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Old 23-01-2012, 15:16   #31
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Re: Sail Area in Regards to Cat Size

The units do matter.

If for example for the same boat you use square yards of sail area for one ratio and square feet for another ratio, divided by the same units for weight, lets say US tons, the number will come out different.

Now if you take that same boat and use square meters divided by metric tons, the resulting number will be different. Don't believe me? Try converting the sail area to square centimeters and see if you get the same number for the same boat.
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Old 23-01-2012, 15:24   #32
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Re: Sail Area in Regards to Cat Size

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Originally Posted by David M View Post
The units do matter.

If for example for the same boat you use square meters of sail area for one ratio and square feet for another ratio, divided by the same units for weight, lets say metric tons, the number will come out different.

We like to use both John Shuttleworth's formula for Stability called Stability Speed Multihull Dynamics, Inc. - News Article

and Derek Kelsall's Kelsall Stbility Index

It is important to realize that Shuttleworth's formula is a recommendation of when to start reefing and Kelsall's is when he says the windward hull of a catamaran and the center hull of a trimaran will be lifted out of the water, both flying on the leeward hulls.


Derek Kelsall's formulas are attached as a pdf from his website.
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Old 23-01-2012, 15:29   #33
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Re: Sail Area in Regards to Cat Size

For whatever reason that PDF is requiring IE8...which I don't do.

In the article Shuttleworth applies a constant in order to correct for the difference in units, which makes sense.
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Old 23-01-2012, 15:32   #34
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Re: Sail Area in Regards to Cat Size

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For whatever reason that PDF is requiring IE8...which I don't do.

In the article Shuttleworth applies a constant in order to correct for the difference in units, which makes sense.
OK, regarding the formulas I need to bring Cal in, he is the Engineer and can explain.

Regarding the need for IE 8, I'll post them on my website with a link to the page.

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Old 23-01-2012, 16:02   #35
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Re: Sail Area in Regards to Cat Size

Kelsall's Formula's
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Old 23-01-2012, 18:06   #36
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Re: Sail Area in Regards to Cat Size

Quote:
Originally Posted by David M View Post
The units do matter.

If for example for the same boat you use square yards of sail area for one ratio and square feet for another ratio, divided by the same units for weight, lets say US tons, the number will come out different.

Now if you take that same boat and use square meters divided by metric tons, the resulting number will be different. Don't believe me? Try converting the sail area to square centimeters and see if you get the same number for the same boat.
I meant that you would have to convert whatever units you were using to the units called for in the formula you were using. Tons and tonnes are always tricky. You got to know which one is which and are often interchanged by brokers and specs which is incorrect. BTW For us Windows7 users there is that calculater in accesories that does conversions, even angstroms. So how big is that Oram 44 in angstroms?
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Old 24-01-2012, 06:33   #37
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Re: Sail Area in Regards to Cat Size

Here is an example of how we organize the results of each formula for review:

Multihull Dynamics, Inc. - Sample Comparison
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Old 25-01-2012, 14:43   #38
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Re: Sail Area in Regards to Cat Size

The units used are absolutely essential in the formulas for performance and stability. The constant 1.58 in Derek's stability formula only works for displacement in pounds (2240 pounds per long ton), other dimensions in feet, and an answer in miles per hour. In the Multihull Dynamics website, we use the constant 13.8578 to get results in knots of windspeed. If you have specifications in metric units, you must convert them to use the formula with these constants. Or you can convert the formulas to use metric units...the constant will be different.

Hope this helps.
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Old 25-01-2012, 14:49   #39
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Re: Sail Area in Regards to Cat Size

PS, I just noticed the original question related to Shuttleworth's formula, while the equations posted were for Derek Kelsall. The difference is that Shuttleworth's constant is smaller because he gives an answer with a safety factor...the wind speed at which a prudent sailor starts to reduce sail. Kelsall's gives the wind speed at which the windward hull will lift.
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