hull bouncing (porpoising?)

[nateb]

Hello, I just bought a 1986 Fourwinns 17' bowrider with a 140hp mercruiser (alpa one drive.) I've had it out a few times and the hull has really been bouncing, at first it was only at lower speeds on plane but now you can't even speed past it. I thought it wasn't trimmed down enough but I looked at it out of the water and the prop points well below parrellel to the hull plane. I'm wondering what the problem could be, it doesn't seem like there is any slop in the trim. One thing I noticed was the limiters on each side of the driver are set differently. Is it possible to lower the drive further? I looked it over and didn't see any adjustments.<br />Thanks, Nate

 

[QC]

Re: hull bouncing (porpoising?)

Trim tabs are the ultimate solution, but you could try changing the boats balance. If you simply take anything heavy and stow it as far forward as possible and move anything light toward the stern it should make a big difference. I would try this before I spent any money. I love adjustable trim tabs and believe you should get them anyway, but balance alone may solve your issue.

 

[crazy charlie]

Re: hull bouncing (porpoising?)

There are 2 solutions but first you can try trimming the drive ALLLLLL the way down until you are planed off.Then just a little at a time trim up until the bouncing starts and then tap the drive back down a touch.If that does not work you can get a Doel fin or a Stingray stabilizer for the drive.It will cost 30-40$$ and you can install in 10 minutes.If that dont work you can try tabs(I am assuming you dont have trim tabs????) That is the most expensive solution but the one that WILL work.Charlie

 

[nateb]

Re: hull bouncing (porpoising?)

I will try to add something heavy to the bow, I'm not sure i can get under there. Charlie, the boat bounces when it is trimmed all the way down, as soon as you start to pick up speed on plane. My thought was that it wasn't trimmed down far enough at the lower limit and I was wondering if that can be adjusted? Or would it be easier to try a stabilizer or tabs?<br />Nate

 

[crazy charlie]

Re: hull bouncing (porpoising?)

Does it appear to be as far down as a right angle????Sounds like you will need a stabilizer either way.<br />Because you will have to trim up after planing and the bouncing will continue.Moving the weight to the front will help you to plane quicker but will not stop the bouncing.Charlie

 

[xltier]

Re: hull bouncing (porpoising?)

those whales tails work great for that

 

[Chris1956]

Re: hull bouncing (porpoising?)

Nate, It does not sound like you are porpoising to me. To porpoise that boat, the bow should be carried quite high(you would likely need to stand up to see to drive) when running. Is it doing that?<br /><br />Also, the "limiters" on the drive are not the same. One is for a trim Gauge and the other is the trim limit. Does the trim limit switch even work? They usually rot away after a few years, and your's could be ~ 20 years old. Also, there is no limit to trimming down, only up.<br /><br />Is your hull true, or does it have a "hook" or "Camber" in it?

 

[nateb]

Re: hull bouncing (porpoising?)

The hull looks fine and the boat rides up on plane pretty level. It will come out of the hole and start off fine, then it will start to bounce and keep getting worse until I have to slow it down till the bow goes up/comes off plane then speed up again. The first time I brought it out it was only bouncing if I tried to drive between about 20-25mph, if I stayed faster it was fine, last time I couldn't even get it up to speed because it was bouncing so bad, there was only one more person in the boat and a little less gas.

 

[QC]

Re: hull bouncing (porpoising?)

Chris,<br /><br />I guarantee that a boat can porpoise even with the trim all the way in AND the bow running down. I had a 20 footer that was just nasty. Weight shift made it livable before I bought trim tabs. Trim tabs made it perfect. This boat ran in the high 50's; my bow was definitely not up. If the hull is right my understadning of the core issue is weight balance. Tabs obviously help, but you would be surprised what a difference weight shifting makes.

 

[DJ]

Re: hull bouncing (porpoising?)

nate,<br /><br />Porpoising, is the up and down "bounce" of the bow. It has nothing to do with bow attitude (up or down). Unless, you are over trimmed-bow down.<br /><br />Usually, it is a weight issue-too much aft.

 

[keelhauled]

Re: hull bouncing (porpoising?)

A dirty bottom will contribute... I had the same problem with my 18' bowrider. The bottom was rough when I bought it. After I cleaned it up and buffed it down, the bounce went away.<br /><br />Just a thought, and easy to check!<br />-KH-