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Old 26-12-2009, 22:17   #1
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Use Bruce #'s to Compare Multihulls

Using the Bruce # has been around for many years. I found a webpage that will run the numbers for you, greatly simplifying the math (as in no math to do) The page is located at:

SA/D Calculator

Just plug in the displacement and sail area, and you can get an actual number and can compare that with any other boats you might be interested in.

This is from the Schionning Designs -

But I am not sure it will pop up so I will paste it here:


5. Sailing Ability and Performance

Power to Weight Ratio

Power to weight ratios show how well a cat will sail in light conditions. As wind strength increases, one reefs the power to stay at safe acceptable speeds (this is different for different people). The Bruce Number is a commonly used value and very useful in comparing cats, displacement is not always reliable and will vary with load.

A Bruce Number =

1 is very slow, 1.3 to 1.4 is a good cruising value, 1.5 to 1.9 reflects a very fast cat. Boats like the French 60ft Tri's and Club Med are running to extremes like 2.3.


Give it a try, can be fun and informative!
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Old 27-12-2009, 00:43   #2
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I have an extreme racing boat????? (On SA/Disp)
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Old 27-12-2009, 13:44   #3
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Quote:
Originally Posted by 44'cruisingcat View Post
I have an extreme racing boat????? (On SA/Disp)

If you are refering to this:


16 to 18 Heavy offshore cruisers
18 to 22 Medium cruisers
22 to 26 Inshore cruisers, racing boats
26 to 30+Extreme racing boats


That does not work with a multihull. That is SA/D......For example, if I plug in my boat at 650 Sq. Ft. and 7,000 displacement I get SA/D 28.42.....same as an extreme race boat (monohull)

But the same numbers give me a Bruce # 1.33 Right in the range of good cruising numbers. Big difference.

If you plug in a real bad boy from Nigel Irens. His race/cruise boat comes in at Bruce # 2.09 ! SA/D of 69....yikes!
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