Is the displacement hull form for you?

By Eric Sorenson

February 27, 2019

There are some important features to consider when you’re choosing efficiency over breakneck speed

While planing hulls, with their speed and sizzle, have dominated the boating scene since the 1960s, things are slowly changing as boaters are more and more concerned about the volatility of fuel prices, and many are gaining a more acute sense of stewardship for the environment.

A planing boat goes faster as more power is applied, with no clear upper speed limit because it flies, its bottom supported by water pressure much as an airplane’s wings are supported by air pressure. The only practical limitation to a planing boat’s speed, in fact, is determined by the size of the waves; the boat’s size, bottom shape, strength and integrity of construction; and the tolerance of its passengers.

The problem with a planing hull is its great inefficiency compared to a displacement vessel. It takes a great deal of power to get that boat up on the surface of the water, and the result is very low “transport efficiency,” as it’s called. Transport efficiency can be applied to any form of vehicle, and it simply refers to the amount of energy it takes to move a given weight of cargo a certain distance.

Looking at common forms of transportation, trains are the most efficient form on land, followed by tractor trailers and, at the other end of the list, pickup trucks and cars. A single train can carry a thousand tons of apples from New York to Iowa (does corn-eating Iowa even want apples?) a lot cheaper than a thousand pickups can, for pretty obvious reasons. The same thing happens on the water. A displacement hull, though slower, may burn a third or less fuel over a given distance than a comparable-size planing boat.

The reason for the displacement hull’s great efficiency is that it expends no power whatsoever climbing up on the surface of the water. It doesn’t even try. Instead, it travels along very easily through the water. The shape of its bottom is such that it generates no significant lift at all; rather, it is designed to minimize wave-making resistance as it makes headway.

If you don’t mind slowing down and enjoying the process of getting there as much as actually reaching your destination, then you’ll be glad to know that the same efficiencies enjoyed by the supertanker apply to the displacement pleasure boat. Nothing’s perfect in life, though, so while the displacement hull is the most efficient, it also has its limitations.

The first is that speed potential is primarily a function of hull length. The longer the hull underwater, the faster it can go. The reason for this is that a displacement hull can only go as fast as an open-ocean wave of the same length before it starts climbing up the wave and overtaking it, which is what a planing hull does as it climbs on plane, over and then past its own bow wave. An open-ocean wave, in turn, travels faster as it gets longer from crest to crest, thanks to the constant effect of gravity.

This wave speed can be calculated as the square root of the distance from crest to crest, which is how wave length is defined, multiplied by 1.34. This gives you the wave’s speed in knots. To make it easy, a 100-foot wave has a square root of 10, resulting in a wave speed of 13.4 knots. This means a displacement hull that’s 100 feet long at the waterline can also achieve a practical top speed of 13.4 knots.

If it’s a very narrow and light hull, the hull can go faster, thanks to something called its slenderness ratio. For instance, a displacement catamaran with two skinny hulls can go a good deal faster than a monohull of the same length. But absent pouring on the power and wasting fuel, the great majority of displacement cruisers are limited to this speed/length ratio of 1.34.

So it’s clear your 25-foot LWL displacement cruiser is limited, as a practical matter, to a top speed of 5 times 1.34, or 6.7 knots, a 36-footer to 8 knots, a 64-footer to 10.7 knots, and so on. While 1.34 gives the theoretical top speed, it takes much less power to cruise at a speed/length of 1.0, which is simply the square root of the waterline length — 5 knots for the 25-footer, 6 knots for the 36-footer, and so on.

For example, the Nordhavn 40, which has an LWL of 36, according to the builder’s Web site, gets 7.7 nmpg at 6 knots (s/l 1.0) and 2.56 nmpg at 8 knots (s/l 1.34). This makes the trawler three times as efficient at s/l 1.0 than s/l 1.34. That’s a great reason for running this boat at 6 knots. And, by the way, this fuel flow, if accurate, means just 15 hp at the propeller. This Nordhavn can go faster — up to 9 knots — but it’s now running along at a fuel-hogging s/l of 1.5 and 1.29 nmpg using all of its 140 horses. Of course, “fuel hog” is a relative term, considering many 40-foot planing hulls get well less than 1 nmpg at cruise speed. All this just shows the influence on speed/length on efficiency.

Seakeeping

Now that we’ve determined that the displacement hull is precisely as efficient as it is slow, let’s look at a few of the elements that influence seakeeping. I’ve mentioned that making a displacement hull very narrow and light will increase its efficiency and speed potential. A tradeoff in displacement hull design is that a certain slackness in the bilges is necessary to reduce wave-making resistance, and this softer hull radius (in cross section) also makes the boat more tender, since form stability is reduced.

Also, having relatively fine ends (bow and stern sections) reduces resistance, but makes them too fine and pitching will increase to uncomfortable levels. There has to be enough buoyancy in the ends of the hull to make it stable longitudinally. So a little more buoyancy outboard in the hull, both fore and aft and athwartships, helps improve seakindliness, but it also somewhat reduces efficiency.

To compensate for the displacement hull’s amplified rolling, most of these boats have one or more means of roll reduction. One is to add mechanical stabilizers — they look and act like rudders — at the bilge, turning automatically to create a force opposite the roll motion. Another is to add paravanes, which are suspended from outriggers and also create a force opposing the rolling force. Displacement boats can also have fixed bilge keels that add rolling resistance underwater, and the builder can also add lead ballast low in the hull to add stability.

