Is BWL an appropriate measure for semi-circular hull sections?

Discussion in 'Boat Design' started by Paddlelite, Feb 4, 2013.

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Simple newbie question here. I see BWL (maximum waterline beam) in so many metrics. But isn't there a hidden assumption that the hull sides are mostly vertical? What if submerged hull sections are semi-circular or even "V" (I'm thinking canoes and kayaks)? Then, the beam becomes significantly less just below the waterline.

So, is there some critical difference concerning the precise point where the hull meets the water's surface, or is BWL more a measure of convenience based on typical shapes? In design terms, the question would be whether there is some advantage to flattening the edges of a semi-circular hull section to narrow the beam right at the waterline.

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Eric SponbergSenior Member

First question: No, there is no hidden assumption that the hull is wall sided at the waterline.

Second question: No, there is no critical difference concerning the point where the hull meets that water's surface.

The Bwl dimension is used when calculating the hullform coefficients such as Block Coef., Cb; Midship area Coef., Cma; and Prismatic Coef., Cp. These coefficients are critical to the performance of a boat, not so much the dimensions themselves that are used to determine these coefficients. Also, the hullform in the middle of the boat and the submerged midship area can be a variety of shapes for different reasons. Generally, the fuller the shape is at the turn of the bilge, the more form stability the boat has. Form stability is a good thing, but that can be taken to the extreme to the point where too much beam or too hard a turn of the bilge can be a bad thing. And it all depends on the boat at hand, whether it is a power boat or a sail boat.

I hope that helps.

Eric

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3. Joined: Feb 2013
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Thanks. So if I understand what you're saying, I'm not going to make my boat faster or better (even though it might appear so by the hull coefficients) by shaving small amounts off a tiny area immediately at the waterline. Such minor tweaks might affect the numbers, but not really how the hull behaves in water. That must mean that the coefficients must be viewed in the whole context of the hull shape, and not by any strict rules.

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Likewise for bow entry half-angle?

Similarly, I assume I should not worry about my bow entry angle if I'm otherwise happy with my semi-circular bow sections. That is, I should not try to square off their very top edges at the waterline just to get a lower measured half-angle of entry.

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Eric SponbergSenior Member

I'd say correct on both counts. I think the differences you would be achieving would be minor.

Eric

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