Boats move through the water faster and can carry larger loads using less power today than at any time since men began navigating the seas. And while the trend toward greater hull efficiency has leveled off somewhat, continual design, technology and style changes keep tweaking the performance of many hulls.
Some of these designs might be considered just a little strange and others positively weird; many seem to fly in the face of classic boat design. But these hulls work because they employ concepts often lost in the rush to add power and amenities. By lowering power and fuel requirements and accepting a slightly different level of performance, we can still speed to the fishing grounds and troll for hours without compromising fishability. To understand why these designs work, we need first to review some hydrodynamics: how a boat travels through water and how the water creates resistance. We also need to look at different methods of countering resistance before we can truly understand how advanced boat design works.
Speed According to Froude
Every boat operates in one of three modes: displacement (moving through the water like a paddling duck), semidisplacement (trimmed up by the bow but with the stern still heavy, like a desperate duck trying to take off) or planing (no longer supported by the water, but rather by the force of the water hitting the bottom of the hull. This force, known as hydrodynamic lift, keeps the boat skimming across the water's surface at a fairly level attitude). As soon as the boat slows down, it falls off plane and assumes the displacement mode again.
Figure 1 shows the speed/power curve for a typical boat. Rather than show speed as a function of the hull or waterline length of the boat, it is shown as the Froude number (named after Professor William Froude (rhymes with "cloud"), an Englishman who in the late 19th century defined the relationship between speed and resistance). This number shows that a boat's speed in knots is proportional to its waterline length, using the formula C x (square root of lwl): a constant times the square root of the waterline length. In displacement mode, a vessel is said to be operating at its maximum speed (called hull speed) when it is moving at 1.34 x (square root of lwl).
A 25-foot boat moving in displacement mode generates a Froude number, or maximum speed, of 6.7 knots (the square root of 25 (feet) multiplied by the constant 1.34). So, before that 25-foot vessel transitions to semidisplacement mode, it will normally max out at 6.7 knots.
In practice, however, the hull extensions (or overhangs) beyond the limits of the waterline may influence the constant slightly. Most designers will use a constant of up to 1.5 to determine maximum speed during displacement.
In semidisplacement mode, the constant's range is 1.5 to 2 times the square root of the waterline. Using a constant greater than 2 usually defines the boat's speed in planing mode.
Hull resistance increases as the speed increases up to point X. As the boat accelerates and the bow rises, hull resistance increases as the heavier stern drags in the water. For a short time, the boat operates along the "hump" of the curve, where resistance is very high. At this point, if the boat has a flat stern and the hull runs straight from midships to transom (known in naval architecture as the "run of the buttocks"), allowing it to plane, hydrodynamic lift will raise the boat, and it will appear to pass its bow wave. If the run of the buttocks and stern sport a more rounded shape (rather like the curved bottom of the duck in our previous example), it is not suitable for planing, and the boat's bow simply rises while the stern digs in further - a situation known as squat.
So what do the Froude number and the graph tell us about the boat? In displacement mode at slow speeds, a hull's main source of resistance comes from water and is known as frictional resistance. At very low speeds, this form of drag might be as high as 80 percent of the total resistance, which can include wave-making drag (the boat loses energy because the hull creates and drags a wave), spray and air resistance. As speed increases, the boat starts to generate waves until there is one long wave along the hull with crests at the bow and stern. At this point, the boat is going at its maximum displacement speed, and the wave-making drag hits its highest point at 60 to 80 percent of the total potential drag or resistance.
As speed increases still further, the boat climbs onto plane. At this point, dynamic lift supports the hull; thus, wave-making drag drops dramatically. Obviously, the easiest way to reduce drag is to go fast. But going fast also requires more power, which in turn demands additional fuel, larger engines and a bigger boat to house larger engines, which necessitates additional fuel, larger engines and so on. So how do we break this cycle of increasing demands and maximize hull efficiency?
As Froude discovered in the 1880s, slender hulls generate less drag and require less power than do wide hulls. But slimmer boats often don't provide the kind of features, accommodations or stability today's anglers require. Modern designers have developed several different styles of boats on the basis of user requirements.
A Surface-Effect Sport-Fisherman?
Fast Cats Ferry Service recently launched a 100-foot passenger ferry capable of running from Fort Myers, Florida, to Key West at speeds of more than 50 knots. The vessel looks like a normal catamaran, but the aft two-thirds of each hull is shaped like an inverted V.
The front of each cat hull remains conventional, but just a few feet aft, a large fan blows high-pressure air into the space formed by the V. In effect, the air lubricates the hull to reduce resistance. The hull rides on a cushion of air that remains trapped until it exits at the transom.
Other vessels currently use air entrapment to reduce drag; the best known is probably the hovercraft. Sidewall hovercrafts use the sides of a catamaran-style hull and skirts at either end of the hull to contain an air cushion. A surface-effect ship uses air either directly vented under the hull or pushed or driven under by the shape of the hull and the speed of the vessel. The new Fast Cats passenger ferry combines these designs by using the hull shape to contain the air, but it also uses huge fans to force the air under the hull.
