I think this would be a lot of fun.

You could look at lateen rigs. I had an old Sea Snark sailboat that had a
styrofoam hull. The mast was supported by sliding it into a 12" long
piece of pipe and tying it down. There was just a price of board across
the hull to hold the top of the support pipe and the bottom was just
sitting in a cup in the styrofoam. If styrofoam will hold such a mast
your aluminum boat will, too. You will want to have the sail a lot higher
off the floor to clear the seats with a bit of allowance for passengers'
heads, say 4 feet high support pipe. Use aluminum pipes or channel bars
to support the top of the support pipe. Some cross bracing on the floor
will hold the bottom of the support pipe. I would use two sails - they
could be beside one another at each side of the boat - simplest
installation with most bits out of the way - but best sailing performance
would be both sails on the centerline, one near the front of the boat and
one about two-thirds of the way back.

If you use a single sail or two side by side sails locate them about 40%
of the way back from the front of the boat.

You need a centerboard to resist sideways sliding and to privide
hydraulic lift towards the wind - required for anything but dead downwind
sailing. One board on the centerline or two boards, one on each side.
Longitudinal location of the board should be at the point one-third of
the way along hte sail from the mast - if you use two sails use the
average of these points.

A centerboard could be clamped onto the sides or hinged below the boat so
they can be raised or lowered by a rope when needed. Alternatively, a
centerboard can be fixed - bolt it onto the boat. You can fasten one onto
the bottom of each hull or make a support for a single board along the
centerline. Fixed boards should be around a foot deep and 3 feet long for
shalowwest depth. Hinged or removable boards should be deep and narrow -
one foot wide and 3 feet deep.

Lateen sails have a horizontal boom that will give passengers a good rap
on the head if they don't duck when you tack or jibe - jibes are more
sudden with more force - enough to maim or kill in the strong wind. Maybe
you should look at simple sloop rig sails without a boom, just a sheet at
the back end of the sail to control stretch of the sail as well as
sideways adjustment. This would be a lot safer for people who are not
accustomed to sailboats and the hazards of booms. A sloop rig would be
installed the same way as a lateen rig - slide the mast into a fixed
support pipe. It may have less sail area and thus less performance but it
would be close enough in your situation. Costs should be similar.

Take a look at the mast mounting here...
http://www.amateurboatbuilding.com/a...der/index.html

On Nov 20, 6:12 pm, cavelamb himself wrote:
Take a look at the mast mounting here...http://www.amateurboatbuilding.com/a...der/index.html

Use the sail only off-wind so you can use a boomless gaff rig thus
keeping the sail low and eliminating the need for stays. I'd use
leeboads instead of a centerboard because with a centerboard you need
more structure under the mast to support the loads on it whereas with
leeboards those loads are on the pontoons.