Spreader and shroud loads- swept aft

[P38]

Hi Brian,

I'm confronted the same problem, designing a modern version gaff rig for a 42 footer. The approach I took was to seperate the transverse and fore/aft loads and then sum them up.

For the transverse loads, there's lots of information available on calculating these. Brion's book addresses it. You start with the righting moment (typically at 30 degrees). You can use vector analysis to calculate the rig load required to heel the boat 30 degrees.

I started the fore/aft loads by picking the static tension I wanted on the forestay. The tension required to achieve a given forestay sag is calculable. You can then calculate the loads on your swept shrouds necessary to supply this tension to the forestay. You do this as a side view vector analysis.

You're going to get some big numbers for shroud tension necessary to keep the forestay taut. Remember that the load is shared between both shrouds.

The end of all this is that you have both transverse and fore/aft loads. These are added together to get the total load on the shrouds, ie the transverse load heeling the boat and the fore/aft load tensioning the forestay are both carried by the shrouds. Calc them seperately and add them together.

You're also going to have spreader loads, etc. But all these things can be seperated into two components, ie transverse and fore/aft, calculated, and then summed up.

Regards, Paul