Bad Porpoise
 

I have a Hallett 270 with the 2 step hull, just added some more power and now at 80-90 mph it has a terrible porpoise problem.
I currently run a Bravo 30p 4 blade, would a 5 blade help ? I am also running a Imco Extreme drive with extender box....

anyone ???

That porpoise should just be the boat losing suction and then grabbing the water again I would think. Maybe the drive is too deep. If you go with a shorty then I would definetly put on a 5 blade. My Fountain gets squirrly at 80-90 but no bouncing at all. I would like to see some expert opinions.

I'm already running a fairly short Imco drive ...

Porpoising can be caused by a few things. The bottom configuration can be built with a rocker or a hook. These situations can be induced by the builder purposely or can be in the bottom due to storage or bad builds. Ultimately a boat will porpoise because the propeller is trying to caryy the bow higher and at some point is unable to do so due to drive height, CG or one of the first mentioned issues. Set up can amplify these results (drive height, set back, propeller selection). Also, some boats just weren't built or tested to run in the speeds we are trying to get them into.

Try a 5 blade propeller. It will lift the stern more and may cause the boat to run slightly flatter as it was intended to do. If this doesn't help, going the oposite way by building bow carrying help into the 4 blade prop is a great option. Ultimately drive heights may need to be adjusted. Lastly, talk to Hallet and see if they have this issue on the 270's with your set up.

Thanks Bblades, I did talk to Hallett, but they weren't as definitive as I would of liked.I too think I should try a 5 blade, but was looking for
a little more direction before I embark on the task of prop testing .

Put a long straight edge on the bottom running surfaces. A hook is the hollow above a straight edge. A hook will cause suction and hold the boat in the water. Generally bottoms going near a hundred or more are perfectly straight. No hooks or rockers. Some hulls usually conventional v's will have a slight rocker to help lift the bow. Others may have hooks near the outer strakes to help the boat run flat. Its possible a slight rocker was designed in to the boat increasing the speed with stock power and now you have passed that targeted speed. Just a guess see what you have and deal accordingly.

Was it hopping before you added the HP? Were you running in that speed range before? What is your red line? What R's were you hitting at WOT?

It didn't happen before because all I could obtain top speed was 80
The bottom appears straight but my slip numbers are a little high,

above 15%...now from 80 on up its hops unless the tabs are dragged
for a while...

I have a Lavey 2750 that had a bad porpoise problem over 70 mph. After I put a Whipple on it it really got exciting. I tried different props, 4 & 5 and no help. Finally had the bottom blueprinted and removed a couple lifting strakes and went with a 2" Imco shortie with a Hering 5 blade. Now I can run low 90's with no bounce at all, almost can run it hands off, and no tabs. Very happy but a little poorer.