Six archetypes of seaworthiness
Posted on 06 September 2011
Written by Eric Sorensen
While my family and I were on summer vacation in Barnstable, Mass., on Cape Cod, I had the pleasure of encountering a veritable smorgasbord of recreational boats. On display from Pleasant Bay to the Cape Cod Canal, they were by turns provocative, charming and inspiring. Seeing them set me to musing about the evolution of boating as a sport. Boats I saw recently on Lake Champlain and that I noted in England also came to mind.
In this month’s column I take great delight in liberally ferreting out the connections between and the singularities among very different craft. I was particularly on the lookout for fuel-efficient boats — displacement and semidisplacement designs and others that might ease the pain at the pump.
A plucky double-ender
Rock Harbor in the Cape Cod town of Orleans hosts one of the largest charter fleets on the East Coast, and it is also home to a retired 36-foot Coast Guard surfboat that was made famous when coxswain Bernie Webber used it to rescue the 33-man crew of SS Pendleton, which broke in half off the Cape near Chatham, Mass., one dark and stormy night in February 1952.
As a teenager, I got to know Bernie in the 1970s. The surfboat, CG36500, features a wooden displacement hull. It’s a double-ender with a single low-horsepower diesel for power. The waterline length is about 35 feet, so this displacement hull is easily driven to 6 knots and will top out at 8 knots. The hull shape is designed to support the weight of the vessel by buoyancy alone, even at top speed, and to minimize wave-making resistance at displacement speed.
The fact that the hull is round-bilged and fine at both ends, which is what makes it a double-ender, is both necessary and virtuous. It’s a necessity because this is a slow displacement hull that can’t keep up with overtaking waves longer than the length of the hull, which is to say most of the waves you’d be likely to see in deeper water. So following seas will regularly overtake the boat.
If the hull were a different shape, that would be a problem. For example, a broad stern would be too buoyant in a following sea, particularly in short, steep waves on shallow bars that would make the hull prone to yawing and rolling, which is otherwise known as a broach. The fine stern’s virtue, then, is in allowing following and overtaking seas to gradually shift buoyancy forward as the waves pass under the midsection of the hull, providing it with a natural tendency to stay on course. In other words, CG36500 has natural course-keeping qualities.
If you want a very seaworthy boat that is limited to speeds of 7 or 8 knots, a boat like CG36500 is pretty much perfect, and not only because of the shape of its hull. It also has practically no superstructure, a huge advantage in terms of survivability in surf. A typical deckhouse would raise the center of gravity, detracting from the boat’s stability, and add sail area, or windage, if you prefer that term, which also robs stability when the wind is abeam. The mass up high adds to the gyradius effect (also sometimes called the pendulum or flywheel effect), which can increase the degree of roll and pitch, decreasing stability and making the boat harder to handle in a seaway, especially when running down sea.
Fortunately, what CG36500 does have in lieu of a standard deckhouse is a whaleback cabin structure (rescuees were strapped securely to their seats inside on the way back to port) that adds buoyancy up high. This attribute, along with a low center of gravity, is what makes the boat self-righting when it’s inverted. Weight down low and buoyancy up high is what you want in a surfboat. The cabin is also rounded in section view, so that beam-wind resistance is very low, as is the vessel’s resistance to righting itself when capsized.
In Bernie Webber’s day, the coxswain of a surfboat would stand out in the open behind a makeshift windshield, eliminating the need for a pilothouse that could diminish the seaworthiness of the vessel and requiring a degree of mental and physical toughness not needed on today’s surf rescue boats. Bernie only had a compass and a depth sounder (if it was working) during the famous rescue, unlike the sophisticated electronics he’d have now.
Freeing ports lining the hull sides adjacent to the boat’s two cockpits quickly shed water over the side. That’s another key element of seaworthiness, given the stability-robbing nature of free-surface effect. CG36500 as an example of an intrepid wooden surfboat is the archetype of the displacement vessels that predominated in the days before high-horsepower engines were available.
Little power was needed to make these boats reach hull speed, and little power means commensurately little fuel is consumed. In that sense, the economy of the displacement hull reasserts itself to the degree that fuel prices go up. We saw a spike in fuel prices this spring, and who knows when we will see another.
