Extraordinarily good head-sea performance

by Lenny Rudow

Cat Advantage 11: Cats love a head sea

Extraordinarily good head-sea performance is the exact reason many people, myself included, choose a cat in the first place.

Just look at a cat's bow, and your eyes will show you the first reason why this myth is as ridiculous as they come. Plenty of monohull brochures talk about a boat's "knife like" entry, but few monos have an entry that's anywhere nearly as sharp as the average cat's. As common sense tells you, a thinner entry means the boat can cut open waves instead of slamming against them. Meanwhile, the tunnel between the two hulls helps cushion the blows of a head sea by compressing air. Remember your basic hydraulics; water can't be compressed, but air can be. As a cat moves forward it crams air in-between the boat's hulls, the tunnel, and the water, creating an air-water slurry that acts as a cushion. The whooshing noises you often hear when running a cat are that compressed air escaping. And this is also why some cats "sneeze," or shoot a fine puff of mist out, as the air gets pushed forward. Yes, sneezing is one of the potential down-sides to owning a cat (most newer designs have eliminated or greatly reduced the problem, but some still do it to one degree or another.) For those of us who have bad backs due to decades of pounding through the seas in relatively small monohulls, however, a puff of mist is a small price to pay for greatly reduce pounding.

The tunnel compresses air, creating an air/water slurry that cushions the blows.

So how could this myth have ever come into being? Maybe overzealous monohull salesmen, or maybe some people charged full-tilt into an eight foot head sea and expected the boat to remain perfectly level. It could also be that people have mistaken tunnel slap for a "bad" ride. So let's clear the air on tunnel slap: this pounding sensation occurs when a wave strikes the underside of the tunnel, and it feels a lot like a monohull slamming into a wave. In short, the sensation stinks. There are a few cat models out there with endemic tunnel slap problems, but most of the time this is the result of an overload situation. You have to remember that cats are more weight-sensitive then monos, and if one's loaded down in the bow, tunnel slap can be the result. For this reason it's a far more common problem on cabin boats then on center consoles, particularly when a center console design has been modified to carry a cabin, thus changing its basic weight distribution. The problem can sometimes be mitigated by trimming the engines to bring up the bow, but the best way to avoid tunnel slap is to distribute the bulk of your weight load aft, and if you're looking at cats with cabins, choose one that was designed from the ground-up to carry one.

One other situation that should be addressed: many cats experience tunnel slap when motoring into a head sea at low RPM. Since the bow doesn't have any lift at speeds under eight MPH or so, the tunnel sits lower then the designed running attitude and may get whacked by a wave now and again. In my experience this is a pretty common phenomenon, but it's easily solved by either speeding up a bit or merely angling the bow a hair off the seas. A 10 to 15 degree course change is usually all it takes to eliminate the problem.

Let me briefly take you back to opening weekend of the 2010 striper season on the Chesapeake Bay. We had a 20 knot wind out of the north, and a 12 mile cruise dead into it to get home. But I knew my Glacier Bay really shines at its brightest when running into a head sea. And on the way home, cruising at our usual 25-mph in comfort, we passed several boats between 24' and 32' which were mushing through the slop at about 15 MPH. So here's my challenge to you: take out any modern reputable powercat and run it on plane into a head sea. Then get on any monohull you like of about the same size, and run it into the same head sea. I'll bet dollars to doughnuts that afterwards you'll agree that the cat rides smoother then the mono. I'll take French Crullers and glazed, please.

As air and water pass through the narrowing tunnel, they combine to create an aerated mixture that becomes compressed and increases in velocity. The ever-increasing compression and acceleration literally sucks the aerated mixture (and spray) aft. The increased velocity and pressure rushing beneath the hull creates an aerated water-cushioned suspension that works like a shock-absorption system, softening the ride, increasing stability and improving fuel efficiency. This is especially true in rough water.