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

Discussion in 'Boat Design' started by avi8r, Feb 18, 2008.

  1. avi8r
    Joined: Jan 2008
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    avi8r Junior Member

    Please take a look at my rudders and PWC video and let me know what type of design I need for a new set of rudders.

    What happens is at speed, I have to steer a great deal to the left to go straight would one large rudder be better then 2 smaller ones perhaps?

    Is there a place on web that has pictures of various rudders for say inboard hydroplane racers?

    thanks for your input

    Here is a youtube video

  2. Frosty

    Frosty Previous Member

    Your problem would appear to be the surface prop walking you to the right at speed. You then need some left to comensate.

    I dont know what rudders you need but I dont like the tabs or what ever on the bottom of the rudders , what are they for?

    Your rudders must be cavitating.

    Its unlikely any one will know exactly what you need , you will have to do some experimentation. I would try with one rudder then the other and find the dominant.
  3. avi8r
    Joined: Jan 2008
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    avi8r Junior Member

    I tried the wings on rudders yesterday as experiment before removing rudders to see if they would generate downforce.
    My thinking was, the rudders on the back create unwanted tail lift, I wanted the wings to counteract that and if propwalking was my problem then the added downforce should diminish it somewhat.

    Yesterday with the wings I was able to run much better with less torque. Which basically took me from an undrivable situation, to where I could do some test runs.

    If the surface drive propwalk is the cause, (which makes sence because its a left rotating prop and ski wants to turn right)
    I cant easily lower my driveshaft.... would a bigger propeller bite deeper into the water and lessen propwalking? I realize engine torque would increase, but it would be worth it to get better bite and reduce prop walk.
  4. TollyWally
    Joined: Mar 2005
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    TollyWally Senior Member

    Hey Avi8,
    I don't know what your problem is. I think a single rudder behind the prop would be more effective but others opinions will carry more wieght. But what I do know, without a doubt, is that is one cool project. Way to push the envelope!

    Fear not, anyone who can build that thing, and then submit such clear pictures and video of the test subject will definitely solve the problem. I suppose some comment should be made regarding safety, proper engineering, etc. but I say ********. Your project is the sort of innovation that made America great. I sincerely wish you lived near by.
  5. avi8r
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    avi8r Junior Member

    :) I really appreciate your vote of encouragement. This has been a very fun project.

    This is actually my 4th set of rudders..
    First in front of propeller like steerable skeg (I had to use paddle to turn even at idle to get back to boat ramp lol )

    second was one rudder behind the propeller, which was very effective at idle speeds, but almost nonexistant as soon as I tried climbing on plane. I think the surfacing propeller was filling the water infront of rudder with air, so it lost effectiveness

    third was just the one rudder on right, then 4th as you see it

    Reason I am looking for a design idea and pull from wealth of information on this site, is that this coming weekend, I will have access to cnc machine, fully stocked machine shop and one very talented welder.

    A guy who lives about 4 hrs away from me saw this project and is donating a saturday of his time and shop to help me with project, so by this weekend, will have to have template of my next best guess.
  6. avi8r
    Joined: Jan 2008
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    avi8r Junior Member

    The point frosty brings up really is a valid issue of propwalking, so let me expand this thread and add this question

    My prop shaft height is 2 inches above the water surface.
    I have a 10 in propeller surfacing propeller. At high speeds, I only have 3 inches of blade in the water. Since I can't lower my propshaft deeper.... Would going to a 12in propeller be the same as lowering my propshaft 1 inch? (to reduce propwalking)
  7. Manie B
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    Manie B Senior Member


    I want to give another senario. If you add more power eventually the prop will have more bite than the weight of the boat = the boat will spin in mid air around the boat. I added a finn / skeg similar to that on a surf board - under the boat in front of the propellor (maybe two). It gave directional straight line stability and counter acted some of the rotational forces. no harm in trying.
  8. fastwave
    Joined: Jan 2007
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    fastwave Senior Member

    Like it has been said, it is a propeller wlaking problem.

    two possible things you can try.

    1. offset your prop so it is off centreline to balance the walk

    2. (if you want to fix it using rudders) leave only the left rudder. The drag will offset the turning.

    Both solutions are probably speed sensitive so might not work through the whole speed range.

    Good luck.

    Ps? what was the speed improvement with the surface prop vs the standard jet???
  9. avi8r
    Joined: Jan 2008
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    avi8r Junior Member

    I can add a skeg in front of the propeller. There is an easy place about 20inches in front of propeller. I thought about adding it, but hadnt yet due to concerns of it adding even more tail lift especially as speeds get into triple digits. with exteme performance of this thing, I try to start small and work way up. The hull naturally has skegs on sides for tracking, as well as strakes on bottom. So with that... should another skeg be added, and if yes, would you point it straight?

