Drivetrain Comparisons

Drivetrain Comparisons

Subject:  Which is better: inboard, outboard, I/O or jet?

Life is an endless series of tradeoffs, and this is another one. First, let's define what we're talking about:

Inboard: this is oldest of the drive systems, having been invented shortly after the discovery of the propeller. An engine (in ski boats, typically a marinized automotive engine) is connected to a transmission, which in turn drives a shaft. This shaft passes through the bottom of the boat hull at some small angle via a stuffing box (which keeps most of the water out while allowing the shaft to turn), then through a cutlass bearing in a strut. The propeller is attached to the end of this shaft, and a separate rudder, with its own through-hull fitting, is mounted aft of the propeller. A variation of the inboard is the Vdrive. In this system, the output shaft of the engine faces forward, and the shaft carries the drive to a Vdrive gearbox, which turns the drive through an acute angle and thus back toward the stern. The portion after the gearbox is the same as for a "straight" inboard.

Outboard: this is the second-oldest of the drive systems, originally invented for powering small boats. The outboard is a single assembly that bolts or clamps onto the boat's transom. The unit contains a motor that drives a vertical shaft, which takes the drive to a lower housing much like a "fat" rudder, where it is turned 90 degrees to point aft and drive a propeller. The entire unit turns to provide steering, and usually tilts for trailering, boat trimming and shallow water operation. The motor is typically a 2-stroke powerplant specifically designed for marine use.

Inboard/Outboard, or stern drive: this propulsion method combines aspects of each of the previous two systems. It uses an engine (typically a marinized automotive engine) mounted entirely inside the hull just forward of the transom. A drive shaft passes through the transom and drives what looks like the bottom half of an outboard motor. This part is variously called the outdrive, the stern drive, or the lower unit. The outdrive turns like an outboard to provide steering, and tilts like an outboard for trailering, trimming, and shallow water operation. Note that some boat salesmen have been calling I/Os "inboards" in recent years.

Jet: A jet boat has a marinized automotive engine driving the impeller of a jet pump, which sucks water in from an intake on the bottom of the boat and pumps it out an outlet that extends out from the transom. A nozzle on the end of this outlet can be swiveled to provide steering. There are no gears in a jet pump. Shifting is accomplished by dropping a "gate" over the pump outlet nozzle which redirects the output stream forward. In "forward gear", the gate is fully retracted. In "reverse", it is fully over the nozzle. In "neutral", it is about half-way over the nozzle. Some jet outlets can be trimmed up or down to trim the attitude of the boat (or to create that really awesome, unlimited hydroplane-style rooster tale we've all seen! :-) ).

Disclaimer: every combination of hull design and powerplant is different. Sometimes the differences are subtle, and sometimes they are large. This makes it impossible to generalize the skiing and operating characteristics of any particular drive type. Now that that's out of the way, let's get on with the sweeping generalizations! :-)

Strengths and Weaknesses, skiing-related

Inboard: In a straight inboard, the engine is typically mounted near the center of the boat, thus putting the largest mass near the center of the boat. This has several effects. For ski boats, the inboard drive is typically used in a fairly flat-bottomed hull. This hull shape, combined with the forward weight distribution and the upward thrust from the angled shaft, results in very short time to plane with minimal bow rise, which is good for popping a skier out of the water. Once on plane, these same factors result in a large percentage of the hull (relative to the other drive systems) remaining in contact with the water, which results in low, rounded, soft wakes at skiing speeds. It also results in lot of drag, which keeps inboards from being very fast (45 mph is a typical top speed) or very economical at high speed (but they are quite economical at low speeds because of their early planing).

Inboard ski boats usually have tracking fins mounted to the bottom of the hull below the engine, and a tow pylon mounted just ahead of the engine. These provide excellent tracking and on-plane maneuverability. Tracking is degraded when the ski rope is attached near the transom, but is typically still better than outboards and I/Os. Steering is typically light because only a rudder is being turned (though some boats are set up with a "bias" in the rudder that provides some torque. Some drivers prefer this, because the torque removes any play from the steering). Because relatively large V-8 engines are typically used, and because the boat is fairly heavy, the speed of an inboard is typically not much affected by skier pulls.