The U.S. Government Office of Naval Research built a smaller, 35-foot experimental prototype of a similar design to prove the concept before Fast Cats commissioned its ferry.
From the company Paragon Mann (www.vsvboats.com) comes a patented hull shape that's advanced yet similar to those seen 100 years ago. In fact, the bow shape virtually matches that of some early steam-powered, high-speed race boats. The slender, elliptical, canoe-shaped hull with a length six times its beam uses its sharp bow to pierce oncoming waves. This hull does not demonstrate a planing hump as outlined above because its wide chines create lift and give the boat remarkable stability.
While high-speed naval craft currently employ this design, the same idea could be adapted for a high-speed sport-fishing boat. Possible drawbacks may include less maneuverability than with a shorter boat and a little more difficulty in backing down.
Tournament fishing boats, such as those built by Contender and Fountain for Southern Kingfish Association anglers, feature long, pointed hulls, but their shape is not nearly as extreme, and the deep-V hull creates different power demands. Because of the V-hull shape, these boats must sport big engines to climb over the power hump. The Paragon VSV hull shape changes the power hump, which translates to smaller engine size and less fuel.
If long, thin hulls are better, then why not make all hulls like canoes? As designers found out in the 1920s, long, narrow boats react badly to prop torque and, like many of the ultrahigh-speed boats of the early motor-boating years, tend to operate with a pronounced list (leaning over to one side, sometimes known as heeling). When pushed really hard, some narrow hulls actually capsized.
Nonetheless, some of these designs attained exceptionally high speeds. For example, Feiseen, a 78-foot-long, 9 1/2-foot-wide mahogany powerboat designed by Charles Dell Mosher in 1893, achieved almost 32 mph with a single 600-hp steam engine. A few years later, an 80-foot boat with an 8-foot 4-inch beam by the same designer achieved 40 mph with a quadruple-expansion steam engine developing about 900 hp, according to D.W. Fostle's book Speedboat (Mystic Seaport Museum Stores, 1988).
Today, with counterrotating props on twin or single engines, you can easily cancel out the torque-heeling problems of yesteryear.
Mono or Multi?
Today, few builders construct such narrow hulls, but long, thin hulls can be seen on catamarans and trimarans. Such a design also was used to create the Cable & Wireless Adventurer, a 115-foot stabilized monohull that set a world record in 1998 for circumnavigating the globe in 74 days, 20 hours and 58 minutes.
The Adventurer's center hull constitutes the main part of the vessel, but pontoons on either side help it stay upright. With an overall beam of 42 feet and twin 350-hp Cummins turbodiesels, the Adventurer boasts a range of more than 3,500 nautical miles with 3,800 gallons of fuel. She ran 18 knots in 9-foot seas.
Could a hull like that be adapted for a serious fishing boat? Perhaps, but the design would pose difficulties; fighting a fish alongside could be problematic. If you wanted such a fishing boat, though, the central hull could contain all the amenities of a normal vessel, and her speed would top out at around 40 mph with twin 250-hp engines. The boat would be about 48 feet overall with a 24-foot overall beam and a hull draft of 12 to 16 inches. By comparison, a more conventional 48-footer might need 900-hp twin Series 2000 Detroit Diesels.
Such a wide beam would provide ample cockpit space. With an engine in each hull, the boat would also be highly maneuverable, and noise levels in the accommodations would be extremely low.
The Wave Trapper would utilize the narrow hull and pontoon stabilization. This design, at 35 feet overall, would use a single 250-hp engine. Its maximum beam would span about 15 feet, and it could attain speeds of about 40 mph.
Bow waves slide under the hull, where the pontoons trap them, providing additional lift for the stern sections of the hull. As a fishing hull, this boat would be fast and comfortable, though probably noisy from wave impacts under the bridge decks. Again, the fishing area would be roomy, with a cockpit easily measuring 8 feet long and 15 feet wide. (Typically, the cockpit of a 35-foot sport-fisherman measures about 8 feet by 9 or 10 feet.)
The boats described above represent one step along the road to high performance. Suppose we wanted to design a boat for greater speed using even less power. In this case, we'd probably look at a long, thin hull, but rather than build buoyant pontoons on either side to stop the boat from capsizing, we might attach stabilizing foils that resemble shallow, V-shaped wings.
As on a hydrofoil, such foils generate lift when the boat is underway, steadying the hull. Unfortunately, when the boat is at rest, the foils provide virtually no lift, so if the crew wanted to drift-fish, they would be confined to the central hull. Depending on the foil size and the lift generated, the boat would be able to troll with crew located toward the outer end of the foil crossbeam.
Taking foil stabilization even further, the central hull could have lifting foils built under it to turn the entire craft into a hydrofoil. But hydrofoils require a lot of power to overcome the drag of the foils until the boat is foilborne. Such boats don?ft troll well at slow speeds because foil drag is very high, and the hull operates just like a normal long, thin boat when at rest or proceeding at very slow speeds.
