Semidisplacement hulls can save fuel and provide the ride you’re looking for, with just enough power
Semidisplacement hulls can save fuel and provide the ride you’re looking for, with just enough power
Timing is everything. No sooner did we have the internal combustion engine than along came the planing boat. However, for the first 100,000 years of human existence (archaeologists and historians may quibble) we had only comparatively lethargic displacement craft of all descriptions, reliant on wind and muscle power for motivation.
Since displacement hulls are shaped to go through the water and can’t climb on top of it, they are limited in hull speed to an open-ocean wave of their own length. This is gravity’s responsibility, but the consolation is that displacement vessels take very little power to propel to just below their maximum hull speed, which is a function of waterline length (the square root of the waterline length times 1.34, measured in knots).
Once we figured out how to pack enough power in a light enough package, in the form of the fledgling gas engine, we tinkered with the shape of the hull until it would fly like a wing over the water. Presto, there was our planing hull. This, of course, evolved into the production powerboats many of us own today.
Since these boats climb over and fly right on past their bow wave, they’re limited in speed only by their horsepower. And what’s more fun than blasting along at highway speed in your boat? On the other hand, when you think in terms of cargo being moved over a distance, by displacement hull standards, planing hulls may be a blast, but they’re anything but efficient.
Unnoticed by many boaters, meanwhile, is a thoroughly delightful alternative to slow displacement and inefficient planing hulls: the semidisplacement (or semiplaning) vessel. This hybrid isn’t limited to hull speed, since it, too, can climb over and past its bow wave, thanks to its hull shape. So where does it get this boost?
While the aft buttocks (the angle of the hull bottom, fore and aft, in relation to the waterline) on a displacement hull sweep up and out of the water at the transom, the bottom of the semidisplacement hull flattens, so it’s essentially parallel to the surface of the water. And the transom is down in the water, rather than above it. This gives the semidisplacement hull the dynamic lift it needs, once it’s moving fast enough, to climb over its own bow wave.
Many semidisplacement hulls have round bilges, as opposed to hard chines (the corner where the side and bottom of the hull meet in a planing boat). This roundness reduces wave-making resistance at slow speeds, so these hulls use less fuel at displacement speed. The same round bilge acts as a less-efficient lifting surface at high speeds, though, since there’s less bottom surface to create lift, so it planes less efficiently. And because there’s no chine to create flow separation at the sides of the hull bottom, there’s more frictional drag at higher speeds.
The good news is that there is a speed range within which the semidisplacement hull comes into its own. That range is — you guessed it — right between displacement and planing speeds. To keep the math simple, let’s take a 40-foot boat with a 36-foot waterline length. A displacement boat of this size will go up to 8 knots and that’s it, because it can only go as fast as an open-ocean wave of the same length as the hull. Since the semidisplacement hull can climb over its bow wave, the boat doesn’t have this gravity-induced speed restriction. Our 40-foot semidisplacement boat can slide right on up to 15 to 18 knots (speed-to-length ratio of 2.5-to-3). So why can’t we make it go faster?
We could, but if you want a 40-footer that can cruise at 21 knots you’re better off with a true planing hull with hard chines. That’s because the low wave-
making resistance of the semidisplacement hull at displacement speeds starts to be undone by its higher frictional resistance at high speed. While wave-making resistance drops substantially once a boat is up on plane, frictional drag increases dramatically. And a semidisplacement hull has more wetted surface and, therefore, more frictional drag than a planing hull.
So why does a semidisplacement hull create more frictional drag than a planing hull? For starters the planing hull, with its hard chines, rises higher and higher out of the water as it goes faster, which means there’s less wetted surface and, therefore, less drag. Plus, a planing hull’s hard-cornered chines peel off the water and separate it from the hull at the waterline, further reducing frictional drag. Finally, semidisplacement hulls tend to have full keels, which are needed for grounding protection and to give these hulls, which tend to have flat sections aft, directional stability. (Deep-vee planing boats tend to go straight precisely because of their deadrise.) A big keel has a lot of surface area, so it creates more drag as the boat goes faster.
The kind of boat you choose to buy will depend on where these wake-making and frictional-drag curves cross. Depending on the hull and its displacement, this typically occurs at a speed-to-length of about 3.
The comfort factor
Here’s the thing. If you can live with a 14- to 18-knot cruise in these days of record fuel prices, a semidisplacement boat is a great way to go. You’ll burn a lot less fuel per mile if you run slower. And there is nothing more comfortable to go to sea in than a well-designed semidisplacement Maine-style lobster boat — a traditional Down East design like a Dyer 40, an old Duffy 42 (Atlantic Marine) or any of the Spencer Lincoln-like designs found at docks and moorings pretty much anywhere in New England.
You’ll be a lot more comfortable in a semidisplacement boat for a couple reasons. The first has to do with the hull shape and the length-to-beam proportions. If it has round bilges instead of hard chines, is not as beamy for its length, and its center of gravity isn’t too low, it’s going to have easier motions. This means the accelerations caused by wave action against the hull are going to be lower, and you’ll have yourself a more seakindly boat. The semidisplacement boat doesn’t wallow in the trough like an unstabilized full-displacement hull, and it doesn’t snap and jerk you around like a low-deadrise, overly wide, hard-chine planing hull.
In fact, a too-wide hard-chine boat at rest or low speed — for example, when you’re playing a fish — will snap-roll you into oblivion. It’s actually too stable. That’s because with all that buoyancy outboard, the boat stays super-glued to the wave gradient. As the waves move, so will the boat.
The semidisplacement boat is more autonomous. It is less stiff than the beamy hard-chine planing hull, but more stiff than the pure displacement hull with its slack bilges and low initial stability. It is often the most comfortable boat to be aboard in terms of seakindliness because of the relationship between hull form, center of gravity and distribution of weight on board.
