Proper shroud tension

[Volkditty]

Hello, sailors. Looking for a sanity check on tuning up the standing rigging on my San Juan 21 Mk1 to get her in race condition.

My shrouds are 1/8" 1x19 stainless. I found this tuning guide that says "Tension the upper shrouds equally by counting turns to 750 pounds tension." That seems high for a couple reasons:

1. The Loos PT-1 gauge I borrowed doesn't go above 500 lbs tension for 1/8" wire.
2. I don't think I have enough room on the turnbuckles to tighten them that much anyway.
3. Breaking strength of 1/8" cable is 1,780 lbs. and I've read the rule of thumb that 15% breaking strength tension is "good enough." 750 lbs would be 42%.

As is, I tuned it up to 320 lbs on the outer shrouds, about 18% breaking strength. I'm just curious on other opinions on how tight is "tight?" I still need to dial in the mast rake and take her out for a test sail to see if all this makes any noticeable difference.

 

[JRacer]

I sent a link to this thread to one of our experienced SJ 21 sailors. Perhaps he will be able to provide some guidance.

 

[Davidasailor26]

I don't know the San Juan 21 specifically, but I agree 42% of breaking strength sounds wildly more than the usual guidance. The tuning guide also seems to say you should further reduce pre-bend after setting the 750 pounds of tension, by adding tension to the lowers. I suspect when you tighten the lowers and pull the mast more in column it's going to exert even more force on the uppers, bringing it to even more than 750.

If you're at 320 now I'd go sailing and see how the leeward upper looks on a beat in 10-12 knots of wind. If it still looks tight you have at least enough tension.

 

[SmallCraftLifeNews]

On my 19' Cruising modified Flying Scot the cable tension is only like 26 pounds. 500 pounds is wild! Regards, Jude #SmallCraftLife News

 

[dlochner]

Generally for keel boats (or keel cb boats) 10-12% of breaking strength works as a starting point. Then go sailing hard on the wind in an about 10-15 knots of wind. The leeward cap shroud should be just a tad loose. Not flopping around, just no real tension on it. If there is a lot tension, loosen the shroud, if the shroud is too tight loosen the turnbuckle. Then tack and repeat for the other side.

 

[pgandw]

The Mariner (ODay/Stuart 19) racing tuning guide suggests 180# for the 1/8" upper shrouds as a starting point, 220-240# for the forestay, back stay effectively 0. Mariner backstay is 3/32, not 1/8. Some racers reverse the tensions for the forestay and shrouds. This year, adjustable, dual terminated backstay was authorized for racing by vote of the Association. This allows elimination of the stock off-center backstay and bending the mast slightly aft in heavy winds (Mariner has a 3/4 rig, with shrouds led aft of the mast).

I notice most cruising Mariners use less tension than the racers, but I found the racing guidelines about right for getting the sag out of the forestay when on the wind. Here is the suggested tuning without a Loos gauge that gives appropriate tension used by most cruisers:

Alternate Less Tension Tuning​

"When you tighten your upper and lower stays, start with your uppers and tighten by hand so the mast is centered. Use the halyard to measure an equal distance from the top of the mast to the chainplate on both sides. Then, tighten the lowers, first one side. Look for the mast to barely be pulled from center at the spreaders. Then, tighten the other side until it pulls the slight bend back out. Tighten the uppers a little more, in a similar fashion. Mast should be straight and centered. The tension should feel like a heavy bass string when you pluck it. Not too tight. Hand tight, then a tiny bit more.

When you are satisfied you have done this correctly, watch the leeward turnbuckle while you sail on a beam reach in winds around 15 knots. Your tension should fall out of the stay, but your turnbuckle should not move from upright or fall over. This way, you put enough tension in your stays to hold everything in place, you won't endanger your mast with overly loose stays during a jibe and you won't put so much tension on your rigging or your hull that you stress the chainplates, or the masthead, or your] compression post and cabin top and keel. Use the least amount of tension to hold the mast steady in 15 to 20 knots of wind."

Fred W
Stuart Mariner 19 #4133 Sweet P