Sheet Load Engineering

[lavery]

Brion,
Yes this is fun, and for me, vital and necessary.

Your comments here do advance the cause:

Appropriate application: Mainsheet shock loads and elasticity just became non-issues. My sails are battenless Dacron cruiser. The text I am using prefers three-strand or single-braid for mainsail sheets (though no reason is given for the preference). I infer from your comments that the preference is about stretch, as appropriate for this application.

Other variables: Cost and durability are important. I’m intrigued by Roblon and its low cost, for example, but at my stage I am more willing to pay the cost of a conventional solution like New England Regatta on the belief that it will spare me from dealing with other variables. I’ll experiment later.

Apparent wind: Yes I chose 25 knots on the theory that I ought to be reducing sail by the time the apparent wind gets that high. Your comments encourage me to go higher (because surprises happen) and this is good. I’ll look at it.

My greatest difficulty in running the numbers was in understanding mainsheet shock loads and appropriate stretch. Also, I needed feedback regarding my selection of apparent wind. I now feel that I have the background information that I need, even outside the veil of mystery, to arrive at rope selection, and then move on to blocks.

Thank you for your help.

Fair Leads to you too.

Jim Lavery
s/v Hoku Ke’a
Southern Cross 31

[Mark Johnson]

Jim, this will not help out your "inner engineer", and I eventually dropped out of mechanical engineering, but...

With larger boats this is a valid point to try to figure out. With your 31'er however, assuming you avoid chafe, if you go with say... NER 7/16" Sta Set, (the smallest size that I like the feel of), the rope is NOT the weak link! Either the blocks, traveler car, boom, or clew of the mainsail is...
You still have to avoid an out of control, crash slam, accidental gybe! I have always been a multihuller, and in my youth, did one of these on my cat, not realizing that the (2 X oversized) traveler was somehow not cleated. It crashed across the 12' track with such speed that the huge traveler car EXPLODED! The 3/8" double braid polyester line was fine. It is hard to engineer a boat for doing something stupid like I did!

A 7/16" mainsheet is big enough for a 31' monohull, even with mid boom sheeting...
Mark

[Brion Toss]

Hi again,
Great traveler story. I once saw a winch explode, one of the gears blowing a hole in the wall. At least I think it was a gear, but we never found it. So yeah, we are dealing with forces that matter, which is one reason to run those numbers.
As you move on to block selection, remember that the angle of the line on the block determines block load, and that you do not scale the block to the selected rope size, but to load and angle.
Fair leads,
Brion Toss

[lavery]

Thank you Mark and Brion,

Mark,
Wow! Yes, operator error trumps good engineering – all part of the sailing adventure. On my 26’ S2 sloop I used a 3-strand nylon (16% stretch) mainsheet to back up my operator errors. I liked the cushioning, but may have degraded performance.

Thank you for your rope recommendation, and too for sharing the diameter that works for your hand. That is another variable for me to consider.

Jim


Brion,
I’ve seen your tratment of angles in “Rigger’s Apprentice.” Good stuff. I look forward to applying it. Thank you for pulling that book together. It has been a big help.

Jim

[lavery]

Thank you Mark and Brion,

Mark,
Wow! Yes, operator error trumps good engineering – all part of the sailing adventure. On my 26’ S2 sloop I used a 3-strand nylon (16% stretch) mainsheet to back up my operator errors. I liked the cushioning, but may have degraded performance.

Thank you for your rope recommendation, and too for sharing the diameter that works for your hand. That is another variable for me to consider.

Jim


Brion,
I’ve seen your tratment of angles in “Rigger’s Apprentice.” Good stuff. I look forward to applying it. Thank you for pulling that book together. It has been a big help.

Jim