Changing pitch

just looked in more detail at some of your pix....judging from what i can see in those few pix posted, i would say there is also a flow problem for the prop, which would reduce the max allowable speed, to somewhere around 13knots, maybe a tad more if youre lucky, or less if you're not.

So to get up over the hump, around 9knots for your boat, there may not even be enough power being delivered at best anyway.

 

Please explain to me what flow problem you see, because I don't. But I'm eager to learn.
The lower half of the prop is below the boat's bottom, the upper half has almost 2 ft of tunnel to draw in water. The exit is in no way restricted, the rudders are positioned to the left and right. Between the tunnel roof and the tips of the prop there is more than 1" clearance.

With restricted water flow I expect a prop to cavitate and the rpm to increase. In the current setup, the opposite is the case.

 

CKD

You still need to address the power to weight ratio and hence the maximum top-end speed you could ever achieve. You need to accept that you wont much more than 16~17knots!, in an ideal world.

Im sure you’re familiar with the concept of the water being slowed down by friction from the bottom of the hull so a propeller is working in a wake, which is being dragged along with the hull? Hence a wide ranging prop efficiencies, of the same prop and installation, at various flows of inclination and obstructions to the prop. To the extent that a boat may doing 8knots yet the as far as the prop is concerned its only doing 6knots, so an estimate of the wake is required. There are rough guidelines, in so far as props in open water, ie below the hull has 0%, behind a skeg, but not below the hull, and slight curvature changes to hull, around 10% and so on, up to figures around 30% which translates into, poor “inlet” flow. It is this reduced water velocity that is used for prop calc's, not the boat velocity. Hence wake is important.

With a duct, the water flow is very dependent upon the flow in the duct, not just the boat and its shape iwo ducting. One of the important parts of duct is the shape of the lip on the fwd and aft edge edges. The ideal is not to cause separation of the flow, because you want as much boundary layer flow from the hull as possible. In other words to draw as much low velocity (not speeded up) boundary layer flow as possible from the hull fwd of the duct. If the shape and angle is too great, separation occurs and you get poor flow into the duct which seriously affects the wake and hence thrust from the prop. Endless studies of angles on flow into ducts have clearly shown that 25 degrees is the upper maximum. Which is why most waterjet manufacturers use around 22.5 degrees, to go as high as possible (so not making the duct too long and problematic) to ensure that there is no, or minimal flow separation and the consequential losses in the duct.

Your duct looks around 45 degrees, ie not ideal and hence all you’re doing is making your own private Jacuzzi!

 

What he is saying is that the water coming down under the boat wants to go in a straight line as does everything. If the tunnel is too quick and short it wont turn the corner and go up into the tunnel. He quotes 22 degrees as the tightest corner it will go into but sooner or later speed will even stop that.

However you are right is saying that you would get cavitation or airiation under these conditions but are getting bogged down.

I assumed from your post your were getting jacuzi type white water and no power to get over the hump but if the RPM is staying down then we are back in pitch problems.

I don't like your 1 inch clearance,--at all,- infact if that is 1 inch as the blades enters the tunnel then that is your vibration.

I think the rule of thumb is 1/3 rd of the prop dia for normal under hull prop position and a bit more for rudder clearence.

By the way you can easily remove 2 of your rudders and gain better steering.

 

If the RMP is low, even under the aerated condition, it is a mechanical issue, not hydrodynamic one. Since the prop would ostensibly be turning in mixture of air/water, but as far as the prop is concerned it would be air, ie free to race.

There are many 'design' aspects that are not right Frosty. But one must deal with the important issues first, such as the lack of power to go beyond 16~17knots, and the flow into the duct/props. The rest are mere details..

 

One of the problems Im having here is information. its seems to be spread over three threads.

How much Hp have you got, length ,weight , gear ratio.? Lets start again.

I have just noticed your diagram of your tunnel, that is not a tunnel, it should start under the gearbox at least. I know I know --you dont have clearance !!!! such is tunnels.

I think you have a huge hump because you have taken away some of you planing surface under the rear of the boat for the "tunnels" -not much but it would act like a negative trim tab. You are down on power and it cant make it. Having the engines moved back would have made it worse

Being at such an angle on the hump means water is getting into the tunnels and the props are just about living with it. I think if you did get over the hump the props would break free into surface mode which of course they wont live with.

I don't know what to suggest but start again, actually you are closer to a surface prop configuration than anything else,--fill in the tunnels and fit surface props!! But even that would be a compromise.

The only thing I can suggest is try some wieght forward and try to get it onto the plane.

Of course as Ad Hoc say you must have the power to do this.

