A catamaran hull handles the ocean differently from a monohull in one fundamental way: instead of a single hull leaning and rolling around a centreline, two narrow hulls sit wide apart and stay flat. That changes how the boat behaves in every sea state.
It also means a powercat rewards being driven properly. Get the trim right and a cat rides beautifully; get it wrong and you’ll get the slap people complain about. More on that below, because it’s the single most useful thing to know.
At rest and at drift
This is where the difference is most obvious, and it’s why most people buy a cat.
With buoyancy carried at the outside edges rather than the middle, a powercat barely rolls when stopped. Someone can move from the transom to the bow without the boat leaning after them. You can land a fish over the gunwale, or get a diver back aboard, without the deck tipping.
A monohull of the same length rolls through all of that. It’s not unsafe — it’s just constant, and it’s what makes people seasick at anchor.
In chop
Short, sharp chop is where a powercat is at its best, and it happens to be the standard New Zealand coastal condition rather than the exception.
Two narrow hulls cut into the chop rather than pushing it aside, and air compressing in the tunnel between them cushions what’s left. Trimmed correctly and carried at a decent speed, a powercat will fly over the top of small to medium chop rather than pounding through it.
Cats like a bit of speed. They come alive once they’re properly up and running — most disappointing rides come from a boat that’s under-trimmed, under-powered, or being driven too slowly for the conditions.
Into a head sea
Two fine hull entries slice rather than slam, and the tunnel does the rest. The result is a softer, drier ride than a deep-V of the same length taking the same water.
The adjustment for a monohull skipper: a cat doesn’t lean into a head sea. It stays flat and goes through. People often need a trip or two to stop anticipating a lean that never arrives.
In a following sea
A catamaran tracks straight where a monohull can want to swing sideways and broach. The two hulls act like a pair of long keels, holding the stern in line. For anyone who’s crossed a bar or run home in front of a building swell, that’s usually the characteristic they talk about afterwards.
Trim is everything
If there’s one thing that separates a powercat riding beautifully from one that slaps and wallows, it’s trim. Cats respond to trim far more than monohulls do — and once you know what you’re looking for, it takes about thirty seconds to find.
How to trim a powercat:
- Start trimmed down, the same as any other boat.
- As you pick up speed and come onto the plane, keep applying throttle and keep trimming up (or out) as you go.
- Keep trimming out until the boat is just about to porpoise — then come back a touch.
- That’s the sweet spot.
What it looks like when you’ve got it right:
- Bow up
- Spray breaking off at midship rather than up forward
- No slap from the tunnel
- The ride goes soft
Trimmed like that, you’re getting air under the hulls, and the boat is doing what it was designed to do.
Why catamarans slap — and how to stop it
Tunnel slap is the most common criticism of catamarans, and it’s worth being straight about: every cat from every builder can slap in the wrong conditions, when a short steep sea gets into the tunnel and meets the underside.
But it’s usually a trim problem, not a hull problem. Too much bow down and the tunnel meets water it should be riding over. Trim the boat properly — bow up, spray off midship — and it largely disappears.
That’s also why a cat that felt “slappy” on a test drive is often a cat that wasn’t being driven right. Worth knowing before you judge one.
Where a monohull still wins
- Traditional handling. Thirty years of monohulls teaches you to expect a lean that a cat won’t give you.
- Narrower. A cat of the same length is beamier, on the trailer and in a berth.
- Simpler and cheaper to buy, power and maintain, size for size.
