Minimum reasonable breaking load for forestay of 31' ketch?

[rhumbunctious]

Recently I replaced all the standing rigging on my '79 Westerly Berwick ketch, including adding into the mix a roller furler system that works with hank-on sails. It basically is a pair of barrel swivels with a drum which is attached as part of the forestay, so that the entire forestay rotates when furling the sail.

The main mast rigging, which is an exact copy of the original rigging, aside from the furler, is 1/4" (6mm) 1x19 316 stainless, which according to the Loos site (http://www.loosco.com/index.php?page=yacht-rigging-cable") has a breaking load of 6900lbs which works out to 3130kg, however, the furler has a breaking load of only 2700kg. The furler manufacturer assures me that it is more than strong enough for the boat, but I can't stop worrying anytime the wind picks up whether I'm going to lose my forestay.

Am I worrying needlessly? How significant is that 430kg difference in breaking load? What is a minimum reasonable breaking load for a forestay for this size of ketch?

Other than the furler, all the clevis pins for the main mast rigging are 3/8" (9.5mm) but the furler swivels use 5/16" (8mm) clevis pins (though the top of the furler drum accommodates a 3/8" pin). Would there be any benefit (or drawback) to drilling out those 5/16 holes to accommodate 3/8" pins and replacing the 5/16 pins with 3/8 pins?

 

[Searush]

I can't give you any technical specs, but I can tell you that the Westerly 31 rig is well over-specced. Compare your mast & rigging to other boats next time you are in harbour. Your mast will be shorter & thicker than the others & the rigging wire will be thicker than most too. I am still using the original rig on my 1973 Pentland despite people telling me it will only last 10 years.

 

[Fr J Hackett]

You should be able to find the original specification for the forestay, I think it would be unwise to change it without the benefit of advice from at least a rigging specialist or better still a naval architect purely from an insurance perspective. Rigging is not all about breaking strain either.

 

[Rigger Mortice]

I've got Ian Nicholson's "Boat data book" in front of me and for a boat your size he recommends a 6mm forestay. He also reckons that the breaking load of 6mm 1x19 is 3000kgs but it doesn't surprise me to see small differences with these things. He recommends the use of 8mm pins with 5mm wire, and 11 mm pins with 7mm wire. If you interpolate between the two for 6mm wire it would end up with the 9.5mm pins that you have.

He also talks about going up one size for everything for serious blue water cruising and going down one size for racing; so there's probably some room for manoeuvre.

Personally I would be reluctant to go drilling out for bigger pins without serious thought. A bigger hole means less material around the pin.

It all depends on the manufacturer though. If the furler is from a reputable manufacturer you should be able to rely on his advice. Odds are you are worrying unnecessarily.

 

[William_H]

One easy answer to your concerns is to have a spin halyard attached to the bow rail or similar hopefully forward and clear of the forestay. This will work as a backup for any failure of the forestay. So at least avert mast breaking. Much of the load on a forestay is from the jib itself so if the furler lets go the load will be removed form the forestay and mast so spin halyard should hold it up OK.
Re searush and his 40 year old stays I feel quite sure from experience around our club that the sidestays at least will let go on you and likely quite soon. This from a very frugal sailor who would penny pinch on many things but not SS stays. good luck until then olewill