 

I would have made much longer tunnels if it were possible. But the existing hull, designed for twin stern drives, did not offer more length.
This design with an oil filled stern tube and thrust bearing in the tube itself, needs to be flanged to the transom, so the tunnel location is dictated. The engine bay length proved to be just long enough for the engine, gearbox, universal joint and prop shaft coupler: there is less than 1 inch between the belt pulley and the bulkhead which separates the engine bay from the fuel tank.

With this setup, fuel consumption is reduced by almost 90 %, so I must have done something good as well. Some 20-30% may be contributed by the VW diesels versus 3.0LX Mercruisers, but the rest can only be the much higher efficiency of the propulsion.

The Mercs - with a cleaned hull - gave this boat 28 knots top speed at 4600 rpm and stable planing at 22 knots, 3500 rpm. Mercuiser claimed 140 hp, but these boats were also sold with twin 120 hp Volvo's which performed equally but needed some more time obtain speed.
Instead of 240 (Volvo) or 280 (Mercruiser) hp I now have between 160 and 180 hp and a more efficient propulsion system. Did I really aim too high when I targeted 22 knots?

A forum member from Turkey (Cemberci) calculated from the supplied data that 14x17 should be the prop size and because I found no cause to contradict him, so that is what I ordered. One clockwise, one counter-clockwise, delivery time 3 months for the latter.

Feeding air in the tunnels and using surface props is an alternative I am seriously contemplating, but with the previous blunt tunnel entrance there was air draw in, resulting in lots of vibration. That might be better with more prop blades but I read a lot about vibrating surface props.

 

A quick browse through the previous links on this project shows that a number of writers have in beforehand correctly pinpointed the issues that would cause trouble. I believe it was ”Ad hoc” who described it as a backwards engineering project. And trouble there were….. . There is no chance that a simple pitch change will be medicine enough for this patient! That much said about the specific tunnel project, I feel that a few arithmetic exercises on the subject of this tread, namely changing pitch, might be of general interest. I might add that we regularly twist and beat propellers and jet impellers in our shop here to suit special demands, so I am familiar with the process…… .

First to the importance of precision regarding blade shape:
Using standard algorithms for propellers of the Wageningen B-type, we get the following results for a 14 x 17 propeller, 3 blades BAR 0.55 in a decent tunnel design (wake factor ~8%), 1500 rpm´s and speed 12 knots: with a shaft power consumption of 44,2 hp it will produce 3132 N of thrust. (The wake factor in CDK´s tunnels is rather something like 30 to 40 % ).

To get an idea on the consequenses of a correcting job done by an inexperienced sledge hand, we assume that 5 % of thrust are lost due to inaccuracy in shape. In order to maintain the same boat speed as before, the engine will have to operate at increased rpm´s. In our example, the operating point is changed to 1526 rpm, where the shaft power consumption will increase to 47,2 hp. With power losses in the transmission, the total power difference will be abt 4,3 hp.

Now, this amounts to a difference in tacho reading of 52 rpm (gear ratio 2:1), which is easily hidden among other dissimilarities between testing runs before/after, not to mention tacho inaccuracy. But make no mistake here, the oil sheiks will still charge you for the increased energy consumed!! This is the dilemma in propulsion technology; the real outcome is very difficult to measure. Nobody has ever seen a horsepowerhour or scratched its belly, but we all have to pay for them! If this one engine spends 200 running hours at this specific load, it will have to produce 860 hp hours extra. With a diesel (indirect injection) specific consumption of 0,24 l/hph, at a cost of ~1 Eur/liter, the total extra cost for this short period is 206 Eur.

This is more than the cost, including VAT of a brand new 14” standard propeller. A professional repitching or minor repair would come at about 60 Eur in our region. With this in perspective, I do not think that a novice to propulsion technology should reach for a hammer as a first priority………!

Then to the leading edge problem when reducing pitch:
Let us just check what happens on the mean effective radius, which is ~0,7 x tip radius. For our 14” prop, the mean radius is taken as 0,125 m. On this radius the chord is about 0,180 m. With a nominal pitch of 0,432 m (17”), the blade has an angle (leading edge to trailing edge) to the disc plane of 28,9 degrees. With 16” pitch this angle is 27,5 degrees. This amounts to an axial difference of close to 4 mm. When the leading edge is rolled down to achieve this change, I often see that the job is done on the first 25 % of chord. In our example this would be something like 40 to 50 mm.

Done this way, the local leading edge incidence will be about 23 degrees instead of the 27,5 we aimed at, often causing a pressure reversal on both sides at nominal chord incidence. The blade chord will have a hook near the leading edge which is a very bad profile, particularly in a cavitating environment, where it will be prone to pressure side cavitation. On the other hand, a profile with a hook in the exit region, due to pitch increase, will cause the blade load to move aft on the blade, whis is beneficial from cavitation point of view. This is my reason to recommend you to start with a pitch on the low side when there are uncertainties regarding dimensioning.

