Forestay loading

Clyde_Wanderer

Clyde_Wanderer

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Any body got an idea of the laodings/stress value on the fore stay to deck plate joint, for a rig with 36.5ft mast and aprox 11mt forestay with roller furling?
Would be very interested to find out, as I am strenghtening the setup at the forepeak.
Thanks.
 
As a rule of thumb standing rigging tends to be set up at between 10-20% of its breaking strain. You would also need to make an allowance for the dynamic loading caused by the sail (say a further 50% of the static load - that's a bit generous)

A very rough figure but is should give you an idea of the forces involved.
 
Not sure if this will help, but may be of interest. It's for calculating sail power but it should be possible to calculate the loading on the forestay by using it.
 
If you really cranked on the backstay you could get up to 25% of the wire breaking strain (the maximum you should ever contemplate), so about 1000kg static load for that size rig.
 
Just about to put the mast back up and read in this months PBO that an 8 inch screw driver gets the rig as tigt as it should go is that right? The rig wasn't that tight before!
 
I think you would be hard pressed to over-tighten the rigging using 8" hand tools, but that approach won't get the rig set up optimally.

Also you have to be careful about the order in which you tighten the rigging - were you to tighten the port cap shrouds fully with the stbd slack, and then tighten the stbd fully you would have an unbalanced set up

I would recommend the Fernhurst book by Ivor Dedekind (that's not quite the right name) - it gives a good basic intro to rig set up and sail tuning.

Also for about £30-40 you can get a rig tension gauge that measures the tension in each shroud
 
Is your mast stepped on the roof of the cabin? Having seen compression in many yachts due to overtight rig, I personally do not use 20% of the breaking strain of the wire as a load factor. As long as the wire does not deflect under load, I am not too worried.
Another consideration is the build quality of the boat, as the lighter layup on some boats coupled with wider beams means that as the boat moves, there is inevitable flexion of the rig
 
i had an innerforstay fitted last season.used a marine surveyor to work out the angles fittings and loadings and to provide drawings for the strenghtening of the bow section.the work was done in local yard.my point is that compared with the cost of the work/new rigging/ sails the couple of hundred i paid the surveyor for his work plus supervising the job was very reasonable.might be worth seeking profesional advice
 
You will also find a useful Hints & advice pdf file on the Selden masts website that describes the procedure for tightening rigs based on the UTS of the wire as part of the whole process of setting up various types of rig.

You do have to be wary of any cases where the wire size has been upgraded and tightening to 15% of the UTS of the new wire size could seriously overstress the fittings and the hull.

Sorry i do not have the url for the Selden file as I have it as a published booklet.
 
I think you will find that the recommended static tension set up for the rig is wildly unrelated to loads in service. (And I don't believe in large static loads anyway)
The strength of your forestay fitting should be better than the breaking strain of the wire however the tension on the luff of the jib should be added to that load. So a properly working jib with a good luff tension should leave the forestay with little load.
The fitting and joint should be overstrength and comparable with similar boats. A strap down the bow for 20 centimetres will
provide a lot of additional support if you think it needs it.
good luck olewill
 
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