Originally Posted by Isotope42
Originally Posted by Jake
If the sail is hooked at the top of the mast and you pull down with 100 lbs of force, you have 100 lbs of tension on the mainsail luff and 100 lbs of compression that the mast has to resist.

If you have a halyard that turns around a sheave at the top of the mast and is cleated at the bottom (i.e., the halyard cleat at the bottom of the mast is what keeps the sail from falling down) and you apply the same 100lbs to the tack of the mainsail, you have 100 lbs of tension on the mainsail luff and 100 lbs of tension on the halyard. You now have two 100lb tensions that the mast has to resist equaling 200 lbs of compression on the mast.

Pardon my bringing this subject back up, but I was mulling over rigging options the other night when trying to get to sleep, and a related thought came to mind:
  1. Is mast bend a function solely of bolt-rope (a.k.a. downhaul) tension, or
  2. is it a function of mast compression?
It seems to me that the answer is "2".

Let's take the following thought experiment. Imagine pinning the mainsail head to the top of the mast. Pull downwards on the downhaul (attached to the mast base) with 100 pounds of force. Measure the mast bend.

Next, imagine that you pinned the head and tack of the mainsail to fixed locations on the mast. Then fasten a giant C-clamp on the length of the mast (from the mast base to the head) and apply 100 pounds of compression. Would you get the same mast bend?

Third, imagine running a halyard from the head of the mainsail, around a turning block at the top of the mast, down to a cleat at the mast base. Pull downwards on the downhaul (again, attached to the mast base) with 50 pounds of force. This induces 100 pounds of compression on the mast. Is the mast bend the same again?

I think it would be. If so, then it means that a halyard isn't necessarily bad. In actual use it doesn't double mast compression. Instead, it halves downhaul force. You only pull on the downhaul until you reach the desired mast bend. The system is, in essence a 2:1 cascade, just hooked up on the opposite end of the sail.

Thoughts?

Eric


A halyard at the bottom would double compression but the downhaul also stretches the mainsail...so yes and no.

Wanna blow your mind? Think about the extra compression of sailors on the wire. Then think about the side forces generated from the drive of the sail plan. When you get a gust, it tries to pull the middle of the mast to the side. The diamond wires provide a truss to resist this motion...but most spreaders are raked so the diamond wires are no longer in column with the mast. Side force on the mast, when resisted by the raked spreaders, will also push the middle of the mast further forward and bend the mast more. This is why the rake of the spreaders is a tuning item. Heavier teams want less rake in the spreaders as the pressure response to the sideloading is lessened. Lighter teams want more spreader rake as the bend of the mast is enhanced with more sideloading.

Jake Kohl