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Author Topic:   fins
leepies posted 11-18-2002 09:00 PM ET (US)   Profile for leepies   Send Email to leepies  
I have just installed some fins on my new mercury 60hp big foot. it goes well but for some reason the fins grabb in the water and nearly over turned the boat, the boat is 5 metres long any suggestions to the cause of this. seems to be dangerous
JBCornwell posted 11-18-2002 10:08 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Leepies.

I suspect that your engine is too low. Your anti-ventilation plate (alias cavitation plate) must be level with or slightly above the trailing edge of the bottom.

Setting trim with a fin on the leg is also different than without it. Always start your accelleration with the engine trimmed in. After you are on the step (plane) then trim it out for optimum performance at your selected rpm.

Good luck.

Red sky at night. . .

lhg posted 11-18-2002 11:22 PM ET (US)     Profile for lhg    
It is essential when using an anti-cavitation plate fin (for quick stern lifting help in achieving plane from idle) that the engine be mounted high enough so that the fin will pop clear of the water surface when planing. Otherwise, particularly with a bigfoot gear case, you will get excessive drag and lose top end, as mentioned.

If it were me, since you're already running a bigfoot gear case, I'd get rid of the fins unless you have a really poor planing hull. This is generally not the case with Whalers except for some of the shorter, now discontinued, Dauntless models.

LMJHB posted 11-19-2002 01:14 PM ET (US)     Profile for LMJHB  Send Email to LMJHB     

I just purchased a 1998 Dauntless 13 with a 4 cycl 50HP YAMAHA, with a 10 5/8" X 12" prop and 8 1/2" fins. The bottom of the cavation plate is about 1/4" below the bottom of the hull and the fins are flat (not arched up like some models) The deadrise is stated to be 13 to 14 degrees. With this prop I get 36 MPH (GPS)at 6200 RPM. No sign of ventilation. The transom adjustment holes on the motor are 3/4" on center.

Prop information indicates that I may be able to run a 10 1/2" X 15" at 5600 RPM. Do I dare try to raise the motor on the transom one hole??

lhg posted 11-19-2002 02:14 PM ET (US)     Profile for lhg    
LMJHB - I relly have no experience with a Dauntless 13, but I would guess that the engine should be raised one bolt hole. I don't think any outboard should be run with the anti-cavitation plate below the hull bottom. I know that "Outrageman" here has one of these boats, and I have heard it's hard to plane, so the Doel Fin may make some sense.
bsmotril posted 11-19-2002 09:49 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
If you run with a fin and the motor trimmed all the way in, you run the risk of having the boat bow steer. This changes the attitude to too much bow down. When turning, the forefoot of the boat can catch a wave causing the back end to skid out and try to swap ends. You have to re-learn how your boat handles whenever you change the trim, or run in new to you sea conditions. Try gradual maneuvers first before anything extreme. I also agree with other posters who state that with the motor trimmed out 75-100%, the fin should be out of the water. That will give you enough leeway to trim in for a hole shot, or rough water slow speed cruising, and still get good top end performance when trimmed for speed.

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