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Tomorrow’s powerboat will adapt to the times

Driven by fuel prices, we'll see designs with lighter and more efficient hulls, better propulsion and a return to simpler accommodations below

When it comes to utter foolishness, there's nothing quite like trying to predict the future. While Old Testament prophets had to bat a thousand to keep their heads, this month's objective merely calls for a reasonably defensible and probably obvious position based on current trends in boating.

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For example, what if fuel prices actually go down over the next 10 years because of vast newly discovered oil reserves, or someone comes up with an engine that runs on bilge water? Fortunately, I'm not going very far out on a limb to predict that fuel prices will climb in the next decade, which leads us to ponder ways to make boats no more expensive to run on $10 per gallon gas than they are today with prices well below $4 - and in some cases below $3 - at most marina pumps.

The obvious solution for the penurious boater is the displacement boat. Put a 30-hp diesel in a 30-foot displacement cruiser, and you'll do 6 knots all day long on a gallon of fuel per hour. That's not bad at all when it comes to counting pennies, but you'll have to be willing to settle for tooling around the lake or bay at a mind-numbing crawl.

As a relatively relaxed Type-A personality, I would soon be certifiably nuts if I couldn't get up on plane and leave a long, low wake behind once in a while. Nevertheless, for some people a displacement tug or launch would be a very sensible alternative to many of the gas hogs on the market today. The key for anyone thinking about a displacement boat is to remember that speed is limited by the hull's waterline length.

For a typical displacement monohull, multiply the square root of the waterline length by 1.1 to find your easy cruise speed, and by 1.34 to get maximum hull speed. The displacement boat produces by far the best efficiency in terms of moving cargo (you and your family) a given distance; you just have to learn to enjoy the journey as much as reaching the destination.

This Steve Danzell design combines an efficient hull with a Spartan interior to reduce weight and improve efficiency.

A well-designed displacement boat is also very comfortable, with easy motion and low noise levels, thanks to a pint-sized and presumably well-insulated engine. This lets you actually carry on a conversation with someone on the other side of the boat without raising your voice, and you can walk from the wheel to the galley without hanging on for dear life. You can also build a displacement hull with heavier, cheaper materials without paying much of an efficiency penalty. This means you can use low-tech - and low-cost - fiberglass boatbuilding methods and materials and still have an efficient boat.

Ratcheting up the speed a notch, we move on to a semidisplacement boat. With this type of hull, you multiply the square root of the waterline length by 2.5 or 3 to get your maximum hull speed. Now our 30-footer (on the waterline) can zip along at 14 to 16 knots instead of crawling along at 6 knots, and the extra speed makes the semiplaning boat a very practical and attractive alternative. You can get to a destination 30 miles away in a couple of hours or cover 180 nautical miles between sunrise and sunset. With a well-designed hull, you will rarely have to slow down offshore when the wind picks up, and the modest diesel or 4-stroke outboard will keep sound levels to a minimum.

The 40-foot Steve Dalzell-designed cruiser on the facing page can cruise at 20 knots with a single 370-hp Volvo D6 while burning a little more than 12 gph, but it has to be built light - just 20,500 pounds at full load - to do it.

Unlike our glacial-speed displacement cousin, weight is the No. 1 driver of speed and efficiency with any planing hull. So the higher tech and, to some extent, the more expensive the construction methods and materials, the more efficient the boat will be. That's because it costs more to build a lighter boat while ensuring that it is just as strong and stiff as a heavier boat.

The key to achieving the kind of efficiency delivered by a boat like the Dalzell design is to not only build the boat light, but to keep it light by limiting the outfitting to an absolute minimum. This boat will have a single 370-hp diesel and make 23 knots at full power, while most 40-footers today make 30-plus knots with more than 1,000 hp. This boat will also have a 230-pound, 4,200-watt generator and no central vacuum, extra icemakers, granite countertops and so on. All this is not much to give up for the benefits, reliability, much lower cost of ownership and simplicity gained. Think of the comparatively Spartan 40-footer as putting the pleasure back in pleasure boating.

Pod-drive propulsion such as Volvo Penta's IPS system has proven to be revolutionary.