And finally: Can the blade be twisted around its chord center line?
With our 14” example again, the hub dia is about 22 to 25 mm. The blade chord is clamped to the turning lever on a radius of about 50 mm. All the plastic deformation takes place between those radii. A fraction of the total propulsion power is spent in this zone, so its shape is not critical to total performance. The result is a blade with its original chord shape in order, good efficiency and no vibrations. It is carried out as a standard procedure on thousands of propellers so far, in sizes up to at least 20” in various materials.

 

Thank you Beackmo, that was very informative, but it also raises some questions.
The term "wake factor" confuses me. There is a Froude's wake factor, a Tailor's one and even a wake factor for vehicles in dense traffic.
In most technical papers, a wake factor is assumed, not calculated or measured and the assumed value is always small, like 0,05 or 0,1. That makes me a bit suspicious about prop calculations because if the assumption is wrong, so will be the calculated result.

I quote from the Propeller handbook, by Dave Gerr from Int'l Marine:

....Determining Wake factor (Wf) as a function of block coefficient - Displacement Vessels.

Vessels with a higher block coefficient are fuller-bodied (tubbier). Accordingly, water flows around their hulls less easy and their wakes are greater than those with finer, more slender hulls. As you can see, the smallest wake factor, and thus the largest difference between V and Va, appears for craft with large block coefficients....
Then follows a diagram where the wake factor is between 0,6 and 0,95, the highest value belonging to a slender hull with twin props.

I assume you define wake factor quite differently because you qualify my tunnel design as having a wake factor of 30%, possibly more. Somehow I get the impression that the higher your factor, the more power is lost.

There is also a minor item at the end of your post, where you assume the hub diameter to be approx. 22-25 mm. You probably mixed diameter and radius: the shaft is 1 inch, so the hub diameter will be about 65 mm at the front and 45 at the back, averaging 50 mm.

 

CDK, I know this may sound silly, but. Is there a chance the propshaft angle is off? I had this happen on an outdrive.The way it was mounted in the transom caused the prop to be slightly aimed at the mud all the time. The speed increased magicly when I corrected this as the engine stopped trying to push a bathtub on end through the water and allowed the boat rise up on plane. I know this sounds simple to all you engineering guys but hey, it's worth a look.

 

One thing that seems to be overlooked here is the relationship between the "hump" you are stuck in, the "hull speed" and the waterline length of your boat. When the bow rises up, the waterline length goes down and takes the hull speed with it. Often more power makes the bow rise more, making the hump problem worse unless the is enough thrust to propel the boat over the hump wave.

As all racers of small hydroplanes know, the solution is to get the bow down. They do that by leaning their weight as far forward a possible. Bigger boats do it with trim tabs to force the stern up and thus, the bow goes down. This not only increases the "hull speed" but reduces the bottom loading on the hull contacting the water and reduces the induced drag so the boat can get its butt out of the hole.

You can test this very easily and cheaply by temporarily adding wedges under the stern. You can make them out of wood and attach with a sealant that can be removed. I have done it on smaller boats with wedges attached with tape. If it works, install some trim tabs. From your drawing, the wedges/tabs will have to be placed out at the chines.

 

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?

 

The prop shaft angle is between 6 and 7 degrees, so there is an upward component that somewhat reduces the tendency of the bow to rise. But you know that getting into plane without changing the hull angle is impossible, even with sterndrives fully trimmed in, the bow will come up.
I always had trim tabs on this boat while it was powered by Mercs, from my calculations with the tunnels I reckoned they could be discarded.

 

What is really impossible is knowing the details of someone else's boat from posts on the forum. Of course, some up trim of the hull is necessary to develop dynamic lift. Too much weight and insufficient power (make that thrust to include wrong props, etc) also makes it impossible to plane. Trim tabs do introduce a lot of drag and need power to overcome that but they will often allow a boat to plane that otherwise would not. It's all about lift to drag ratio and how to balance the under boat stuff to maximize that. Sometimes you need to increase one at the expense of the other and sometimes vice-versa. I don't think any of us know which is your problem, assuming there is sufficient thrust available. With enough thrust, most problems can be overcome but it is a poor way to design.

 

Tom,
Could you expand on the trim tab and bottom loading thing. If this is not the appropriate thread I would gladly start another one. Does the surface area of the trim tabs help to lower the bottom loadong by supplying more surface area? Ever since reading about your Bluejackets, I've had the notion of large trim tabs increasing the surface area and lightening the bottom loading. I do like the attributes of those boats of yours. If I didn't already have one mistress and had more time I'd love to build a 28'.