Diesel or gas, inboard or outboard, choose a setup that’ll get the job done the way you want it to
It may well be the case that you’ve found the boat that best suits your needs, but the propulsion package is not what you really want. If you look at the most common propulsion drivetrains — outboard, conventional prop and shaft, sterndrive and waterjet — then add the possibility of gas or diesel engines, you suddenly have lots of options. Let’s look at their relative merits, expressed in general terms.
In terms of sheer numbers, outboards are the most common power plant. A big part of the reason for their popularity is that they have the highest power-to-weight ratio of any engine, except the gas turbine. For instance, the Yamaha F250 4-stroke weighs in at 604 pounds for a pounds/horsepower ratio of 2.4-to-1. By comparison, the lightweight power-to-weight champ Yanmar 315 diesel weighs 899 pounds (2.85 pounds/horsepower), and that’s without the transmission, shaft, struts, rudder, packing gear and propeller (all part of the outboard package), making it closer to 4.5-to-1 pounds/horsepower.
The outboard can be run at nearly full throttle all day if gas consumption isn’t important, though the Yamaha 250 4-stroke, which turns a little more than 6,000 rpm at full power, runs most economically (on my boat, anyway) at 4,000 to 4,500 rpm. The outboard can be tilted up so you can beach the boat or raise it clear of corrosive salt water when the boat is on its mooring or at the dock. It can be serviced or replaced easily since it’s out in the open, bolted to the transom. And you don’t have to worry about things going wrong below deck — the engine is right out there in plain view.
The new 4-stroke (and direct-injected 2-stroke) outboards are remarkably quiet and smoke free, and they burn less fuel than the old smoky carbureted 2-stroke. The outboard is expensive for a given horsepower, but that’s balanced by the higher continuously usable horsepower and its light weight, which, along with its trimmability, make it the fastest power plant of a given horsepower.
Trim tabs let you raise the stern to lower the bow, but the outboard (and sterndrive) lets you lower the stern and raise the bow to decrease the drag of wetted surface, increasing speed. Trimming the engine also is a handy capability running down sea when a bow-up attitude helps with course-keeping, or when too many people are riding forward.
Since the entire outboard turns, you redirect thrust instead of just deflecting it, like an inboard’s rudder. This means the boat turns with more zip and slows down a lot less in a hard turn, and it also handles much better around the dock, since you can both turn and back with more control. Finally, the outboard’s lower unit is streamlined, so it has less form drag compared to an inboard.
The inboard is the simplest drivetrain and the least expensive to buy and maintain. It has none of the trimmable lower unit’s operational advantages, but it is as reliable and durable as they come. A transmission with a reversing gear is bolted to the engine, and from there we have a propeller shaft, stuffing box where the shaft passes through the hull, one or two struts to support the shaft, a propeller and a rudder. All this underwater hardware is pretty bulletproof and, being made of bronze and stainless steel, will last pretty much forever, given proper galvanic corrosion protection.
It also creates more drag than the sterndrive or outboard, so it’s less efficient, but it doesn’t make much difference at speeds below 25 knots. Draft is fixed, so you can’t beach the boat in shallow gradients. Then again, there’s none of the outboard or sterndrive’s obnoxious gear whine at high speeds. Other than the waterjet, the full-keel inboard is the only design with full drivetrain grounding protection.
The slower the boat, the larger and slower-turning the prop should be. A fast-turning, too-small prop on a bigger boat will be sluggish around the docks, since the prop’s water column is just too small to provide much traction. While the engine manufacturer does half of the decision making by choosing the gear ratio (leaving the builder to simply choose from a few standard prop pitches and cups), it can be amateur hour at a number of boatbuilders when it comes to inboard gear ratio and prop selection. There also are cost issues; the larger, slower-turning propeller that digs in dockside and works so well at semidisplacement speeds costs more, and it requires a larger, more expensive shaft, struts and shaft log, all motivation for the builder to go small and cheap.
With any inboard, make sure the engine is accessible, both so you can change the oil and do other routine maintenance easily but also so you can swap out the engine without extensive prep work. A boat with a hard-to-get-to engine will be less fun to own, and it won’t be as well taken care of. If you fish, dive or just like to have a clear transom, nothing beats an inboard for accessibility over the stern.
