!...hence why i wanted to see how it looked in profile too, see how far it extends (aft) and how the transition is done up fwd, into the hull.

However, the lower figure left has view (the boxy one) looks best. But again, depends upon the profile and transition.

The extension can be made to work for you...ie have it as a storage for fenders and such like, as well as a nice diving platform too.

You just need to make sure you have good structural connectivity to the main hull.

[pistnbroke]

I thought I posted this before .KISS ..just turn the boost pressure up to 12/15 psi and up the fuel until it smokes ...thats all you are going to get ..after that you can fiddle with trim tab ideas.....

[CDK]

With an average figure of kq=0.12 for this kind of prop, water density 1000 kg/m3, 1050 shaft rpm and a transmission efficiency of 94 %, we have a power consumption of 33 hp. This is for a prop in an unlimited volume of water; the flow restriction of the tunnel will increase power further, say at least 10 %. The resulting total developed power is then ~36 hp (~27 kW), which is very close to the preliminary engine data collected earlier!



Baeckmo, are you absolutely sure that a flow restriction before the prop increases the absorbed engine power?

[baeckmo]

Baeckmo, are you absolutely sure that a flow restriction before the prop increases the absorbed engine power?

CDK, with increasing age, my inclination to give unconditional answers is reduced.......but generally yes, the propeller/impeller does not know what is causing the reduced inflow velocity!

There are two common exceptions though:

A) If the head (~pressure) loss due to the restriction is great enough to reduce the static pressure on the blade down to the vapour pressure of the fluid; then there will be cavitation. Roughly speaking: If vapour cavities are covering about 10% of the blade surface, you will have a performance reduction on conventional blade sections. Using cavitation criteria due to "Burril", your props if in free water (1050 rpm, zero advancing speed), would have about 6% cavitation. If the boost pressure of your engines had been lower, I would have suspected propeller cavitation.

B) If the disturbance is causing a pre-rotation, same way as prop rotation, there will be less work induced by the propeller, thus less power absorbed.

PS, Might mention that the power characteristics we see with axial impellers/propellers (increasing power with decreasing velocity) is reversed, when it comes to diagonal/centrifugal impellers. There we have increasing power with increasing flow!

[powerabout]

Baekmo
pretty simple when explained like that
Cheers

[pistnbroke]

if you put in all the power you have availabe and its till not on the plane then you know where the problem is ......god stop looking in the mirror KISS.

[CDK]

Is there any data or a rule of thumb about the wet area of a planing hull?

My estimate is, that under stable planing conditions the surface will be approx. 6 sq.m., so with 3.6 tonnes the load will be 600 kg/ sq.m.
The "trim plates" on the left side will be 0.25 sq.m. and get an evenly distributed vertical load of 150 kg. That applies to optimal conditions only.
For use at sea there will also be a dynamic load when planing through waves. Although most of the beating will be taken by the front of the planing hull surface, the trailing trim plates will also receive their portion, say another 150 kg.

In displacement, the plates will have no lift load, but there can still be a dynamic one either upward or downward in large waves.

With an assumed distributed load of 300 kg, half the load will be absorbed by the triangle formed between the trail end of the tunnel and the stern. But the other 150 kg is taken by the unsupported other half. If I construct the parts from 8mm GRP with a 50mm rib around the circumference and two diagonal ones it will be strong enough and doesn't need a diagonal support tube to the stern.

That is, if my assumptions are correct.

[baeckmo]

For pressure guessing: let's say we get her to 16 knots, ~8 m/s. That gives a stagnation pressure of 32 kPa, seen close to the spray root forward. At the aft end we see something like 30-40 % of this, acting at a trim angle of ~6 deg, ie a dynamic pressure of 1340 Pa normal to the surface. The flat portion of the bottom is operating at ~0.4 m draft, ie a static pressure of ~3920 Pa. In total 5260 Pa.

So your estimate, 6000 Pa looks reasonable enough in my eyes; including allowances for transients et c. And yes, make the square version, but give the chine edge a slight inwards taper (~5 deg) in top view, to avoid spray.

As said before, make the addition a continuous "stretch" of the bottom; no additional strakes, no horizontal or vertical strips, just an added bottom area inside and outside the tunnels! The inner tunnel sides must reach vertically down to the bottom surface. I would cut away the rudder balance in way of the vertical extension, so that there is a solid wall all the way to the rudder post. The outer t-sides should preferably meet bottom surface with 90 degrees, not vertically. This because the water is turning up into the tunnel with the bottom inclination as a base. Give edges bottom/tunnelside a generous radius.

[CDK]

Thank you Beackmo. I never got used to Pa instead of kgf, but I understand the reasoning.
Attaching the extra surface to the hull is something I still have to think about. In displacement and heavy seas there will also be downward forces due to banking and rolling. Probably the best is to laminate the plates to the full and the tunnels, embed a metal strip in the flange to the hull and add a row of screws just in case...

[powerabout]

Dont you just love the metric system...all these years and they still cant decide on a std.
fruit pascals and kilo grams per foot...what next?

Dont get me started on metric nuts and bolts...