Boat engines take many shapes, sizes, and forms, ranging from the common outboard to rarely seen surface drive systems. But sorting through all of the different options can be quite difficult. Which is best for the type of boating you enjoy? Is newer technology necessarily better?
No matter what sort of boat you're interested in, choosing the right powerplant is a key factor in becoming a happy boater. Obviously with a powerboat, the engine is key, but even with sailing boats, in most cases your engine will be a key back-up for emergencies and times when sail power needs a bit of help (think no wind or a difficult marina to navigate).
Strictly speaking, the engine is what creates power and the drive system is what transfers that power into thrust. So logically, you want to distinguish between the two. There’s just one problem: many power and drive systems are integrated. The term 'outboard engine', for example, commonly describes both the powerhead and the lower-unit drive system that's attached to it. On the other hand, a 'jet engine' comes in both outboard and inboard varieties. So let’s start off by dividing engines and power systems into their most basic forms: outboards and inboards.
An outboard engine sits on the transom of a boat and has its own self-contained drive system. The powerhead (or motor, in the case of an electric outboard) is protected by a removable cowl. A motor mount just below the powerhead bolts or clamps onto the transom of the boat, and the midsection of the outboard, which encloses the drive shaft and exhaust housing, has a lower unit bolted onto the bottom. Most of the time the lower unit swings a propeller for propulsion, but in some cases the lower unit houses a jet drive. A lower units with a propeller also incorporate a skeg, which offers it some protection from striking hard objects.
Outboard engines boast a long list of advantages. They generally have an excellent power-to-weight ratio; they’re essentially portable (especially smaller models) and can be easily replaced; they can be tilted to remove the drive system from the water; handling is excellent since the entire system articulates as one; outboards don’t take up room on the inside of the boat; many are designed from the ground up for marine use (as opposed to being automotive engines that are marinised); their cooling systems self-drain for easy winter storage; and in many cases they’re less expensive than other options.
A huge range of sizes are available, from small electric models which produce a fraction of a horsepower and are powered by a 12-volt battery, all the way up to the largest currently offered, a semi-custom Seven Marine 627 HP outboard. Between all those different choices, it’s possible to power everything from a canoe to a 50ft centre console powerboat with outboard engines. Outboards are also used by sailors, as auxiliary power and are a popular option for smaller cruising yachts or racing keelboats. In some cases inboard boats also have an outboard 'kicker' motor that is used for extended slow-speed operation (such as trolling for fish) and/or get-home power in case the main powerplant breaks down.
What about the poor reliability, awful pollution controls, and high noise and vibration levels that older salts may associate with outboard engines? It’s ancient history. Stricter rules and regulations forced outboard manufacturers to either shift to four-strokes or redesign cleaner, more efficient two-strokes years ago. Today, virtually every outboard on the market not only offers excellent reliability, but also has vastly improved efficiency and is far more pleasurable to run.
Naturally, there are some downsides associated with outboard engines. In some classes of boats, they aren’t quite as efficient as inboards with stern-drive propulsion systems. Another downside to having an outboard is the fact that you can’t have a full-beam swimming platform. At best, you can have two small platforms, one to either side of the engine, and on many watersports boats this is a significant disadvantage. Finally, when it comes to large, heavy boats, at some point outboards simply aren’t an option. Even the biggest outboards aren’t large enough, for example, to power a 60ft motorboat.
See Outboard engine buying: 10 top tips and 10 best outboard engines.
Inboard marine engines
Simply put, inboard engines are those that reside inside the hull of a boat. But here’s where things often get confusing: straight-shaft drives are commonly just called 'inboards' by most people, yet stern drives and pod drives, which are all radically different propulsion systems, are technically also inboards. We’ll address these different drive systems later, but for now, bear in mind that all of them require an engine inside of the boat—so they all count as inboards.
When it comes to sheer power, outboards simply can’t match inboards. They’re available in thousands of horsepower; come in both petrol and diesel models (though when considering the models that out-size the largest outboards, diesel is the only option, and petrol inboards are less common); and since they can be adapted to mate with many different propulsion systems they work well in a variety of different types of boats.
In some cases, inboards also have an edge when it comes to weight distribution. Many boat designers would argue that having inboards deep in the belly of a boat makes for a lower center of gravity and a better running attitude. That said, all hulls are different and different boats with different power systems can be designed to accommodate different weight distributions.
As a recreational boater, what’s important for you to remember about this issue is that a boat should be designed specifically for the power system you’re getting with it. In some cases a manufacturer will modify a boat to sell it with inboards when it was originally designed for outboards, or vise-versa. If great care isn’t taken to mitigate the resulting changes in weight distribution, the boat’s running characteristics may be sub-par.
There are, of course, some significant down-sides to inboard engines. Handling is rarely as good as with an outboard (though there are exceptions, most notably inboards paired with pod drives); their power-to-weight ratio isn’t as good as it is with outboards; they’re more difficult to work on and replace; and in many cases they eat up a significant amount of room inside the hull which could otherwise be dedicated to seating, cabin space, or stowage.
