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Old 24-02-2009, 17:28   #1
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Load on Halyards

I have the formula for calculating the load at the clew of a headsail (or the end of the boom) given the sail area and wind speed. However, I'm trying to figure out the approximate load on the main and jib halyards for my Bristol 19. Other than placing some kind of measuring device inline in the halyard, I have no idea as to how to figure it out or what a good estimate would be. The reason I'd like to know is 1) to make sure the jib halyard block is strong enough and 2) to see if one of those Forespar marelon winches would work for the main halyard.

Thanks in advance for any guesses on the load or for advice on how to measure it without purchasing a special piece of equipment.

Jay
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Old 24-02-2009, 17:36   #2
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I forgot I had another question (although I think I know the answer to this, I just wanted to make sure). If you have a four-purchase mainsheet setup with two fiddle blocks, the maximum load the entire set up could handle would be the maximum load of block 1 plus the maximum load of block 2, right? Because each block is only taking half the load?

Also...would you divide the strength of the line by four, since it's a four-part purchase? In other words, if the strength of the line is 1,200 pounds, you could theoretically put a load of 4,800 pounds on a four-part mainsheet setup? (Not that you probably ever would...) Just curious.

Thanks.
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Old 24-02-2009, 18:06   #3
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Jay--

The load on the halyard can be roughly approximated as twice the vertical load on the mainsheet. There's actually more to tha calculation but that's close enough for government work. Add 10% for every pass of the line over a block.

The total load carried by the mainsheet is divided between the number of parts plus 10% for every block. If you have two blocks on the boom, with a total of four lines to the traveler, the point load is divided evenly between the lines. In theory one should should divide that by the cosine of 1/2 the angle between the two blocks on the boom and the fiddle block carrying the lines on the traveler but that usually doesn't make much difference given the fact that the safe working load of the line is only 20% to 25% of the breaking strength. For example, if the theoretical total mainsheet load is 1200 lbs and you've got four parts attached to the boom, passing over two sheves, each "part" is carrying roughly 360 lbs = [(1,200 x 1.20)/4]

Not a heck of alot of load in the grand scheme, eh? In such cases one chooses line based on what's easy to handle verses the loading, with 3/8" being about a minimum.

s/v HyLyte
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