The 34-knot cruiser or sportfishing boat will also benefit from lighter construction and improved hull designs. Unless you're cruising at 60 knots offshore, a hull with 24 degrees of deadrise at the transom is pouring your money down the drain by wasting power. A more modest 20 degrees at the transom, with more forward where the waves impact the hull, produces improved efficiency and stability. This is one of the reasons I like the Hunt hull forms used by Grady-White, Grand Banks (Eastbay series), Hunt Yachts and others so much - a superior ride and improved efficiency. The way the hull is shaped makes every difference in the world when it comes to a comfortable and safe ride, down-sea tracking and propulsion efficiency.

Another fundamental that many production builders have lost sight of today is the excessive bottom loading that makes their planing hulls inefficient dogs once the lines are cast off. It's much better to extend the waterline aft under the swim platform, or increase the chine beam, to spread out the load over a bigger bottom, making the boat more efficient and reducing the speed at which it will get and stay on plane.

For decades, boats have been designed around available engines. The norm has been to put twins in anything larger than 30 feet for the simple reason that people like the redundancy, maneuverability and speed afforded by two engines that will fit under a low deck. In my opinion, this attitude will have to change as well, since twin engines are inherently wasteful compared to a single larger engine.

This is true with purchase and installation cost, and also to propulsion efficiency because the amount of drag created by the running gear underwater is effectively doubled. It's much better to have a single larger diesel in an inboard boat, with bow and even stern thrusters to help with dockside maneuvering. Diesels and modern outboards are so reliable that having two of them adds unnecessary weight and takes up unnecessary space when it comes to efficiency and reliability when one engine will do.

So beyond the basics discussed so far, what can we look forward to in the next decade? The most remarkable propulsion advance in recent memory is without question pod drives - Volvo's IPS and Cummins MerCruiser Diesel's Zeus, for example. Pod propulsion reduces weight and the amount of space taken up in the engine room and increases efficiency by 30 to 35 percent over conventional inboards.

Pod drives are certainly not cheaper than conventional inboards, but - at least originally - Volvo's IPS pod drives were priced to pretty much match what the equivalent amount of inboard horsepower would cost to produce the same speed. And these drives are no more vulnerable to high-speed grounding damage than conventional inboards, though that perception remains in some quarters, which in any event serves to underscore the compromises represented by any power plant.

The ultimate build

If you want the very lightest hull and, therefore, the optimum efficiency for a given drive train, then prepreg epoxy is the way to go. My July column on MJM Yachts built at Boston Boatworks covers this technology in detail. There is simply no way to build a lighter, stronger, stiffer boat than with this method, but it is not cheap.

Consider an old adage many of us are familiar with: We can build your boat 1) cheaper, 2) faster and 3) better - now just pick two. The corollary to this is that the slower and lighter the boat, the less strong and stiff the hull has to be. That means our semiplaning 18-knot cruiser can be built lighter to lower strength standards (scantlings) and less expensively than a 35-knot sportfisherman hull. Being slower and lighter means the dynamic loads on the hull from wave impact will be a fraction of what they will be on the faster, heavier boat, which might have four or five times the horsepower and carry three times as much fuel.

Materials matter; epoxy resin is by far the best in terms of adhesion, elongation, strength and osmosis resistance. But there's another corollary: Even the best materials can produce a poorly built boat if quality control is neglected. I've seen "wood-free, all-composite" boats with the deck pulled loose from the stringers, their thin bottom skins delaminated and separated from the thick foam core substrate, and with osmotic blistering in the all-vinylester resin. So don't take anything for granted and pay more attention to the builder's reputation than their marketing fluff. You can build an excellent boat with wood, fiberglass composite, aluminum, ferro cement or some combination of these materials.

The only way simpler, lighter, more reliable and less expensive boats are going to catch on is if fuel prices continue to rise and people continue to want to spend less to be on the water. Otherwise, we'll continue to see builders offering loaded boats that meet unchanged market demand. Whether the chicken (market demand) or the egg (boatbuilder foresight and innovation) drive the equation remains to be seen.

Green bucks

The "green" machine sees opportunity, and it's cranking up and gaining momentum in a number of industries, including in the marine business. Steyr, for instance, makes what it calls a hybrid drive train - essentially a 250-hp diesel with a 165-pound, 7-kW motor/generator bolted between the engine and the transmission. The unit can be used as a motor to start the diesel (which itself is no reason to buy the system) or to turn the shaft (which may be), drawing current from the batteries. It can also be used as a generator, driven by the diesel, to charge the batteries.

