Main Sheet Rigging

Moderator: GreenLake

Re: Main Sheet Rigging

Postby GreenLake » Sun Aug 17, 2014 1:10 am

bnnauti wrote:Attempting to modify my 3:1 mid boom mid sheet to utilize a traveler set-up like Phil shows. My question is should I use the single block and becket on the boom end and keep a 3:1 set-up at the mid-boom to centerboard set-up?


Because the effort required to pull the boom is double when you pull at the center, you need twice as many parts there for the same purchase as at the end of the boom. However, because the boom only moves half the distance at the center, the amount of mainsheet you pull in or pay out is the same. It's only dependent on how far the boom moves and the overall (or effective) mechanical advantage in your setup. One thing that is different is the number of blocks that the mainsheet has to go over, and that will, eventually result in some difference in friction. There's another difference that may matter more, I'll get to it.

First, let's walk some scenarios. My setup is comparable to a 5:1 center-boom purchase. That is, for every inch that the center of the boom moves away from the centerboard fitting, I have to pay out 5 inches of sheet.

If I had a 3:1 setup at the CB, as you suggest, I would pay out 3 inches for every inch of movement (measured at the center). If, instead of tying the sheet to the center of the boom (or to a becket on a block at the center of the boom) I were to continue to lead it aft, then down to the traveler, I would have to pay out 2 additional inches (the amount the end of the boom moves away from the traveler when the center moves 1"). So, I would get an overall purchase of 5:1.

However, that's not how my setup is done. I have a 2:1 purchase at the end of the boom and a single purchase at the mid-point. Same effective purchase of 5:1, but the difference is that I'm pulling twice as hard at the end of the boom than on the center (factoring in the lever arm, I get 4 times as much effect at the end than in the center, that is 4:1 ratio between pull from the traveler over pull from the CB).

Why does this matter?

Well, the traveler and the CB fixture aren't totally equivalent. If you look at pictures (or make a diagram) you'll see that the angle of pull on the boom is different. By keeping most of your purchase at the CB, you reduce the effect of the traveler. Instead of a 4:1 ratio, you get a 2:3 ratio of traveler vs. CB bracket. It should be obvious that that would largely negate the effect of having the traveler in the first place (the effect would be 6 times smaller in your setup).
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