I have extensive experience in "high seas, offshore" conditions with outboards on beachcats.
Mounting Bracket:
First I tried my 1.5 hp Cruise N Carry (12 pound, 2 cycle, Shindaiwa weed eater motor head) on the rudder blade. I lifted the rudder and tied the casting so that it would not fall down. Then I applied a layer of suede leather over the rudder blade and clamped on the motor to the raised rudder blade. This worked pretty well (would be fine in a rare emergency) but caused excessive helm due to the motor being off-center.
Next, I went to a lot of time and trouble to build a mount. I won't bore you with the details since my findings were that my mount was not very good. When sailing downwind, it spanked every wave and sent a splash reflection onto the sailors' heads.
Then, I used a Cheetah Bracket and can not say enough good about the wonderful design of the Cheetah. Mounting an outboard on a catamaran is much more of a problem than the shear weight of the motor. The Cheetah is a very nicely thought out design. (except for the soft aluminum 1/4" block pivot part that needs to be replaced with a stainless steel bolt, immediately, upon purchase)
Engine:
The worst part about trying to use an outboard motor on a cat is that you have no transom in front of the motor, blocking the seas from striking the motor as you would on most boats. The motor not only gets struck by seas, but gets struck by the bow waves from both hulls. This causes repeated dunking.
I have run a Cruise N Carry 1.5 horse, a Cruise N Carry 2.7, a Gamefisher 2, and a Honda 2. Not being in the mood to type out all the details of my experience I can just say that the Honda won the contest.
The Honda is a whopping 27 pounds, nearly twice that of the 12# Cruise N Carry. But when your running motor goes under a three foot wave, 35 miles from the mainland, while trying to get to the shore of an island for dry comfort, you are happy to have a Honda on board.
Eventually my Honda's intake valve got stuck in the open position. When I rebuilt it, there was a lot of salt build-up in the intake port. But after going under water about thirty or forty times when it was running, this was not a big surprise.
Interesting facts:
The Cruise N Carry 1.5 gives me about two to three miles per PINT of fuel. The Honda has never failed to run when on the water. (gas mileage is about half as good)
The best container for either gasoline or pre-mixed gas with oil is a plastic coke bottle. I have tested them, even the older ones that had a rubber membrane liner in the cap, for 6 months in my garage. They tuck nicely into trampoline pockets and if you do not have gas inside your enclosed hulls, you don't have to have a fire extinguisher aboard. (USCG rule number lah dee dah).
The one pint size coke bottles fit in the fuel tank filler hole perfectly. You can invert the bottle in the fill hole, just like a race car, and never spill a drop.
A cavitation plate, made of Lexan or whatever you choose, added to the motor will increase your performance.
Falling off of a catamaran, when testing a motor, in the ocean, miles from shore, while alone, on a weekday when there is nobody around, while the propellor is turning, is a test of survival skills. If you find yourself dragging behind a boat that is driving at 3 knots into the wind you will learn new tricks. I learned how to steer the boat into the wind so that it would not capsize, then to operate the "kill switch" while having to pull myself dangerously close to an engaged propellor.
Tether leashes are ultimately important, when soloing!
GARY