Why double purchase halyards?

Hi All

I have been wondering why all modern catamarans have double purchase halyards? Except for the general trend of manufacturers using cheaper everything and being able to use a smaller winch?
Drawbacks I see:
- Problems with twisting
- Cannot (easily) move the points of chafe by cutting a little off the end
- Changing the halyard is a real pay in the a***

Thanks
Adrian

 

A double purchase decreases the compression of the mast, generated by the halyard tension, by half.

 

????so where does the compression go………yes, the fittings and everything else can get smaller…..but compression is compression, the tension in the sail doesn’t disappear, each side of the halyard has 50%…..that still comes to 100%……

 

Draw yourself a diagram of a two-part tackle and it will become apparent.

 

Diagram shown here Leopard 42 42' Leopard - Do I Really Need a Double Halyard ? - Page 3 - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f151/42-leopard-do-i-really-need-a-double-halyard-57968-3.html

 

More exactly, with a single line halyhard, mast compression is twice the mainsail luff tension. With double purchase , mast compression becomes
1.5 time the luff tension ! So, the compression is lessened, but not halved, but 3/4 of single line halyhard, which still remain an appreciable benefit !

 

I'm a bit confused, to be honest...I think these figured are correct for an unstayed mast? But a stayed mast is already compressed by the stays, how much difference does the halyard make there?

 

You are correct.

 

Reread, it's spot on to me.
He did not go to total mast compression.

 

The acurate sentence would be double purchase reduce the compression induced by halyhard and luff tension to 3/4 of single line !

 

Hi mudsailor. Now after reading these further opinions have you changed your mind or do you still stand by your above statement?

 

I had issues with this too. I think its the tension between top and bottom of the luff, not the mast that causes issue. Example, I have a load cell in my luff and i haul my main up with a 4-1 tackle and tie off the tail on a mast cleat at deck level, for all intents and purposes, that head is now a fixed point whether it is a 4-1 halyard or a single. If i now tension the luff using a tack downhaul on the boom, it might take less effort with a 4-1 downhaul than it would with a single line, "for the same luff tension". I explained it to myself by using a single string bow to a re-curve bow with multi-rollers, you may have a 90lb drawstring on a single line, but on a recurve 20lb. It still seems counter-intuitive at times , but the science says the load at each end of that bow (mast) has now been reduced, to impose the same curve ( luff tension).

I may have confused further......

 

I'm still trying to figure this one out, so I have some questions. I don't know proper terminology so could use some help with that. There is a pulley wheel at the top of the mast. Where is the other pulley wheel located that creates the double purchase? Is it attached at the head of the main sail? Where is the static end of the halyard attached?

 

Correct

To the top of the mast.

There is a diagram in the thread I posted in #5
Leopard 42 42' Leopard - Do I Really Need a Double Halyard ? - Page 3 - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f151/42-leopard-do-i-really-need-a-double-halyard-57968-3.html