    1 offsetting prop thrust would be difficult due to solid shaft from engine with no flex joints. It wouldnt be impossible, but would like to exhaust other ideas first unless they will be complete waste of effort.

    2 just left rudder is excellent idea..... the drag it creates to left of prop would want to counter act the right drifting. Will have to redo steering cables where they pass through hull, but could be done :) What deminsions on a single rudder? would 8 in long, 1/4 wide 4 in deep be too big or small?

    as far as speed improvements, know I am a speed freak :D
    Before running propeller, I had tried many attempts to get close to 100mph (my goal)
    Just on throttle, my best gps numbers were 78 mph
    I added fogger nitrous with methanol (plus extras in shoe,rideplate, impeller) and got 88 mph.
    very dissapointingly short of my goal

    so I added the propeller and reset my engine combination to pump gas, with no nitrous and have already hit 93mph at about 60% throttle opening before the torque was soo bad even with full left steering I was drifting right.

    I feel like there is ample power (200 hp @7400 rpm) to run well past 100 if I can get this steering under control.

    the best benefit of it is that it sips fuel compared to the guzzling it did with jetpump
  10. charmc
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    charmc Senior Member

    Neat project. Like any project breaking new ground, each step unveils new problems. Be sure you keep a detailed log; you want to know everything you did and the results, and there might be a time when you need to go back to an earlier setup.

    I honestly can't predict what will work best. As frosty said, keep experimenting and measure the results. There is no doubt it's propwalk creating a strong offsetting force, complicated by cavitation. I suspect the larger prop will help somewhat, but not eliminate the problem completely. I'd try Manie B's skeg sugggestion, and fastwave's idea of a single rudder, offset to the left. If the rudder idea shows some improvement but not enough, then experiment with a deeper (vertically) rudder, and with a longer rudder chord (blade not quite as deep, but longer from front to back). I believe the longer chord rudder blade will generate enough force to offset the drift, but at high speeds it will be sensitive, so be careful.

    What you've accomplished so far is impressive. I'm sure you'll break 100 mph. Then I bet you'll set a new goal.......:) Good luck!
  11. FranklinRatliff

    FranklinRatliff Previous Member

    Relocating the rudders

    The only other thing I can think of to consider is installing a new subframe on the transom so that you can set the rudders farther back.

    There may also be some things you can do with ballast to reduce torque reaction.

    Some adjustable tabs on the bottom of the transom to trim the craft's attitude may not be a bad idea either.
  12. Kay9
    Joined: Oct 2006
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    Kay9 1600T Master

    Frank is on the right track here I think. I would image your steering problem is due to the cavitation that prop causes so near the rudders. Rudders dont work well in bubbles. So either move them back or move them forward.

  13. Guest625101138

    Guest625101138 Previous Member

    To steer you need to produce a moment. This requires two oposing forces separated by some distance.

    With your hull virtually flying on the prop there is no means of producing a force couple.

    You can generate side force at the stern by placing a rudder in the prop stream or shrouding the prop with a steerable tube. As noted above aerating props are not very effective so you need to direct the prop stream. This will give some benefit but you will still tend to skid sideways in a turn.

    You could mount a fin roughly midship to reduce sideways slip but beware it could also trip the boat in a turn and cause a rapid roll over - something to be really concerned about with that open prop. You would not want authorites to have a close look at the craft with the exposed mincer.

    Rick W.
  14. avi8r
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    avi8r Junior Member

    OK, sounds like great suggestions

    Here is what I am considering doing, and will be driving 8hrs sunday to have access to cnc machine.

    removing the 2 rudders that I have now make one out of 1/2 thick aluminum (so I can drill out for water passages instead of welding to bottom)

    I will have it tapered at front, and add lip to right rear of rudder (like a torque tab)

    I will mount it on a bracket thats bolted solid to the transom and move it forward just a bit and further left ( to get away from propeller)

    Something like this picture....
    How does this design look?

  15. avi8r
    Joined: Jan 2008
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    avi8r Junior Member

    Rick, this ski has skegs for directional stability and a keel ( not flat like hydroplane, or smooth rounded bottom like 90% os jetskis)

    If I turn and ski doesnt sideways slip (it hasnt yet that I have noticed, but this project is still soo new) do you think the fin would still be a good idea?
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