Why are my rudders so ineffective ? (pwc converted to propeller)

I was just under ski and measuring ... its 9 ft long

about 29 inches infront of the rudder is an excellent place to mount such a skeg it its needed. Would that be far enough forward or would it need to be in middle of skiThe further forward I go , the deeper it would have to be to reach down to water due to it mostly riding on prop and rudder and last few inches of hull at high speeds

 

As long as it is not slipping sideways it will turn if you can get some side thrust in the stern. The fin would only be required if you a skidding sideways in a turn. It also brings in the risk off too much grip.

Surface props work as exposed pump impellers. They produce a high velocity stream. You will get effective turning if you can direct some flow sideways and the hull grips. The rudder in this case is a flow deflector and not a lifting foil. I can see no advantage in have the rudder actually being deeper than the prop. It will just add a bit more drag.

Rick W.

 

Ok, just so I am understanding what your saying... If the prop is running with 3 inches of blade under the hull, thats all the rudder I would need?

What do you think mounting it on the left for the drag to help counteract the torque?

 

Yes - no deeper than the prop.

If you mount the rudder in the free stream outside the prop you are defeating the main feature of a surface prop that aims to reduce appendage drag. You are better off redirecting the thrust slightly to offset the side walk rather than adding unwanted drag to provide steering. Bulldozers and tanks use offset braking for steering. Offset braking is the most inefficient means of steering that comes readily to mind.

There could be a slight advantage in mounting the rudder off centre because it will robb some thrust from the stream. However it only needs to be a flat plate because it will not provide lift in the aerated stream. So it should be as thin as you can get but still survive the deflection force.

If you do a complete redesign you could mount the prop at a slight angle to the centreline so the thrust is skewed to counter the prop side walk. (Have you got a universal joint in the drive that would easily allow a few degrees of skew so you can tweak it now?) Another option is to mount the prop shaft slightly off centre in the next version.

Rick W.

 

I maybe misunderstanding what your suggesting, if I am , let me apologize. Where are you talking of mounting the rudder? It sounds like your saying mount it behing the propeller?

if thats the case, here is my results when I had it there.
It was undrivable, at idle speeds it was fine, very effective, as soon as it climbed on plane, the rudder had no effect, I could steer it lock to lock with very little change in direction. so I had to idle back to boat ramp

I figured it was due to the prop aerating the water infront of the rudder causing to to act as though it was spinning in air.

When I started looking at how hydroplane racers with surfacing props do their rudder, it seems like from the pictures I saw, 90% of time they mount rudder to left or right of the prop thrust keeping it undisturbed water.

 

I did not realise you had already tried the rudder in the prop stream.

This link shows what I am proposing:
http://www.powervent.com/

You may need to shroud the flow ahead of the rudder. Similar to the tunnel on the hull shown.

The rudder you tried may have simply been too small. A deflector has around 25% to 30% area effectiveness of a lifting foil. So your rudder needs to be larger if it is aerating.

If you can get a rudder in front of the prop that does not aerate then it could be quite effective. Placing it under the transom or locating a cavitation plate flush with the hull at the transom to prevent air entering the blade from the shaft should be effective.

Other surface props like Arneson are steerable and they DO have a small rudder just in front of the prop but being able to direct the thrust will assit the steering from the blade.
http://www.arneson-industries.com/page.php?type=products&id=drives
Note that the rudder is not much deeper than the prop.

Rick W.

 

Yeah, those are different setups from how mine is. The powervent has the prop almost completely underwater like vdrive so wouldnt introduce as much air infront of rudders as my setup would

and the arneson has a skeg in front of prop, not a rudder. Its not a steerable device. On an arneson drive, the entire outdrive steers and changes prop thrust direction up , down side to side

My setup is closest resembles an inboard hydroplane with surfacing propeller. Those guys run offset rudders, i just need to find out whats the reasoning and how they decide

Thanks for ideas, anytime i can look at it from various angles it gives me food for thought

 

It is a fine point but the fact that the whole unit turns means the "skeg" is in fact a rudder. I expect this rudder will contribute more to steering force than the the actual directional change in thrust from the prop. I would need to do the sums to be sure but don't underestinmate how effective that little skeg will be. A blade 6" deep by 4" long could produce a couple of hundred pounds of side force at something like 40kts. I suspect this force exceeds the thrust required to do that speed.

The skeg/rudder on the Arneson is also protected from aerating by the prop shaft support it is mounted off. This would act as a cavitation plate to prevent air bleeding into the skeg.

With a little thought you could probably get close to the Arneson set up apart from the redirected thrust.

Have a close look at the Hydroplane rudders and see if they do not have a clever means of avoiding rudder cavitation. It would not take much at high speed.

Rick W.

 

Yes, I agree, the skeg/rudder on arneson could provide a trendous amount of force and I didnt mean to sound contrary, was just pointing out that the entire drive was steerable, not just the fin portion.

 

Hydroplanes

http://www.missmadison.com/photos/Detroit07/1B/P7150001.jpg

Typical rudder setup on an Unlimited hydroplane.

 

a picture is worth a 1000 words...

He has a right rotating prop and mounted the rudder way offset to the right.. I wonder if this was to help with torque?

Mine being a left rotating, I am thinking that mounting to left side will help

great pic

 

av, go through the information available from the flat bottom drag boats of the 60s and the single step hydros of the 40s.

Are you solely interested in drag-type situations of straightline speed and only turning to return to shore, or something a bit more all-round?

The 50s and 60s flat bottom drag boats seem to give the best perspective for your project. They have square sterns and hard chines, giving some bite to the water. You are lifting up on a pad initially designed for much less speed and a pump, which adds displacement and pulls the stern down. The hull is designed around vectored thrust for steering, so it needs basically nothing to bite against the water. The opposing forces example was very good above, and a forward skeg option is a very easy one to test (20 min with a hot batch of vinylester and the evidence is gone).

Have you considered changing hull shape at all? Taking advantage of the speed knowledge from the drag racers and the old crackerbox boats might be of benefit, and give you a decent chunk of extra speed.

Cool project, but dont fall off it!

E

 

The single offset rudder is a proven concept. The photos are from Slo-Mo-Shun IV and Miss Budweiser, 50 years apart. This exerpt from an article about the Slo-Mo-Shun hydroplanes addresses your situation exactly, I believe: "Having solved speed issues, the remaining challenge was that of control. The team tested a single rudder amidships, then tried a dual rudder setup, then finally settled on a single rudder offset to starboard." http://www.pugetmarine.com/jensenmtrboat.html

RF mentioned a setback rudder, as shown in these photos. I mentioned earlier that you should try a deep rudder blade, but the depth necessary should be just enough to give you a bite when riding on the prop. The hydros have their sterns often 8"-10" out of the water, hence the very deep rudder blades

 

I think they may be off set just to get it out of the prop wash.

Which side it is offset on maybe not critical.

 

It really looks like a common theme to have rudder on the side of rotation (left rotating left rudder, right rot, right rudder) The more videos I watch of them running and pics I see, I really get the feeling that a left rudder will help.

The rudder needs to be out of the prop wash on my application, and it will cause tremendous drag in the water at 80+ mph speeds. So I can use that drag to my advantage and put on left side to help counteract the torque. If I mount it on right side, the drag will make drift to right worse.

I know the rudder has a great deal of drag just by looking at the BEEFY rudder mounts on 100mph inboards using them.

Now all I need to do is come up with best geuss on rudder design

Unless someone sees a flaw with what I drew out, this will be one I will make.
any suggestions on improving it?