Stability Question

[phishown]

I have a 26´ 6" custom built boat that had a 210 hp cummins and has been repowered with a 330hp bta. The cruise was 17-18 knots and now has a cruise of 22-23 knots. The hull is very similar to a shamrock with a 8´6" beam. The only difference is the keel is 18" at it´s deepest point from hull aft and does not protrude aft under prop...it stops around 3´ from prop. The shaft comes out of the bottom of the hull and not out of the keel. The stability issue is that the boat is very tender while on plane (especially when walking from side to side at the stern. Also you have to really steer the boat....The boat wants to lay over sometimes while on plane. I have to constantly adjust tabs. I think the tabs are a little small at 12" wide and 8" deep. I understand I am exceeding the hull speed for this design. Do you guys think it would help by trimming off 6" of keel or more? From the research I have done to this point the boat is wanting to ride on the keel with the faster speed. This is a modified v hull and is fairly flat in back.

Thanks for any input.

Mark

[gonzo]

You pretty much answered you own question by admitting the design speed is exceeded. Any "fix" will be a compromise and create other problems. The real question, is what can you live with. Adding wide lifting spray rails at the chines will add beam and can be used to correct chine walking.

[phishown]

If I were to trim the keel a bit....I could live with taking away low speed tracking if it would help the the chine walking. I'm not sure that taking away keel depth would create any issues aside from this do you? The boat is pretty flat in the back so stability at idle wouldn't be a concern. I'm not sure that trimming the keel would help if it would I would be all about it as soon as tomorrow. I do have spray rails and I could extend them aft all the way to the stern like the Fortier design.

[gonzo]

The horizontal surface of the bottom is the same with or without keel. In other words, there won't be any significant difference in lift. The lateral resistance of the keel is likely part of the problem and may help to make it shallower.