The ABCs of Hull Design
Or what sex, rock, and roll have to do with your boat.
As part of my research for a story I was writing, I once asked a fellow marine writer, who was also a well-known naval architect, to explain the basics of hull design. His answer was short and pithy. “Hull design is like sex: Everyone knows the basics; it’s the details that separate the good efforts from the bad.”
Well, whether you subscribe to this metaphor or not, you can’t deny that the subject of powerboat hull design starts out pretty simple and quickly gets complicated. A layman could easily spend hours, if not days and years, poring over textbooks and tomes that explain such arcane (at least to the average boater) concepts as warp and prismatic coefficient.
Top Speed: 39 knots
Displacement Speed: 11.3 knots
Krogen Express 52
Top Speed: 22.4 knots
Displacement Speed: 9.6 knots
Top Speed: 9.8 knots
Displacement Speed: 10.1 knots
But if all you really want is a working knowledge of those basics so you can be a more informed boater, a good place to start is not erotica but rather with a knife and a box. Envision placing each in a pool of water. Notice how easily you can move the knife through the water, even if it’s choppy, and that the knife doesn’t rock or roll much in reaction to wave motion. But of course a knife doesn’t float, and being very narrow, has little internal volume in which to carry anything. The box, on the other hand, if watertight, will float and can carry a lot more to boot. You can move it through the water fairly easily when it’s calm. But when it’s choppy, the box will begin to rock and roll, especially at rest, and as you move it forward, it will bounce, slap, and wander to either side.
Clearly this comparison has limitations, but hopefully these two objects illustrate the basic concerns hull designers must deal with to create practical, efficient hulls.
You can intuitively see that the easiest boat to power through the water would be one with a fine, sharp entry with minimal frontal area that therefore produces minimal resistance. And you can also see that the boat that could carry the most people and cargo would be a box—a barge. So the obvious solution to traversing the water with maximum comfort and cargo is to combine the two shapes—a box-like body with a sharp bow. Such a vessel would not only carry a lot of stuff and slice through the water, it also would be able to achieve relatively high speeds because its flat bottom will be pushed up out of the water as the speed and thus the force of the water increases. In other words, it would plane.
But a pointy box has serious and obvious limitations. First of all, since the entire bottom except for the very front is flat, it’s still going to pound and slap when it is headed into waves of any size, especially if it achieves any significant speed. Second, because the bottom is flat, such a hull would be hard to control; it will wander from side to side, requiring constant correction to the helm. And third, when it’s going slow or even just sitting still, a flat-bottom, barge-like box will rock and roll in reaction to even a small wave.
So the next step in the evolutionary process of hull design is to bring the pointy part farther aft so that the front part of the hull isn’t totally flat but rather somewhat V-shaped. This will not only provide a smoother ride going into chop and waves, but if the V is extended far enough aft, it will create a ridge—a keel—that will help the hull track in a straight line. But only up to a point, for as you extend the V aft, you reduce both the hull’s interior volume and the lift that was created by the flat surfaces passing through the water. So now your hull doesn’t plane quite so easily, and you’ll probably have to add power to achieve the same speed. At the same time, increasing the V increases the total surface area of the hull coming into contact with the water, and that increases resistance. Now you need even more power.
This explains why today only a few builders offer true deep-V hulls—24 degrees or deeper—preferring instead to blend fine foresections for slicing through waves with flatter after sections that will get the boat up and out of the water so it can plane. Probably the best example of this combination is South Jersey and Carolina sportfishing boats, which employ very sharp—or “fine”—frontal sections that gradually bleed into what is, for all intents and purposes, a flat surface at the transom. The best of these boats go fast and ride well because their designers have figured out how much V to employ forward, exactly when it should begin to flatten out, and how quickly.
Most builders don’t opt for such a solution, preferring to rely on more V at the transom and compensating for the loss of lift by adding lifting strakes—small horizontal flats running fore and aft that help push the hull up out of the water. But whatever the choice, every solution implies compromise. A shape that generates lift also produces resistance, and a wider hull that has more interior room also has more hull surface in contact with the water, and therefore experiences more resistance.
However, hull design isn’t just about speed and interior volume. There’s also stability and tenderness, which is a boat’s tendency to rock. The boats described above have planing hulls, which means they have hard chines. This creates port and starboard flats that create lift and help plane the boat and keep it on an even keel. But these flats also react to the lift created by waves moving under the boat, causing it to rock and roll. If you must have a boat that goes fast—a planing boat—this is something you’ll have to deal with, at least to some degree.
But suppose that you’re not as interested in speed as you are in a smooth ride and efficiency. In that case you probably don’t want a planing hull but rather a displacement design. Instead of that flat hull bottom and hard chines, it will have a round bottom and rounded chines that don’t react so severely to pressure generated by waves. Because this design is slippery—it produces relatively little resistance—it’s typically more fuel efficient, but that slipperiness also means it won’t track well. So virtually every displacement hull also has a prominent keel that keeps it headed in a straight line.
Maximum speeds for displacement hulls are determined by this formula: the square root of the waterline (not overall) length multiplied by 1.34. Generally speaking, the resulting number is the fastest in knots a displacement hull can go, regardless of the amount of horsepower applied to it. So if you have a displacement hull with a waterline length of 36 feet, its maximum speed is 8.04 knots. (6 x 1.34 = 8.04) Add more horsepower, and you’re basically just pushing water ahead of the boat. As you increase waterline length, you also increase the potential top speed, which is why multihulls and boats with bulbous bows are able to go faster than simple monohull displacement boats.
As noted above, the lift that a planing hull experiences due to the upward force of the water on its flat surfaces is a double-edged sword. At slow speed, a swell or chop will also exert lift on these surfaces and make the boat rock and roll. Lacking these flat surfaces, a hull with round or “soft” chines doesn’t react as much to waves and usually rides much more softly—it may also roll but it won’t “snap roll” like a hard-chine hull will. This is important because the upward force the water exerts on a planing hull’s flat chines is equal on both sides, which generates what is called dynamic stability—as long as the hull goes fast enough to plane, it will maintain even trim and head in a straight line. Dynamic stability doesn’t operate at slow speeds or on a soft-chine hull.
Finding the Balance
Some hull designers are loath to accept the limitations inherent in these two hull types and so attempt to combine the best features of the planing hull and the displacement hull by creating a design that can exceed theoretical hull speed yet provide the kind of stable, soft ride in waves that a displacement hull is famous for. The result of their effort is the semi-displacement hull form, and it generally has either truncated hard chines or other relatively flat hull surfaces on the after part of the hull that generate some lift as the boat moves through the water. Once again the secret of a good semi-displacement hull is how the designer balances the lift required for more speed against the lift that makes the boat rock and roll.
Obviously this explanation is highly simplified, and there are plenty of other design nuances that I haven’t covered. So it’s helpful to think of the three types of hull forms—planing, displacement, and semi-displacement—as pure types. Most hulls you come across will be variations on them. So here’s a final observation that might be worth keeping in mind: Overused though it may be, the maxim holds that there is no free lunch. In hull design, as in so much of life, the addition of something—speed, say—presupposes the subtraction of something else—perhaps stability or a smooth ride. Everything has its price, and in the end it’s up to you to decide what you must have and what you are willing to give up to get it.
This article originally appeared in the May 2013 issue of Power & Motoryacht magazine.