Inboard ski boats typically have a deep, flat full-width swim platform just above the waterline that is easily accessible because there is no motor in the stern area.

The interior of an inboard tends to be somewhat cramped because of the central motor box.

Outboard: The outboard's advantages stem primarily from its excellent power-to-weight ratio. Outboards are (with some exceptions) two-stroke motors (no valvetrain) and are made mostly of aluminum, yet develop fairly high power for their size. Because the power package is lighter, comparable performance to the other drive systems can be obtained with lower horsepower. Hulls available for use with outboard power range from specialty ski boats with shapes similar to the inboard ski boats, to family boats with relatively deep V hulls, making it hard to generalize. In general, though, outboards offer excellent acceleration (better than the inboard ski boats, in some cases), higher top speed, poorer tracking than an inboard (when pulling a skier in the slalom course, it can be difficult to keep an outboard headed straight down the course), very low, narrow, but perhaps firm, wakes, and poor fuel economy. Because of their acceleration, top speed, narrow wakes and room for a full complement of judges, outboards are preferred by barefooters.

Outboards don't maneuver at speed as well as inboards, but their lowspeed maneuverability is much better (e.g., for docking). Big outboards can be hard to steer, since the whole motor must be turned and engine and propeller torque can have a significant effect. Power steering is available on some outboards to alleviate this.

Outboards usually have a small swim platform on either side of the motor. Often there will be a swing-down step on one side. These platforms are useful for climbing in and out of the boat, but in most cases you can't stand on them while putting your ski on like you can on an inboard's full-width platform.

The outboard sticks up and gets in the way of the driver's and observer's view of the skier, particularly when he's in the water, and it often also gets in the way of the ski rope.

Outboards typically have lots of interior room, because of the engine being entirely outside the boat.

Inboard/Outboard: The I/O, as you might expect, is a mixture of some of the strengths and some of the weaknesses of inboards and outboards. However, few of the strengths have anything to do with skiing (see the next subsection). Because of the relatively heavy engine mounted right at the transom, and because the mounting of the outdrive requires a certain amount of deadrise, or V, at the transom, I/Os are typically the slowest of the powertrain types to get on plane, and they usually have the worst bow rise during acceleration, as well (but again, there's lots of variation between different hull designs). They are more fuel-efficient than outboards at low speeds, and are more efficient than either inboards or outboards at high speeds, in part because of the ability to trim the boat's attitude by tilting the outdrive (an ability shared with outboards, but the outboard's engine is less efficient). I/Os generally have higher top speeds than inboards, but lower than outboards.

I/Os generally have the highest wakes, because of their V hulls and rear weight bias, and the wakes usually have a crest, rather than being rounded. This can be very good for wakeboarding and kneeboarding, and for "hot dog" skiing, but is not usually desirable for slalom skiing and can be an impediment to beginning skiers.

Tracking and maneuverability of I/Os is similar to outboards. Most I/Os nowadays have power steering, so steering is easy and torque steer is not a problem.

I/Os generally have roomier interiors than inboards, thanks to the motor box being right at the transom, but are not as roomy as outboards.

Most I/Os now have a full-width swim platform, but it is usually somewhat shallow and some distance above the waterline. It usually has a folding step to aid in climbing out of the water.

Vdrive: Vdrive boats typically have wakes that are more like those of an I/O than an inboard, because of their rear weight bias. However, they also typically have flatter bottoms than I/Os, which mitigates this to some extent.

Vdrives get on plane slower than straight inboards, but have higher top speed, again due to their rear weight bias.

V-drives are somewhat less maneuverable than a straight inboard at speed, because they can't "pivot around the engine". They share an inboard's low-speed handling characteristics.

V-drives are about as roomy as I/Os, in general.

Vdrives have the "clean" transom of an inboard, and thus are able to have the same sort of full-width, deep swim platform mounted just above the waterline (although not all V-drives take advantage of this). Access to the platform is like an I/O, however, because of the motor box.

Jet: Jet boats are typically fairly flat-bottomed (not counting the new mini jet boats), but have a rear weight bias. Therefore, they get on plane quickly, have a high top speed, and low wakes.

