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Old 10-08-2012, 10:25 AM   #1
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Boat name: Viper
Boat make: Broom-Viper 16ft closed bow
Engines: F90hp Mercury

Join Date: Mar 2011
Posts: 13
Severe Porpoising Problem

I am encountering porpoising on my 16 ½ foot speed boat. It is a closed bow boat and is running with a 90 Hp 4 stroke Mercury. Before purchasing it I was concerned that it was stern heavy with the 4 stroke lump and on the first run (test drive) it ran great with no issues but then again I had only my youngest seated in the rear and didn’t take a lot of time to play with trim etc as it was a rough day on the water.

If I trim out at all the porpoising can be quite pronounced and with the 2 kids (6-7 stone each)in the back, 2 adults in the front I need to get to about 3200revs to get on the plane and there again even the slightest touch of trim and I am porpoising. In terms of weight distribution the only thing in the stern is the battery and I have moved all forward that I can.

At present the fuel tank is in the bow (7o litres) and the fuller it is the better but if I give it any trim at all even a miniscule amount I get some porpoising particularly at lower speeds.

If I don’t trim all is well and she will run fantastic, nice plane, pull skis and toys and will deliver 5900 rpm (WOT) with a nice top speed.

My queries are:

1. how can I stop the porpoising (other than not trim at all)
2. should I be able to plane before 3000-3200 rpm

Thanks
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Old 10-08-2012, 10:27 AM   #2
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Country: Guernsey
Occupation: Postie
Interests: Heavy Heavy Metal
Boat name: Slayer II, Slayer, Pen15
Boat make: Phantom 19G-R, Bernico F2 Extreme, Ring 15
Engines: Evinrude 115HO, Yam 90
Cruising area: Channel Islands

Join Date: Aug 2009
Posts: 1,460
How high is the motor set in relation to the bottom of the 'V', and what prop are you running?
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Old 10-08-2012, 11:17 AM   #3
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Boat name: Viper
Boat make: Broom-Viper 16ft closed bow
Engines: F90hp Mercury

Join Date: Mar 2011
Posts: 13
The cavitation plate is about 8 mm below the hull and I beleive the prop is a 13.5 x 15P
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Old 10-08-2012, 11:24 AM   #4
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Country: Guernsey
Occupation: Postie
Interests: Heavy Heavy Metal
Boat name: Slayer II, Slayer, Pen15
Boat make: Phantom 19G-R, Bernico F2 Extreme, Ring 15
Engines: Evinrude 115HO, Yam 90
Cruising area: Channel Islands

Join Date: Aug 2009
Posts: 1,460
Raise the motor if you can, hopefully you're on the top mounting holes at the moment and have scope to come up a few inches.

Having the cavitation plate below the V is causing you an awful lot of drag, and definitely won't be helping your problem!!
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Old 10-08-2012, 11:34 AM   #5
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Boat name: Viper
Boat make: Broom-Viper 16ft closed bow
Engines: F90hp Mercury

Join Date: Mar 2011
Posts: 13
Yes I am in the top hole and have some scope to come up and will give this a try!

Thanks
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Old 10-08-2012, 11:48 AM   #6
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Country: Guernsey
Occupation: Postie
Interests: Heavy Heavy Metal
Boat name: Slayer II, Slayer, Pen15
Boat make: Phantom 19G-R, Bernico F2 Extreme, Ring 15
Engines: Evinrude 115HO, Yam 90
Cruising area: Channel Islands

Join Date: Aug 2009
Posts: 1,460
Bear in mind that raising the motor and trimming out will raise your RPM, I don't know what your motor should go to but you may end up needing a bigger prop.
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Old 10-08-2012, 12:11 PM   #7
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Boat name: Viper
Boat make: Broom-Viper 16ft closed bow
Engines: F90hp Mercury

Join Date: Mar 2011
Posts: 13
Max rpm is 6000 and as you say if this occurs I may need to go up a size in prop to bring the revs down!
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Old 10-08-2012, 12:23 PM   #8
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Country: UK
Location: Guernsey
Occupation: Network Engineer
Boat make: Argo 21
Engines: 2.5 EFI
Cruising area: Channel Islands

Join Date: May 2009
Location: Guernsey
Posts: 426
Pictures of the set up would help
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Old 10-08-2012, 02:38 PM   #9
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Boat name: Viper
Boat make: Broom-Viper 16ft closed bow
Engines: F90hp Mercury

Join Date: Mar 2011
Posts: 13
Any particular pictures and to be honest it a fairly straightforward setup e.g. engine bolted to transom, the cavitation plate roughly 8 mm below the hull (when checked with a sprit level) and everything else pretty much basic boat from there on in.
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