Congratulations on your acquisition of one of the best beach cats ever built.
Catamarans sail very differently to monoslugs.... they do not point as well, they run like crazy.
We have had no problems in winds of 15 knots, gusting 30. After that, life can get 'interesting', as it would on any craft only 18 feet long. I am not recommending that you explore nuclear conditions, just be aware that there are folks on this Forum who regularly go out to sea and jump waves, (right Mike !). The H18 can handle about as much as you can throw at it.
You would be well advised to take your boat to an area where you can do a controlled capsize and capsize recovery, (with a power boat if required for help in lifting the tip of the mast), and practice that a couple of times, see elsewhere on the Forum for 'how to etc'. I need to do that with my 20 year old son, as he has never helped me right my SX18, and we sail together all the time. It will give him confidence and experience for when we really need it.
You may also want to make sure that your mainsheet and main blocks run 'free and easy'. We run the stock Harkens with Robbline. You will need to learn to read and feel the wind and to react accordingly by sheeting out VERY rapidly, far quicker than on a monohull. Otherwise, you will enjoy lots of swimming. So for the first season, I suggest that you NEVER cleat the mainsheet. The jib is not as critical.
For upwind work, we normally set the traveler dead centre. If you are concerned about being over-powered, set the traveler out 8" or so. That will 'soften' your ride enormously. Agreed, SRM and others?
Depending on wind angle and wave height on the downwind run, I'll often set the traveler centred, or at 8" or at 20". then I sheet in the main. The H18 has tons of volume in the forward part of the hulls, so downwind should never be a problem - very hard to pitchpole an 18.
If the wind really pipes up, how about furling the jib, and running purely on the main? I've often done that.
Cats are also sensitive to trim, so your fore-and-aft positioning on the boat can be more important than leaning out.
When you feel comfortable, you can move up to using the trapeze and hiking out. Read up more on this aspect first...
as it has its own set of challenges.
Whatever happens, don't be nervous when the Cat rockets ahead... this boat will give you a great ride.
1989 Hobie SX18 Sail # 1947
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'