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Thread: Wooldridge Alaskan and Cavitation problem

  1. #21
    Moderator LuJon's Avatar
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    I disagree with you on this. I think that beveling the other side would form a vortex at the end of the bevel and cause air bubbles to form along the impeller blade and induce cavitation.

    In the end I guess whatever works for you.

  2. #22
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    Quote Originally Posted by Club220 View Post
    lujon,
    Re:the picture of the impellar being sharpened, the bevel on the impellar should be on the other side of the blade, you want the bevel to draw the water into the pump, if you bevel it this way it pushes it away. I saw the pdf from outboard jets and with all due respect it works better the other way, DD

    I agree with Club220. I believe the OB Jets PDF is incorect. I have mentioned this to them before. Club220 is my Uncle and has been working on, installing and testing OB Jets for us here at Wooldridge boats for 28 years. He knows jet boats better than most in my option. If you think about it when you sharpen or bevel the opposite side than what is shown in the picture then it will grab and dig into the water better therfore forcing water up into the jet with less cavitation. If you bevel the side shown in the photo then some of the water that hits that edge is not forced up into the jet but rather deflected back down by that flat spot created by the bevel and therefore causes cavitation. The only reason i ever file the side shown in the picture is to just get rid of any burs, not to put a bevel into it. Just give it a try and let us know what you think. You may have a hard time getting rid of the cavitation now though that there is such a bevle on what i would consider the "wrong side".


    I also aggree that it would be good to get the fins which are welded to the jet foot back to the 45 degree angle that they should be at. Dents in the bottom in front of the tunnel will also effect this.


    I hope this makes sence,

    Grant

  3. #23
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    Holy cow, that is a huge piece of knowledge. I wonder if my stuff has been sharpened wrong. Not that I am going to dig 10 feet down there to find out right now.

    I lost that piece of paper long ago, but had it in my shop for quite a while. I will be super careful from now on to bevel it the right way (or wrong way depending on how you look at it). You can't argue with a Wooldridge about how jet units work. Thanks.

  4. #24

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    Impellers

    This debate is extraordinary. The company that invented the outboard jet and has manufactured it for half a century says put a gentle curve in the bottom of the impeller blade.

    The Wooldridge family, who have been installing outboard jets, running them, and designing excellent boats for outboard jets since the units were invented argue that the manufacturer is incorrect. Itís quite a disagreement.

    When viewed in profile, an airplane propeller has a gentle curve (camber) on the forward side, and prop blast goes aft.

    A helicopterís main rotor blade has the camber on top and the rotor blast goes downward.

    So for an outboard jet impeller which moves water upward along the axis of the impeller, why would it not similarly have the camber on the bottom? Thatís how it works for aerodynamics.

    But of course weíre talking about water. Air and water both have fluid characteristics, but unlike air, water is essentially incompressible. So is the incompressibility of water pertinent?

    Also, perhaps an impeller works differently than a propeller because it is a helix configuration rather than a discreet airfoil. Still, I would think that a helix (actually four of them on the same axis) would behave like stacked airfoils.

    What do the highly regarded businesses who professionally restore impellers do?

  5. #25
    Member Rob B's Avatar
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    When I used to have my Yamaha 40 Jet, I remember I sharpened it once from the "easy" side and boy did it cavitate bad. It's seemed like it was just stirring up all kinds of air. Then I read that I sharpened it wrong. After redoing it the correct way, the "hard" side, it ran 100% better. So, the way I remember it is this, if it's too easy to use the file, it's wrong. You should have to use short strokes due to the tight space between the twists of the impeller. Sort of like a knife sharpened on one side. It cuts at an angle. As the impeller cuts or scoops water up into the intake.
    2005 Hewescraft Alaskan 260 MLC.
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  6. #26
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    One more question.
    Is there a way to take out big dents in the hull without pulling the floor and banging them out? I have been trying to brainstorm a way to pull them out without the labor intensive process of pulling the floor and reinstalling.

