Porpoising can be a problem with many of these smaller boats for reasons Ray
pointed out. It think that two things can make the biggest difference.
- 1. The center of gravity is to far aft. This is very similar to the CG of
aircraft. The CG affects the attitude at which the boat will ride and the
planing surface location. I once saw a 17' Barrel back that a guy totally
reworked. He decked over the front cockpit making it look like some kind of
gentlemen's racer and put a 350 SB in the engine compartment. The boat
porpoised so badly that he could not even get up on plane. It was a sad
sight for a once beautiful 17' Barrel.
2. The planing surface and aft is not fair. If there is hogging and
warping of the planing surface this acts like a wedge and puts down forces
or the opposite on the attitude of the boat.
In the case of the 1949 18' Sportsman there were several factors to
- 1. The reconstructed boat was lighter than the original due to a light,
strong multi-lamination bottom.
2. The owner wanted to replace the KL 105 HP with a strong V8 (325 HP).
3. The hull was not designed to go that fast with that particular CG even
though the KL and V8 were similar in weight.
So we were close to the original CG but with the loss of weight and the
higher speed we developed a porpoise above 40 MPH. It was not dangerous
just annoying to the owner because he liked to get out and run her up on a
smooth day. During test trials with no interior in the boat and two in the
front the boat did not porpoise at speeds up to 50 MPH. Once the boat was
completely assembled and with a tank of gas the porpoise showed it's ugly
I used a 6" X 6" X 1" block of Lexan (Clear) and cut it on a diagonal to
produce a 1/2" rise. I used epoxy to glue to the bottom in the location I
mentioned. The cool thing was the wedge was clear so it literally
disappeared and it does not catch your eye. The owner then added 100 LBS of
weight to the nose in the form of two 50 LB free weights and no more
She tops out at just under 50 MPH because of the prop I used. I did not
want him going any faster than that. I used a lower pitch so it will come
out of the hole like a comp ski boat.