Okay engineering genius-types, how do I figure out a good halyard size/strength for my mainsail?

Looks like I've got 5/16" (8mm?) class II double braid on there now..

But I have no idea what kind of loads the sail/halyard generate. This main is cleated at the base (not hooked at the top)

Mainsail Specs:
Luff 29'6" (9 M). I think this is measurement "P"
Foot 11'6" (3.5 M) "E"
Size 243 sf (22.5 sq.M)

Would load=(1/2E)*P work for a halyard? I think that's for end-sheeted mainsheet?

And if I read correctly, the working load should be 20% of breaking strength, so once I figure out the load(s) I need to multiply by 5 to get the break strength I need to buy, right?

Does your main halyard take any load other than raising your main and lifting it off the hook for lowering?

Yes, his mast has no hook at the top, so the sail is held up by the halyard only, so that is why he's asking about breaking strength when down hauling it to the "Wow" setting.

Jay,

Measure
1 - the boom to the top of the mast
2 - top of the mast to the base
3 - base to the clutch
4 - clutch to the winch

Add these up and that will be your halyard length
I would recommend a 6mm vectran based doublebraid such as V-100 or equivilent.
While you are at it, take a look at the clutch cam and consider replacing it at the same time.
pm me if you have any Q's

I was just going to copy the current length, but your method may prevent too much extra. I think I'm at about 72' right now which leaves about 8 feet to coil once I've stowed the main.

I suspect 6mm line would be too small for the rope clutch I have. The 5/16" I'm using now has an additional cover attached right where the clutch grabs to keep the halyard in place when tensioning the downhaul/sheet.

What type of clutch do you have? You can size the cam to the rope dia. If the teeth look shiny, then they need replacing.

Spinlock, but not sure what size (it's got a handle-type lever). Will see if I can get an ID measurement tomorrow

Should be stamped on the cam
ex cam-0610 is for 6mm-10mm line.

oooh... that's downright sneaky... And very helpful. Thanks!

can't seem to find the part # on top or sides...

How much of a project is it to make it a masthead lock?

And would it make a significant difference in sail handling and performance?

I guess I could figure out how to mount one up there, but not sure how difficult it would be to engage/disengage. The mast does rotate...

As to the advantage/disadvantage of the mast hook vs. halyard clutch, that may be better left to those engineering types..

I think there was lively discussion about this on other threads, but I'm still having trouble understanding the physics/forces. I would have thought that the total (down) force on the mast would be the same in either setup...

For instance, if you put 100 kg of downhaul force on the mainsail, the mast would be resisting that 100kg whether it's attached at the top, or runs back down to a halyard clutch.

Because the halyard runs through a pulley down to the clutch, perhaps the load on the clutch itself is higher though....?

Perhaps a good model would be a compound bow? Is the bending force (let's say the force needed to deflect the bow 3") on the bow itself the same whether it's a simple or compound draw?

Instead of comparing the forces to a compound bow, let's look at a 2:1 jib sheet system - which is something we all understand better. Imagine you attach a single block to the clew, and run the jib sheet from the boat through the block, and back to the jib car. Now, if you put 100 kg of tension on the running end of the sheet, you also put 100 kg of tension on the standing end. The force on the turning block, however, is 200 kg. That's the whole purpose of a 2:1 purchase - it doubles the force of your pull.

It may not be obvious at first, but a masthead sheave does the exact same thing with your halyard. When you fasten the halyard and put 100 kg tension on your downhaul, your clutch also sees 100 kg of force. The sheave, however, experiences 200 kg of downward pull. Even though you don't see any movement, a traditional main halyard doubles the compressive load on the mast. A hook or clutch mounted at the masthead, however, does not. Without the turning block (sheave), there is no 2:1 purchase. A 100 kg pull on the downhaul produces only 100 kg of compressive load on the mast.

That is one advantage of a hook over a halyard. The other is that you don't have to contend with halyard stretch. You can use a lighter, cheaper line because it only has to take the load of hoisting/lowering the sail. A traditional halyard, on the other hand, makes for simpler reefing.

Getting back to your original questions though, if your current halyard is class 2 double braid, why switch to a different diameter? You can even get abrasion resistant cover to stitch on where the clutch holds.

I hope that helps,
Eric

Thank you Eric.. that helps a little. I guess since I do have reef points (and roll the main on the boom for additional sail shortening) I may need to keep the halyard/clutch arrangement...

As to switching diameter, I was wondering if they have over-sized the halyard for the anticipated load(s) and therfore spending more money than I need to for an appropriately sized halyard.