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Thread: aqua lark not planing

  1. #21
    Team Member fs5's Avatar
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    Quote Originally Posted by modman View Post
    how far up should i be with cavatation plate from bottom of boat sorry its 1-2 inches below bottom i can raise up about 4 more inches not sure where it should be
    start off with the centre of your "prop shaft" 2 1/2 below the hull .keep an eye on the pisser/telltale and keep going up untill it stops sucking water.then lower it a 1/2 inch.
    it might pay to fit a new impellor kit ,if the one thats in there is pretty old

  2. #22
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    ..."First of all the boat is very short and it will never ever get out of the hole fast with anything bigger than a 15hp. i had the same problem with a 30hp on a apollo 9ft 6in boat."

    Too true! My 9 foot dink boat crawls onto plane with me aboard, and it needs "help" (push the bow down) with a passenger aboard--and that's with 70 hp!

    Short, relatively heavy boats plane off at a steep angle. If my boat was twice as long it would plane easier!

    Jeff

  3. #23
    Team Member capnzee's Avatar
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    a couple of additions such as "whales tales" bolted to the cav plate will assist the boat getting on plane. They are used extensively on small/short "blow" (rubber dinghies) boats. Once on plane, these additions to the cavitation plate will usually be out of the water reducing the drag they would cause if the engine was mounted lower. They will also dampen out a tendency for your boat to "porpoise". These "plates" are often used in outboard drag racing set-ups. They will help in acceleration by getting the boat on plane quicker. Rod

  4. #24
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    Agreed (and thanks--got trim tabs on mine and they DO help).

    jeff

  5. #25
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    Quote Originally Posted by capnzee View Post
    a couple of additions such as "whales tales" bolted to the cav plate will assist the boat getting on plane. They are used extensively on small/short "blow" (rubber dinghies) boats. Once on plane, these additions to the cavitation plate will usually be out of the water reducing the drag they would cause if the engine was mounted lower. They will also dampen out a tendency for your boat to "porpoise". These "plates" are often used in outboard drag racing set-ups. They will help in acceleration by getting the boat on plane quicker. Rod
    The problem with a whale tail is that only provides lift when he is getting up on a plane. As the OP noted earlier, once he does get it up on a plane the boat is porpoising. I know if he goes really fast the porposing will go away, but I don't think he is going to get that much speed out of it. If the whale tail is out of the water (since he should jack the motor up to the point where it isn't even close to the water when underway), it isn't going to do much to address porposing. The only thing it is doing is redirecting some spray, and that isn't going to create much lift back there.

    His real problem is that the CG is too far aft relative to the planing surface. For that reason he'd probably be better off with fat trim tabs that not only provide lift to get him up on a plane, but also would get the CG further forward relative to the back edge of the planing surface and prevent the boat from porposing. The drag from the trim tabs isn't any higher since he doesn't have to angle them down, he can just set the flat to the bottom of the boat. Most likely he will gain speed with the trim tabs.

    JMHO

  6. #26
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    Agreed. I've installed "automatic" trim tabs on mine to cure that irritating porpoising actin at low speed. Haven't had a chance to try it yet (long story), but it looks like it should work. Trimming the LU in pushes down on the...you know!

    Jeff
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  7. #27
    Team Member capnzee's Avatar
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    The trim tabs are probably the way to go. What about mounting them just an inch above the bottom, in that manner they would only touch if the bow tries to come up? By the way, that is a good looking Mark 50 or is it a 55? The method you have devised to control the trim tabs is certainly unique, hope it works! Rod

  8. #28
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    Thanks, Rod. Dying to find out how the tabs work, but the wife is pretty sick right now so I have to stay close.

    The motor is actually a 70 hp triple mounted on a mark 55 type tower. Built three of them to date and like them (unlike most Merc guys!) Weighs only 150 pounds with electric start and a FGS.

    Jeff
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  9. #29
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    Those are great looking units! Great paint jobs. Hope they work out well. Did the 3 cyl blocks "bolt" right up to the Mk 55 mids? Rod

  10. #30
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    Thanks! It's a fairly easy conversion, and the power output is far more than my old 44 cube 50 hp. I also built external reeds for it over the winter, and that's worked out really good; a noticable power improvment over the stock triple (that has rather crappy reeds). Makes nearly 1,000 rpms more with the same prop!

    Jeff
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