boat porpoising

[jims3000]

I have a 17ft bumble bee bass boat with a mariner 135 at mid speed it porpoises I have read a number of things can cause this.it has a 6 inch jack plate on it and heard that maybe a 10 inch jack plate might help.i have tried about 7 different props so far and raising and lowering motor with no change and I also move trolling motor batteries to the front.running out of things to try.

 

[275prostratos]

im going to say, not enough hp for size boat with that jp. take the plate off and start all over, will need all your numbers to, rpm, gps speed , prop size . its like picking up the end of a 10 ft 2x4 with one hand if you don't have enough muscle it will fall down to ground . some people put hydrofoils on their motors to help improve but I think its a bandade

 

[jims3000]

I know I am going to have to put a floor in it and the foam may also be water logged..rpms hit 5400 and boat runs 54mph when you get past the porpoision

 

[stumpknocker]

If you need new floors, chances are high .. you need more than just the decking replaced.. if your lucky the rot is confined to the area around the drains.. but expect to find rot in the stringers as well.. and if it has sat long enough.. could extend to transom. I know yo don't wana here that but be prepared.. u can do all your self .. but very time consuming and use epoxy resin.. epoxy resin water proof standard resin is not, and don't use anything but exterior plywood, never pressure treated.

 

[BulletTJ]

If you have transom problems that could be part of your problem. Get that checked before you put the boat back in the water. Some older boats have a certain speed where they will porpoise. My old Stratos did it around 40 mph. I had to trim way down at that speed. It smoothed out at around 42 mph. If everything checks out good, a jackplate or transom wedges with a negative angle will help. I also added a cavitation foil to my boat. It helps with holeshot and helps me stay on plane at lower speeds. Good Luck.