Owner, Green Bay Packers
Join Date: Sep 2006
Location: SW Michigan
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T34C, If I have you right, I think what you'd end up with is a six to one purchase with the addition of the extra block at the mast. If you end up, somehow, at 8 to one you should be aware that each sheave added should be considered a 10% gain in friction under load. For instance: Your two-fold purchase, with a 4 to 1 mechanical advantage, has a theoretical power of four times the pulling power you apply to it. The actual power is 2.86 times the force applied to it, making your mechanical efficiency 72%. As the number of sheaves in a purchase go up the mechanical efficiency goes down. The easiest to observe illustration of this is your halyard. If your load to hoist on the halyard is 100 lbs the actual force needed is 110 lbs, making the halyard, a single whip block, 90% efficient. If you add a block at the head of your sail, and run the standing part back to the head of the mast, you have a gun tackle with a mechanical advantage of two, but your actual power is 1.67. Instead of taking 50 lbs to hoist the load it takes just under 60lbs. The gun tackle is 83% efficient. So, there is no free lunch.
Modern sailing blocks will yield a coefficient of friction less than the traditional 10% used for calculations, but friction losses still should not be ignored. Anyone who's fairlead his lines through multiple sheaves back to the cockpit will know the feeling of trying to heave in a line that they previously handled with ease on a straight pull.
Back to your attachment point question. Regardless of the number of parts in your purchase the following holds true; if the load on the tackle is 100lbs and you are heaving it in the load at the deck block in the vang case is equal to the 100lbs and the strain you are putting on it. For a 2 to 1 M.A. your load at the block would be 167lbs. I mention this because, when encountering difficulties in heaving in a line, the reaction is often to lead it to a winch. And with the high loads involved that could result in popping a block out of the deck and a head-ache. Make sure the attachment point, whether on boom for cunningham, or mast or deck for vang are secure. This usually calls for through bolting, not screws, and may well be the determining factor in how you rig either of the two.
The cunningham can be rigged to the boom, but you probably won't be happy with that arrangement unless a vang is rigged also. If you have padeyes at the base of your mast either, or both, could be rigged off of there with a block and jamb cleat.
Play around with a snatch block and shackle and you'll come up with the best layout for your boat, before you start drilling holes. Eight to one does seem like a lot of purchase, and will be prone to fouling, but I don't know your boat.