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Technical Recommendations for Choosing Engines for a Boats

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To make the right choice of engines for boats, you need to consider many factors, including the size and type of boat. Select a motor size and power that matches your boat size and intended use. Consider speed, fuel efficiency and handling requirements. The type of fuel is important. Choose gasoline, diesel or electric fuel based on cost, availability and environmental requirements. Estimate maintenance requirements and costs for different engine types.

Consider the initial cost, fuel consumption and long-term maintenance costs. Choose engines from reputable brands known for their reliability and customer support. Seek advice from boat dealers, mechanics and experienced boaters for recommendations based on your specific needs and preferences.

If you buy a new production boat it will probably have engines installed, but you can order a boat with your choice of engines. This assumes the fore and aft location is the same for which the hull was designed. We will briefly discuss each engine type and installation so you can make an intelligent choice for your particular mode of operation. First, let’s define what we want the engine to accomplish.

Any engine converts the chemical energy of the fuel into mechanical energy to turn the propeller shaft. The measurement of the turning force on the shaft is called torque, and is defined in foot-pounds. Mathematically, torque equals engine horsepower multiplied by 5 252 and divided by the rpm (revolutions per minute) of the shaft. (T = 5 252 HP/RPM) We can see torque increases with higher horsepower and decreases with higher rpm. Most boats use a reduction gear in the same case as the reverse gear in order to reduce the engine rpm to an acceptable level, which increases the torque.

The propeller converts the torque of the shaft to thrust in order to move the boat. The larger the diameter of the propeller (more blade area in contact with the water), the more thrust it provides, consistent with maintaining the maximum engine rpm. One might reason that more torque is available with low shaft rpm and that more thrust is available with a large propeller diameter so this is what all boats should use. As with all design engineering, there are limits to what one can install. A large diameter propeller on a Key Points for Buying and Selling a Boatrecreational boat would increase the draft to an unacceptable depth and the increased frictional drag would limit the speed on fast hulls. All good design is a compromise of objectives.

The engine only develops the maximum rated horsepower at the maximum rpm and thus the propeller must not be so large, both in diameter and pitch, that the maximum rated rpm is not attainable. In order to get a large amount of thrust in displacement speed (slow speed) boats, a large diameter propeller is desirable, and a slower shaft rpm is used. Ships may have a propeller rpm less than 100 and tug boats may have a propeller rotating at 300 rpm. Cruising powerboats may have a propeller rpm of 1 200 and fast powerboats might have a shaft rotation of 2 000 rpm. Each type of hull, engine, and method of operation has to be analyzed before selecting the correct reduction gear.

Gasoline Engines

Engines burning gasoline are found in many types of installations; outboard motors, inboard engines driving outdrives and conventional inboard engines. Gasoline engines are basically lighter in weight and less expensive than diesel engines, but they do use more fuel per horsepower. Table below shows the approximate fuel consumption for both gasoline and diesel engines. Fuel consumption can be estimated by dividing the horsepower by thirteen for gasoline engines and dividing by eighteen for diesel engines.


Generally, gas engines attain their horsepower at a higher rpm than diesel engines and thus are subject to more wear per operating hour. Automobile gas engines do very well over many years as they are usually run at half the maximum rpm with much time spent accelerating and decelerating between moderate speed and idle speed. The time spent on an interstate highway at high rpm is small when compared with the total operating hours. Therefore, the car gasoline engine operates in a far different manner than the same engine in a boat.

The engine in a boat is generally run at constant speed except when leaving or entering the dock. This constant speed is especially apparent when cruising or heading some distance offshore for fishing. Maintaining this relatively constant rpm is far different from the car operation and there is less wear on the internal parts if the required horsepower can be achieved at a lower rpm.

All engines should have fresh water in the cooling passages to prevent corrosion by saltwater. A heat exchanger is used with saltwater to cool the fresh water jacket. Of course, in fresh water there is no corrosion problem. The Evolution of Boats with Outboard MotorsOutboard motors, and the lower unit of an inboard/outboard installation are usually made of cast aluminum that has been treated to resist saltwater corrosion. When at the dock in saltwater, outboard motors and inboard/outboard installations should be tilted up out of the water. Flushing with fresh water is always recommended.

Outboard motors are desirable as the installation costs are reduced and replacement is much easier. Of all the boats sold in the USA, about one-third have outboard motors. With the engines exposed outside the hull, they are easy to work on, but are also easily stolen. Chains that secure the engine to the hull may prevent unauthorized removal. In many comparisons, outboard motors may be much less expensive to purchase than an equal horsepower inboard gasoline or diesel engine.

