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Old 21-04-2009, 14:11   #1
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Cavitation at Plane

i have a little 8ft10 flat bottom inflatable with wood slats and a new 3.5 merc outboard. im having an issue where that when i start to speed up the engine and feel like im getting up on plane the engine starts to buck and almost feels like its cavitating. anybody have any experience with this and if so what should i look for. thanks
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Old 21-04-2009, 16:37   #2
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With a rig that small balance is critical. Try changing the tilt on the engine, and make sure the tubes are pumped up to very rigid state. If that doesn't help, you're probably stuck with it. The bottom makes a huge difference in performance, and if your wood slat bottom undulates when you get up to speed, I don't think there's much else you can do about it.
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Old 21-04-2009, 19:53   #3
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I agree with Hud. Proper inflation of the tubes is important. I have a soft-bottom with an inflatable keel. If the keel isn't rigid, the motor will cavitate at higher rpm's.
Try different settings on the motor tilt (as Hud suggested), and also try re-distributing weight in the dinghy. Sit farther forward (attach a tiller-extension if necessary).
Perhaps investigate "Dolfins" or some other brand of planing fins for your motor.
HIH,
John
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Old 22-04-2009, 13:35   #4
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A 3.5hp will only be able to get you on plane if you perfectly balance the boat, as mentioned above lean as far forward as possible. Once on plane you will have a hard time keeping the flat bottom dink stable, once it starts to hunt it will be very hard to straighten it back out. A 3.5hp is really only a kicker motor, running the dink on plane a lot will put a lot of strain on this little engine.
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Old 22-04-2009, 13:41   #5
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thanks everyone my wood slat floor does tend to flex and roll a fair bit. do you think it would be worth putting in a solid plywood floor on top of the slats .... any benefit here. will check the tube pressure as i think they might be a little low.
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Old 22-04-2009, 13:49   #6
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rigid transom?
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Old 22-04-2009, 13:52   #7
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3/4 in thick poly transom attached to the tubes
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Old 22-04-2009, 14:03   #8
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Don't add any weight. 3.5 hp is way small for planing. Pump up the tubes to spec. and get forward until it goes on plane. This is sort of a balance dance. You can feel the boat react and you have to move back when it goes. sometimes you can use a wave to get it going.
Oh yea, don't eat lunch before going out.
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Old 22-04-2009, 16:46   #9
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I recently bought a "Dol-fin" (or some such spelling) for my outboard, it attaches to and extends the horizontal plate and, surprisingly, held up to it's advertisements and I no longer get cavitation when planing on my little inflatable dinghy (and a 9.8hp engine).
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