Inboards work quite efficiently up to speeds of 25 knots, at which point their increased hydrodynamic drag really starts to kick in. You can make an inboard go 45 knots or more with enough horsepower, but you’re burning more fuel just dragging all that running gear through the water as speed increases. Much better to have sterndrive, outboard or pod power, at least from a drag perspective.
Volvo Penta came up with the idea of joining a car engine to a transom-mounted propeller in 1959, and the idea stuck — for good reason. You get all the handling and maneuvering advantages of the outboard but at less cost, since the car engine is mass produced and, therefore, inexpensive.
Compared to an outboard, the sterndrive weighs more per horsepower, but the weight is lower in the boat, increasing stability. The sterndrive’s profile also is lower, since the engine is under the deck (or inside an engine box, depending on the boat). The only thing sticking out past the stern is the lower unit, and it’s mostly below the waterline, so there’s less of an obstruction to fishing lines. In fact, many sterndrive-boat builders cover the lower unit with a swim platform, so you hardly know it’s there.
Single-engine accessibility in an express cruiser (up to 28 feet or so) is usually very good, as it is in many runabouts on the market today. By far the best drive is the one with counter-rotating propellers, introduced first by Volvo as the Duoprop and soon followed by Mercury’s Bravo 3. These are superior for any boat operating in the 20- to 45-knot range for a number of reasons. The second propeller turns in the opposite direction of the first, taking the helical twist out of the prop wash. This effectively directs more of the propulsion energy straight aft and thrust forward.
The twin-prop setup also has more surface area, so it acts like a bigger rudder at slow speed, improving course keeping. Since side force is cancelled out, you can back the boat in any direction, including downwind, without hesitation. The single prop has a strong tendency to walk in the direction of propeller rotation.
The sterndrive, with through-hub exhaust, is very quiet, and fumes are practically non-existent. The biggest news with the sterndrive is the economy. While a gas sterndrive and 4-stroke outboard are in the same ballpark economy-wise, the highly efficient diesel sterndrive really opens up a boat’s range. Depending on the design of the lower unit, many sterndrives work well up to 45 or 50 knots, with specialty racing drives capable of speeds to 150 mph or more.
The biggest objection to sterndrive use in salt water has been corrosion, and that remains a valid consideration. However, both Mercury and Volvo have introduced anti-corrosion systems for their drives that have earned kudos from dealers and consumers alike.
The waterjet excels if shoal draft and imperviousness to lobster pot buoys is a priority. These boats, especially when equipped with joystick controls, are very maneuverable dockside, nearly as much so as a pod drive (more on pod drives below). If you want to be able to beach your boat, routinely operate in shallow water, or want the ultimate in swimmer safety, the waterjet is worth a close look.
These power plants do not do well in midrange; you’re either going to be running at displacement speed or at a high cruise, but not in between. That’s because the waterjet impeller is much smaller than a propeller, and it cannot absorb the horsepower that the engine is capable of putting out until it’s running at a high percentage of maximum rpm. (Most, if not all, waterjet setups use diesel engines.) Once the engine reaches 85 or 90 percent of its maximum rpm, the impeller starts absorbing horsepower and generating thrust. Jets come into their own at speeds of 25 to 45 knots, so the engine has to have enough continuous horsepower to sustain those speeds at full load to work well.
Waterjets have a small niche, so they are more expensive than sterndrives and inboards, but they’re a wonderful alternative for applications putting a premium on shoal-water operation.
Surface-piercing drives are very efficient at high speeds, since the only immersed hardware is the bottom half of the specially designed cleaving propeller. These are low-volume specialty propulsion systems and commensurately expensive, but they are often made of stainless steel and bronze, so they are relatively impervious to corrosion.
Surface-piercing vessels are designed to cruise at speeds of more than 40 knots, where their low-drag advantage really kicks in. At slow speeds, they don’t handle well. Many surface-piercing designs project well past the transom, with the prop shaft at the waterline, so they are inherently hazardous, but some builders cover them with large grated swim platforms.
Propeller immersion with units like ZF’s SeaRex or Twin Disc’s Arneson drive can be adjusted up and down to control prop loading, as well as side-to-side to steer. Other surface drives are fixed, with rudders for steering, which reduces cost and weight.
