The first point is that the same basic rules of sailing apply. A tack is still a tack, a gybe a gybe, a mast, mainsail, jib, sheet, etc. Most of the differences come from the characteristics of the design of the hulls, which result in a different approach to sailing multihulls than monohulls, especially in a race.
Boat Speed and Trim
In light wind, when monohulls are not at full speed, cats are again fast for several reasons: They tend to have taller masts with bigger sails, they have less friction due to wetted surface area, and they are usually lighter for their length--especially compared to keelboats.
Now stop here for a second and think about what this means in terms of a race. When you consider monohulls, supposing there's enough wind to reach hull speed, the entire fleet will all sail at the same speed (if it's one-design). Your trim can even be a little off if its' windy enough and it won't slow you down. So how do you win? Taking current out of the factor, tactics become the key. In a multihull race, there is no margin for error regarding trim regardless of the wind speed. The difference is huge, the speeds are high, and every second you are not sailing to the max, you are loosing lots of ground on those who are. You can always make it go faster! That's the challenge. If you're not 100% committed to all-out speed, catamaran racing is not for you.
Another point to remember is that because catamarans are so light they carry little momentum -- the wind is their only source of forward motion. If the sails stop working, it’s like putting on the brakes. We'll touch more on this later.
Let's consider some less obvious aspects of multihull sailing, first the idea of apparent wind (windsurfers will be used to this): Most sailors know that when a boat is moving forward, the apparent wind also shifts forward a bit from the true wind direction. On a catamaran, because the speeds are so high, the apparent wind shifts forward a lot. Once you get moving, you have to re-adjust your trim and/or sailing angle to a point where your sail trim is completely useless in relation to the true wind. You're riding your own wind, and this is tricky to maintain. If anything happens that affects your speed, it will affect your apparent wind angle (and vice versa), this can all but stop you dead, and then you'll need to sheet out and/or head up to start building your apparent wind all over again. Sound like a lot of work? This is how a catamaran is sailed and is especially significant downwind.
Another less obvious but important characteristic affecting catamaran performance is the fact that it doesn't heel. Going upwind, when a gust hits a monohull, the boat heals to leeward in order to compensate. This is caused by two things: the increase in wind pressure causes more heeling force in the sails, and the fact that, assuming the true wind direction is constant, the apparent wind of a gust will always move aft (if you think about it, it makes sense -- you're moving slower in relation to the true wind). If the sails aren't let out, a monohull will be tilted sideways. Though not optimum, a monohull still sails forward when heeled over in a gust. A catamaran, however does not heel, and unless the gust is strong enough to lift a hull, it does not compensate for the gust and simply sits there, out of trim (stalled), basically being pushed sideways to leeward. It is for this reason that catamaran sailors have to pay meticulous attention to their sail trim, working the sails and the helm in the gusts. The boat won't help them out.
This rapid acceleration factor also makes starting a catamaran race a little different. The whole trick to starting any race is to be at the favored end of the line when the gun goes off, in clean air, at full (or better than full) speed. Monohulls will swarm around and do all sorts of fancy approaches so as to be at optimum speed when the gun goes off (many will claim this is fun). Catamarans, which take all of maybe 10 seconds to go from full stop to full speed, can pick their spot about 50 feet from the line, sit there, then about 10 seconds before the gun goes, sheet in and go! (Easier said than done, mind you.)
The lack of momentum of a catamaran is also a hindrance in really light wind (or drifters). Here, the monohull will have some momentum built up to help power through the flat spots between the patches of wind. The catamaran will likely just stop dead in the middle of the flat and sit there until the wind comes to it. If there's any chop it's even worse. It's quite frustrating watching monohull boats that you blasted by ages ago when the wind was up, catch up and drift by while you sit in a hole (on days like that, a good catamaran sailor will remember to bring beer!). Likewise, on a catamaran it is difficult or impossible to use boat momentum to power through dirty air, such as that of a boat's wind shadow. The idea on a catamaran is to not get yourself in dirt in the first place. Considering this, it is easy to see why the start of a catamaran race is tremendously important. Even if you're a faster sailor with better trim, it is very difficult to pass someone upwind in disturbed air. You need clean wind.
Note If you're sailing on the same course with monohulls, take extra caution. They tend to sail at completely different angles to the wind, and if they're not used to it, it can be difficult for them to judge your speed. Several times I have been in situations like coming up hot on starboard to a pack of Lasers all sailing 6 inches apart, by the lee, on port tack. Of course, they don't expect you to get there that fast, and as soon as you get under their shadow, you slow down, and have to heat up even more to maintain way. What a disaster!
No Tactics; All Strategy
Downwind, now this is a completely different story. All the rules can be exercised. Because catamarans will sail downwind in up to 80 degrees of apparent wind(!), the gybing angles are very hot (roughly 90 degrees -- same as tacking upwind). This combined with the fact that you can literally double your speed the instant you hop up on a wave and start surfing, presents a large field of opportunity for tactical moves, passing lanes, etc. If you can time a wave so you get it right about three seconds before the guy directly in front of you, you're past him on the high side before he has a chance to defend! Actually, if you do the math, you can see that cats never actually sail downwind at all: IE, on a broad reach with the true wind at 160 degrees, your apparent sailing wind is around 70-80 degrees, so you are actually trimmed slightly for upwind (this depends on the boat and/or spinnaker). On the beam is usually optimum, and except in a drifter, catamarans seldom sail with the apparent wind aft of the beam.
Boat length, boat weight, sail size, crew weight
While all this may seem obvious, the extent of the effect these factors produce may not be immediately evident.
Upwind on a monohull, you want to stay with the fleet, slice and dice your opponents, and take advantages of their tactical errors. On a multihull, not only are you not really maneuverable enough to pull off most of that stuff, you can't afford to be anywhere near dirty wind, so get a good start, go clean (often to the corners), go fast, and sail your own race! Take advantage of your opponents strategical and boat handling errors upwind. Downwind: surf surf surf!
Chris Jackson, 2003