Loads on my turning blocks? - Page 2 - SailNet Community
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post #11 of 16 Old 09-13-2011
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Thanks to knuterikt for correcting my mistake on counting mechanical advantage of a purchase (I SAID it was a long time ago...). However, gut feeling tells me that the load on the head of a mainsail is a lot less than mainsheet load? And does the boom act as a lever? Oh my, time to go back to the books...
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post #12 of 16 Old 09-13-2011
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There is another consideration here, in terms of shiv load on turning blocks. Regardless of sail size and wind speed, there is a functional limit to how much force can be imparted.

Consider this:

You are sailing with full rig in a hurricane. Good times! Wind speed, lets say, 3000 knots. Why not? In this situation, your turning blocks will be under no more stress than, say, 35 knots of wind.

The boat heels. So, in that hurricane, your boat isn't going to accept all that extra wind force.. it's just going to lay over and take a little rest in the water. So the maximum loads on your turning blocks is not just a function of sail size, but also how much force your boat can exert on the wind to keep itself upright. That force has been called "righting moment" by people much smarter than myself.

I only bring this up to suggest that one can do a lot of math - which we all agree is a ton of fun! - but in the end, it's more of an exercise in mental acrobatics more than sailboat mechanics.

Don't get me wrong.. I do love me some good mental acrobatics. But lets try this.. what type and size rope is your current halyard? Determine the max load on that line, assume it's the max load for your rig (safe assumption, since the halyard hasn't snapped in two on you (I hope)) and double that number. The result is the size of turning block you need.

I know it's not sexy.. but it'll get you on the water, if that's where you want to be

... or I'm wrong.

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post #13 of 16 Old 09-13-2011
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I see some folks did remember basic physics!

I guess not everyone hid in the back!

Another thing to consider is the maximum force the main sheet is likely to add. After all, often the maximum force boils down to the maximum the sailor can crank. Say, the purchase times 50 pounds. For design purposes, 80 pounds. See how that adds up. Again, I suspect the mast will be in the water before that. In general, the reefing tackle is matched to the mainsheet tackle. On my boat, at least, the loads are equal.

(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




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post #14 of 16 Old 09-13-2011
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Despite what you think of SA there was a great thread with some good info in a civil form i cant find it right now BUT

There were some really good pictures of this posted on SA and the purchase depends on what is anchored wear and the load on the block does depend on the angles

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post #15 of 16 Old 09-14-2011
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Great points, folks.

The physics and mechanical engineering involved in arriving at a definitive answer are obviously beyond what any of us have covered here.

The bottom line is that I would tend to trust the recommendations of a reputable manufacturer like Harken when it comes to selcting block sizes. I'd bet that they've done the research, run the numbers , and added a safety factor to their calculations before making any such recommendation.
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post #16 of 16 Old 09-14-2011
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Harken used to have all these numbers on line for hundreds of designs - CompuSpec. But they have apparently removed it all because they say it was dated - now they say email or call for info.

Brian
Living aboard in Victoria Harbour
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