The A-class with spi is THE dream boat, and I have one!!!!

You gain about 30% of downwind VMG in higher winds if you compare an A-cat+spi with a normal A-cat. In lower winds the gain is a bit more.

Upwind you loose a bit, 5% maybe? How much you loose depends on the type of snuffer you are using and if you're using external or internal halyard for the spi. The "gain factor" between upwind and downwind is approx 2, that means if you gain 5% of downwindspeed with some extra gizmos and loose 2.5% upwind due to the gizmos you haven't gained anything during a complete lap.

You start to trapeze downwind in about 8 knots of wind, and in 18 knots of wind you must stand with the rear foot close to the rudder to keep the bow up. Use a chicken line or footstraps to stay attached to the boat.

I have several plots of downwind sailing with the A-class using the spi and in 16-18 knots of wind you are doing around 17-19 knots of boatspeed with an angle of 30° to a direct downwind course (15 knots VMG).

If you go wild thing without spi you will sail around 45° angle to a direct downwind course and the speed would be around 14-16 knots (11 knots VMG).



The course, one lap plotted:



/håkan