On the static calculations that I have performed in the past on my own boat I found :

The forestay can see 600 kg loading, the bridles are much the same

The sidestays are loaded up less to about 450-500 kg. (when both crew are sitting on the hull in a blow. )

This assumes that you can pull on the mainsheet line with 50 kg and are using a 8:1 tackle setup. It also assumes sailing hard in 2-up mode.

When the crew is double trapezing then the side stay load drops to about 350 kg. As such the sidestay loads respond differently to going double trapeze then the forestay. You unload the sidestays by going double trapping when you load up the forestay even more (max about 600 kg). However by far the most of the forestay loading comes of the mainsheet system. Pulling harder on that increases the forestay load, the sail forces on the sails are not a large component in this setup. Going into a violant dive actually decreases the forestay load and increases the sidestay/mainsheet loading. As such this is a positive thing as the latter can easily take the increases when the forestay can not.

I've chosen to use 4 mm steel 1x19 all around. Basically because I had to choose between 2.5 mm dyform (625 kg), 3 mm 1x19 (720kg )and 4mm 1x19 (1200 kg). I found the breaks strengths of the first two unnervingly close to the real loads and I couldn't get 3 mm dyform (900 kg) that I really wanted. I also planned to race my F16 doublehanded hard and my calculations were based on that.

If you are only sailing singlehandedly then 3 mm 1x19 may well be enough but for hard 2-up racing I would choose 3 mm dyform or 4 mm 1x19 for the forestay and bridles. I feel that on the sidestays you can take some more risks.

In engineering it is common to use a factor of 150-200% as a safety margin.

I also discussed this with Greg Goodall in the past and he mentioned that they could get away with the older 1/8th inch 1x19 wires on the Taipan 4.9's. That was before they started using 3 mm dyform. However, he was convinced that any smaller diameters would break.

The older stype 1/8 wires are actually a little stronger then the new metric 3mm 1x19 wires. These tow sizes are very close together as 1/8 inch = 3.175 mm, but this difference still allows the older wires to carry 12% higher loads before breaking. 720kg (new) to 810 kg (old) and everything helps. This also makes clear how attractive 3 mm dyform is (900 kg).

It must be noted that the F16's, with the large squaretops, need more leech tension then the older Taipan mainsails. As the mainsheet is such a large component of the whole I would not take to much risk on the forestay.

Difference in weight between 20 meter 3mm 1x19 and 20 meter 4mm 1x19 = 1.060 kg - 1.885 kg = 0.825 kg.

I think the difference between 20 meter 3mm 1x19 and 3mm dyform is about 0.350 kg

Those are the downsides of 4 mm 1x19

The advantage being of course that you are totally secure about this wire not breaking AND more importantly having a stiffer rig !

If indeed keeping the spi luff tight is so important and the mast movement is sufficiently large to upset this then the 4 mm 1x19 wire will by far have the least mast movement of all the alternatives discussed below. Similar things will apply to the forestay stretching and sagging (=upwind pointing ability). 4mm 1x19 is still 50% stiffer then 3mm dyform.

I hope this helps.

Wouter