Jl, try contacting Hakan
hakan.frojdh@bostream.nu He sailed the M18 with spi to a 2nd or 3rd round texel a while ago. more remarkable he published a video showing speed differences on an A with and without spi (gps reading). he probably can answer you how to build the marstroem system. stay away from the spi-drum. upwind it might be better but malfunctions , spi life and limited angle for rise and lowering of the spi are not worth the better windage.
those who say you don't miss a spi on the A for open class sailing or long distance races just never tried it. you are so much faster. you can beat the M20 with an A+spi on line honours. you laugh about F18. In less than 10 knots Tornados become swimming obstacles. Raised some rating discussions once when beating the new crowned F20 European champions by half a lap in a three lap race in 10-15 knots of wind. Without spi they would eat you before the first downwind mark. The advantage of the spi decreases with stronger wind on triangle courses as hoisting and pulling in take (relative) more time.
If someone dont share my view, please don't feel offended, its personal view based on personal experience.
