Sailing ANY multhull with a resonable performance downwind with spi is about steering and sheeting, not how much you weight. On several pictures and videos with singlehanded boats with spi they don't even bother to use the trapeze!
If you use a really big spi on the A-class you could get a boat that is difficult to handle downwind, but if you are on the light side use a smaller spi. I use 16 m2 on my A-class and that makes me go a bit deeper on the downwind than other cats even though I'm flat out on the trapeze, if that is a problem, use a smaller spi. The big issue if you are light is sailing upwind.
About voiding warranties, this is not an issue for the Marstrom A-class. You can buy a spi kit with pole, snail and support rods from Marstrom and add them to your Marstrom A-class without voiding any warranties. The latest snail design from the M20 is now being scaled down to the A-class spi size of 16 m2 and is soon avaliable from Marstrom. You need to drill a hole in each bow for the support rod, glue a fitting for the spi pole on the main beam and rivet an eyelet on the mast for the spi block. The sheeting blocks are tied to the shroud fitting and the cleats for the spi halyard is mounted on the spi pole. The boat is a blast downwind and if you are using a chicken line and trapeze as far back as possible there is no problem with submarining even in higher winds thanks to the fullness of the Marstrom hulls.
About M18!
The ONLY thing that differs between the M18 and A-class is the width. The mast, hulls, center boards and rudders are the same.
/håkan