A semidisplacement longliner
As larger and lighter engines became available, hull shapes changed to accommodate them. A great example is the hull of a longliner typical of the type that operates out of Chatham (see top photo above). About the same length overall as CG36500, this boat can go faster with its bigger engine because the buttocks aft flatten out to provide dynamic lift at cruising speed. The immersed broad and flat transom lets the hull climb over its bow wave rather than being limited to the speed of a wave of the same length.
The picture is instructive because it shows the boat moving at twice the speed of CG36500 (about 11 or 12 knots), but the bow is just a few inches higher, and the stern is at essentially the same elevation in relation to sea level.
The longliner is going faster than displacement speed, but even with the clean wake astern it’s still not fully on plane. The hull is supported by buoyancy as much as by dynamic lift at this speed, which is why we call this a semidisplacement or, just as accurately and more optimistically, a semiplaning hull.
This Maine-style round-bilge full-keel hull also can be quite seaworthy on a bar, if properly handled. It is not as forgiving of errors in judgment as surfboats like CG36500, but in capable hands it can use its speed to stay out of trouble, whereas in inexperienced or careless hands that same speed can get the operator into plenty of trouble.
When coming home across a bar, this boat has a much better chance of being able to climb on the back of a wave and stay there, yet not so far back that the next wave would poop you and not so far ahead that you’d pitchpole off the crest into the trough. The full keel is played up in builders’ marketing copy as a directional stability asset, but it can also get you into trouble if an overtaking wave catches the keel and throws the stern sideways, which is just another way to get into a broach.
When avoiding a broach situation, it’s important to match hull speed to wave speed when crossing a bar as you head in. Obviously, many a speed-challenged full-displacement hull can’t do that.
The Maine-style hull may be wet in a bow sea and might get squirrelly when running down sea, but it is a comfortable sea boat and it’s valued by commercial fishermen for that reason — it doesn’t beat you up in a seaway. Its round bilges create a gentle motion, with none of the snap roll you see in overly wide and flat hard-chine hulls that stick tenaciously to the wave gradient.
What it’s not good for is going fast efficiently. If you want to go faster than semidisplacement speeds — call it a speed-to-length ratio of 3, or about 20 knots for a hull that’s 40 feet at the waterline — you really ought to be in a hard-chine planing hull, as I’ll discuss. The Maine-style lobster boat hull, with its round bilges, is unable to create flow separation from the hull sides, and the full keel adds a great deal of surface area underwater. The result is you’re saddled with lots of frictional drag, which, by the way, increases with the square of the hull speed.
The semidisplacement hull is a good choice if you want to run along easily at displacement speed to save on fuel but would like to be able to cruise relatively efficiently at 14 to 18 knots when the mood or necessity dictates. Or if you value comfortable motion over burning less fuel, which is a perfectly valid position.
A planing sedan
Let’s take a look at an Eastbay from Grand Banks Yachts as an example of a full-fledged planing boat with hard chines and spray strakes to break the water and spray away from the hull, which lowers drag and increases efficiency, speed and range.
In the photo on the previous page, the bow of the sedan is mostly clear of the water, and the stern is at or slightly above its static level. Because the bottom continues all the way out to the chines, rather than rounding up to meet the hull sides, there’s more bottom area to create lift than with a semidisplacement lobster boat.
That’s not to say a hard-chine hull can’t be designed as a semidisplacement boat. That’s exactly what you see with many trawlers, such as those from Grand Banks, which can amble along at 6 knots, though less efficiently than a full-displacement Kadey-Krogen at the same speed, and then pour on the horses — and the fuel burn — to run at planing or near planing speeds.
Assuming the Eastbay weighs the same as a Maine-style hull of the same size and that its center of gravity allows it to run at an optimum angle, it will burn less fuel at, say, 22 knots because its shape produces less drag at that speed. At displacement speed, it will produce more wave-making drag than the round-bilge displacement hull, so you decide what matters most to you — efficiency at 6 or 7 knots, or the ability to get up and move when the urge strikes. (I am definitely in the latter camp.)
