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Bertram31.com General Bulletin Board
Re: Need some help from the smart guys
Posted By: Capt Patrick McCrary In Response To: Need some help from the smart guys (Bruce)
Date: Tuesday, 11 January 2005, at 8:31 p.m.
Bruce,
Most hull manufacturers will have stats on what speed they've designed a particular hull to operate at, so the first source of info would come from them.
Hull type plays a significant part. Generally a flatter modified V hull is capable of more speed than a deeper V, but will pound the daylight of you. The deeper the V, the smaller the wetted surface is at higher speeds and the more top heavy the hull will become as it rises to a higher plane.
The next big factor comes from length to beam ratio. The wider the beam and shorter the boat, the less capability the boat has to develop higher speeds without more horse power. Conversely, the longer the boat and narrower the beam, the faster the boat will go with less HP.
A narrow beam and low center of gravity, combined with a deep V is the accepted setup for most GoFast boats. The Deep V gives a more comfortable ride in heavy sea conditions, the narrow beam offers less outboard weight to topple over when higher speeds are obtained.
Probably the most dangerous conditions are confronted with center of gravity being too high when high speeds, (above 35 kts), are attained.
One pretty well known set of variables are those for the B31. Beam to length ratio is 0.37:1 & a 24° constant dead rise, that yields a 35 kt hull speed. This speed on a B31 without a tower and as much weight kept as low as possible can be handled relatively safely, but the boat is very tender and reacts very quickly to the steering or factors such as cross wakes & beam wind. A stock setup on a B31 FBC probably has the Center of Gravity is approximately 2' above the water at rest.
Add a tower, two big guys on the bridge, another guy in the tower, batteries out board of the engines, lots of electronics up above the guard rail, and that same 35 kts will become down right dangerous...
Another set of variables might be an off shore race boat with a beam of 10 ft & a length of 38 ft, coupled with a 24° dead rise. The beam to length ratio is now 0.26, there is very little weight above the guard rail, passenger are below the guard rail, engines are aft, and a safe speed, in terms of chine walking, may approach 55 - 60 kts.
Again, the depth & length of the dead rise will determine the amount of power necessary to achieve a planning attitude. The B31, along with it's relative lightness & lift strakes has no problem popping onto a plane at 13 kts, even with small power plants. But there is a point that weight and length of the boat become too great for lifting strakes to efficiently raise the hull out of the water.
At that point, most designers start decreasing the dead rise angle and shortening it to the forward part of the hull. This is typically the case with larger fast boats such as Jim Smith, American, Tribute, and that lot. All capable of pushing 45 kts safely in the right sea conditions.
I don't have any dimensions on the Smith type hulls, but guess work brings the B:L ratio in around 0.26. Since they are considerably heavier & have a higher CG, the shaft angle is being kept at around 10° and bottom is more of a modified V with a quick dead rise forward and very low angle aft.
I guess if there is a "rule of thumb" hidden here, the faster the boat is pushed, the smaller the beam to length ratio has to be. But CG and hull design still play a big part on what ever the equation is,
Also, the faster the boat is pushed the lower the shaft angle has to be, since it'll take more power to push it harder, which equates to larger props to load the engines. The greater the shaft angle & larger the engine, the more tenancy for the stern to be pushed up, resulting in the bow going down. Ya' can't get fast if the bow is plowing water...
The last,but very important consideration is the construction of the boat. It doesn't matter how fast the hull configuration is if the construction won't take the pounding, vibration, warp, twist, and bend that comes as a by product of speed.
Case in point was an old 35 Viking that was a wooden hull with a pair of GM 892 naturals in here. A Coonass Hot Rod mechanic talked the owner into letting him goose the engines up to full TA status, max HP, & bypassing the buffer boxes. Originally the boat was a 16 kt cruise with about 18 kt on top. Came the big day for the sea trial, and as expected, the old Viking was a damned rocket ship.
Only problem was that buy the time they got back to the dock, the boat was falling apart at the seams. The interior had as much saw dust in it as my table saw has under it, and the bilge pumps were cycling far more frequently than the did before leaving the dock...
So, the bottom line to this is first call the builder and get his opinion as to max safe speed, then survey the boat & construction methods used, & calculate the B:L ratio.
If a B31 has a B:L ratio of 0.37 & gets you to 35 kts, a 65' Smith has a B:L ratio of 0.26 & gets you to 45 kts, it looks like that for every kt above 35 kts, the B:L ratio has to fall by roughly 0.01. Also the heavier, wider, and longer the boat, the less dead rise it can have.
See what happens when I get too much time on my hands?? Jim's boat left Saturday, up to date on Gerry's boat, almost finished with George's boat, & Tommy's boat doesn't show up till Thursday.
I'm in the eye of the storm...
Br,
Patrick
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