Length : Beam Ratio's

[Willallison]

It's generally accepted that for a planing hull for normal service one shouldn't employ a length : beam ratio of less than 3, and that thinner is generally better in terms of efficiency and seakeeping.
And yet a wider boat will plane at a lower speed than a narrow one.
Obviously, once speed increase significantly wetted surface would hamper the wider boat. But what about a boat not designed to travel particularly quickly - for arguments sake a 33 foot boat that travels at 15 - 20 knots.
Will short and fat work? And how would you calculate how it performs against a narrower, but longer boat with the same volume...?

[jehardiman]

Yes, you can go lower than L/B =3, but you buy yourself more problems with insignificant gains. From Series 62 data, L/B < 3 models had a significant first hump, sometimes exceeding the power requirements of a speed 3 times faster. This causes uncomfortable surging and loss of speed control in operation. Additionally, pitch angle was excessive to step up over that hump and the vessel becomes sensitive to lcg location. At higher speed, there is some small advanage in resistance, but at volumemetric Froude numbers between ~3 and 3.5, but these decrease as you begin to climb the second hump.

[tom28571]

Will,

I can't offer a definitive answer to your question but am interested in the answers you get. I wonder just what you mean by beam length ratio though. I assume you are using the waterline length and waterplane max beam. I like the term "aspect ratio" which takes the shape of the hull into account, sort of. Divide the waterplane area into an aft rectangle and a triangle forward. Take the length of the rectangle and add half the length of the triangle to that to get the length function of the beam length ratio. I settle on a ratio of about 3.4 plus or minus a bit as a good one that works for me in a moderate speed planing hull.

I don't know how the efficiency over a speed range of a boat like this compares to Dave Gerr's needle boats which he says are very efficient.

[Willallison]

Yes, you can go lower than L/B =3, but you buy yourself more problems with insignificant gains. From Series 62 data, L/B < 3 models had a significant first hump, sometimes exceeding the power requirements of a speed 3 times faster. This causes uncomfortable surging and loss of speed control in operation. Additionally, pitch angle was excessive to step up over that hump and the vessel becomes sensitive to lcg location. At higher speed, there is some small advanage in resistance, but at volumemetric Froude numbers between ~3 and 3.5, but these decrease as you begin to climb the second hump.

hmm - yes, I can see how lcb would be more critical and also how trim whilst climbing the hump might increase.... But a boat with very low transom deadrise - maybe even a tiny bit of hook in the bottom should overcome this to some extent.....
The biggest advantage of course, is that you maximise interior space

can't offer a definitive answer to your question but am interested in the answers you get. I wonder just what you mean by beam length ratio though. I assume you are using the waterline length and waterplane max beam. I like the term "aspect ratio" which takes the shape of the hull into account, sort of. Divide the waterplane area into an aft rectangle and a triangle forward. Take the length of the rectangle and add half the length of the triangle to that to get the length function of the beam length ratio. I settle on a ratio of about 3.4 plus or minus a bit as a good one that works for me in a moderate speed planing hull.

I don't know how the efficiency over a speed range of a boat like this compares to Dave Gerr's needle boats which he says are very efficient.

Tom - not many answers so far! Yes, I was referring to the WL length and beam. The problem with very narrow boats - as I suggested above - is that they tend to be somewhat awkward to live aboard - everything's a bit of a squeeze. The idea of a short(ish), fat(ish), flat(ish) planing hull has always intrigued me, so I'm simply fishing for thoughts.
Perhaps I'll email Dave and get his thoughts too....

[mmd]

If one is planning to be on board at other times than underway at planing speed, one must also consider habitability. A beamy hull with low deadrise and hard chines will have a very short roll period accompanied by lurches and sudden decelerations that would be very uncomfortable while trying to barbeque in an anchorage where there was a small chop.

[jehardiman]

Originally Posted by Willallison

hmm - yes, I can see how lcb would be more critical and also how trim whilst climbing the hump might increase.... But a boat with very low transom deadrise - maybe even a tiny bit of hook in the bottom should overcome this to some extent.....
The biggest advantage of course, is that you maximise interior space

Tom - not many answers so far! Yes, I was referring to the WL length and beam. The problem with very narrow boats - as I suggested above - is that they tend to be somewhat awkward to live aboard - everything's a bit of a squeeze. The idea of a short(ish), fat(ish), flat(ish) planing hull has always intrigued me, so I'm simply fishing for thoughts.
Perhaps I'll email Dave and get his thoughts too....

Short, fat, and flat....I cannot think of a worse hull shape for a habitable speed boat. Racing boat maybe, not a crusier. As mmd says, motions at rest would be intolerable (do the math on waterplane force to inertia for any reasonable weight), power requirement would be large, even accomadations are difficult because of the hull shape.

The only thing I could see that type of hull shape for is one of those "marina mansions" with everything forward of the engine compartment (which shows how "manly" the owner is btw ) done in red velvet tuck-and-roll and a huge round bed only usable while tied between finger piers.