Because jets squirt water out of a transom-mounted pump, skiers are generally more comfortable if they stay farther away from the boat. 100-foot ski ropes are typically used with jet boats to keep the skier out of the spray and wash.

Because they have no appendages in the water (no rudder, no fins), jet boats have poor tracking against skier pulls. Their maneuverability can be excellent, given an experienced driver who knows how to make use of the special abilities of a jet drive. However, they cannot be steered without using the throttle.

Jets are typically slightly less roomy than an I/O, because the engine must be mounted farther forward to make room for the jet pump.

Jets typically use a split swim platform similar to those of outboards.

Strengths and Weaknesses not directly related to skiing:

Inboard: Inboard ski boats, as mentioned, typically have relatively flat-bottomed hulls. This can result in a rather firm ride, although the newer designs are better in this regard. They are also typically moderately low-profile boats, which makes them less seaworthy than designs with higher freeboard.

If an inboard runs aground, it will almost certainly sustain damage that will require out-of-the-water repairs (e.g., bent prop, bent shaft, bent strut). However, inboard ski boats typically have the lowest draft of the drive types (compared to outboards and stern drives in the "down" position), so running aground is somewhat less likely.

The inboard drive train is relatively simple and reliable. Compared to an outboard, it doesn't have a power trim/tilt system, bevel gears, cooling water running through the "transmission", and has a simpler steering system. Compared to a stern drive, it lacks all those things plus universal joints and another set of bevel gears, a more complicated power trim/tilt system, and rubber bellows through which the drive shaft and exhaust are routed. The engine is easily accessible and is familiar to most driveway mechanics.

Inboards have notoriously poor low speed handling. With practice, one gets used to it and can handle most situations, but docking situations will always arise where an outboard or stern drive could easily get in or out and an inboard can do so only with difficulty, if at all.

The prop of an inboard is well under the boat, which it makes it safer for skiers boarding or exiting the boat than an outboard or I/O (but you should always turn the engine off during boarding or exiting anyway).

If an inboard runs over a ski rope or other line in the water, you have to either pull the boat out of the water, or dive under the boat, to disentangle it.

Inboards can be fairly noisy (although it's a low "V-8 rumble" that I find fairly pleasing).

Outboard: Outboards, having been completely designed for marine use, are generally more resistant to corrosion than the engines of inboards and stern drives. The downside is that all the parts for an outboard are specific to marine use, so they tend to be more expensive.

The outboard sticks out a fair distance from the stern of the boat, restricting the length of boat that can fit in a given space (e.g., your garage). Because of the greater interior room of an outboard, a shorter boat may be quite adequate (and lighter to tow, too).

An outboard will typically be lighter than an inboard or I/O of comparable performance. Thus, a smaller, lighter tow vehicle may be used.

If an outboard runs aground, the engine may tilt up, protecting itself from damage. This doesn't always happen, though. The prop typically has a rubber hub, which allows the shaft to become decoupled from the prop if the prop strikes something, thus preventing damage to the motor. And finally, if one knows one is operating in shallow water, one can tilt the motor up ahead of time and thus prevent grounding (but beware of accelerating in this configuration, as the angle of thrust will tend to push the stern down).

Overall, outboards are more complicated than inboards, but less complicated than I/Os. An outboard has a power trim/tilt system, bevel gears, and cooling water running through the "transmission", none of which the inboard has, and it has a more complicated steering system than an inboard. The 2-stroke powerplant is unfamiliar to most home mechanics, but is actually simpler than the 4-stroke powerplant in inboards and I/Os.

Outboards, being two-strokes, never need oil changes. Conversely, they do need oil mixed with their fuel, either manually or via an automatic oil injection system. Most modern outboards ("modern" excludes the Force line) have oil injection, but there are those who distrust it and disconnect the automatic systems, preferring to mix it themselves.

Outboards put the entire engine outside the boat, where it can be easily reached for servicing while the boat is on the trailer, but is more difficult to reach while the boat is in the water.

Despite advances in oil injection technology, outboards still smoke at idle.

Outboards have a relatively high-pitched, rattly sound that some find annoying. However, the absolute level of the noise is typically lower than that of an inboard, and lower than that of many I/Os.

Outboard motors are quite expensive.

