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I am in charge of building a new mainsheet for a 52' offshore race boat (think TP52), with a "German/Admiral's Cup" mainsheet system (a la Farr 40)

The old mainsheet was somewhere between 2/3 and 3/4 of an inch, very stiff and inflexible, and almost too big to ease through the line tunnel under the deck.

What is the appropriate line diameter for this application? The Harken calculator with my ballpark numbers at 45kt said the boom loading would be somewhere around 17000lb. If that is doubled up then do I look for a MWL around 9k, or have the redundant safety margin of 18-20k? The difference for, say, Samson Warpspeed II is 9mm for a 9000lb working load but 12mm for a 20000lb MWL

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If you’re thinking of dropping down in size consider clutches or winches which might limit or restrict what size you use, don’t want new line that’s too small for existing hardware 

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Remember that you probably won't have the full main in 45kts. At least I won't. I just worked the calculator for my boat with a reefing point at 20kts and, surprise, came up with a much lower number like 1700 instead of 10,000! Also 9mm is pretty small for handling. My Lewmar clutches will be sized for 10-12mmn (3/8"-1/2") for reefing lines. Mainsheet stays on the self-tailer. According to Samson, Warpspeed II size 11mm has a breaking strength of 14,000. That should provide an ample safety margin for working load, fit the clutches, and be easy to handle.

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4 hours ago, onepointfivethumbs said:

What is the appropriate line diameter for this application? The Harken calculator with my ballpark numbers at 45kt said the boom loading would be somewhere around 17000lb. If that is doubled up then do I look for a MWL around 9k, or have the redundant safety margin of 18-20k? The difference for, say, Samson Warpspeed II is 9mm for a 9000lb working load but 12mm for a 20000lb MWL

We use 10 or 11mm on our 38’ boat, so for a 52’er, you definitely need larger diameter. The Harken calculator isn’t considering the shock loads of a gybe, which is what the line, blocks, etc should be sized for.  (Also With your ST50 winches, 9mm will slip in the jaws anyway.)

The other thing is that this line weight is all low down, so it’s not hurting your stability hardly at all, might as well go up a size.

 

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...one more thing, you want to minimize stretch, so going up a diameter size reduces stretch by a lot.
 

Samson Warpspeed 2 is a great price/performance line, but its stretch at 30% break load is .8%, vs only .44% at 10% breaking load.  Trimming will be more consistent with a larger diameter.

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2 minutes ago, Salona said:

...one more thing, you want to minimize stretch, so going up a diameter size reduces stretch by a lot.
 

Samson Warpspeed 2 is a great price/performance line, but its stretch at 30% break load is .8%, vs only .44% at 10% breaking load.  Trimming will be more consistent with a larger diameter.

The issue we are having is that it is difficult to trim the main because the line is already too big (14mm?) and does not ease smoothly through the tunnels, so bigger is not an option.

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I wouldn’t go under 12mm though.

You could also investigate stripping the cover where it goes through the tunnel and around the blocks on the boom (German mainsheet, right?).
That will reduce friction while still giving you the width and grip needed for the self tailers, at a cost of slightly reduced lifespan.
 

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The 11mm figure comes from the Samson catalogue for high tech racer. Cruiser and cruiser/racer are less.

I think Will1073 is correct in that the system will be hardware limited.

edit: I just realized that as my mainsheet is 2:1, the load on the line is halved.

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As far as hardware limitations go I think the blocks are all Harken Black Magic 75's so 5000-7500lb MWL (seems really low?), the German mainsheet is functionally a 4:1 so 0.44% stretch is pretty much negligible on a winch that's constantly being trimmed, even with 2:1 the margin is pretty great for shock loading, no?

Tapering might be an option but the taper would be so short that I wonder if it's even worth doing.

11-12mm seems to be the winner, I'm not married to Warpspeed but that was what we discussed with the chandlers

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