To reduce pitching, which becomes much more pronounced as speed increases, some displacement-trawler builders add bulbous bows. These are intended to increase speed potential by interfering with the development of the bow wave, and they also have a dampening effect on pitching motion. The designer has competing interests here, because the most slippery shape is not the one that best reduces pitching motions, so this is one more design compromise.

Generally, you want a bulb shape that reduces the formation of the bow wave while cleaving smoothly down into a wave and creating some resistance to the trip back up on the next wave. This, in turn, calls for a flatter top and sharper bottom to the bulb. A short, wide hull will pitch more than a long narrow one, all else being equal, and this is something to watch for, as some designers, like with many planing hulls, try to fit too much into a 10-pound sack.

The height and size of the superstructure also bear directly on seaworthiness. This is because a larger superstructure raises the center of gravity (CG), which makes the boat less stable. A large superstructure also increases sail area up high, and this can be as depleting to stability as a higher CG. When you combine wave action with the heeling force of a beam wind, the combination puts any vessel to the test in a survival situation.

While stability curves take into account the boat’s hull form and center of gravity, there is no accounting for the amount of vertical surface area exposed to the wind. Just as efficiency works at cross purposes to form stability, seaworthiness decreases as interior superstructure volume increases. For a coastal vessel, it may not make a lot of difference, as long as you stay tuned to the weather and are close to a harbor or lee shore. However, for an oceangoing vessel, a smaller, lower, lighter superstructure and lower CG make for a more seaworthy platform.

The placement of the superstructure also makes a big difference in seaworthiness and seakindliness. The motions in the aft third of the hull are more comfortable than in the forward third, so from a vertical-motions perspective, farther aft is the best place for the wheelhouse, or bridge. On the other hand, visibility is better farther forward in the hull, closer to the bow, though if the helm station is high enough in relation to the bow a moderate-size, aft-oriented deckhouse is a good way to go, in my view. Also, the deckhouse is less exposed to boarding wave impact when working your way into heavy seas.

The layout

Fact is, there are few pleasure trawlers with long, open foredecks and aft-oriented superstructures for the simple reason that people naturally want lots of inside room. That’s why you see king-sized deckhouses that stretch much of the length of the hull. One thing to look for on any trawler is the traffic pattern — how easy is it to get from the pilothouse to the saloon, for instance — and does the layout accommodate the lifestyle you’re looking for.

One reason the pilothouse motoryacht layout works so well is it puts the pilothouse forward and just a few steps up from the saloon. And it’s only another four or five steps up to the flybridge, which is usually tucked behind the pilothouse and above the saloon. Everything is close by the wheelhouse, including the forward accommodations a dozen or so steps down and forward. If the boat is big enough, over 45 or 50 feet, there’s also room for a midships master suite (stateroom and adjoining head) directly below the wheelhouse.

Back aft, look for a cockpit that’s big enough to suit your purposes. At the very least, there should be plenty of room for handling lines and for getting on and off the boat, either through a door in the hull side or via the swim platform. Railing height all around the weather decks should be at least 32 inches; 39 inches is even better, especially forward where the motions are more harsh and your exposures to boarding waves is increased.

Some builders like the so-called Portuguese bridge, which adds a bulwark and a walkway forward of the pilothouse. This is especially useful — actually it’s a necessity — in superstructures that nearly reach the anchor locker, as you’re much more likely to encounter green water on deck the farther forward you are.

The scuppers should be large enough to drain water almost immediately overboard. If you have lots of water sloshing around on deck and waiting to drain overboard through microscopic scuppers, then just know this creates a free-surface effect that greatly diminishes stability. Also, the taller the bow, the less chance of dipping it, so lots of freeboard forward is your friend, as long as the builder doesn’t get carried away and prevent you from seeing where you’re going from the wheel.

The galley should be fully outfitted, and there should be enough storage space, whether in the galley or elsewhere on board, to accommodate the amount of time you expect to spend at sea. If I were going to spend a lot of time cooking at sea, I would definitely want a window or big porthole in front of me to see outside the boat. I’d hate living in the cave-like surroundings you see on many cruising boats. The accommodations layout that puts a saloon, or at least a long passageway, between en-suite staterooms is most conducive to couples maintaining friendships over a week or longer at sea, since an extra layer of privacy is afforded.

Standup engine rooms are great; it’s much more comfortable to be able to stand and walk around the engine and other equipment. I’m a fan of single-diesel power, because the boat is more economical both to purchase and operate. Some people, in spite of the thousands of commercial vessels running hundreds of miles offshore for thousands of hours each year, understandably like the fuzzy feeling they get from having two propulsion engines, both for the redundancy and for the added maneuverability dockside.

A small diesel with a deep gear ratio and a large, slow-turning propeller moves a lot of water and has tremendous traction and low-speed thrust for the horsepower. Don’t skimp on propeller diameter, whatever else you do. The large prop’s traction is nice to have not only around the dock, when you’re hoping to see some result the same day you throw it into reverse, but also offshore in heavy weather, when you want to keep punching through big waves at a steady pace.

We’ll talk more about trawlers in another column. Hopefully, for now, we have enough to chew on, and look for when considering a displacement hull under your next boat.

Eric Sorensen 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 [email protected].