Marine Engines

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Marine Engines

Diesel Engines 

Diesel engines are used in mid-sized cruisers, trawlers, large yachts, workboats and commercial vessels. In the diesel engine, only air is introduced into the cylinder head. The air is then compressed to about 600 pounds per square inch (psi), compared to about 200 psi in the gasoline engine. This high compression heats the air to about 1000 degrees Fahrenheit. At this moment, fuel is injected directly into the compressed air. The fuel is ignited by the heat, causing a rapid expansion of gasses that drive the piston downward, supplying power to the crankshaft.

Advantages of the diesel engine are numerous. It burns considerably less fuel than a gasoline engine doing the same work. It has no ignition system to attend to. It can deliver much more of its rated horsepower on a continuous basis than can a gasoline engine.

The life of a diesel engine is generally longer than a gasoline engine. Diesel fuel will not explode. However, it will burn!

Some disadvantages to diesel engines are that they're very heavy for the horsepower they produce, and their initial cost is much higher than a comparable gasoline engine.

Whatever type of engine you choose to operate, always follow the manufacturer's recommendations and procedures, as those folks know their product better than anyone else does. Also, always use marine rated replacement parts. Using automotive parts can be very dangerous!


The Diesel Engine

In 1892 Rudolf Diesel developed and obtained the German patent for the diesel engine. His goal was to create an engine that was highly efficient. Much more efficient than the gasoline engine that was invented in 1876 and was not very efficient at all. Especially at that point in time.

 There are two main differences between a diesel engine and a gasoline engine.A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously. A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.  Gasoline engines use either a carburetor or a fuel injection system to deliver the fuel to the cylinder. With a carburetor the fuel is mixed as it enters the intake manifold, long before it gets to the cylinders. In a fuel injection system the fuel is injected just before the intake stroke at the intake valve Diesel engines use direct fuel injection (DI), that is to say the diesel fuel is injected directly into the cylinder. The diesel engine has no spark plugs. The air it takes in is compressed and the fuel is injected directly into the cylinder where the heat caused by the air compression ignites the fuel. In the old days this meant that it exploded and expanded very quickly, making a noisy engine. This is why most diesel cars were IDI (indirect injection); the rough behavior was fixed by injecting the fuel into a small pre-combustion chamber that is connected to the cylinder by a narrow passage.   This slows down the explosion, as the gasses have to escape from through the narrow passage into the cylinder. This gives a softer bang and a smoother engine, but the gasses have to work harder, which lowers the efficiency a little. However the newer breed of DI engines use other techniques to tame the behavior of the engine, such as two stage injection, electronic control, and acoustic shrouds and shock absorbing engine mounts to mask the rattle. The injector on a diesel engine is its most complex component and has been the subject of a great deal of development and innovation. On any specific engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process. One major difference between a gas engine and a diesel engine is in the injection process. Most car engines use port injection or a carburetor rather than direct injection. In a car engine all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine. If it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated. In the United States and Canada, diesel engines are most commonly found in trucks and buses. In Europe, where fuel prices fluctuate around $3.00 to $4.00 per gallon, the fuel efficiency of the diesel has made it a popular choice for cars, and most European-market cars are available with a diesel or a turbo-diesel engine. Compared to gasoline-powered vehicles, diesels are more fuel efficient, and they can travel significantly farther on a tank of fuel than their gasoline counterparts. Diesel engines produce more torque, and they tend to be more durable. They don't need an electric ignition system, which reduces their complexity. However, they also create more noise, they can be difficult to start in extremely cold weather and they sometimes require more frequent routine maintenance than gasoline engines. Most passenger car diesel engines have a glow plug of some type. When a diesel engine is cold, the air compression may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electric heater that glows red-hot and helps to ignite the fuel when the engine is cold so that the engine can start

Gasoline Engines - Four-Cycle

Unless you plan to propel your boat with a steam engine or turbine, your engine will probably be some type of internal combustion device. Either gasoline or diesel fuel power these engines, and the engines themselves are of either a two- or four-cycle design.4-cycle gasoline engines power everything from ski boats to mid-sized family cruisers through drive systems such as inboard/outboards, Vee drives, straight drives, and so on.In the gasoline engine, a mixture of gasoline and air is introduced into the cylinder head (or heads). The mixture is then compressed to about 200 psi (pounds per square inch) and ignited by a spark plug. The resulting heat expansion drives the piston downward, supplying power to a rotating crankshaft. SOME DIFFERENCES BETWEEN AUTO & MARINE ENGINES 

For those of us in the marine industry, a common occurrence is to be questioned by customers about auto vs. marine parts.  A common scenario is to have a customer ask for a replacement part for his boat.  The part in question is a starter for a 350 GM block.  You give the customer a price, and he looks at you with a blank look on his face.  He, then states (very angrily) "I can get one at the local auto retailer for $29.95".


In the marine industry, this little scenario occurs all the time.  Many boaters are unaware of the difference between an automotive and a marine engine and their respective accessories.  The most notable differences include the exhaust systems, the cooling systems, the electrical systems, and the fuel systems. Additionally, items such as heads and cams are usually different.   For the purpose of this article, we will only be hitting the highlights on select systems.


In regard to the cooling systems, one of the major differences is found in the water-circulating pump.  This is especially noticable when you have a raw water cooled engine.  Unlike their automotive counterpart, a marine pump works in an open cooling system.  This type of system is extremely corrosive to the pump.  Therefore, the pump must be altered for longevity.  A marine pump has a special ceramic seal, stainless steel backing plate, and a bronze impeller to resist corrosion.  An automotive style pump, with its stamped steel impeller, would fail due to corrosion in a short time.