The Eastbay in the surf would be a more capable boat by three distinct measures: It can move a lot more quickly to find and stay on the back of a wave, it can accelerate out of harm’s way if a wave starts breaking astern, and it can get back to an inlet and into a harbor a lot sooner than the other slower boats. The Eastbay owner will be sipping a piña colada back at the dock while a surfboat such as CG36500 or the 12-knot longliner are still slogging their way home.
On the other hand, a foolish, inexperienced or careless skipper can get into more trouble faster in the Eastbay in very rough conditions, especially those involving breaking surf. And the Eastbay’s higher and heavier deckhouse also diminishes seaworthiness in extreme conditions. Ah, decisions, decisions. Also, make sure the rudders on your inboard are large and responsive, rather than small and ineffective just so the builder can squeeze another half-knot out of the top end.
Saving on weight
Whether you choose a hull with hard chines or a round bilge, or you select a hull that has flat buttocks aft so it can get up and plane, what matters even more than shape is weight. Add weight, and a planing hull slows down. Subtract weight, and it speeds up because form drag (caused by the hull and appendages displacing water) and frictional drag (the underwater hull drags a boundary layer of water with it) go down. As a boat gets heavier it displaces more water, even at very high speeds, and more hull is wetted, so frictional drag also increases.
The bottom line is that the lighter the boat is, the less fuel it will burn at a given speed. Naval architects and other boat designers speak (or should speak) in terms of bottom loading. This is simply the weight of the boat, divided by the water plane area of the immersed hull. Make the boat smaller and keep the weight the same, and bottom loading goes up. Take weight out of the boat, and bottom loading goes down.
Here’s the harsh truth: There are many boats on the market today that have absurdly overloaded bottoms, a result of heavy hulls that require large engines to go as fast as the market seems to demand and large fuel tanks to feed those engines. It might take four times the power to make a boat cruise at 30 knots than it would take to make it cruise at 20 knots.
A 20-knot boat requires that much less power because it can weigh so much less, and resistance climbs sharply. That’s right — it’s a function of the square of the speed. A lightweight 42-footer can do 20 knots with just 280 hp at the prop, which is an easy cruise at a fuel flow of 14 gallons an hour for a 400-hp (that’s 20-gph wide open) diesel. To cruise at 30 knots, the same size boat would need a pair of 600-hp diesels, each of which weighs nearly twice as much as the single 350. Then you have to double the fuel capacity to accommodate those engines, raising it to 600 instead of 300 gallons, for example. That’s an additional ton of fuel to haul around the bay. Speed is not cheap.
On the other hand, although more weight equals less speed, a boat that is too light will be too reactive to wave action. For a given hull shape — deadrise, angle of entry, strake and chine flat geometry — a lighter boat will ride harder. It is a simple matter of mass, that of the boat and each wave with which it encounters and reacts.
Moderate bottom loading combined with an intelligent hull design delivers the best possible blend of speed and comfort. High bottom loading also makes it harder for a hull to get on plane. I’ve run 30-footers that planed at anywhere from 11 to 17 knots. The latter boats had too much transom deadrise, usually 24 degrees, they were heavy for their bottom area, and the center of gravity was well aft. These boats are dogs because they only run reasonably well at high speed and do so with a lot of bow rise, which increases pounding and form drag, and so on.
When you test-run a boat you’re considering buying, make sure to find out the speed at which it will plane when fully loaded with fuel and water and with a full complement of passengers. If it’s 11 or 12 knots, your operating range is much greater, so you can come home in very rough water at slow speed without dragging half of the bay along with you. If it’s 16 or 17 knots, you will pay a price in terms of midrange speed efficiency, to say nothing of decreased visibility over the bow from the helm.
A speedy RIB
I took an HBI 30 rigid hull inflatable out for a spin this summer on Rhode Island’s Narragansett Bay. The boat planes at 11 knots. When at its 30-knot cruise speed the hull sides above the chines are dry (well, maybe not actually dry, but they’re out of contact with the water), which is an indication of moderate bottom loading and an efficiently driven hull.