Gasoline inboard engines are sometimes adaptations of car engines and some are available to about 400 hp. These top performers are usually limited to light weight racing craft where maximum horsepower and rpm are used only for a short period of time. Higher horsepower is available and more than eight cylinders may be found, but at a very high price.

Diesel Engines

Rudolph Diesel, developed and patented the engine in 1892. It has grown to be the most accepted type of engine in the world. Diesel engines for ships are built to almost any size and are rated at a certain horsepower per cylinder. Cylinders are then added to produce the desired power. Small diesel engines are used frequently, but fewer than four cylinders result in a great deal of shake, rattle, and roll, as seen in small cars or gensets with diesel engines. You can find an engine for most any application.

Diesel engines are widely used in Europe for both cars and boats as high taxes make diesel fuel more affordable. Most commercial boats use diesel engines for economy, safety, and durability. Diesel fuel is less flammable than gasoline and thus, safer. All engines require good maintenance, and diesels are no exception. With good care, diesel engines usually last for a large number of hours. Keep in mind, a recreational boat may only be used on weekends, for a total of about 250 hours each year, but a commercial craft may see 2 000 hours per year.

Diesel engines are more expensive and are heavier than gasoline engines when compared with equal horsepower, but cruising boats shouldn’t be concerned about a heavy hull. The cost of engines varies widely and may not include installation. Used engines are readily available and again the cost varies widely with the condition and whether the engine has been overhauled by a factory trained mechanic (see table below).

50$ 9 000$ 3 000-4 000
100$ 13 000$ 4 000-6 000
150$ 17 000$ 5 000-9 000
200$ 21 000$ 7 000-11 000
300$ 29 000$ 10 000-17 000
400$ 37 000$ 15 000-23 000
500$ 46 000$ 20 000-30 000
Approximate cost of one marine diesel engine in US Dollars in 1999.

Number of Engines & Location in the Boat

Whether you have one or two engines is a matter of preference. Many owners feel two engines provide a factor of safety in case one fails, especially when going well off shore. In inland waters, you can usually find a tow from a sympathetic owner who hopes you will tow him if the situation is ever reversed. Some single engine boats also carry a small outboard motor for use if the primary engine fails. Single engine boats are harder to maneuver around the docks and the skipper who docks his single engine vessel surely shows everyone his excellent boat handling skills. A single engine reduces cost, weight, and maintenance.

Many cargo ships and most Use of Fiberglass in Boat Constructionsailboats have only one engine. Ships do this for reasons of economy, sailboats to reduce drag while under sail. Large powerboats that move at high speeds have two engines and the high costs associated with high speeds. The table below shows approximate boat speeds.

3 00022314453626975
4 00019273846536066
5 00017243441485358
6 00015223138444953
7 00013202835404549
10 00012172429343841
14 00010152025293235
18 0009131822252831
22 0008,211,51620232628
26 0007,610,81518212326
30 0007101417192224

If you like to be economical and have good boathandling skills, a single engine may be the proper choice.

Approximate boat speed in knots for selected boat weights and horsepower. Weight is in pounds and the advertised horsepower is used.

Outboard motors and inboard/outdrive installations are necessarily at the stem of the boat, which keeps the remainder open for other activities. They also provide the desirable feature of having the fuel tanks toward the middle of the boat. This puts them closer to the center of buoyancy where there is little change of fore-and-aft trim from full to empty tanks. Having any tank at the stem is poor practice. When underway at half throttle, all powerboats should have only a small trim angle, with the bow about three degrees higher than the stem.

Customary Inboard Engine Installations

Some inboard installations have the engine at the stem with the use of a V-drive. This has the engine reversed with the drive shaft at the forward end so the gears reverse the shaft direction and the tail shaft goes through the bottom of the hull in a normal manner. This type of installation is more expensive and it is used in larger custom powerboats and in some sailboats so the interior accommodations can be arranged to fit the owner’s requirements. See Figure 1.

Boat rig design
Fig. 1 Straight and V-drive installations

Actually, there are new products on the market that allow the inboard engine to be placed at any location in the boat. These are similar to the inboard/outdrive, which may be pictured as a «Z» drive. The lower drive unit with the propeller goes through the hull bottom at any desired location, and the engine is directly coupled to the top of this drive. This type of installation has been used in sailboats for many years.

In many cases, the engineering of the type and location of engines is determined by the necessary interior arrangement.

