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Posted from Archive: Porpoising by DonJF

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Posted from Archive: Porpoising by DonJF

Post by Club Archivist » Tue Nov 29, 2005 1:51 pm

I apologize for bringing up this topic again as it was discussed back in August 2000. I just need a follow-up on the problem of pospoising of a '63 CC ski boat. I got my boat in the water three times last summer and had a terrible issue of porpoising and must address it now.

Comments were made by Don Ayers that using two 6" X 6" wedges tapered to 1/2" placed approximately 14" on each side of the rudder greatly reduces this problem on a 1949 18' CC. Ray Barber also commented.(where are you Ray?)

I'd greatly appreciate any additional comments.

Many thanks...........DonJF
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Posted from Archive: Porpoising by Don Ayers

Post by Club Archivist » Tue Nov 29, 2005 1:54 pm

Don;

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 consider.
1. The reconstructed boat was lighter than the original due to a light, strong mulit-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 ugly head.

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 porpoise. 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.

Thx

Don
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