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Old 12-07-2008, 06:40   #1
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Physics, specs, and wind...

I was browsing various hardware at West Marine the other day and sort of ruminating over the different breaking thresholds on the labels.

Starting with mast height and % sail how can I calculate the force on the boat for a given wind speed?

I guess one of these questions is sort of implied... How can I get square footage of the sail from the mast height and the label on the sail... Then, how do I use that with wind speed to calculate force?

If this is a case of me being so dumb I can't even ask the right question just let me know... It won't be the first (or last) time that happens...

Happy Saturday,
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Old 12-07-2008, 07:40   #2
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"How can I get square footage of the sail from the mast height and the label on the sail... "
Well, you don't. There's no size label on a sail and the mast height doesn't matter. You get a tape measure and measure the sail, then use grade-school geometry to multiply out the area of it. Length of the foot times height of the leech roughly times two will get you close. But since the wind is usually sliding past the sail not broadsiding it, the force will usually be lower too.

You'll find formulas on the web for calculating wind loads per square foot (or meter) based on wind speed, they're the same for all applications, architecture, sailing, whatever.

That still won't give you the 'force on the boat' since the boat heels (spilling some force) and sideslips through the water. But it will give you some rough numbers. Depending on what you want to do, you might want to buy a text on marine architecture or engineering, where the issues are examined and treated in more detail.
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Old 13-07-2008, 07:38   #3
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..... You get a tape measure and measure the sail, then use grade-school geometry to multiply out the area of it. Length of the foot times height of the leech roughly times two will get you close. .....
I would try: length of foot times length of luff divided by roughly 2. I reckon this will be a whole lot closer
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Old 13-07-2008, 10:23   #4
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Luff, yes. Must find espresso....
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Old 13-07-2008, 11:53   #5
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A good sailmaker could get you your answers.

A back door way of doing this is to get your boats stability curves and see how much moment is being applied to your rig in order to heel the boat a given number of degrees.

For example if your in 10 knots of wind and your boat is heeling 12 degrees then you know how much force 10 knots of wind applies to your boat heeling at twelve degrees by looking up 12 degrees on the stability curve. The answer will be a moment (force times distance)
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Old 13-07-2008, 23:53   #6
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Hhhmmm....

Actually, theoretical maximum based on size of sail and speed of wind is probably a good number. The context of this is that I want to know how strong of stuff I need to buy.

Or, perhaps I should step back more.

How do you determine how big of gear you actually need?
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Old 14-07-2008, 00:41   #7
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Hi Jack, this might seem silly but why donít you look at other boats with similar displacement and design characteristics that have crossed an ocean or 2 to give you a realistic feel.

Why try to finesse the numbers when there are so many good (and bad) examples out there?
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Old 14-07-2008, 01:48   #8
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The deck hardware catalogues usually have selection info based on size and sail dimensions. My Harkin certainly does

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Old 14-07-2008, 05:28   #9
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Since it hasn't been pointed out you can also use Harken's CompuSpec site Harken Compu Spec as a starting point.
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Old 14-07-2008, 05:33   #10
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Jack,

Even if you calculate the wind's force on the mainsail, it won't tell you what the rig will experience in an uncontrolled gybe, or when slamming into square waves in a heavy blow. There's much more to spec'ing gear than static wind forces.
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Old 14-07-2008, 09:34   #11
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How do you determine how big of gear you actually need?
If you don't want to design a boat the simple answer is to replace stuff with what is on there.

That's what we did. We took old spring blocks and snatch blocks to West Marine with us and bought the same stuff.

If the stuff isn't there I would say for a 30 foot boat you want sheets that are 3/8" with a working strength of 4,000 pounds or so.

After that you go buy blocks that are good up to 1/2 inch. These will have a working load rating around 2,500 pounds. Get the largest wheel diameter that's practical.

We've broken lead blocks on our boat. Some were old and ready to fail but we also broke a new replacement one (I didn't check the size and got a smaller diameter one - 170 boat bucks down the crapper) The line size was right but the wheel was small. We replaced with basically the same size block but a larger diameter.

BTW - I also agree with Pelagic that you should look at some other boats of your size.

BTW - I always like to provide interesting search results.

Go to this site - Scroll down and select a Coronado. Then it will tell you your rope size options. After that you can go to the blocks sectino and look at blocks knowing what line size you are dealing with - pretty cool I thought.

Running rigging: custom sheets and halyards
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Old 14-07-2008, 16:53   #12
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Try this site:

Harken Compu Spec

They only have the Coronado 25 listed, but you can probably find a similar boat. Click on the boat, then on the "View the loading report...", and it will give you ALL of the specs you will need!
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Old 14-07-2008, 21:23   #13
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They only have the Coronado 25 listed, but you can probably find a similar boat. Click on the boat, then on the "View the loading report...", and it will give you ALL of the specs you will need!
That's sweet - Thanks!

You can also add a custom boat - Mine wasn't listed so now I have to get all my numbers and plug them in!
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Old 14-07-2008, 21:41   #14
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A guy at a local boating store in Corpus Christi turned me on to it. Problem is, HE found my boat on their, now I can't find it!
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Old 14-07-2008, 21:59   #15
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Found my boat! If you don't find yours at first, go to the same link, but click on "I can't find my boat". You will then find a longer list of "user maintained" boat types.
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