While the Steyr hybrid results in a compact system, eliminating the necessity for a separate generator, it is not an efficient diesel-electric propulsion system since such a large diesel driving such a small generator is inherently inefficient. A diesel should be loaded to 90 percent of its rated power to be efficient, not idling along at a load of around 4 or 5 percent.

However, if you want to be able to shut off the Steyr diesel and turn your shaft with a silent electric motor fast enough to hit 4 or 5 knots down the Intracoastal Waterway or glide unheard through the marina, that's fine. However, you are doing nothing to improve the environment, unless perhaps you only go a single battery charge's distance down the river and then plug back into shore power overnight to recharge the batteries. As soon as you fire up the diesel to charge the batteries, you're burning a lot more fuel than a separate 7-kW genset with a 10- or 11-hp diesel prime mover would be burning, so there is a loss of efficiency, not a gain. Just make sure you're buying the Steyr hybrid for the right reasons - perhaps as a novelty, but more practically for its silent, slow-speed, battery-limited range capability.

The catamaran hull form, such as this Lagoon Power 43, has low resistance to forward movement, which will reduce wave-making resistance.


Also in the news these days are catamarans with their decks and pilothouses plastered with solar panels and touted as being solar-powered. From all reports, however, these have seen limited success, since only under the most ideal conditions - bright sunlight and clear cloudless skies in tropical waters, with calm seas to minimize hull resistance - can such a boat make any real, reliable progress toward its destination. While this concept of solar propulsion power may need to wait for the next generation of solar panel and battery technology to be viable, there is an instructive element to the story - the use of a catamaran hull form.

A cat has very low resistance to forward movement since the cross section of the twin hulls is so low, which reduces wave-making resistance. If you want to get an idea as to the resistance, or drag, being created by a hull, you have only to look at its wake. A 40-foot semidisplacement cat moving along at 10 or 15 knots leaves hardly a ripple behind, while a 40-foot monohull at the same speed is digging a very large hole in the water, burning a lot of fuel in the process of creating all that wake. For that reason, a cat or trimaran is an excellent way to improve efficiency regardless of the propulsion system.

The catamaran hull form, such as this World Cat 330 TE, has low resistance to forward movement, which will reduce wave-making resistance.

On smaller cats especially, the interior layouts are a real limitation, and, with few exceptions, they are not easy on the eye. Perhaps the biggest limitation is the cat tunnel's tendency to bottom out and pound when running off plane. Once you get to the 40-foot range, though, these limitations start resolving themselves. The accommodations can be tailored to suit the twin hulls (a stateroom in each hull, for instance), and the tunnel height above the waterline can be high enough to eliminate or at least minimize the pounding issue.

But there is still the issue of marinas being designed to accommodate 3-to-1 length/beam monohulls rather than 2-to-1 length/beam cats. In a future column, I will explore with naval architect Eric Sponberg the potential for trimarans with retracting outer hulls, or sponsons, in the powerboat world. These hulls are usually found only in sail applications, but with their splendid slenderness ratios and low drag, I think there is great potential for powerboat applications where efficiency and comfort and a lack of hump speed are important. My kind of boat (among others).


There is at least one potentially marine propulsion-related development beyond pod drives that is worth mentioning. A company called EcoMotors ( is developing a new opposed-piston, opposed-cylinder, inherently counterbalanced engine that the manufacturer says is 30 percent lighter, one-quarter the size and has up to 60 percent better fuel economy than a conventional state-of-the-art turbo diesel engine.

The company is developing two versions of the OPOC engine, the EM65 and the EM100, model names that refer to the cylinder bore in millimeters. The EM100 has a dry weight of 296 pounds and measures 22.8 inches long, 41.3 inches wide and 18.5 inches high. It produces 325 hp at 3,500 rpm and 664 foot pounds of torque at 2,100 rpm. This comes out to .91 pounds/horsepower.

Yanmar is the reigning power/weight champ in the marine diesel world, so let's compare the Eco-Motor EM100 with the Yanmar 6LPS-STP2. This 899-pound turbo diesel is rated at 315 hp and is 38 inches long, 26.2 inches wide and 29.1 inches high. This puts the Yanmar at 2.85 pounds/horsepower, more than three times that of the OPOC engine, so repowering a boat that has a pair of Yanmar 6LPA-STP2s with two EcoMotor EM100 OPOCs would save, in theory, 1,206 pounds. (Incidentally, these engine weights are bobtail - the gear adds another 100-plus pounds, depending on the gear ratio.) Add the EcoMotor's purported fuel efficiency to the lighter weight, and the improvement in range and speed would be substantial - again, theoretically speaking.