    I appreciate all of your help. I am new to the jet boat world and have enjoyed it so far! Definitley a thril when running up a 5" deep shoal and not hitting anything.

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    Taking the floor out of an Alaskan is not all the bad, it can be done in an hour or so. The fuel tank might be another story. Might be a good time to replace rotting floor boards if you have them, check hoses and replace the sending unit in the tank. I have to replace my sending unit this year. When I do so, I probably with make peace with nice dent towards the back of my boat. Hopefully you do it before I do so you can post how it went and I can learn from your mistakes instead of my own.

  8. #28
    Moderator LuJon's Avatar
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    I doubt that there will be a "cheaper" way than pulling the floors, but an aluminum stud welder and a slide hammer may work. You would still likely have to heat up the metal which would be less than ideal without being able to see what is under there.

  9. #29
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    There is a method of removing dents in the bottom of a boat with out removing the floor. When you put a dent in aluminum the metal stretches in order to shrink the metal all you do is cut slots in the aluminum and weld it back together. The welding of the Aluminum will shrink the metal or cut out a section and replace it.

  10. #30
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    Taking the floors out is probably the best way to get the dents out. When your ready to start pounding them out, back the dent up with a piece of wood on a floor jack so the dent just doesn't bounce when you hit it, and try to hit it with the flat of the hammer or you will just make small dents the other way, oh yea, and ear plugs. if the dent is in the formed corrigation then get a piece of pipe that size and lay it in the groove then hit the pipe. Sounds fun. DD

  11. #31
    Moderator stid2677's Avatar
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    To access the tunnel area on my Alaskan II, it would require removal of the welded in fuel tank, I tried to pound out a dent near my tunnel.
    "I refuse to let the things I can't do stop me from doing the things I can"

  12. #32
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    Yea that for sure is more work but if you have access to a welder you can cut the welds holding the tank in, remove it (easier if not much gas in tank) fix the dents then reweld tank in. I know welding on a tank sounds like trouble but you will be welding the brackets that hold it in, just use boards or leather to cover tank as best you can. DD

  13. #33
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    small dents-- heat, then dry ice... worked for my car and all the dang door dings..

    If you cant stand behind the troops in Iraq.. Feel free to stand in front of them.

  14. #34
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    I have had my boat out running a few times since talking with Glen. I have sharpened the impeller, shimmed the impeller, and bent the tabs on the side of the foot about 20 degrees down. Could not go futher due to cracking the welds, which Gln said would happen. I definitely have less cavitation. Only had it in rough chop and hard turns. Still rides a little funny due to a big dent on the port side of the hull causing more drag on that side. Now I need to fix that dent and level out the rear trim tab and it should be running great.

  15. #35
    Moderator LuJon's Avatar
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    Glad to hear some progress is being made! Keep us posted as you make the other repairs and I sure hope you are able to get her all smoothed out.

  16. #36
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    My 17.5 AK cavities with a good chop unless it is really loaded. It cavities in a hard "slide", but not when it is actually "turning" hard. Unless I encounter some chop in a turn then all bets are off. My boat has a goodly dent on the port side as well and I notice it does not track well now. I have compensated by changing the fins inside the jet nozzle and bent down one trim tab more than the other. It runs pretty straight now unless a trim tab gets bent back up because of a dry water boating experience. Best of luck and good to hear the little AK is running smooth. It is a real fun boat to kick around in.

  17. #37
    Moderator LuJon's Avatar
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    Well I am still uneasy about sharpening styles but I went with Grant's call and sharpened my stainless impeller opposite of how I have been doing my aluminum one. I guess we will see how it does. I have tons of respect for the Wooldridge crew (heck they invented stainless impellers!) but I think I will stick with the outboard jets diagram for aluminum impellers. The idea that it would grab more water if sharpened from underneath seems possible. This would however also put the leading edge at a steeper angle than the OB jets method. I can't help but think that may induce cavitation, especially on a thick bladed aluminum impeller.

    I slapped this together to help people visualize what I am talking about. Now to come up with a controlled environment and a method to test...


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