Sheet Load Engineering

[lavery]

Sheet Load (of) Engineering

Sailors,
Does anyone actually calculate their sheet loads in order to determine their sheet tensile strength?

Would anyone care to entertain my inner engineer with how you get from mainsheet load to sheet tensile strength?

Harken provides a mainsheet load calculator at
http://www.harken.com/calculators/MainsheetLoading.aspx

Plug in your E and P boat dimensions, which can be found at:
http://www.mauriprosailing.com/techi...cs/Rig%20S.htm

You will need also the distance between aft boom end and sheet attachment point, as well as a guess about relative wind speed. The calculator cranks out Mainsheet Load in pounds.

The equation is a doozey and can be seen at
http://www.harken.com/charts/mainsheetsysloading.php
along with a caveat about the equation being controversial. If Harken puts it out there though, I’m thinking it is probably reasonably good.

Whatever your path to peace with your mainsheet, whether it be number crunching or not, I’d like to know how you arrive at a mainsheet tensile strength with which you feel comfortable.

Thanks all,

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

[benz]

Like Ian said on a recent thread, it's less about strength than about what you can easily grip. I use 7/16 Regatta braid for my CG 31', very easy to pull with bare hands; hasn't broken yet. It gets fatter with use, so be sure your sheaves can handle a size or two bigger. I have sheaves for 1/2" and wish they were just a tad fatter.

[lavery]

Very good.

Thank you Benz,
Experience and empirical evidence is exactly what I am looking for to bounce against my formulas.

Thank you also for the tip regarding diameter expansion. Also invaluable is your list of specifics regarding diameter that you use, "model" of rope, and your boat size (that all puts my inner engineer at ease).

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

[Brion Toss]

Hi there,
Your inner engineer is far too easily appeased. "Hasn't broke yet" is not engineering, though in this case the rope size is right. Regarding this and the previous thread, knowing what the load is and responding appropriately is a good working definition of good rigging.
For sheets, the loads are highest when the runs are shortest, so elasticity, while it matters, is less of an issue than with halyards; handling, and dealing with shock loads matter more. Also, it is a bit erroneous to think purely in terms of tensile strength; what matters more is that the design load (typical working load) is a fairly small (20% or less) percentage of the tensile strength. This is not to provide a safety factor, but to keep elasticity down. That's why you can usually afford to tie knots in sheets. The result also usually happens to be a line that is comfortable to hold, but diameter is something to engineer in, as there are lots of lines that will be absurdly strong, and too (shock loads) inelastic if they're comfy to hold.
I can't stress it enough: run the numbers. Play them off against cost, handling, elasticity, strength, and any other variables that you can think of. Block and winch selection, as well as rope selection, can then be rational exercises.
Fair leads,
Brion Toss

[lavery]

Brion,
Not appeased, but I have found value in all responses. I’ve gone out to 2 owner’s groups as well, with similar results. You can lead a sailor to a calculator, but… even a calculator requires judgement and experience.

I’ll get right back with my specifics if you are willing. Is it possible to upload a Word or Excel document (those formats become cranky in your text editor)?

Thanks,

Jim


Line 1 Foresails (feet) 1 Mainsail (feet) 1multiplier 1 Total Sail Area (feet2) relative wind speed (knots) 2 Harken Genoa Sheet Load (pounds) 3 Harken Main Sheet Load (pounds) 4 Lewmar reference round up to nearest 500 pounds
I J P E
Main 31.5 13.5 0.555556 236 20 1,146 1,100 2,000

100% jib 36.5 15.5 0.5 283 25 762 2,000 2,000

Halyard 1,400 1,500

[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