Hull Bounce?

[stuee]

Hi.
I have this problem that when my boat gets to around 60kph on the water the front starts bouncing so i have to pull back to about 55kph.

i have a 21ft fibreglass halfcab with a johnson 235hp on the back.

I have had the trim fully down and removed the rest pin to see if it will go further but it wont.

How do i know if its going down far enough to get the right angle or could it be something else.

I have a battery and 25l fuel at the back and a battery and spare 25l under the front seats at front and anchor too.
When the boat is still in the water it sits flat and doesn't appear to be heavy at the back..
Any ideas / suggestions?

thanks
Stuart

 

[pn]

Re: Hull Bounce?

is porposing the correct spelling

 

[82rude]

Re: Hull Bounce?

hope this doesn't apply to you but mine was doing the exact same thing and when it was all said and done she was waterlogged.all the foam was soaking wet.

 

[stuee]

Re: Hull Bounce?

Hi 82rude. Its just have new stringers and floor fitted and no foam under it. thanks

 

[Silvertip]

Re: Hull Bounce?

Porpoising is a trim & load issue. Basically it amounts to the thrust from the engine trying to raise the bow of the boat but it cannot hold it so the bow drops only to rise again. Trimming higher may help, redistributing the load may help, a prop with more stern lift may help and of course, water soaked foam as mentioned may be contributing to or may be the sole cause of the issue.

 

[stuee]

Re: Hull Bounce?

Hi Silvertip,. So with the trim completely down im probably going the wrong way, should i be trimming it up instead which should lift the nose up more?? am i correct in thinking this?

Cheers

 

[James R]

Re: Hull Bounce?

Try putting some weight up front, a body or two, or taking some weight out of the stern.

 

[82rude]

Re: Hull Bounce?

if you got trim all the way in at those speeds the boat will dig in the front and god knows what will happen .trim out when going fast.you will find the sweet spot somewhere.s.that may not be the total issue but you do not go fast with trim full in.

 

[MaPaHa]

Re: Hull Bounce?

I'm not sure exactly what type of hull that is but several years ago I fixed (improved significantly) three different boats that were doing the same or similar bouncing by using the following method. Two of the boats were tri-hull bass boats with 115 hp mercury motors and one was a higher performance v-bottom bass boat with the same motor.

At the time I noticed that the newer model bass boats had a lip at the back of the boat on the bottom rear edge that was cupped downward and it seemed to me that it would help with porpoising. So, having worked with fiberglass and bondo-filler on car-truck bodies I tried to create this type of cupping. I started by sanding the last 4-6 inches of the boats bottom so the material would stick. On the first layer I used a product called "tiger-hair" which is a high grade bondo/fiberglass with fiberglass hair in it. It's made to build up in layers (unlike regular bondo body filler) so I crafted this wedge to turn the water down as it left the rear edge of the boat. This stuff does not finish smooth so I used a thin coat of regular filler on the finishing coat. It was a lot of work but it worked. I figured there was a reason the most of the high performance bass boats had this lip and guessed that's what it was for. Mine was more like a 3-4 inch wedge rather than a curled lip. It allowed me to trim out more without bouncing and I gained some speed. The last boat I did it on was a 1980 Ranger v-hull with a Mercury 115 and a Laser II prop.

You might try a fin on the motor also (much easier to install). I've used them off and on over the last 30 years on different boats and they work well to help get the butt of the boat out of the water and settle down the bouncing. As previously mentioned, I would look at a stern-lifting 4 blade prop also. If you can drop the motor a notch it may help but trial and error is about all you can do there. Too deep and it will drag more.