The thing about any planing or semidisplacement boat is that in order to maximize efficiency, you have to minimize weight. A really light, resin-infused, moderate-beam, semidisplacement boat is the way to go from a strict efficiency perspective. That’s because more displacement means more wetted surface, and that slows you down. If the designer comes up with the right calculus to minimize rolling (by keeping topside weight down and not making the bilges too slack) and reduce semiplaning drag (by minimizing weight everywhere and, again, not making the bilges too slack) you’ll have yourself a nice sea boat. However, a little more weight in the boat will make it less reactive to wave action, which can make it more comfortable. But weight, as mentioned, slows you down, so there’s the tradeoff.
Over the hump
Don’t succumb to the temptation to overpower a semidisplacement boat. That’s almost always a bad idea, and here’s why. You can make a brick plane if you put enough power in it, since the brick has a flat, lift-inducing surface on the bottom. Our hypothetical 40-footer described above is a great 13- to 20-knot boat, but it’s a fuel-guzzling, wet and likely dynamically unstable horror show at 25 to 30 knots.
I was asked to figure out what was wrong with a 44-foot Down East hull a few years ago — why it didn’t perform the way the owner expected it to, based on the builder’s representations. The problem was simple. This poor boat had a pair of 660-hp diesels, more than double what it needed for its design speed of 14 to 20 knots. Its full keel also dragged a layer of water along with it (the frictional drag mentioned earlier), soaking up horsepower. Adding to the problem on the hydrodynamic side, its round bilges had about half the effective lifting surface (think wing area) of a hard-chine boat of the same size. Plus, the windshield wipers were wearing out because these overpowered Maine-style semidisplacement boats are just plain wet, and wetter still from being overloaded from the weight of those oversized diesels and the big fuel tanks they require.
Back to our discussion. While the displacement hull is supported completely by buoyancy and the planing hull at high speed completely by dynamic lift, the semidisplacement boat is supported by some of each, the exact ratio depending on how fast the boat is going, the hull shape and how much it weighs. Things get more complicated when you consider that bottom loading matters a lot when it comes to performance. (Bottom loading is displacement divided by hull bottom surface area.)
Consider that while a lightly loaded 40-footer may be able to plane at 11 or 12 knots, with 6,000 pounds of codfish in the hold it may have to make 17 or 18 knots to plane. A boat starts to plane when its center of gravity starts to rise, a phenomenon that can be seen and felt when it goes fast enough to climb atop and get past its own bow wave.
When a boat leaves a clean, flat wake astern with water separation at the transom, it also could be on plane, with a lightly loaded hull leaving a flat wake astern at 9 or 10 knots. In this case, you can quibble about whether it’s really planing, at least according to the speed/length rules, but if it’s going faster than hull speed, it’s at least partially supported by dynamic forces and, therefore, semiplaning. But rules are made to be broken. Consider that a displacement catamaran can go five times its hull speed because of the hull sponsons’ slenderness ratio — but that’s another story.
Anyway, the semidisplacement boat starts to semiplane at a very distinct point — when it climbs over the hump. This is the point in the speed curve when propulsion resistance climbs steeply (the hump in the resistance curve) and just before it drops off dramatically. This bow-wave summiting is experienced as acceleration from displacement speed, bow rise, more acceleration and then the bow dropping back down again. When the bow starts to drop, you’re just getting over and ahead of the bow wave. Once over the hump and on plane, it takes a lot less power to push the boat than when it’s wallowing along and climbing over the bow wave. Wave-making resistance becomes negligible, and frictional resistance comes to the fore.
A nice thing about well-designed semidisplacement boats is that their center of gravity is farther forward, so they are able to climb over the hump easier, at lower speed and with less resistance than a typical planing hull. Some of these lighter boats are pretty much humpless, in fact, thanks mostly to light bottom loading and a forward CG. These boats can operate from 0 to 18 knots with hardly any bow rise, and this open-ended operating range is a great thing to have, as you can run at pretty much any speed without digging a hole in the water.
That makes coming home in snotty weather a lot more comfortable, and at hump speed it also makes it safer to run the boat; you’ll have a better view forward with less bow rise, which matters when running the boat from a lower station. A conventional constant-deadrise deep-vee with its center of gravity well aft is pretty sluggish between about 8 and 16 knots.
There are more reasons why semidisplacement boats are so huggable. At semidisplacement speeds, in addition to burning less fuel, you’re getting thrown around a lot less, since you’re doing maybe 14 or 15 knots instead of 25 or 30. Imagine running at cruise speed, standing up, walking around, carrying on a conversation without screaming, and not hanging on for dear life — all at the same time.
The noise levels are also a lot lower because your 40-footer has a single 250- or 300-hp diesel under the deck instead of a pair of 500s. This is balanced against more than a few diesel-powered planing sportfishermen that register 90 dBA up on the flybridge at cruise. On these boats you risk permanent hearing damage after less than eight hours per day exposure. Keeping in mind that a diesel consistently produces about 20 hp for each gallon per hour of fuel flow, you can decide whether you prefer 2.5 mpg at 12 knots or 0.8 mpg at 28 knots.
So now you may have a better idea as to whether a semidisplacement boat is for you. If you, like a FedEx package, absolutely have to be there overnight, get the planing boat. But if you like the idea of traveling at a more civilized pace in more comfortable environs, want to be able to hear your mate without slowing or stopping the boat (OK, that’s a stretch), and don’t own more than 1 percent of Exxon Mobil, the semidisplacement boat may well be your cup of tea.
Eric Sorensen was the founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at email@example.com .