Boat Speed and Trim
The key thing to remember about sailing a multihull is that you're not limited by a maximum hull speed like most monohulls. Where as (non-planing) monohulls can really only go as fast as when their bow wave reaches their stern (sqrt(lwl x 1.34)), the long skinny cat hulls (<8:1) do not create much of a bow wave, so the water-line-length is not really a factor in limiting speed. You are limited only by how fast your sail can take you. Longer catamarans tend to go faster than shorter ones because they can carry more sail before submarining the bow, though sail area to weight ratio is more indicative of performance. More sail generally needs more crew weight to keep the hulls down, which in turn propel the boat even faster. This can be realized when you consider windsurfing; pretty much the fastest genre of sailboat, and has the smallest sail. It's fast because it's light (and it planes).

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.

Boat Handling
In order to be fast, boat handling of a catamaran requires perfection. There is little momentum in a catamaran, so when the sail looses power or becomes out of trim, such as while tacking and gybing, the boat de-accelerates at an astonishing pace, sometimes to the point of stopping dead in the water! Though gybing is not too bad, tacking, to a beginner catamaran sailor, is nothing more than a necessary evil to be avoided at all costs. To an experienced cat sailor, it's an opportunity to gain ground, as an excellent wire-to-wire tack and quick re-acceleration gains boat lengths over a mediocre one. Tacking is an art for both skipper and crew: they have to come in off the trapeze, scramble across 8 or more feet of trampoline, whip a 10-foot tiller extension behind the sheet, the crew has to release and tack the jib, they hook back in and push themselves out to the other side. (The position of their heads moves about twenty feet across from tack to tack!) And, don't think you can get away with leaving the sails in tight while tacking either -- catamarans have great acceleration, but you have to trim into it to make it happen. If you don't slacken your sails, precisely over-tack to a close reach, and re-trim as you accelerate up to close hauled, you are going to loose boatlenghs on someone who does.

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.

Less maneuverability
Although catamarans tend to respond faster than most monohulls, and are generally more high-precision, they have much less maneuverability than the monohull. This is partly because of the momentum issues discussed previously, as well as the insurmountable fact that you have 2 big long hulls spaced 8 or more feet apart that you have to move around. Close-quarter maneuvering is just not going to happen to the same degree as with monohulls. It is for this reason that you should generally stay well clear of boats that are right-of-way, and make early calls to those who aren't -- just in case! Protest or no, a high-speed collision can ruin your weekend!

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
Because they tack so slow, it's counterproductive to tack upwind on every header. Most often, you loose more ground by tacking than you would gain. It has to be one pretty obnoxious shift to make it worthwhile. Likewise, upwind boat-to-boat tactics on a multihull are almost a non-issue. You can sometimes pull off something like a lee-bow, but you'd have to be like ten boat lengths ahead before tacking to make it stick! Who would do a lee-bow if they were already ten boat lengths ahead?? (crossing and slam-dunking to a tight cover would be way more affective).

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.

Capsizing
The good news is, except maybe for when you're really humming, capsizing on a catamaran is way less likely than a non-keeled monohull. The bad news is, when you do go over (and it happens to everyone) it’s much more difficult to right. Unless you're on something like a Hobie 14, you usually need two people to get it back up. The trick is, if there's wind, you can get the boat on it's side with the mast pointed into the wind, and as you try to leverage the sail out of the water, the wind catches underneath it and it sails itself upright. If there's no wind, then you can't really do this, but you're very unlikely to capsize in the first place.

Pointing
It's a well accepted fact that catamarans don't point as well as monohulls, and for some catamarans this is certainly the case (asymmetrical hulled cats like the Hobie 16, for example). Whether or not this is true for all catamarans is another question. Although you'd have to try right beside you're buddy in his Laser to find out for sure, one thing you can bet on is, boat length for boat length, the speed difference of the catamaran is almost always going to make up for the fact that it may not be able to feather up as high as the monohull.

Boat length, boat weight, sail size, crew weight
Optimizing these factors is what will make you the fastest. Boat weight is a hindrance. Any weight is a hindrance unless it is being used for ballast. Therefore, unless it’s hanging off the trapeze wire, working to keep your windward rudder down in the water, or keeping your leeward bow from doing a nosedive, you're better off without it. Too little weight: If you have too much sail for your weight (IE you're overpowered), you have to spill a bit of wind off the sail to keep the boat down. Your speed will suffer compared to heavier crews who can provide more ballast and don't need to spill. Longer boats can carry a bigger sail, and therefore usually need a heaver crew. They're not necessarily faster in all conditions; it's the weight-to-sail ratio that's important, and whether the boat is long enough to handle the size of the sail.

Summary:
Aside from the obvious difference in number of hulls, the three biggest differences between the two schools are these: Multihulls to not have an effective hull speed limit like a mono, they don't heel, and they don't have a heavy keel (or, even compared to centerboard dinghies, they are lighter for the length).

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!