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Hull-form factor

Jan 1, 2003

How far" and "how fast" are the two most important questions when you decide to voyage in a powerboat. In general, the faster you go, the more limited your range is, and the rougher your journey will be. There is a definite trade-off in speed, range, and comfort. If speed counts, a deep-vee hull is certainly the fastest (some power catamarans are faster, but that's another story). If a slower, more comfortable speed is your desire, you might choose to make the trip in a semi-planing hull. Or, if you are an inveterate voyager and time isn't as important, you could choose a displacement-style boat. Displacement-style yachts have speeds generally in the eight- to 15-knot range. Their motion is much easier than higher-speed boats, but they take longer to get to their destination. Each hull shape has different characteristics, and it helps to know a little about them before determining what type is best for your kind of voyaging.High-speed deep-vee hulls If you want to cruise fast, this is the hull form to choose. Deep-vee hulls are called so because the deadrise angle (the deadrise angle is the angle between the hull and a horizontal line) exceeds 24°. (If you ask about the deadrise at a boat show you will always be given the angle at the transom because designers armed with line plans are the only ones who can find the deadrise at any other part of the boat.) Boats with high deadrise angles generally have a "soft" ride. That is, they tend to bang down onto the water more softly than boats with lower angles. Imagine a knife cutting into butter. If you use the sharp edge it slides right through the butter. If you use the flat of the knife you have to use much more force and the butter spreads out farther. The same is true of boats. A sharp-bottomed boat slides into the water easier than a flat-bottomed boat. However, because deep-vee hulls slide into the water more easily, they don't lift out easily to get onto a plane. To help them lift out, strakes are used. Strakes are generally triangular and run the length of the hull. They provide the lifting surface needed to get the boat onto a plane quickly. Some boats of this type have flat pads on the bottom of the vee-shaped hull for the same purpose. A further refinement of the deep-vee hull is the stepped hull. In general, a speed increase of 10 to 14% can be expected for a stepped hull over a non-stepped hull for the same power train. Steps are breaks in the hull intended to reduce the amount of hull in contact with the water. Steps can run transversely across the hull, or they can be V-shaped facing forward or aft. Steps have large apertures on the outboard side of the hull to allow air to be sucked down into the step area and ventilate the step. In a seaway some forms of stepped hulls may get the step blocked on one side by a wave. This can lead to loss of lift on that side and that may throw the boat into an unexpected turn. Planing and semi-planing hulls The difference between a planing hull and a semi-planing hull may come down to the power plant. A small engine package that cannot quite get the boat onto a plane results in a semi-planing hull. A boat that is too heavy for the designed power plant may also result in a semi-planing boat. Often, though, a builder might feel that a boat will give too hard a ride in the planing mode and so will run it at semi-planing speed to get an easier ride. Most sportfishing and lobster-style boats plane fully or semi-plane when they are lightly loaded. That is, they use dynamic lift to raise the boat up out of the water. Only a small part (the last third) of the boat remains in the water. Planing reduces the wetted surface of the boat, enabling it to go faster for the same horsepower or to go farther at a reduced horsepower. Planing boats have several features that make them more efficient. A vee-shaped bow gives them the ability to meet a wave head on and not stop dead. Flare above the bow throws spray to one side rather than having it come aboard the boat. The chine keeps the spray to one side farther aft and reduces wetted surface. Spray strakes provide additional lift. All these features help to keep the boat planing and to give it a good ride in most sea states. The two major hull types in the planing and semi-planing modes are chined hulls and round-bilge hulls. A chined hull gets additional lift from the chines, but without careful design some chined hulls can loll onto one chine or the other when they are planing. When this phenomenon occurs it is known as chine walk and is uncomfortable for the boat user. However, it is comparatively rare among today's chined hulls. Round-bilge or lobster boat-style hulls have a rounded or soft chine hull instead of a hard chine and tend to be better boats in waves. This fact was recognized as far back as WW II when the German navy built E-boats using a rounded hull shape, while the British and U.S. navies built chined-hulled boats. In heavy waves the German boats rode better and were able to power faster than the Allied vessels. In low wave conditions the chined hulls were faster and more maneuverable, although the ride was slightly harder. Since that time, variable deadrise hulls have softened the ride for chined hulls somewhat, but the rounded hull forms still have a softer ride in heavy seas.Displacement hulls Probably more voyaging is done at displacement speeds than at any other, so it is only right that we devote most of our discussion to that mode. A boat moving at displacement