Considering the strength and weaknesses of inboards and outboards, it shouldn’t come as much of a surprise that in most classes of boats under 30ft outboards are more popular. (Some exceptions include dedicated wakeboarding or water skiing boats, and some watersports-oriented runabouts). In most classes of motorboats over 40ft inboards rule the roost. And there’s a gray area in-between, where both types of powerplants thrive.
Now that we’ve differentiated between outboards and inboards, we can drill down a bit farther and look at the differences between drive systems. Those for inboards consist of straight-shaft, stern drive, pod drive, V drive, and in rare cases surface drives. Jet drives, you might remember, can be used with both outboards and inboards. Here are the basic characteristics of each...
The shafts come out through the hull-bottom of a motorboat, and steering is accomplished by directing thrust over rudders. That means handling is commonly poor as compared to boats with articulating drives. This deficit is more noticeable at low speed, but can be mitigated if you have twin engines whose fore and aft thrust can be opposed to spin the boat, or with the addition of bow thrusters and/or joystick systems.
Both petrol and diesel straight shafts are available, but aside from dedicated water ski and wakeboarding boats (where wake-shaping and locating the propellers as far as possible from people in the water are significant concerns) and a few mid-sized cabin cruisers (where price is a big factor - diesels cost significantly more than petrol engines), there are relatively few petrol inboard straight shafts on the market today.
Straight-shaft boats have relatively deep drafts, and if any of the underwater running gear gets damaged, the boat must be hauled out for repairs. The engines in straight-shaft boats are fairly far forward, which can be advantageous when it comes to weight distribution. However, this does tend to eat into cabin space.
With stern drives, the engine is mounted near the transom of the boat and is attached to a drive system that goes through the transom. Stern drives offer comparatively good fuel efficiency and good handling, especially at high speeds. At low speed, single-engine stern drives with single propellers (as opposed to DuoProp or Bravo III drives, which have multiple, counter-rotating propellers,) tend to wander and may be difficult to steer.
Since the drive unit on a stern drive can be trimmed, running angle can be adjusted. At slow speeds, draft can be reduced, and you can bring the prop all the way out of the water when beaching or mooring your boat.
Stern drives have an awful lot of moving parts, are fairly complex, and have a well-deserved reputation for requiring more maintenance than most other drive systems.
The newest drive system around, these articulating drives go through the boat’s bottom. Compared to straight shafts they usually allow for more cabin space, and compared to stern drives they usually allow for better weight distribution.
The articulating drives, mounted forward of the transom, offer the very best handling of any type of inboard drive system. They generally come with joystick controls and can spin in their own length or move the boat sideways.
Pods aren’t an option on smaller boats; very few under 35ft have them and most pod-equipped boats are 40ft or over. The drives can’t be trimmed, so draft is always relatively deep and trim tabs are a necessity on most pod-driven boats.
Pod drives offer an efficiency gain as compared to straight shafts. Many boats net a 15- to 30-percent fuel economy advantage.
V drives allow for the engine to be located far aft, yet face forward, and still have shafts that exit the boat's bottom facing aft. This is advantageous since, being located all the way back near the boat's transom, they don't take up any cabin space.
Since the engines aren't located forward as with traditional straight shaft drives, the weight distribution advantages of straight shaft inboards are largely forfeited.
V drives are mostly seen on either mid-sized cabin cruisers and motoryachts (where an emphasis is placed on maximizing the cabin's interior volume,) or on wake surfing and wakeboarding boats (where locating the weight aft helps create a large wake as well as eliminate the need for a motorbox in the center of the cockpit, yet inboard propulsion is still preferred over a stern drive).
Sometimes called surface-piercing drives, these systems come through the transom close to the waterline and swing a propeller that’s only half-submerged when the boat is on plane. This reduces the propeller’s drag, resulting in a speed and efficiency increase. The most common is an Arneson drive.
Surface drives are usually found on extremely expensive performance-oriented dayboats (like the Magnum 51 Bestia, or the Revolver 42). Some, but not all, surface drives are articulating. Some others require a rudder. Handling for either tends to be difficult at low speeds, and handling in reverse is particularly problematic. Draft is significantly reduced, as compared to other propeller-driven propulsion systems.
Surface drives are designed to operate with half the propeller(s) out of the water, but at pre-planing speeds, the propeller(s) are fully submerged. As a result, some boats with surface drives have a very difficult time transitioning from pre-planing speeds to planing speeds.
Jet drives utilise a water impeller and nozzle to create thrust, instead of swinging a propeller. Jets are favored by some people on small runabouts since they eliminate all the dangers associated with propellers.
They may be utilised on larger motorboats where draft is a serious consideration; unlike most other drive systems they require water no deeper than what is required to float the hull.
Jets aren’t as efficient as propellers through most of the power-band, and although the difference often slims as you get closer and closer to top-end, at most reasonable cruising speeds and certainly at slower speeds, MPG will suffer as compared to propeller-driven boats.
Handling a jet drive, which steers with thrust directed by the nozzle and (usually) a “bucket” that swings down over it to redirect thrust, takes some getting used to. There will be an adjustment period for boaters who are accustomed to other systems.
See our guide to marine diesel engines.
That’s quite a bit to digest, isn’t it, but understanding the high and low points of each type of boat engine and drive system is important. What’s even more important? Finding the system that’s best matched to your needs — so you have even more fun out on the water.