How deep of lake for 18 foot cat?

"Averages" 3' that must mean that it is only 1' in places? I'm not quite sure what you are asking. If you mean. can you sail a cat that has boards, but sail it with the boards up, the answer is not very well. You need the resistance of the boards to counteract lee drift. With no resistance you would move downwind like a balloon tossed into a lake.
If you mean. can you sail a board less design in shallow water, certainly. Those models depend on the hull shape to provide resistance. The windsurfers & kite boarders love shallow water, especially shallow water between sandbars. They can have big wind,(lots of speed) but flat water.
It also depends on the layout of the shallows. A "saucer" that is deeper in the center is better than random channels & weedbeds, remember an 18' Cat can't maneuver like a Laser.
The downside of really shallow is when you crash the mast will hit bottom pretty easily, possibly causing damage. Worse, when you flip your cat in 1'-3' of water, people can go from over 8' in the air headfirst into 1-3' of water, can we say broken bones & motorized wheelchairs.
I only have 3' at my dock near the end of summer, I sail conservatively til I get out to where I know I won't be slammed into the bottom if I tip it. Paraplegics do many things, but I think Cat sailing would be tough.

It is a saucer with 6 feet in the bottom and 3 feet surrounding then getting shallow for about 50 feet to shore. The lake is about a 1.5miles across and 5 miles long in the area I want to sail. I have sailed a H16 but never crazy enough to tip.

If it gets weedy, that can be a pain with boards. I went into a weedy bay with my Invitation once. I had to pull the center board up every 50' & dump the weeds. Same for the rudders.

Look for a boardless 18 footer like a Prindle 18 or Nacra 5.7 or 570. There are others but those are probably the most likely to find.

Oh, also the Hobie Getaway if you are looking for a newer boat.
http://www.hobiecat.com/sailboats/getaway/

That is a SMALL lake to sail on. I would certainly not get a boat with boards. An 16' or even a 15' boat would work and pop up rudders are a must. But man are you going to be tacking lots. My lake is around 10 mi by 10 mi and in a blow it seems I just get my rig set and I have to tack again.

And sticking the mast is a horrible pain. I once got the old H16 stuck for like 3 hours before we could pry it out of the mud with the help of a boat. Weeds aren't as much of a problem, just sharpen your boards really well! 😆 Truly there is nothing quite like dragging 200 lbs of weeds around the lake.

D.

Edited by Wolfman on Jun 22, 2011 - 09:15 PM.