A catamaran hull is one of two slender, widely spaced hulls joined by a bridgedeck. Each hull carries roughly half the boat’s displacement, so it can be much narrower than a monohull of the same weight. Hull slenderness, bridgedeck clearance and the choice between daggerboards and mini-keels decide how a catamaran sails and how it rides.

That is the whole design in one paragraph, and it explains why catamarans behave the way they do. A monohull carries its weight in one fat hull and stands up to its sails with ballast. A catamaran carries the same weight in two thin hulls and stands up to its sails with width. Every consequence follows from that: shallow draft, no ballast to drag around, level sailing, a lot of interior volume for the length — and a structure that has to bridge the gap between the hulls without being pounded apart.

The characteristic feature of a catamaran hull, then, is its length-to-beam ratio. A cruising catamaran hull typically runs somewhere between 8:1 and 12:1 — long and narrow. A performance multihull pushes higher. The narrower the hull, the less wave-making drag it creates for a given speed, which is why two slim hulls can be faster than one wide one despite having more wetted surface between them. It is also why load matters so much, a point we come back to below.

Two hulls is the definition, not a variable: a catamaran has exactly two. Three is a trimaran, and the practical differences between them are worth a separate read if you are still choosing a platform. What varies between catamarans is everything above and below those two hulls.

Sail a catamaran upwind into a short chop and you will hear it: a hollow bang under the saloon floor as a wave crest meets the bridgedeck. That is slamming, and it is the most common complaint owners make about boats that looked perfect at the dock. It is loud, it is tiring on a night passage, and over years it is a structural fatigue load. It is also largely predictable from the drawings.

The working rules among multihull designers are expressed as a fraction of overall length. Around 4% of LOA is low and will slam readily. 5% is acceptable for coastal work. 6% or better is what you want for ocean passages. A 45-foot catamaran at 6% has about 32 inches of clearance; the same boat at 4% has about 21 inches, and those eleven inches are the difference between a quiet night and no sleep. Richard Woods on bridgedeck slamming makes the point that clearance is a primary predictor of slamming behaviour and that more of it simply produces less of it.

There is a second rule for unusually wide boats. Where beam exceeds half the overall length, designers also check clearance against the span between the hulls, looking for a minimum of about 20% of that span. The older workshop version of the same idea — one inch of clearance per foot of beam, rising to 1.3 inches on wider boats and 1.8 on very square ones — gets you to a similar place with no calculator.

Two cautions when you use this table. First, measure the boat you are actually buying, loaded the way you will actually cruise it — not the specification sheet. Second, clearance is not free: raising the bridgedeck raises the centre of gravity and the windage, and it puts the saloon floor further from the water. Designers trade it against other things deliberately, which is why a lower-clearance boat is not automatically a bad boat. It is a boat with a different intended use.

Below the waterline, a cruising catamaran does one of two things for lateral resistance. It either has fixed, shallow mini-keels moulded to each hull, or it has daggerboards that drop through trunks in the hulls. This is the most consequential specification difference between two otherwise similar boats, and it divides the market cleanly.

For Caribbean cruising the calculus usually favours keels, and it is worth being blunt about why. The trade winds blow, the reefs are shallow and hard, and much of the sailing is a reach rather than a beat. A boat that gives up two degrees of pointing and never asks you to think about a daggerboard trunk is the easier boat to own here. The best catamarans for liveaboard cruising in this region are, more often than not, keeled boats for exactly that reason, and the picture changes again if you are weighing power versus sailing catamarans, where lateral resistance stops being the question at all.

That is a preference, not a rule. If you sail somewhere with light air and deep water, and you care about windward performance, boards earn their keep. Charter fleets specify keels almost universally because they are forgiving of guests and of coral, which has a downstream effect on what appears on the used market — the ex-charter catamaran prices that dominate Caribbean listings are overwhelmingly keeled boats, so a boarded cruiser here is a deliberate hunt rather than a shelf item. It is also one of the sharper contrasts in any honest comparison of a catamaran versus monohull, where a fin-keeled monohull simply does not have the option. Hull form drives price as much as length does, and Caribbean catamaran prices show how much a bridgedeck and daggerboard specification adds over a simpler production layout.

Slender hulls are efficient and unforgiving in the same breath. Because each hull is narrow, it has relatively little waterplane area — the horizontal slice of hull at the waterline that resists sinking. Add weight and a catamaran settles noticeably further down than a beamy monohull would for the same load. Designers quote this as pounds per inch of immersion, and on a mid-size cruising cat it is a smaller number than owners expect.

The consequences compound. Sinking the hulls increases wetted surface and displacement, which costs speed. It also reduces the bridgedeck clearance you carefully checked at the dock — a boat with an honest 5% of LOA empty can be down to 4% once it is provisioned, tankage full, with a tender on davits, a generator, a watermaker, ground tackle, tools, spares, dive gear and two people’s belongings aboard. The boat you sail is not the boat you surveyed unless you account for that.

So treat the payload figure as a real specification, not a footnote. Ask what the quoted displacement includes. Weigh the big items honestly: full fuel and water, the tender and outboard, chain and anchors. Then look at where the weight sits — heavy items in the ends of slender hulls make a catamaran hobbyhorse, and weight high up in the bridgedeck raises the centre of gravity. Owners who load thoughtfully keep most of the performance they paid for; owners who treat a catamaran like a warehouse do not.

A catamaran gives a surveyor twice the hull to look at and several structures a monohull does not have. Interiors sell boats and hulls cost money, so spend your attention below the waterline and at the joints.

None of this is a reason to hesitate about a multihull; it is a reason to survey one properly. A competent pre-purchase survey on a catamaran takes longer and costs more than the same job on a monohull of similar value, precisely because there is more to examine. Understanding what drives pre-purchase survey costs in the Caribbean before you make an offer keeps that expense from arriving as a surprise mid-deal, and a haul-out survey is the only place several of the items above can be assessed at all.

Our brokers walk these hulls for a living, in a market where most of the inventory has spent its life in salt water and sunshine. If you want a candid read on a specific boat’s clearance, appendages or laminate history before you spend money on a survey, ask.