Powercats: You either love them or hate them.

You probably already know that twin hulls offer many advantages over monohulls, and that they offer some significant disadvantages as well. The cat-versus-mono debate has been driven into the ground through years of discourse.

But whichever hull design you favor, the basic argument can be boiled down to this: Powercats usually display superior efficiency and stability, a softer ride and more deck space than a monohull. But, they also look ugly (well, some people think so), feel strange when turning, may "snap roll" in escalating seas, and generally cost more than monohulls.

Which is better? Neither, either and both. It depends on which factors are important to you personally. Yet there's no denying that catamarans – both power and sail – have continued to grow in the marketplace, seducing interested boaters. If you are one of those flirting with the idea, read on.

SEA CAT BOATSPowercats like this Sea Cat are roomier than their monohull counterparts because they can carry the beam all the way forward.SMOOTH RIDE

For most boaters, the biggest attraction to powercats is their soft ride. But, is it really softer? Some would argue no, but they probably don't have much cat time under their belts. The fact that cats can run smoother than monohulls has been proven numerically, as well as anecdotally.

In one cat-versus-mono test I was involved in, we rigged a 26-foot Glacier Bay catamaran and a 26 Regulator, a monohull well known for its smooth-riding deep-V hull, with an accelerometer. This gadget measures the severity of impacts, and interfaces with a laptop computer, which then translates the impacts into a graphical format. We ran the two boats side by side in identical conditions: into, with and across the prevailing seas. Time and time again, the powercat posted drastically reduced impacts. The degree of reduction varied a bit, depending on which direction the boat was going, with the most impressive results in a head sea. In some cases, the cat experienced fully half the impact of the monohull.

How is this possible? Look at the tunnel on most powercats, and you'll notice a surprising feature: they grow smaller as you move aft. These are called compression tunnels, because they use the forward motion of the boat to compress air as the space between the hulls grows smaller. That compressed air then acts as a cushion, which absorbs a great deal of the impact when the boat hits waves. Combine this with the fact that cat hulls are much finer at the entry than monohulls, and thus have less impact when the fiberglass meets the waves – and a much smoother ride.

So it's true: We have numerically proved, at least in this case, that a cat is smoother than a monohull – when, that is, it was being operated properly. When driven improperly, the same tests prove that a cat significantly loses its ability to smooth out the ride. What do you do differently when driving a cat? More on that later. First, let's look at some other advantages and disadvantages.

SNAP ROLL

Stability is another big factor that's notably different between the two. Cats are far more stable, which seems like an advantage – and usually it is. But in certain conditions, when the seas overcome the inherent stability of twin hulls, the shift from stable to rocking is abrupt and severe.

In other words, the monohull rocks a little, then a little more, then a lot. But a cat will go from an essentially static position in little waves into a sudden roll when those waves really increase. That roll motion is commonly called a "snap roll." Still, on nine days out of 10, the super-stable platform will make you glad you're on a cat.

Modern powercats also win the space race. Since the two hulls run parallel to each other all the way from the transom to the stem, the boat enjoys its full beam for its entire length. Take a few deck measurements, and you'll discover that in most cases, this gives cats a 20- to 30-percent deck-space advantage when compared to monohulls of the same length. That also translates into more stowage space, higher passenger capacity, and larger cabins.

GLACIER BAY CATAMARANSA powercat's fine entry has less buoyancy then a monohull's, so weight distribution is a factor.The final advantage of running a cat is improved efficiency. Remember the comparison between the 26 Glacier Bay and the 26 Regulator? The cat needed just 300 total horsepower to cruise in the mid 30s and top out in the low 40s. The Regulator requires another 100 horsepower to post similar performance numbers. And fewer horses translate into a lower fuel burn.

What gives? There are several reasons, but to understand them, first we have to look at the major difference between cats: Some are planing, some are displacement, and some are semi-displacement. Just exactly which boat is which, however, is tough to identify.

As a rule of thumb, slow, trawler-style cats are commonly thought of as displacement. A Glacier Bay is often pointed to as a classic semi-displacement hull; a Twin Vee is known as a fully planing hull. There's just one problem: There is no mathematical formula that provides "hull speed," which is the hull's maximum displacement speed.

When it comes to monohulls, it's easy to do the math: 1.34 x square root of LWL (length at the waterline) = hull speed in knots. Take a boat with a 25-foot-long waterline as an example. The formula is 1.34 x 5 = 6.7. When that 25-foot-long boat hits 6.8 knots, it is, at least to some degree, planing. But there is no similar formula that works for powercats. Since no one can prove exactly when a cat hits planing speed, the determination between planing, displacement, and semi-displacement is usually left up to the marketing gurus.

The bottom line: How efficient a hull is depends on many factors. But when it comes to displacement hulls, cats gain an advantage over monos because they have less resistance going through the water. And when it comes to planing hulls, they have the edge because that cushion of compressed air helps raise the boat higher, minimizing the amount of hull in the water, thus reducing resistance. Either way, the cat's a winner when efficiency is in question.

CAT CONS

Oddly, the biggest complaint one hears about powercats is that thy're not as pretty as monohulls. Fair enough – if that's all it takes to turn you off to cats, so be it.

Another common complaint is that they lean outward in a turn. This feels strange and can make an experienced monohull boater feel uncomfortable. It takes quite a while to grow used to. Occasionally, it even causes some people to fall down. Then there's the snap roll, which we discussed earlier; this also can occasionally cause someone to lose his or her balance.