Updated: Mar 1
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.
Inboard marine engine
When the engines reside inside of the boat, regardless of which drive system is utilised, the boat is considered an inboard.
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.
Straight shaft inboards are the norm on large sportfishing boats, like this Bertram 64.
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.
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 a manufacturer to mount the engine far aft, but still utilise shafts that exit the hull bottom.
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).
Surface drive Magnum 51
The surface drives on this Magnum 51 Bestia are clearly visible, and they help this boat break 70 MPH.
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.
Propellers? Who needs propellers? This Yamaha jet boat gets by just fine, without them.
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.
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.