The vessel's highest speed depends on the foil size, sea conditions and power at the prop shaft, but it could top out at well over 60 mph.
Designers have created two distinct wave-piercing styles. The pure catamaran-style hull has wave-piercing appendages that resemble long, tapering knives, as typified by the large ferries built in Australia by Advanced Multihull Designs (www.amd.com.au). The powerboat-style hull sports twin appendages protruding forward of the hull, as in the pleasure yachts and ferries designed by Craig Loomis of New Zealand (www.cld.com.nz). Both of these companies have designed and launched large wave piercers, but as yet haven't drawn a boat small enough to be used as a recreational fishing boat.
The design concept here defines boats that operate at high speeds, thanks to protuberances extending forward of the hull that lance through large waves and reduce pitch (bow up, bow down) and heave (entire boat lifting up and down) as the boat passes through waves. In other words, the appendages enhance the ride, and the boat can travel at higher speeds in high seas.
While appropriate for larger craft such as ferries, the action might not be suitable for small boats that face many more and larger waves relative to the size of the boat. However, if the technology could be worked out, significant gains in speed may be made for the same power on a boat as small as 40 feet.
A small wave piercer using a New Zealand-style hull design - about 35 feet overall - lists more conventional hull and powering statistics than other boats in this article.
Cutting a SWATH
The theory behind Small Waterplane Area Tunnels (SWATH) is the same as that of a submarine: A submarine does not create waves when it is submerged. Putting deep, podlike pontoons underwater and connecting them to the main hull with thin struts reduce wave-making drag, and powering the vessel becomes much more economical.
Let's look at that in the light of our earlier discussion of wave/hull dynamics. On a conventional hull, as speed increases so does wave-making drag. A simple glance astern when you're out fishing shows you how much energy you're losing. All the waves trailing behind your boat represent lost energy. If these waves could be eliminated or reduced, less energy would be lost.
Waves occur only at the water's surface. So by making the parts of the boat (the struts) that actually cut the water very thin, you reduce wave-making drag dramatically. If the underwater pods are made large enough to carry the weight of the boat, very few waves result.
Unfortunately, keeping the submerged part of the boat below the water in a satisfactory way proves difficult. Submarines must be perfectly balanced to stay beneath the waves. So, too, must the pods of a SWATH vessel. Where submarines have an operator trimming the hydroplanes, a SWATH vessel uses a computer to monitor the water's surface and keep the pods at a constant depth. These computer systems can be expensive, and operators need training.
The significant wave height at which the boat will operate creates another drawback. If the boat's platform above the pods does not ride high enough off the water, waves will impact the bottom of the platform and could cause instability.
But here's the biggest problem from a fisherman's point of view: The pods need to support the entire weight of the hull; consequently, much of the vessel is underwater, making the boat slow and very deep. You won't be sticking your rod tip into the water when a fish goes under the boat on a SWATH. The shape represents some design refinements that attempt to reduce wetted surface and increase maneuverability.
Conversely, the SWATH provides vast amounts of deck space for fishing. Our mini-SWATH hull design could be 30 feet long by 28 feet wide. With the operator housed in the center console, a 4- or 5-foot-wide strip of deck remains clear all around the boat. Maneuverability promises to be excellent with the aftmost pods able to turn and control the vessel's direction. If desired, all pods that house the engines and prop systems could be made to rotate; then, the boat would be able to move sideways, backward or forward easily.
Unfortunately, the SWATH's platform still must be several feet above the water to reduce wave impacts. This would make it more difficult to lift a 150-pound fish to the deck or to revive a fish you plan to release. But a longer gaff or net or an attendant dinghy could be part of the overall design.
Adding It Up
Any of these high-performance, high-efficiency designs could become tomorrow's premier blue-water fishing boat. But because weight plays such a large part in the performance of these designs, all would need to be built from high-tech materials, making them fairly costly at this point.
For designs such as the SWATH, height, displacement and stability controls require constant monitoring, making software a major part of the investment. Sea conditions also affect the designs: Long, thin boats handle most sea states, but multihull forms become limited to wave heights with no significant impact on the undersides of the bridge decks. Of course, foil-stabilized vessels do best in flat water. High seas can load the foils unevenly and cause the boat to heel or slew unexpectedly to one side.
By determining the most common wind and sea conditions in your area, you could choose the optimum design to suit them. Who knows? Perhaps your grandchild's first boat may not be a conventional fishing boat after all.
Roger Marshall designs sailboats and powerboats. After completing a program in naval architecture at Southampton College in England, Marshall worked at Sparkman & Stephens, Inc., in New York for five years, then established his own yacht-design business in Jamestown, Rhode Island. As an independent designer, he has designed boats ranging from 15-foot dinghies to a 55-foot powerboat and an 85-foot schooner. He was project engineer for Courageous Challenge during the 1987 America's Cup campaign in Australia. He has cruised on both powerboats and sailboats in many parts of the world, especially the European, Mediterranean and North American coasts.