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I did some calcs on my mast head rig with self furling also covering the stay. I figure the fore stay will see about 125% of the tension on the back stay due to the smaller angle between the forestay and mast as compared to back stay and mast. So if you angles are similar, then take back stay tension and multiply by 1.25 to get approximate forestay tension. If you send me you mast height from deck, and distance from mast to bow and also mast to the stern, and your backstay tension, I will calculate exact number, otherwise 1.25 x should be close.
Dedekam says: "Forestay length determines the mast rake. Backstay tension has only a small effect on mast rake. On a masthead rig, backstay tension mainly controls forestay sag. On a fractional rig the running backstays (runners) do the same. On a fractional rig without runners the tension of the cap shrouds mainly determines the sag."
Notice he said Forestay length (pre determined in the case of my furler), not tension
. BTW, he also mentions several other factors that relate to Mast rake, but the issue here was Forestay adjustment.
As I said, it's not rocket science (I wish I could even attempt the calculations you did!). However, I strongly recomend we find a credible source of information and understand how the system works BEFORE adjusting our rigs.
Last edited by L124C; 01-26-2012 at 02:16 PM.