pitching problems on powerboat

[massimo]

I’m enfacing often porpoising problem on powerboats stern drive and outboard powered size 20/25 ft
as in the drawing enclosed. They look to be traditinal hulls with LCB about 2 mt from stern 28% of LT and weight about 2/3 ton.
If someone have experience on this problem and can help me

[Scott]

I don't know the answer, but it occurs to me that some of the people who might have some good feedback for you might not have AutoCad on the computer they're reading this forum from, so I thought I would also post a thumbnail of your drawing as an image.

(If you posted the dwg format to keep your drawing more private, just say so and I'll remove my post. Nice drawing by the way.)

[DavidG]

Seem to remember that Savitsky did some work on Porpoising, looking through my old lecture notes this is rather more complex than I can post. However you need to solve a moment balance equation, where the trim angle and deadrise angle are critical.

The latest edition of Larsson also covers this. Another excellent reference is Dhows and Deltas by Levi.

[Evolution Yacht]

We had a boat in our shop with a similar problem.28', 8' Beam, 25dg deadrise an transom w/ stern drive. The boat fully loaded sat 4" above the designed waterline. when on plane it would roll over on one side and ride there. We did a weight study and added 2100 lb. We also felt because the engine and tanks were mounted on center there was no moment to right the boat. So we kept the weight low and outboard as possible.It solved 99% of the problems. I dont know if your problems are the same.

[Stephen Ditmore]

My remarks at
http://boatdesign.net/forums/showthr...p?threadid=335
might be more applicable here than to that discussion. I think we need to know if the behavior described as "pitching" is what is known in the technical literature as "porpoising," which I think would be defined as oscillating trim above a certain speed...