# Help, how do you calculate BRUCE NUMBER ?

Discussion in 'Boat Design' started by mcm, Nov 20, 2014.

1. Joined: Sep 2006
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### mcmSenior Member

I once had the formula for calculating the BRUCE NUMBER, but now i can't find it.
Can you help me out ?

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### bhnautikaSenior Member

google bruce number and you will get a calculator for that

3. Joined: Sep 2006
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### mcmSenior Member

thanks bhnautika,

i missed that first time round on google; but got it this time !

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### Richard WoodsWoods Designs

Before you get too carried away with Br No you might like to read this article (extract below)

http://sailingcatamarans.com/index.php/articles/12-to-be-published-mainly-technical/51-hull-shapes

"People try to simplify hull design and performance predictions, formulae like the Bruce Number and KSP spring to mind. These coefficients rely only on basic sail area, displacement and length dimensions yet purport to give an accurate indicator of performance. It's easy to show that these formulae cannot be relied on if you consider that a Tornado would have the same rating whether it was sailing forwards or backwards! I suspect the latter is slower! Its probably as accurate as predicting car speed from the kerbside weight and engine horsepower."

And in fact since the BrNo has no length factor the answers are especially meaningless. For example, try a boat with length 1ft, sailarea 1sqft and weight 1lb. How fast will it really be?

A better simple multihull performance indicator is the Texel rating

http://www.texelrating.org/pub/Pagina.php

Other articles you might find of interest

http://sailingcatamarans.com/index....-articles-technical/40-multihull-rating-rules

http://sailingcatamarans.com/index....ormance-questions/107-performance-predictions

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### Steve WSenior Member

Richard, you are absolutely right, same applies to the online sailboat data websites that people treat as gospel when comparing monohulls.

Steve.

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