Adjustable backstay refit

I am thinking of changing my backstay to an adjustable with a pair of blocks. I have a 24' boat with a 30' 8" masthead rig. I am not going for mast bend but I would like to be able to adjust the tension on the forstay for wind conditions. I was wondering if anyone out there is using this system and how much purchase is enough. I am thinking 4 to 1 is enough but would like to hear from anyone using this type of set up. I do not have a split stay and I think it would be difficult to make it a cascading system.

 

Size it to your backstay. Set the parts so that ~200 lbs pull on the tail gives ~50% breaking strength of the wire rope. Thay way you really have to want to to bring the rig down.

 

Good advice ... but it's not too much backstay load that will bring the rig down. It's when someone uncleats the backstay and the rig goes over the bow that will bring it down.

In general you want to include some way to maintain a minimum of 5-10% of rated strength on the backstay when the adjustment is slack. Then, as jehardiman suggests, set the purchase so 200# pull = 50% of rated strength.

If your backstay is:
1/8" wire you need 4.4:1 to get 50% at 200# ... you lose 5-10% for every turn, so 6:1 or 8:1 would work.

5/32" wire you need 7:1 to get 50% at 200# ... with losses 10 or 12:1

3/16" wire ... don't even try.

The beauty of a split backstay system is that it provides uncleated safety and a large and variable purchase for adjustment. The advantage is large when the stay is lightly loaded, then the advantage decreases as the load increases. Something like 32:1 at the beginning of the travel and only 8:1 when the split legs are deflected 60deg.

 

I believe the stay is 1/8 inch. I was thinking a pair of fiddle blocks, one with a becket and cam cleat and some 5/16 sta-set x with some stopper knots to help prevent any accidents (and a little better grip when wet). That would give me 4 to 1 I believe. I did not think about over powering the rig though. My other thought was a pair of triple blocks with 1/4 inch line. What do you think of either of these set ups, assuming 1/8 inch stay? I am curious what the weakest link in this system would be though. Thanks for the advice!

 

The 1/8" backstay @ 2100# (302 alloy) should be the weak link. I'd use triples to get a 6:1 or 7:1 purchase. Harken 40mm Carbo Triples have the strength you need. I would also add a 3/16" 7x19 safety wire from the backstay to the chainplate to limit how far the tackle can release the stay.

Most people can't get anywhere near 200# pull on 1/4" line. That , combined with the losses in multi-part tackles should keep the adjustment within the elastic limits of the wire.

 

I race a 24ft boat and the standard purchase for backstay tension is 16 to 1, with a double ended tail going to each side of the cockpit. There is no way you can apply 200lbs of pull to a line without having someone put their feet/back/ total strength into it, and even then I think 200lbs is on the high side. Load up a bag with weights and find what you are comfortable lifting one handed in the sitting position.

16 to 1 allows for a one handed tug to have some noticeable effect on forestay sag. Having a way of making rapid and repetitive adjustments is a really effective tool that can do more than the classic 'more wind, more luff tension' school of trim.

Sagging the forestay is a really great way of powering up when there is more waves than wind, especially as you come out of a tack and are trying to get back to speed. But you will have a flatter genoa entry that makes finding the groove harder than with a nice rounded luff, but the power and speed will be impressive. Once back up to speed increased luff tension will help you point.

The backstay has developed into a really important control for us replacing the genoa cunningham in importance (although I'm not sure what the curent J24 thinking is.)

Ultimate mast stability (and the effectiveness of the backstay) will be affected by the shroud and spreader configuration. In order to have maximum mast movement we only have single lowers with the chainplates (and hence spreaders) all in line with the back face of the mast. Although some people have wire strops to stop the block purchase overhauling itself, lots of us don't, and masts seem to stay up, even though only deck mounted. You will be amazed at quite how far you can move the masthead to good effect.