Running backstay Swan 411 Hello to the list - it's just me beating the same subject to death as always -
Well, I installed my running backstays this spring, and immediately saw the truth of what I had been told here (you hardly need them for most easy cruising and they are best brought to the base of the mast and kept out of the way until needed - like the baby stay forward).
But my question now relates to the size of the hardware I bought and installed. The boat came equipped with Harken fiddle blocks that can take 1/2" line easily, maybe even 9/16" line, consistent with what Harken now calls its mid range line of blocks. Compared to the breaking strength of the 4 mm wire, the line that goes through there is serious overkill, whether at 1/2" or 9/16" - on the other hand, being of larger diameter, the line is easy to grab and get a good grip to snap it out of the cam.
I'll just get to the point - I ended up buying Ronstan block pairs that take 5/16 line, which in spectra has a tensile strength of 3,000 lbs (close to the wire), and the blocks have a max (working) load of 880 lbs, which seemed to be high enough when I bought them (giving me close to 4:1 margin - if the 4 mm wire has a breaking strength of 3,300 lbs, I am not sure Swan ever intended the load, even in rough conditions, to exceed 800 lbs in the blocks. With the block system providing 4:!, the line really doesn't have to give me much at all (220 lbs or less).
Here's the BUT - the 5/16 line and the Ronstan blocks just 'look' small, like toys on this boat. Even though they do look small, they have a higher load capability that the next size up Harken blocks (3/8" line, max load 750 lbs).
Now I am sure the original big Harkens were overkill, I am also sure the line quality and load capability is higher now by far than in 1978 - and the degradation of load capability with time is much less now than before - but saying all that, did I size the blocks and lines too small?
I think I should I move up, but to what size - to 3/8"? or 1/2"? Part of what I get is more margin, which doesn't hurt, I also get a larger line to hold, which is good, and finally I get rid of the appearance of the undersized hardware, which I have learned over the years, if it looks small, it probably is.
I could use the big Harken blocks I have (or maybe replace them with new, same size - they are of unknown age) and just be done with it, but am I really doing myself any favors by going to that extreme - bigger, heavier gear, bigger, heavier piles of line laying about, wether in use or in stowage -
Any ideas or advice, that's all I am seeking - I attach pics of the Ronstan block that is now aboard, 5/16 line, working load 880 max, and the next size Harken for 3/8 line, but working load is only 750. I know if you go up to 1/2" in the Harken line, the working load goes up to about 1,800 lbs, which really should be high enough (I think).
Lars - can you speculate on what Swan expected that running backstay to see as a max load, given they used 4 mm wire with a 3,330 lb breaking strength? 5:1 would say 666 lbs....
The blocks should be sized according to the wire strength, and you could either use the breaking strength, or SWL as criterion.
In the first case blocks with at least 3300 lbs breaking are needed.
SWL for 1x19 wire can be assumed to be 1/3 of the breaking, and the blocks should have at least 1000 lbs SWL.
It can be commented that when the runner is under high load releasing the cam cleat may become difficult. On bigger yachts the runner is usually taken to a winch without a tackle.
