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Bertram31.com General Bulletin Board
Re: HULL SPEEDS
Posted By: Richard Miller In Response To: HULL SPEEDS (craig mac)
Date: Wednesday, 14 June 2006, at 12:48 p.m.
Since the last time I got involved in a discussion on this website, a lot has happened - married, two kids, etc., all of which reduced the time I have available for Bertram31.com. But I can't resist adding a few thoughts on this subject.
I repowered with Yanmar 6LP-STEs in either 1997 or 1998, I forget which. When I did so, as many of the old timers on this site may remember from old postings, I talked to C. Raymond Hunt Associates about weight distribution. They advised me to move the center of gravity as far aft as I could during the repower, and I did. The result is that my boat rides with a little more chine in the water at the stern, and it has no tendency at all toward "bow-steering." It is as stable at WOT as at cruise, and it responds easily to the wheel, turning exactly the way it should at WOT. It also responds to the helm noticeably better at cruise. Interestingly, before the repower, with the original gas engines, I noticed the squirrelly feeling at WOT that others have commented on.
Twe key points about the race boats: they have no cabin structure, obviously, and therfore their centers of gravity are further aft than on the production boats. Also, in addition to having no superstructure, the original Moppie was built of cold-molded conrtuction. She weighted thousands of pounds less than the production boats, which was a big factor in getting those higher speeds with converted Lincoln car engines. Several years back I found an equation for estimating speed for deep-V hulls in a book about naval aechuitecture, and I solved from horsepower and speed to get to Moppie's estimated weight. The formula indicated that, in order to reach 50 mph with about 800 shp, she couldn't have weighed more than about 7000 pounds.
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