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Old 08-12-2009, 22:52   #1
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Load per Square Foot of Sail

I am doing some sanity checking on the max loads for a cutter stay.

My trusty old Skeene's suggests 1 lb/1 ft^2 of sail. That doesn't seem too unreasonable as a crude rule of thumb, if we only take wind into account. With that assumption the force calculations become trivial.

What I am wondering is what a more realistic number would be if we were also going to take into account the shock loading from waves breaking across the bow/deck. Maybe the water/wave loading location is such that it most likely would blow out the bottom of the sail rather than part the cutter stay or tear out the deck attachment point ? Maybe that is why none of my design books seem to mention it ? (I say "seem" because they are paper books so doing a "find" is a manual task)

Thoughts ?



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Old 09-12-2009, 03:52   #2
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Wind load is a square function with wind speed...1lb/sq ft at 15 knots but 8 lbs/sq ft in a F8 gale (43 knots). Shock loads on the stays in heavy seas may be many times that...

Having lost a two stays and a shroud, in moderate to heavy weather over the years, I can attest to the fact that it was not the wind alone that broke them but the combination of wind and wave shock loading the boat. And not wave on sail as that would probably just blow out the sail.

In retrospect the real cause was rigging over 10-15 years old...All mine is now less than 2 yrs old.

Wind Load Calculator
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Old 09-12-2009, 07:57   #3
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Quote:
Originally Posted by s/v Moondancer View Post
Wind load is a square function with wind speed...1lb/sq ft at 15 knots but 8 lbs/sq ft in a F8 gale (43 knots). Shock loads on the stays in heavy seas may be many times that...
Thanks Phil. That was exactly the kind of answer I was looking for and it brings back hazy memories from texts I read too long ago.

Thanks !



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Old 09-12-2009, 08:32   #4
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Rather than estimating wind loads and then doing the engineering calcs to determine your rig scantlings, you may want to examine what others have done for similar successful rigs. There is no need to reinvent the wheel.
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Old 09-12-2009, 08:48   #5
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Hi David,

Quote:
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Rather than estimating wind loads and then doing the engineering calcs to determine your rig scantlings, you may want to examine what others have done for similar successful rigs. There is no need to reinvent the wheel.
I'm doing that too but there were only twenty E39Bs ever made and I still haven't found one with the cutter stay option. I have the blueprints but they don't show the structural detail for the cutter stay attachment point.

A bit of structural engineering analysis is a fun trip back down memory lane anyway

Thanks,


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Old 09-12-2009, 08:50   #6
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And consider the type of craft. Multihulls don't heel as much to the gust to spill the force - they accellerate, instead. Generally, they require one size larger rigging wire to keep the mast where it belongs.
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