>>Nothing will discourage them more than sitting out on water not moving and thinking they are doing something wrong. When the problem is not much weight. I say 18 or 20.<<

A Hobie 16 with 325lbs of crew may not be ideal for top-level racing, but to say that it won't move is a bit of an overstatement. I've seen pictures of H-16s with four adults on board flying a hull.

Here's something that will discourage them more...
Spending three or four hours on the beach trying to figure out how to rig their new (used) overly coplicated Hobie 20. Still getting it wrong. Then going out for a sail in too much wind (which for a newbie on a H-20 would be anything over about 12mph) and flipping 5 times and being totally out of control. Then crashing their new to them $5000+ boat into the beach and damaging it.

I don't see how anyone with experience can possibly recommend a Hobie 20 as a beginner boat. Seasoned racers have trouble handling it even before the wind hits 20mph. If you're recommending a Hobie 20 to a total newbie, it's most likely because you have never sailed one- if you did, I think you would feel much differently.

sm