The beginnings of this I got from the Harken site. I’m trying to design a mainsheet system for a sail of around 155 sq ft. By my reckoning, in strong conditions, one needs up to 15 sq ft to one purchase [for comfort] but in light conditions, considerably less.
Therefore I was looking for a “two speed” system which didn’t involve rethreading the blocks as conditions changed. I came up with this below and have some questions for you, if you’re not bad on physics [6th grade level or higher].
M and N are blocks on a traveller hawse running across the deck and fixing either side at each hull. They have block A attached between them. There is a double [or could be triple] purchase system hauling them port or starboard. They are at deck level.
In the air, attached to the boom, are blocks B and C. The sheet P runs through the blocks and wants to lead [through the air] back to each cockpit but it’s too short – maybe 15 feet.
Sheet J is an attachment sheet via snaphook H, which would join snaphook E. The other end of sheet J runs to the cockpit.
Floating around on sheet J is a block L with becket [attachment point] K. Letter I has no purpose – it was an error in the pic.
F is a u-bolt attached to the hull. G is a block which is open one side to allow a sheet to be clipped in – it is bolted to the hull as well.
OK, let’s begin. By my reckoning, if D is attached firmly for the nonce, then sheet P is in a 3:1 purchase, if you were to haul directly on E.
Question 1: is that so or is it 4:1?
In light conditions, assuming it’s 3:1, then that is about right for that sail and were I to attach H to E directly, then it does not alter the purchase.
However, if conditions become moderate, I need to increase my purchase. So I clip H to F and block L to E. By my reckoning, I’ve turned 4:1 into 6:1.
Question 2: is that so or is it some other purchase?
If conditions become bad, then I need 9:1. I now remove H from F, run it through G [which is open and doesn't require any unthreading], go back up and attach H to becket K. By my reckoning, I’ve just created 9:1.
Question 3: is that so or have I created 12:1?
If conditions become really bad,then I reduce sail.
Thanking you in anticipation.