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

[masalai]

Excellent lateral thinking, Best of fortune... this time have the "officials" there to ratify each step.... Then you have something to present to sponsors and get their money

[SaltOntheBrain]

On high speed rudders, the flat sides are supposed to be dead flat to avoid cavitation. If you sharpen the bottom, how can you do it and not cause cavitation?

You said you chopped the bottom off at 8 degrees, it looks like you went 8 degrees from the front edge, not from the shaft angle, so it's more like 12 right?

I'd use a straight-edge to make sure both faces are dead flat, then REDUCE the bottom angle a degree at a time, then I'd face the back edge 'til the transition is a sharp corner. (Making the back edge a little concave would be even better)

You never told us what you did for a water pick-up.

See if you can find a source for rudder info rather than listening to us. Know anyone racing inboard hydro's you can talk to? Do you have Dave Gerr's book "The Nature of Boats"? Look on pages 395 and 397 at the stuff on wedge rudders. I don't have a better rudder reference handy, but it's a start.

Lance.

[avi8r]

I tried it and it still felt draggy (as I am sure you knew ) I went back and cut it at 3 degrees so along with the slight angle in shaft put me about 5 degrees

here is a pass from yesterday

http://www.youtube.com/watch?v=zHdrg3mYSgk

the ride attitude looks and feels much better...... Now when I am going straight, it tracks straight... when i lift the throttle it wants to go left ( I presume from drag on left side) rudder athority is not as much as I was hoping for, so havent cut anymore off. I did mount it 1 1/4 inches higher.

Really considering going to a gearbox to lower shaft deeper in water, but worried that the surfacing propeller will still try to climb on top of water and push nose down...

here are pics of new rudder ... what do you think? I put it in lathe and made nice transition from point at front to .680 thickness in rear. I also added water pickups in side

[c-cat]

Iv'e watched the vid, and it seems that the prop is lifting the transom,It's what cleavers do.If the rudder were making substantial lift the hull would lean stbd.as speed increased.Have you tried talking to some prop guys like mazco,or hill, they will have higher rake props capable of glueing the transom down. I would check blade rake on current prop,(probably 5-10deg.) and try near 15-20deg. rake for comparison. They will more than likely be round ear props.I would also make sure all sterring components are tight,leaving only enough play to be operational,this will enhance high speed handling.

[masalai]

You looking to break the sound barrier? For me THAT IS MOVING! - I will stick to sail and enjoy your efforts, Thanks - not allowed to give more points yet, but here are some tokens ::::: ::::: ::::: :::::

[SaltOntheBrain]

Man, David, that's a slick job on the water pickup!

You do nice work. What do you do for a living?

Thanks for the update, I've been checking every day for a progress report.

Lance.

[SaltOntheBrain]

How hard would it be for you to try a longer shaft? That'll get you deeper into greener water.

I don't have the slightest idea how far you can have the shaft unsupported past the bearing, but I'd either try the max length with what you have, or build a strut and add a bearing. Either way would be less expensive and time consuming than adding a gearbox.

Keep those updates coming!

Lance.

[avi8r]

thanks for comments fellas

I may extend the shaft... but what would be easiest way to install a one way bearing or clutch on a 1 in or 25mm shaft? So when I let off throttle less drag. I would like to extend the shaft, and go with 8 in round ear propeller with more rake but started on building a gearbox... will try it first as it will be able to adjust prop thrust and trim angle with wedges

[charmc]

C cat has a good point, David. We've all focused on your initial rudder questions exclusively, with a sort of unspoken agreement that the prop was good. Based on your speed runs, the prop is damn good ... but it might not be optimal. Talking with an experienced high speed prop shop might yield some good advice. A small change in pitch or rake could give big results.

[avi8r]

I will do some prop testing in future and let you know how it goes

anyone know where i can get a one way shaft coupler like this?

I saw it on ebay for right hand rotation prop and thought it would be a good thing to reduce drag when lifting off throttle (which causes a lot of weird handling issues)

http://cgi.ebay.com/ebaymotors/ws/eB...ayphotohosting

[tuantom]

Still alive?

[Gilbert]

I haven't read all the thread, but I believe someone should have told you before you started to stick with the jet drive. They are more efficient at speeds over 35 knots. If you can prove me wrong, that's great.

[charmc]

Originally Posted by Gilbert

I haven't read all the thread, but I believe someone should have told you before you started to stick with the jet drive. They are more efficient at speeds over 35 knots. If you can prove me wrong, that's great.

While it's true that jet drives are more efficient under certain circumstances, high speed racing, in which the goal is to get as much of the hull out of the water and reduce drag as much as possible, is not one of them. There may be some innovation with which I'm not familiar, but for this application, he went with prop drive to reduce drag and enable absolute top speed.

[avi8r]

Yep , still alive and having summer fun with this project. 106mph is best speed so far, but by far the best benefit is I get double the mileage on tank of gas

Gilbert, I am almost 30mph faster with propeller then i was with jetpump (same hull, same engine, same hp) I think the surface drive drop is much much more efficient in my application.

[tuantom]

106 mph - Wow! I just saw a pwc skipping across the water at ~ 60 - 70 mph and thought That looked somewhat insane - then I thought of this thread.