Inboard engine installations try to keep the shaft angle less than twelve degrees to the horizontal, in order to maintain propeller efficiency. This requirement puts the engine well forward of the stem and near the middle of the boat. In order to keep a watertight seal, there is a stuffing box on the shaft where it passes through the hull. Underwater, a bronze strut and rubber bearing support the shaft and propeller. The rudder is just aft of the propeller. In twin Self-Survey Criteria for the Engine and Electrical Systemsengine boats the rudders are usually set two inches off the shaft centerline so the shaft can be removed without removing the rudder. On single engine boats there may be a hole in the forward portion of the rudder for shaft removal.

Propellers normally have three blades on average powerboats, but hulls over forty-feet may use four blades for greater efficiency. Sailboats use two-bladed propellers to reduce drag while sailing, but may change to three blades when they face a long passage made under power. Your local propeller shop can assist with correct sizes for your engine and reduction gear, but it is generally true a large diameter is more efficient, if other factors are considered. This diameter selection must allow a space between the propeller top and the hull (tip clearance) of at least twenty percent of the propeller diameter. For example, a 30-inch diameter propeller should have six inches between the tip of the propeller and the hull.

Other Propulsion Methods Surface Drives

Manufacturers offer a surface drive system where the engine may be located in any part of the hull. The drive shaft penetrates the transom (at the stem) and not through the bottom of the hull. Aft of the transom, the shaft and propeller are positioned with two hydraulic cylinders so the top of the propeller is just below the surface of the water. The special propeller is designed for high-speed operation and approximately twenty percent greater speeds are possible with the same horsepower. Steering is accomplished by moving the outboard shaft left and right with the hydraulic cylinder. A rudder is not used.

The surface drive system has higher efficiency as there is lower frictional drag from the lack of any appendages below the hull. Since the shaft, strut, and rudder are not projecting from the bottom of the hull, the water flow is smoother and is not interrupted by this hardware. This system does produce a large propeller wake at high speeds and does not offer any slow speed advantages except unusually shallow draft. This system was invented and patented by Howard Ameson and has proven to be very popular for high-speed, shallow draft boats.

Water Jet Drive

Another system that provides very shallow draft is the water jet. This uses an opening in the bottom of the hull to take water into a duct that encloses a specially designed propeller. This duct bends aft so it exits the hull at the transom just above the waterline. The propeller moves the water aft at high velocity and the reaction moves the boat ahead at high speed. To reverse, a door is hinged down over the exit opening and the water flow is deflected forward. This reaction moves the boat aft. Steering is accomplished by hydraulic cylinders moving a deflector plate in the water jet stream.

Read also: Future of the Boat Development – Insight from 1970s

A conventional inboard engine, or a gas turbine, turns the propeller for fast operation without gear installed below the hull. All of this is achieved at high initial cost in both the purchase and installation of the water jet casing. These water jet drives have been used successfully in all types and lengths of fast boats for many years. They gained popularity in the USA after being used in the US Navy thirty foot patrol boats during the Vietnam War. The small jet ski boats (personal watercraft) use water jet drives to avoid the possibility of a person being injured on a propeller. If a boat is not frequently used, you may get barnacles and marine growth in saltwater on the water intake duct surface, which may be hard to remove. For shallow draft operation, this is a popular system.

Summary of Engine Selection

If you want a very economical boat, it may be suitable to have a single outboard motor in a boat under thirty feet and a single inboard engine in a larger boat. It is also practical to select an engine your local mechanic is accustomed to servicing. Don’t buy an engine that will require parts from another part of the world. If you can do minor maintenance on the engine, it is always necessary to have water pump impellers, oil filters, fuel filters, and tools on board.

Figure 2 shows the equipment in the engineroom of a cruising boat.

Engine room diagram
Fig. 2 Typical engineroom plan. A/C is air conditioning compressor and RJO is reverse osmosis watermaker

Smaller boats, and those intended for just an afternoon ride will not have the luxury of hot water, an AC generator, air conditioning and a watermaker. Each owner, and their budget, must determine what is necessary and what is optional for the intended area and type of operation. If the boat will be used for a large number of hours, it may be a good investment to buy a diesel engine so fuel costs may be reduced. This is the reason most commercial boats have diesel engines.

Whatever the engine and type of installation, it is always desirable to have a keel on the bottom of the hull a few inches lower than the bottom of the propeller. The keel will protect the propeller when in shallow water, and will allow the boat to follow a straighter course, especially in rough water and when using an autopilot. In addition, the propeller shaft strut on two-engine installations should be in the shape of a Y so the lower arm provides some protection for the propeller.

Author photo - Olga Nesvetailova
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  15. Coles, Adlard. Heavy Weather Sailing, 3rd rev. ed. Clinton Corners, N.Y.: John De Graff, Inc., 1981.
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