Charter operators, sports fishermen and ferry owners prioritize function and bottom line when choosing a power catamaran boat. When finding out what’s new in this arena, there’s no one better to ask than the team at Gold Coast Yachts in St. Croix, USVI. Over the last 25 years, the company has custom built over 100 sailing vessels and power catamarans. Roger Hatfield, born into a sailing family and an early convert to multihulls and now power cats, leads the design team. All At Sea spoke with Hatfield about trends in power cat design and build and how design features are making for greater fuel efficiency.
AAS:Why have power catamarans become so popular?
GCY:Power catamarans have several advantages over standard power boats. Fuel efficiency is one component. Standard power boats, especially in the Caribbean, usually cannot operate at a full planing speed where that might approach the efficiency of a power cat especially at speed above 30 knots. Below this speed, and when it is rough, the power cat simply slices through the waves with greater comfort for its passengers and at up to half the total fuel consumption.
AAS:What are evolutions in hull design for power cats?
GCY:Most monohull power boat designers who switched to multihulls retained the hard chines. The hard chine slender cat works on the same theories as the monohull. However, little extra lift is provided by this slender shape plus penalties accrue. For example, the minimum surface area created by a round bottom cannot approach that of a V-shape. A V-shape with the same area as a semi-circular hull would have at least 10% more perimeter and thus more drag. I think the popularity of the hard chine cat will linger on for a while as choice of building materials frequently affects hulls shape. Aluminum and plywood are popular plate materials that are difficult to bend and, when used, slightly compromise any efficiency that might be gained when using a pure round hull shape.
AAS:How do you build maximum efficiency in your power cat designs?
GCY:By making choices such as in the slenderness ratio and the prismatic coefficient. The slenderness ratio is the ratio of the waterline length to the beam. Any ratio below 10:1 will start dragging that nasty transverse wave, which are those that follow along behind you perpendicularly to your travel direction, and they also make a slightly larger divergent wave, which are those that get plowed out from either side by fatter-hulled boats. Hulls with near 10:1 will have less wetted surface than very slender hulls and are an okay choice if the intention is to go at a low to medium speed. Hulls exceeding 16 or 17:1 will be excellent at the upper end of medium speeds. These vessels are slimmer and can cut through the waves more easily for a softer ride but can be susceptible to dropping further into waves and need to have high under wing clearance. This has been our first choice in craft for the Caribbean conditions.
AAS:How does the stern shape of a power cat affect its efficiency?
GCY:Sterns have been our largest evolution in the last twenty years. These slender craft do not plane, therefore they need a stern that closes the water behind them rather than pulling it along with them. There are two choices. The Malcolm Tennant-style stern uses a vertical transom like a canoe from which the propeller shaft exits at the bottom corner. It has a flat extension that hangs out beyond this and holds the rudder and helps reduce pitching. The Lock Crowther shape shows the hull having a reverse rocker such that the propeller can be placed up higher and the stern still allows some pitch dampening. They both have their special attributes.
AAS:Could you offer an example of efficiency using the concepts you’ve spoken of above?
GCY:My favorite example is the 83ft power cat we built in May of 1999. She has done well over a half million miles serving the Key West – Dry Tortugas – Fort Jefferson route. She averaged 21.2 knots on her maiden voyage to Florida and used exactly a gallon of diesel per mile carrying a 2/3 passenger load of fuel for the delivery. That’s pretty green! She is a wave-piercer, which is a normal power cat that got stretched out a little further to be more slender. Wave-piercers are our first choice for fuel efficient comfort with good speed in the rough conditions of the Caribbean.
AAS: What do you foresee in the future of power cat design?
GCY: I doubt there will be large design evolutions unless it is with foils. Foils will always come with problems as do all appendages. Clearly the use of carbon fiber will grow, reducing the weight of the craft. Many of our power cats have left with major components having been built with carbon fiber. The cost of the materials can be compared directly to the fuel saved over time usually with good payback. One of the next catamarans we are going to build will be all carbon fiber.
AAS:What else is on the horizon for you?
GCY:We have been approached to develop a maximally comfortable and fuel efficient boat to compete with our own catamarans. We have spent considerable time creating a 70ft wave-piercing power trimaran to compete with our 60ft wave piercing catamarans. We are confident that the vessel will use at least 30% less fuel than our cats and be slightly more comfortable due to its extra length. It is intended for the toughest inter island passages.