If the hull sides are wet well up from the chines when at a fast cruising speed, you’ve got yourself a heavier-than-ideal boat. This is also one of the reasons the hard-chine hull is more efficient than the round-bilge hull at cruising speeds. There is less of the hull surface in contact with the water. The bottoms of the HBI’s tubes are well above the on-plane waterline, so there’s no added drag there, and the boat will be able to heel properly into a turn, unlike other RIBs I’ve run, including some of the orange ones that the Coast Guard uses.
It would be a great surfboat and would be the one I would want to be in of all the boats presented in this column, largely because of the boat’s speed and agility. The small sail area (lack of windage), the ability to shed water almost instantly over the transomless stern, and the reserve buoyancy and stability that the big tubes provide also make the boat quite seaworthy.
Rangeboat: moderation in all things
The other thing that matters a great deal to efficiency and comfort, or seakindliness, is the ratio between the hull’s length and beam. A ratio of 3-to-1 is pretty typical today, and that’s about as wide as you want to go, although waterline length and chine beam are what count the most here.
Nigel Irens’ 39-foot Rangeboat is the best example I know of when it comes to pure efficiency. The boat has a modest beam, just 10 feet 10 inches, and it is very light for its size, just 11,000 pounds, making for modest bottom loading. Another driver of efficiency that powerboat designers almost always ignore is the displacement-to-length ratio. The heavier a boat is, the longer it should be to support the weight. It’s much better to spread out the buoyancy and dynamic lift over length than beam.
The Rangeboat in the photo above tops out at 21 knots on just 240 hp. It slides up on plane with just a couple degrees of bow rise, or trim, and there is no discernible hump speed — the point at which most planing hulls strain to climb over their own bow wave. The Rangeboat just eases right up on top until it’s running at its 16-knot cruise speed.
Now Irens’ trick to getting a boat to run like this isn’t really a trick as much as it is a fastidious application of discipline. You just can’t get performance like this if the boat is loaded up with two big diesels, 400 gallons of fuel, two heads, three TVs, a 12-kW genset, teak decks, granite countertops and other heavy items. You get the picture. But keep the boat light, and amazing things happen, including infrequent or abbreviated visits to the fuel dock.
The round-bilge Rangeboat is designed to operate in a sub-21-knot speed range, but the same cost advantages of purchase and ownership would accrue to the 30-knot cruiser that weighs 40 percent less than its overloaded counterpart. The penurious owner will find similar cost savings in a semidisplacement or planing vessel pushed along by outboard, sterndrive, inboard or waterjet power. Propulsion systems produce thrust, and hulls produce weight-driven resistance. Really pretty simple.
Dolphin, a 1929 Consolidated commuter yacht
Lastly, I was fortunate to come across the exquisitely maintained Dolphin, a 66-by-13-foot double-planked mahogany-over-cedar (read lightweight) Consolidated commuter yacht built in 1929. With the original twin 200-hp Speedway gas engines, the boat was capable of making 20 knots.
This fabulous yacht with Bristol-condition mahogany topsides is the epitome of low displacement/length and moderate beam/length. With diesel power, the boat’s sister ship reportedly makes 24 knots on 21 gph of fuel, for 1.14 nautical miles per gallon, which is better than most 35-foot diesels get.
Before yachts started being designed around marina slips, a long and narrow boat such as Dolphin was the only sensible way to go. It still is if seakeeping, comfort and cruising efficiency are top priorities. But the only practical way to get high horsepower and moderate weight in those days was with a gas engine, as the World War II PT boats had, so the option of just adding more and more lightweight diesel horsepower was simply not available. Of course, that limitation forced builders to produce boats of more moderate proportions than is commonly seen today.
Maybe it’s just me, but a return to the thinking that Nigel Irens and Consolidated showed has a great deal of appeal, financially and aesthetically. Although plenty of people will want new 30-knot cruising and fishing boats for the foreseeable future, perhaps the reality of higher fuel prices and an increasing sensitivity regarding the consumption of natural resources will drive demand for both the exquisite Rangeboat and a 21st-century version of the commuter yacht built of modern composite materials. Let’s all stay tuned.
Eric Sorensen is a consultant to boat- and shipbuilders and to the government. He was 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
This article originally appeared in the September 2011 issue.