speeds will be unlikely to exceed 1.5 to 2.5 x the square root of the waterline length (LWL). This makes it very easy to predict how much fuel your engine will use per hour. Since you also know how far your boat will go in an hour, you can figure out refueling stops along the way. For any voyaging powerboat these calculations are essential. There is very little difference between a chined hull and a rounded hull in terms of speed potential. In the displacement mode both are locked into a wavelength that is slightly longer than the LWL. Boats with a transom cut abruptly across the stern, known as a destroyer stern, are likely to drag more water behind them and will use slightly more fuel to go the same speed as a boat with a rounded or canoe-style stern. The bow. On both hull forms the bow entry half-angle is likely to be much higher than a conventional sailboat or a planing powerboat. Typically it might be as high as 40° rather than the more normal 25° to 30°. A wider angle of entry makes the bow waterplane more blunt but also a little more stable. A wider angle pushes more water in front of the hull and requires slightly more power to propel the boat at the same speed as a hull with a narrow entry angle. Because of the "plowing" action of the bow, the hull may have spray strakes fitted to induce the bow wave to leave the hull. These spray strakes provide a break that throws water to one side rather than having it slide up the boat and increase wetted surface, adding drag. Above the waterline, the displacement-style hull should have plenty of freeboard. Typically, in bad weather this type of hull is powered slowly into a sea, and, without adequate freeboard forward, green water will be breaking continuously on deck. Typically, freeboard is increased by a bulwark forward to make it safer to walk around. As there is usually a substantial (and heavy) anchor-handling system, this type of bow should have plenty of reserve buoyancy in the form of flare above the waterline. The stern. A hull that is designed for maximum efficiency in the displacement mode has a stern that allows water to flow smoothly around the hull, without a large amount of turbulence at the stern. This often means that a rounded or canoe-style stern is more efficient than a transom stern shapea fact that old-time designers recognized when they designed fan-tail launches to be powered by the low-speed engines of the time. This type of stern is also a good heavy-weather stern, although its use appears to have dropped out of favor. Sterns on lower-powered hulls tend to look a little more like sailboat sterns for less resistance through the water. As engine speed and power increased, most builders used transom sterns on their hulls. This type of stern is easier to build and to maintain but requires slightly more horsepower to push along. It often has low freeboard to permit easy boarding from the transom platform, and, by setting the rudders and props under the stern in relatively clean water, propeller efficiency is increased. This is close to state of the art for the average displacement voyaging powerboat today. Keels. Low-speed powerboats have none of the dynamic stability that planing or semi-planing powercraft do. Consequently, they tend to roll in most sea states. A rounded hull is also more likely to roll than a chined hull is. In order to resist rolling, both types of hull should have some form of keel. It doesn't appear to make a lot of difference if it is a centerline keel or a bilge keel, although centerline keels are seen more often on a chined hull and bilge keels are more likely to be found on a rounded hull.Surviving heavy weather A displacement hull is relatively slow for voyaging, but its motion is a lot easier, it is less fatiguing on the crew, and heavy wave impacts are fewer. However, against that is traded the longer time spent at sea and the greater likelihood that the boat will run into rough weather. There are several factors you should take into account before the onset of heavy weather. First the engine needs should be taken care of. Heavy waves tend to make the boat pitch and heave. This stirs up the fuel in the tanks, and any sediment in the bottom of the tank also gets stirred up. Sediment gets sucked into the fuel lines and blocks filters or clogs lines. How can you avoid this situation? The first step is to make sure that only clean fuel goes into your tanks. Filter all fuel, without exception, even if the process is slow. If you have any doubts about the state of your tanks, drain and clean them to eliminate a build-up of sediment. Carry enough spare filters to keep your filters clean. Check fuel often for water, bacteria, and dirt build-up. In heavy weather a powerboat is under most control when it is moving faster than the oncoming waves. Going upwind is not a problem in that the boat speed can be adjusted to suit the wave conditions. Go moderately fast in these conditions to keep the boat's head into or just off the wind. If you try to go beam-on to the sea you will roll heavily in large seas. This also exposes the boat to a capsize. If you have to run downwind in heavy seas, you may have to go fairly fast to maintain steerage as the waves overtake you. This puts you at risk of burying the bow in the wave in front and broaching the boat. A better way is to run downwind trailing a drogue to keep the stern into the seas. Having the right gear on board to face heavy weather is a precaution that any slow-speed voyaging powerboat should take. Make thinking ahead a way of life on your voyages.