Ask the Experts: Power Multihulls
“Without tradition, art is a flock of
sheep without a shepherd. Without
innovation, it is a corpse”
Modern power catamaran and trimaran designs are creeping into many maritime applications from eco-tourism to the Zumwalt-class destroyers of the U.S. Navy. These new power multihulls are considered cutting edge by many in these fields, but why? If they are so great, where are the fleets of recreational power catamarans poking around the San Juans during the summer?
To sate our curiosity, we turned to Larry Graf, the founder, chief designer, and “lead adventurer” of Aspen Power Catamarans, a Burlington, Washington based boat building company. Aspen Power Catamarans has begun to make a name for itself with patented, ultra-modern power catamaran designs. The company, in cooperation with a proud Aspen owner, is even taking a 40-foot Aspen on an adventure /pilgrimage from Alaska to the Annapolis Boat Show (via the West Coast, short Mexican trailer hop, and up the East Coast) this year to strut their flagship design. We toured Graf’s bustling factory before settling into the conference room to shoptalk multihulls.Q: Is there anything you’d like to say to the monohull crowd?
new boat buyers, maybe as much as 80% of them, have always dreamed about buying a pointy bow boat (monohull). But I’ll tell you, we tend to sell interested buyers a boat after we get a chance to get them on the water. Our roll stability is 78% more than a monohull, our fuel efficiency is on the order of 50-70% more efficient, and the motor is out behind the bulkhead so it’s super quiet, and Aspens are great in heavy seas.
Q: What are the different types of catamaran hulls?
All catamarans are definitely not created equal, and there are big differences in terms of performance. Essentially, there are three shapes; planing hulls, planing hulls with rounded bottoms, and high speed displacement hulls.
The first catamaran hulls in the Pacific Northwest were Livingston-type planing catamarans. The design features a fairly flat hull bottom at the stern, and as one moves forward, the hull shape gets a little more v-shape to it. This planing hull isn’t much different than what they’re doing in Australia these days. A lot of the cats in Australia are planing hulls, and as one moves forward, the hull shape gets sharper and sharper (i.e. greater deadrise angle) until the bow. They run and track ok and come up on a plane no problem, but you have flat surfaces on a planing hull which by definition means you’re at the mercy of the shape of the ocean surface. So when I hit a wave, what has to happen? I either bang over it or turn it into spray.
The round-shape planing design, used by companies like World Cat, is another family. Instead of a flat-hull bottom, it’s a round shape, even at the transom. They are basically trying to plane on a round surface. In some ways it’s ok, because it does give a little more cushion as you run through waves, but it doesn’t have any tracking to it. In large seas, sometimes those round shapes slip out. I don’t care for that feeling when the boat suddenly goes sideways on me.
Then you have the high-speed displacement hulls. The Glacier Bay shape was modified so, as one moved forward along the hull from the stern, the shape got finer and deeper much quicker before coming up again, almost like a lobster boat shape. There was 55-degree deadrise forward, very “v.” That hull slices through the water and has big chines on it; 20% of the lateral stability at speed came from the lateral chines. The trick for displacement boats is to not have the boat plane for the sake of rough seas. Once you’re planing, you have to pound, and I don’t like pounding. The Aspen power proa shape is basically a full displacement design.
Q: All this complexity may come as a surprise to the uninitiated.
There are a ton of features to catamarans, and especially my Aspen designs. An important physics concept to remember is buoyancy. With our 40-footer, for every inch the boat is pushed deeper into the water beyond the resting waterline, it gains about 2,700 pounds of buoyancy per inch. The tunnel section has about 22 inches of clearance toward the stern, so if the boat is pushed 10 inches downward into the water, the resulting force translates to 27,000 pounds of buoyancy. Here’s the key, the boat only weighs 24,000 pounds. See what I’m getting at? Thanks to buoyant forces, the boat naturally bobs through the waves as a full displacement boat. The driver never gets that slap feeling like with a planning design. Aspen hull designs always get travel beyond buoyancy that allows the boat to bob through seas. There’s a lot of thinking that goes into how that tunnel section is laid out.
Another thing about an Aspen hull is the height of the tunnel vs. the width of the tunnel. There are a fair number of catamarans out there that don’t understand the importance of the ratio. Some think, “I’ll just spread it out and get a whole bunch more interior space.” The problem is that if you have the ratio of the tunnel height vs. tunnel width wrong, one hull drops into one wave and the other hull drops into another wave during rough seas. The bridge deck in-between slams.