Volvo Penta’s IPS and CMD’s Zeus are the two pod drive systems available today. The propulsion units are mounted under the hull, so they don’t project past the transom. They’re continually submerged, so they’re made of corrosion-resistant bronze and stainless steel (rather than aluminum). Like a sterndrive or outboard, the pod drives are very streamlined hydrodynamically, and they have counter-rotating propellers (IPS forward facing, Zeus aft), so efficiency, acceleration and overall handling are outstanding — much better than an inboard.
They also take up little volume inside the hull (light and compact IPS, in particular), making for larger accommodations in a given LOA; burn less fuel for increased range; and have very low vibration and noise levels. They work well in their operating range to around 40 knots, and their superiority over inboards increases markedly at speeds of more than 25 knots, because of the form drag issues discussed earlier.
Both IPS and Zeus have joystick controls, with the pods turning and speeding and slowing automatically as the joystick is moved and twisted by the operator. This makes docking a pod-powered boat child’s play, without exaggeration. IPS is available with both gas and diesel power (soon to the equivalent of 850 hp), while Zeus is diesel only.
Gas engines are relatively inexpensive to buy, quieter at low speed, simple to work on, cheaper to fix when things go wrong, and are reliable and durable, given proper care. Gas engine life span is usually limited more by saltwater corrosion than by wear, for recreational users.
Figure on being able to cruise at a little more than half the engine’s rated horsepower. For example, a 375-hp gas engine will typically (varies by propeller load) develop about 190 hp at an easy continuous cruise speed of 3,000 rpm, or 240 hp at a fast cruise speed of 3,500 rpm. Best economy is often found between these rpm settings. You can run a gas engine harder than that, but fuel consumption goes up dramatically, and engine life shortens because of the added strain on the internal components.
A gas engine is made for light-duty operation — think of a car going down the highway — so it’s not well-suited to heavy hulls that take a lot of power to push at cruising speed. Figure on a gas engine producing about 12 hp for each gallon per hour of fuel flow.
Flushing the engine with fresh water after use, or having a freshwater cooling system, tends to prolong any engine’s life in salt water. Big-block gas engines are seen on 35- to 40-foot inboard cruisers, but they can struggle to move the boat at planing speeds and are really only available to keep the purchase price down. On lighter, faster hulls in the 35- to 40-foot range, gas sterndrives and, of course, today’s high-tech outboards are certainly viable options.
Diesels are the only way to go for most inboard cruising boats 30 feet and larger. They develop more horsepower on a gallon of fuel — around 20 hp per gallon per hour — making them much more efficient than gas inboards. This is a great way to tell how much horsepower your engine is developing: multiply gallons per hour by 20, and you can also sniff out con artists selling boats who say their 40-footer does 20 knots at 5 gph, which comes out to an absurdly low 100 hp.
You can cruise a diesel at a much higher percentage of its maximum horsepower than a gas engine. Figure on a continuous 90-percent-rpm cruise producing a little more than 80 percent of the engine’s max-rated horsepower with the typical propeller load curve. Properly cared for, and if not overstressed at their horsepower rating, a diesel can run anywhere from 5,000 to 10,000 hours — or even more in derated commercial applications.
Diesel fuel is inherently safer than gasoline, so a leak below deck, though obviously still a problem, is not as potentially catastrophic. Diesels cost more to buy and to have fixed, but they’re worth every penny if you use the boat on a regular basis. And resale value and demand will be commensurately higher.
Modern common-rail diesels are very quiet and practically smoke- and fume-free; forget about diesel fumes smoking you out in the cockpit. Diesels also keep getting lighter and more compact, many with power-to-weight ratios superior to gas engines when measured by continuous horsepower. And some of the common rail engines are quieter at cruise and top speeds, though not at idle speed.
So there’s a lot to choose from when it comes to power options. The other good news is that it’s getting harder to find a poorly built engine. There’s diesel sterndrive power for economy, speed, range and maneuverability; gas inboard for low cost and simplicity; waterjet for shoal draft; outboards for speed, light weight and accessibility; and surface drives for high-speed efficiency. Choosing your power is half the fun — the other half is getting out on the water and using it.
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.
This article originally appeared in the September 2008 issue.