Or instead of increased range, you could carry less fuel for the same range, reducing weight further while opening up internal volume for more accommodations or storage space with smaller fuel tanks. Besides power density, it is important to keep in mind that the Yanmar is a proven engine, while the EcoMotors engine is still very much an unproven prototype.

For the time being, EcoMotors says it plans to demonstrate a five-passenger car that gets 100 mpg. This will be a milestone to watch for, as the implications for boat propulsion are obvious. I look forward to hearing more about it being installed soon in a boat.


Back to the present - who knows how long it will take for a potential game-changer like the OPOC or some other engine to come to market in the marine industry. In the meantime, we've been seeing incremental improvements in existing technologies.

During the last four years or so, all four major manufacturers of 4-stroke outboards - Mercury, Yamaha, Suzuki and Honda - have made significant improvements to their engines, reducing weight and increasing speed and fuel efficiency, as well as adding new models. The knocks against the 4-stroke - heavy and lacking acceleration and out-of-the-hole power - have all but disappeared.

Mercury launched its high-horsepower 6-cylinder Verado 4-strokes in 2004 with 200-, 225-, 250- and 275-hp engines and has since fine-tuned their efficiency. In 2005, it introduced 135-, 150- and 175-hp 4-cylinder outboards and in 2007 added 300- and 350-hp models. Mercury no longer offers the 135-, 200- and 275-hp 6-cylinder 4-strokes.

Hunt Yachts' deep-vee hull produces a superior ride and improved efficiency.

Honda and Suzuki have focused on midrange power. Lighter, more compact and better-performing 4-strokes from Suzuki include the DF8, DF9.9, DF70, DF80, DF90 and, most recently, the DF40 and 50. Honda has a BF40/50, BF60 and this year introduced the BF115.

Last November, Yamaha unveiled nine second-generation 4-strokes from 4 hp to 300 hp. This group includes three F-Series V6 offshore engines in 225-, 250- and 300-hp models, a new 70-hp engine and 4- and 6-hp kickers.

Evinrude has kept pace, too, offering E-TEC 2-strokes to 300 hp. This year, the manufacturer introduced a 15-hp engine.

The result of these steady improvements in 4-stroke efficiency is that a 36-foot Grady-White powered by twin Yamaha F350s is precisely as efficient as a smaller and lighter Cabo 35 powered by twin 450-hp Caterpillar diesels. Both boats get 0.75 nautical miles per gallon at 27 knots, while the Grady actually gets better economy - 0.71 nmpg versus 0.63 - at 32 knots, which is the Cabo's top speed. For a traditionalist, talk about a topsy-turvy world - a gas outboard that's more efficient than a diesel.

The increased efficiency is thanks to the trimmable outboard lower units, which allow optimal hull trim to be dialed in, and their lower resistance (hydrodynamic drag). And it's all despite the fact that a diesel produces considerably more horsepower at a given fuel burn than a gas engine. The outboard-powered boat also makes more than 38 knots at full power, compared to the Cabo's 32-knot top speed, another bonus when speed is of the essence.

Bottom line

I don't think predicting an increasing need for greater efficiency is much of a risk. Fuel costs will go up - it's a question of how much and how soon. Plus, people are simply becoming more environmentally conscious, which for this old Reagan fan is naturally a good thing, just like building windmills is a lot smarter than just buying more foreign oil.

On the horizon, I see a combination of lighter boats; slower speeds that allow less-stressed boats to be built lighter with smaller engines and less fuel; improved length/beam ratios; less stuff in the boat, which also reduces weight, build cost, repair costs and complexity; and, of course, more efficient propulsion. My view is that the increased cost of lighter construction can be more than offset by accepting 20-knot boats that are more simply outfitted than today's 30-plus-knot standard.

In the meantime, I'll keep an eye on developments at EcoMotors and give you an update when the time comes and, as a bonus, we'll soon talk with Mr. Sponberg to get his thoughts on multihull efficiency.

Eric Sorensen is a consultant to boat and ship builders 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 eric@sorensens

This article originally appeared in the September 2010 issue.