If an outboard runs over the ski rope or other line in the water, you can tilt the motor up and disentangle the line fairly easily.

The outboard places an exposed propeller at the back of the boat, right where skiers are boarding or exiting. This is a safety hazard. You should always shut off the engine when a skier is boarding or exiting.

Inboard/Outboard: I/Os offer the familiarity and somewhat lower engine parts prices of an automotive-based engine combined with the maneuverability of an outboard.

I/Os are the most complex drive system available. Compared to an inboard, the I/O has a power trim/tilt system, two sets of bevel gears, cooling water running through the "transmission", and more complicated steering system, universal joints, and rubber bellows through which the drive shaft and exhaust are routed, none of which the inboard has. It has the complication, and also the familiarity, of a 4-stroke automotive powerplant, but the engine is usually much less accessible than either an inboard or outboard engine.

I/Os are generally quiet, since most use through-prop exhaust. They have the familiar, low-pitched automotive engine sound.

I/Os have a very clean look. They don't have an outboard powerhead sticking up to spoil the lines of the boat and get in the way.

If an I/O runs aground, the outdrive may tilt up, protecting itself from damage. This doesn't always happen, though. The prop typically has a rubber hub, which allows the shaft to become decoupled from the prop if the prop strikes something, thus preventing damage to the motor. And finally, if one knows one is operating in shallow water, one can tilt the outdrive up ahead of time and thus prevent grounding (but beware of accelerating in this configuration, as the angle of thrust will tend to push the stern down).

Like an outboard, the outdrive sticks out some distance from the stern of the boat, which must be accounted for when calculating required parking space.

If an I/O runs over the ski rope or other line in the water, you can tilt the outdrive up and disentangle the line fairly easily.

The I/O places an exposed propeller at the back of the boat, right where skiers are boarding or exiting. This is a safety hazard. You should always shut off the engine when a skier is boarding or exiting.

V-drive: V-drive hulls may or may not be deeper V hulls than typical inboards, so their ride may be firm like an inboard, or soft like an I/O.

The V-drive has the same vulnerability to grounding as an inboard, and usually also has deeper draft (because of the engine being aft, right over the prop).

Mechanically, the V-drive adds only the V-drive gearbox to the drivetrain of a straight inboard, so the comments about the simplicity of an inboard drivetrain apply. Engine accessibility is similar to an I/O.

All other comments on inboards apply.

Jet: Most jet boats have fairly flat bottoms, so the ride is usually quite firm.

Running aground is virtually a non-issue with a jet, which is why they are preferred for running in shallow water (I don't know about you, but I don't want to ski in water that shallow!). They have very low draft, and no appendages extending below the hull. Sucking up sand can damage the jet pump or the cooling system, however.

If a jet runs over a ski rope, it is possible for the rope to be sucked into the jet pump, from where it is quite difficult to remove (have you ever seen 75 feet of ski rope being pulled out of a jet pump in 6-inch pieces? I have).

Jet pumps are fairly inefficient at low speeds (and become "jammed" with water at very high speeds). Thus, a jet will typically use more fuel than an I/O or inboard.

Because there is no exposed propeller, a jet boat is very safe for swimmers (but I'd still shut off the engine during boarding or exiting, because somebody could slip and bump the shift lever into gear, causing the boat to run over the skier).

Jets are typically at least as noisy as inboards (because through-prop exhaust isn't an option), and are often noisier, because over-the-transom exhausts are often used.

Summary of most important characteristics. Of course, this assumes boats of the sort used for waterskiing. Obviously, an inboard cruiser doesn't have "poor" seaworthiness:

                Inboard      I/O        Outboard    Jet        VDrive
Seaworthiness   poor         good       good        poor       good
Handling - low  poor         good       good        poor       good
Handling - high excellent    good       good        good       good
Tracking        excellent    poor       poor        poor       good
Wakes           low          med-high   low-fair    low        fair-high
Skier access    excellent    fair-good  fair        fair-good  excellent
Interior space  fair         good       excellent   good       good
Economy         fair-good    good       poor        poor-good  fair-good
Reliability     very good    fair-good  good        good       very good
Noise           med          quiet-med  quiet-med   med-loud   med
Bill Walker - - QUALCOMM, Inc., San Diego, CA USA