Sheet Load Engineering

[lavery]

Brion,
For your reference I have sent a calculation sheet to rigging@briontoss.com.

BASIC SHEET CALCULATION:
I have run numbers using Harken and Lewmar. At 20 knots of relative wind the Harken calculator is about equal to the Lewmar reference. So here is the first area requiring judgment:

Harken results vary significantly with windspeed – as you have pointed out.

Harken and Lewmar are credible references, but results vary widely for main and 100% jib.

I went to 25 knots worst case, then selected the highest of either Harken or Lewmar, and settled on 2,000 pounds main and jib.

How would you play these calculations?

RUN THE NUMBERS
Exactly. Having calculated steady state load (2,000 lb.), would not one add pounds of shock load to arrive at design (working) load? Then divide by 20% to arrive at rope tensile strength?

Or are shock loads included in the Harken and Lewmar calculations?

Or are shock loads absorbed in the 20% calculation?

Like shock loads, elasticity is not quantified. Apparently elasticity is like the 3 bears’ story: not too big and not too little. But where are the guidelines to find just right?

Many thanks,

Jim

[Brion Toss]

Hi again,
And first, note that I started this board as a place to answer questions that were too simple to charge for; you are well past that stage, but this is fun, so I'll go a bit further.
Shock loads are relative to acceleration and elasticity. They can be quantified and included in design load calculations if desired. The short form here is that the materials you are likely to use appropriately are also appropriate for any shock loads, if they are scaled to the design load, times a safety factor. As a counter-example, I've seen main sheets and vangs break gear when owners ill-advisedly replaced Dacron with HM line.
Elasticity can also be quantified, as it is directly related to relative load, and to length. The specific effect of a given level of elasticity is, like all specifics, a bit more involved (he said, retreating behind a veil of mystery). One piece of advice I might offer is to introduce other variables, like cost and durability. Cost/benefit, though sometimes subjective, can be helpful to look at. You might also look at the cut and material your sails are made of, to see if they are consonant with the proposed rope.
25 knots might be right for inshore sailing in some waters and boats, too darn low for others. Remember, this is apparent wind we are talking about, which is relative velocity, a question of boat speed and actual windspeed. So two boats with identical sail areas will have different rig loads, if their hulls can move at different rates in the same actual windspeed. I'd run the numbers at several windspeeds, both to gauge max loads and to calculate things like purchase and winch powers.
Genoa sheet loads are usually higher than mainsheet loads, but it all depends... I can't interpret the rendering of your numbers at this end. Maybe when they show up on email. But then it's a consultation.
Fair leads,
Brion Toss

[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