This blog is part of a series for new boaters. To read more from the full article, see Boat Buying for Absolute Beginners, Part III.
Powerboats can run on several types of engines: twin or single, gas or diesel, outboard, conventional inboard, pod drive, or stern drive. And different options work better with certain types of boats. So how do you decide which combination is best for you?
The fact is, not every engine and drive package in say, the MerCruiser line, is offered for every boat. Far from it. Naval architects and engineers use complex formulas to determine the correct engine options for a given boat.
So you don’t need calculus or a super computer to make good boat-buying decisions. The number crunching, at least in terms of engines offered for the boat that piques your interest, has been done for you before you walk into a dealer’s showroom. Your job is picking the best power options from those offered for a given boat that meet your needs.
Gas or Diesel?
You have two basic choices: gasoline or diesel. Diesels offer outstanding durability under frequent use. In fact, many mechanics say the primary reason for diesel engine problems is lack of use: “Diesels like to work.”
If you plan to use your boat often, spending the extra money on diesel power if it’s available might be wise. This is especially true if you buy a heavy boat that demands lots of muscle to push it through the water.
However, diesel power isn’t offered for the overwhelming majority of powerboats — those from 18 to 23 feet — sold each year, as diesel wouldn’t make sense for these boats.
In addition to costing less (at least initially), gasoline engines offer maximum flexibility in drive configuration. They generally offer superior bursts of speed and throttle response, and they don’t “mind” going unused for longer periods of time, as long as the fuel is treated with an ethanol-friendly stabilizer. Plus, gasoline is available just about everywhere, whereas sometimes diesel fuel can be hard to find.
Don’t be tempted to make a direct comparison of horsepower between gasoline and diesel engines. They are measured differently. The horsepower of a gasoline engine is rated at or near wide-open throttle, which may be at 4,500 rpm or more. The actual horsepower of any engine drops quickly as the rpm drops. Diesel engines are rated at continuous duty, typically in the 2,000-to 2,200-rpm range. This is why a diesel will outperform a gasoline engine when hauling heavy loads. Of course, the gas engine has the edge when it comes to quick bursts of speed.
Although not all propulsion systems are offered with either gas or diesel engines, powerboat buyers have a wide selection of options, including:
Outboard motor — An outboard engine is a self-contained package of engine, gears and propeller that attaches to the stern of the boat. Two-stroke outboards up to 300 hp are available; four-stroke outboards commonly up to 350 hp and in one case, 557 hp. Direct-injection two-stroke outboards are far more environmentally friendly than their carbureted ancestors and, in recent years, once-temperamental DI technology has become far more reliable. (See our DFI Tutorial for a more in-depth look at modern two-strokes).
Conventional inboard — Sometimes called “straight inboard,” this configuration has power transmitted from the engine through a shaft which exits the bottom of the boat. The propeller is attached to this shaft and rotates beneath the hull, usually directly in front of the rudder.
V-drive inboard — A V-drive allows the engine to be placed in the rear of the boat. Power goes forward to the V-drive. From there, a shaft leads out of the bottom in a manner similar to a conventional inboard boat. Using a V-drive allows boat designers to create more interior space than you’d find in a conventional inboard, which has a motorbox in the center of the cockpit.
Stern drive — Not so long ago, this drive package was called an “inboard/outboard” because this configuration combines an inboard engine with the lower unit of an outboard motor. This combination allows the use of higher horsepower engines. The lower unit can be tilted up for trailering or shallow-water operation.
Jet drive — In this system, water is sucked from beneath the boat and shot out of a nozzle in the stern to provide forward thrust. Jet drives can operate in shallow water and have no exposed propeller. Personal watercraft (PWC) are jet driven, as are a number of compact runabouts (read Jet Boat Renaissance to learn more about their new-found popularity).
- Pod Drives — Pod drives consist of a contained drive unit which is located beneath the hull, directly below the engine. The drive unit articulates independently, eliminating the need for rudders, shafts, and other running gear. This type of drive is rapidly replacing conventional inboards on large boats, because it's much easier to handle around the docks and can deliver efficiency gains of up to 30-percent.
Stern drives come as a full package that includes the engine, gimbal housing and lower unit. The boat builder simply cuts an opening in the back of the boat and installs this package. Combining inboard engines with the outboard lower unit has its advantages:
A stern-drive boat is easier for beginners to maneuver than a conventional inboard craft.
Adjusting the angle of the lower unit allows the bow to be trimmed up or down when the boat is moving.
Shallow-water operation is possible at slow speeds by tilting the lower unit upward until the propeller begins to break the surface.
The lower unit can be tilted up and out of harm’s way when the boat is being trailered over the highway.
In a stern drive, power from the engine has to go through a universal joint and then make two right angle bends to reach the propeller. All that twisting and turning add complexity, and complexity means more parts to break or wear out. Even so, the stern drive has proven itself to be popular.
Inboards and V-drives
Conventional inboard and V-drive transmissions are mounted inside the hull. They may be attached directly to the engine via a large fitting called a “bell housing,” or they may be remotely located and connected to the engine by a short length of driveshaft called a “jack shaft.” Forward, neutral and reverse are usually accomplished by gears, which work in conjunction with an oil-filled clutch.
The driveshaft of a conventional inboard boat goes through the hull in a fitting called the “shaft log.” A “stuffing box” attached to this fitting allows free rotation of the shaft but keeps water from leaking into the boat.
Inboards and V-drives are most commonly offered for tournament-style water-ski and wakeboard boats. Center-mounted inboards contribute to ski-boat balance and especially flat wakes. In the V-drive configuration, the aft placement of the motor helps create larger wakes for boarders.
Other installments in this series: