# Calculating L/B ratio uses beam at waterline?

Discussion in 'Boat Design' started by abosely, Aug 16, 2023.

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### aboselySenior Member

I tried looking up answer to this question but didn’t find it specified.

When calculating length to beam ratio, beam used for calculating is waterline beam, correct?

It sorta seems obvious, but when looking for the answer, didn’t see beam at waterline specified.
Kind of an embarrassingly simple question, but would like to verify that L/B ratio uses beam at waterline.

Edit: Guess it might be useful to mention that wondering about a V hull, Wharram Narai actually, just curious what the L/B ratio is.
Waterline length is 32’-3” and beam at WL is 35-½” so if I did that correctly it would be L/B of 10.9.

Cheers, Allen

Last edited: Aug 16, 2023
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### TANSLSenior Member

You can use the full beam or the beam at the waterline but the length must be measured accordingly.
If you want to find out something related to the stability of the boat, for example, you should use the dimensions of the waterline.

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### fallguySenior Member

yes

it gets a bit more complicated, because even the waterline beam is changing, but it is usually widest point of beam

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### aboselySenior Member

Read an article commenting about L/B ratios, (generally speaking) and it was mentioned that about 10-12:1 was usually considered a long slender hull form.

So was curious what the L/B ratio was for a Narai.
Since it is a V hill was guessing the waterline beam would have the most affect on hull speed and how much wake it generates, at least in general.

The lightly loaded waterline isn’t much higher than the heavier loaded waterline so the WL length doesn’t change dramatically.

I’m not trying to calculate any specific parameters, just wondering what it’s L/B ratio is, in round numbers so to speak.

Appears to be about 10:1 L/B ratio, from what I can determine.

Cheers, Allen

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### TANSLSenior Member

That value (L/B) varies greatly depending on the type of hull you are dealing with, from very high values for a kayak, to much lower values for a fishing boat, a tugboat or a barge. (If we talk about multihulls the relationship is totally different)

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### aboselySenior Member

I was just curious if the Narai has what might be considered a relatively long slender L/B ratio.

Relative to other catamarans common nowadays.
Wasn’t comparing any performance perimeters really.
From what I understand 10-12:1 L/B ratio and fairly pointed bows usually indicate hulls that usually are relatively easy to propel through water.

Edit: Should have mentioned that the Narai is a catamaran.

Last edited: Aug 17, 2023
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