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Old 28-05-2014, 03:27   #1
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Ballast/displacement ratio, and capsize screening ratio

Hi all,
When talking about off shore sail boats, the two above terms keep coming.
Can anyone tell me what these two terms are in a simple way.
And if I have to buy a blue water sailboat, what is considered a good number or range to look for.

Thanks,
Andrew
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Old 28-05-2014, 04:27   #2
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Re: Ballast/displacement ratio, and capsize screening ratio

Ballast/displacement ratio (often expressed as a percentage) is just the ratio of your ballast to your total displacement.


Capsize screening ratio (normally expresses as a number) is your beam (in feet) divided by the cube root of your displacement (in cubic feet).

CSR values less than 2.0 are considered OK for offshore.
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Old 28-05-2014, 04:45   #3
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Re: Ballast/displacement ratio, and capsize screening ratio

Although this boat is a bit light, it has a decent Bal/Disp Ratio of 48%.

MOORE 24 sailboat specifications and details on sailboatdata.com

This guy Webb considers it a fine Bluewater Boat

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Old 28-05-2014, 05:29   #4
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Re: Ballast/displacement ratio, and capsize screening ratio

Ted Brewer explains the boat design “numbers”:
Ted Brewer Yacht Design
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Old 28-05-2014, 14:18   #5
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Re: Ballast/displacement ratio, and capsize screening ratio

While both the numbers tell you something, they are also somewhat misleading.

As an example. A boat with a low ballast number doesn't tell you much alone. If all that ballast is carried in a bulb at the end of a deep keel she may have massive amounts of righting moment compared to a boat with huge amounts of internal ballast. This boat will also have a higher CSR since it doesn't carry the ballast weight, and so has a much lower displacement.

All despite the fact that the boat could very well be a much better offshore boat than these numbers indicate.


Taken in isolation any of these numbers are at best an imperfect guide, and need to be understood in light of the other issues surrounding the boat. Even worse the numbers can be highly misleading when comparing two very different types of boats.
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Old 28-05-2014, 19:23   #6
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Re: Ballast/displacement ratio, and capsize screening ratio

Quote:
Originally Posted by Stumble View Post
While both the numbers tell you something, they are also somewhat misleading.

As an example. A boat with a low ballast number doesn't tell you much alone. If all that ballast is carried in a bulb at the end of a deep keel she may have massive amounts of righting moment compared to a boat with huge amounts of internal ballast. This boat will also have a higher CSR since it doesn't carry the ballast weight, and so has a much lower displacement.

All despite the fact that the boat could very well be a much better offshore boat than these numbers indicate.


Taken in isolation any of these numbers are at best an imperfect guide, and need to be understood in light of the other issues surrounding the boat. Even worse the numbers can be highly misleading when comparing two very different types of boats.
+1. You definitely have to compare apples to apples. In other words, don't compare a 1980s Tayana 37 to a brand spanking new Bene 38. Compare similar boats to each other.

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Old 28-05-2014, 19:43   #7
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Re: Ballast/displacement ratio, and capsize screening ratio

Quote:
Originally Posted by GordMay View Post
Ted Brewer explains the boat design “numbers”:
Ted Brewer Yacht Design
Thanks, Gord, I would have suggested he buy a BOOK, or Google it.
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