Porpoising, is it pitch or motor height?

[FishnMike]

I have a 17.5' Alumaweld Stryker which is a windsheild forward fishing boat with a moderate V. I have a 2004 Merc 60hp four stroke. Boat came with a 14 pitch Turning Point prop which I was able to get about 29 or 30 mph at 5200 rpm before the boat would start to porpoise. I purchased a 12 pitch Turning Point which I can get 33 to 35 mph at 6000 rpm without any porposing. I still have throttle left and can easily over rev the engine by giving it the rest of the throttle and continuing to trim up.

Would a 13 pitch be a good place to go since there is such a big difference between the 14 and 12 on my boat or do I need to make a motor height adjustment to get rid of the porpoising with my 14 pitch?

Thanks,

FishnMike

 

[tallcanadian]

Re: Porpoising, is it pitch or motor height?

i don't think it's a prop issue. sounds like too much trim. it's one thing to get the most speed out of your boat but you can certainly over trim it. when it starts to porpoise bring your trim down until it stops.

 

[steelespike]

Re: Porpoising, is it pitch or motor height?

If your going from a regular prop to the turning point you can probably raise the motor may help the porpoising and add a little speed.Raise it to just before venting(cavitation) becomes a problem.Be sure it pumps water.Or try moving a little weight forward.Anchor,gastank, battery etc.If stuff is allready there try moving some back so it can hold the bow up better.Just to experiment you might try driving from behind the seat.Give you an idea if a weight shift will help.Theres a slim chance pushing faster my move beyond porpoising.

 

[hwsiii]

Re: Porpoising, is it pitch or motor height?

Mike, porpoising is actually caused by the center of lift on the boat being moved back and forth form the center of gravity of the boat, a lot like chine walking. On some boats if you throttle up slower when you come on plane it will slow the effect down a lot, but on others it doesn't matter if you do it slow or fast it still happens. It is a basic instability in the boat that can be caused by many reasons, including hull features but usually it is caused by weight placement in the boat. Moving some weight forward or backwards some of the time can totally cure the problem, but not always. This problem can be exacerbated at times by using a prop that has some rake but not enough to lift the bow as required, and at other times it is best to use a zero rake prop that lifts the stern only and thus eliminates the tendency of the boat to propoise because it keeps the center of lift on the boat in the stern and it never passes the center of gravity farther forward in the boat.On some boats you can give it more speed and it will actually stop porpoising. I know that is a very lengthy and detailed explanation, but that is the root of the problem with porpoising.

H

 

[FishnMike]

Re: Porpoising, is it pitch or motor height?

Thanks guys. I was wondering why the porpoising was a problem with the 14pitch prop but not the 12. I can trim the motor much further with the 12 than the 14 so I don't think its a problem of over triming. Also with the 12, I cannot make the boat porpoise.

Mike

 

[hwsiii]

Re: Porpoising, is it pitch or motor height?

In the past I have seen that raising the motor some has decreased the amount of porpoising form a prop, and one time it even let the boat drive through the porpoising because of the higher RPM and speed attained by doing this.

H