DIY tunnel drive

Discussion in 'Surface Drives' started by CDK, Nov 29, 2007.

  1. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Good news and bad...

    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!
     
  2. CDK
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    CDK retired engineer

    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.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    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.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    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.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    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....."
     
  6. apex1

    apex1 Guest

    Baeckmo
    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!?;)
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    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.
     

    Attached Files:

  8. baeckmo
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    baeckmo Hydrodynamics

    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.
     
  9. apex1

    apex1 Guest

    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
     
  10. CDK
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    CDK retired engineer

    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.
     

    Attached Files:

  11. baeckmo
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    baeckmo Hydrodynamics

    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.
     
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  12. CDK
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    CDK retired engineer

    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?
     
  13. PetterM
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    PetterM Senior Member

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


     
  14. CDK
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    CDK retired engineer

    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.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    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.
     
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