With Aspen, there are a number of details on the tunnel section of the hull. The inner chine grows quite a bit. If you are going into a head sea and you want to bob, there’s quite a bit of lift there because the inner chine is about three times the size of the outer chine, which allows the boat to lift very well. There are also some features in the top of the tunnel as the vertical wall rolls into the tunnel. There is a step there that breaks off the water and creates a bubble water zone that helps with the cushioning at different speeds. There’s also a large asymmetrical wave breaker down the center of the tunnel that helps a bit in a tight chop. The wavebreaker also stiffens up the hull and gives us a wonderful place to run wires and cables to the dash.
Also, the propeller shaft is inside a big keel. Additionally, my rudder is 50% bigger than the design books tell you to do. I wanted a boat that that tracks well in heavy seas.
In general, I think almost all catamarans are about 20-30% more fuel efficient than a comparable monohull of similar LOA. The catamaran simply has a finer shape to slide through the water, and you can see that in action if you witness the wake that comes off a multihull. On any boat, if you’re making gentle, soft-shaped waves, then the fuel bill will be less. If you look at some of these boats that, when underway, their bows are just covered in spray and the wake has this tight, curling, whitewater wave, that boat is going to use up a lot more fuel than an Aspen. If you look at an Aspen while underway, there’s no bow wake at all. All you see is a little rooster tail, and you can’t even see any wake at all from 1,000 feet away. All that energy that a boat puts into the wake represents wasted energy that wasn’t expended to propel the boat forward; it’s wasted energy that is used to push water out of the way instead. With our catamarans, we perform at 50-70% more fuel efficiency than comparable monohulls. It’s a very slippery hull shape; the proa side of the hull is almost free to go through the water. Above the water, the windows and everything are designed to be very aerodynamic.
For example, our 28-footer with a single 150-horsepower engine at 16 knots only uses five gallons an hour. On the Aspen 32, picture it, a 32-foot boat with a 10-foot beam and king-sized master stateroom, dinette for four, and standing head with shower. That 32 uses six gallons an hour at 18 knots. A ski boat uses about 12-15 gallons an hour, and we’re using six with a 32-footer. Our 40-footer is 23,000 to 24,000 pounds depending on fuel and water on board, complete with a big flybridge, 14-foot beam, three staterooms, two heads, two showers, dinette for six, etc. According to one of our owners who was fully loaded with fuel, water, and adventure gear for a summer of cruising, he averaged 17 knots at 11 gallons an hour. In our magazine testing, if you carry half the fuel and half the water, it’s only 10 gallons an hour. That’s 1.7 miles per gallon, which is competitive with any other powerboat design in the world.
Q: I suppose you need less engine, which means less weight, which means less fuel needed.
Exactly! The single engine on the Aspen 40-footer is a Volvo 435. To run a diesel engine at 100 horsepower requires five gallons an hour. So if we’re using 10 gallons an hour at 17 knots, the inference is that it’s about 200 horsepower coming out of the motor to push the boat at 17 knots. That’s 200 horsepower to go 17 knots with this 40-foot boat. And again, the motor is at less than half throttle, because it’s 435-horsepower at full throttle. The motor should last forever at that speed. The same follows with the 32s. There it is only using about 94 horsepower to do 17 knots, and the motor is a 220 horsepower motor. The motors are very lightly driven, which has all kinds of benefits with regards to longevity and maintenance.
Q: Let’s move above the waterline. The master suite on the 40-foot Aspen we saw today is huge, and even has a king-size bed.
Yes, every Aspen has a king-size bed. I like it because it’s comfortable. There are no triangular v-berth shapes monohull guys are forced to deal with.
Q: What are some design features that one should stay away from when looking at a multihull form?
Always think about the tunnel height vs. tunnel width ratio. The most common power catamaran design flaw is when the height of the tunnel is, for example, one foot and the width of the tunnel is five feet. A one-to-five ratio will guarantee a boat that will get the tunnel section banged in about 12 inches of chop. You need travel; think about the hull shape as a suspension system kind of like a dirt bike. A dirt bike has a real long travel suspension, right? In my view, the monohull and a lot of the other catamarans have a suspension kind of like a skateboard, basically none. My boats are two and a half to one, height of the tunnel vs. the width of the tunnel.