The electrical systems in a marine application are also extremely specialized.  In an automobile, any gasoline vapors that accumulate will readily dissipate through the bottom of the engine compartment.  However, a boat with its sealed engine compartment, does not have that luxury.  Therefore, any spark could literally cause a boat to explode.  All electrical components in a boat are either completely sealed or specially vented to prevent such a catastrophe.  This includes the starter, alternator, distributor, and many other engine electrical components.


Carburetors typically have what is known in the industry as J type fuel bowls.  These allow for the extreme vibration, pitching and yawl experienced on boats.  This allows fuel to be drawn in exteme conditions and keeps fuel from being ejected from the carburetor barrels.


Heads and cams are set up for a completly different torque curve.  A marine engine must develop most of its torque on the low end of throttle range.  This allows for the ability to use a single gear transmition system.  Think of it this way:

You get in your car

Start it up

Drive to the highway

Put your foot to the floor and hold it there for an hour

Its a little different, well for most of us.


Although we have only briefly touched on some of the differences between an automotive and marine application; hopefully, it will serve to remind us that there is a difference, and that the reasons for these differences need to be shared with our customers and friends in the boating community.  Hopefully, when shared, this information can eliminate some of the "blank stares" in regards to parts, and maybe prevent some dangerious situations.


Most gasoline inboard engines are a version of a truck or heavy duty industrial engine. Remember, your car gets to coast going downhill, but your boat engine is pushing uphill all the time. Also keep in mind that a boat engine is driving the hull through a heavy fluid, not just air.

Some advantages of 4-cycle gasoline engines are:
  1. They produce more horsepower per pound than their diesel counterparts,
  2. with the new ignition systems, they are very reliable, and
  3. they are much less expensive in initial cost.
Disadvantages are that they burn more fuel than a diesel engine and will not last as long. And let's not forget the inherently dangerous nature of gasoline! Although gasoline has a lower heat producing ability than diesel fuel (124,000 BTUs per gallon for gasoline, versus 138,000 BTUs for diesel fuel), it has a lower flashpoint. In other words, gasoline will explode when exposed to an igniter. Because gasoline fumes are heavier than air, they can sink to the lowest part of your boat (the bilge) and stay there until something sets them off. Forget to ventilate and Ka-Boom! Voyage terminated!

The Life Expectancy of the Marine Engine 

The average marine gasoline engine runs for 1,500 hours before needing a major overhaul. The average marine diesel engine will run for more than three times that long and log an average 5,000 hours under the same conditions. The number of hours that a marine engine runs is very dependent on the amount and quality of maintenance over the years.

The typical gasoline marine engine will run fine for the first 1,000 hours. It is at this juncture that the engine starts to exhibit small problems. If these small problems aren't addressed, they can turn into major problems which may make the last 500 hours of life difficult to reach.

Interestingly, an automobile engine may run almost twice as long (3,000 hours) as your marine gasoline engine. The reason is that marine engines normally work harder and under worse conditions than automobile engines.

A well-maintained gasoline engine run under the best conditions may well run for more than the 1,500 hours without major overhaul. However, many that operate under the most atrocious conditions of salt air, damp bilges, intermittent operation and pure neglect will certainly die early.

Diesel engines are built to finer tolerances than are gasoline engines. They will accept much more abuse and often deliver, if well maintained, 8,000 hours of hard work before need a major overhaul. Theoretically, a well-maintained diesel may last the life of your boat. Since the average recreational boater logs only about 200 hours per year, the 8,000 hour diesel would last 40 years.

Although diesels can add considerable cost to a boat, they should be seriously considered because of their durability, economy of operation and safety concerns. Diesel fuel has a much higher flash point than gasoline and does not present the same threat of explosion that gasoline fumes carry.

Engines like to run long and steady. The shorter the running time between stops, and the longer the idle time between runs, the fewer the hours they will deliver before needing major repairs.

The adverse conditions under which marine engines operate have a great deal to do with their longevity. What they really need is rarely what they get. Naval architects recommend that engine compartments should be supplied with lots of dry, cool (50 degrees F), clean air. The very minimum fresh air vent area (in square inches) for natural ventilation without blowers is found by dividing engine horsepower by 3.3.

Two of the most important rules of thumb for engine compartment blowers on gasoline engines are that they should always be set to exhaust, not to blow air in, and they should be run for a minimum of 5 minutes before starting the engine.

Two indicators that can alert you to potential trouble are the color of exhaust smoke and changes in the appearance of your oil when you check it.

Exhaust gases from marine engines should be clear. Any color of smoke can warn you of potential trouble.

Black smoke is the result of engine overload, a restricted air supply, or a malfunctioning fuel injector in the case of a diesel engine. Improperly burned particles of excess fuel are blown out the exhaust.

Blue smoke is formed by combustion of the engine's own lubricating oil. This can be the result of worn piston rings, valve guides, or oil seals. The oil can come from an overfilled air filter in the case of a diesel engine or excess oil in the crankcase.

White smoke indicates either water vapor from dirty fuel, a water leak into the cylinder or atomized, but completely unburned, fuel. Air in the fuel can also cause white smoke.

You can not check the level and condition of your oil in your engine too often. You should check it at least once a day and preferably before every start. It is also a good idea to wipe the dip stick clean with your bare fingers and feel the consistency of the oil. Use the paper towel to wipe your fingers. You should rub the oil on the stick lightly between your thumb and index finger and feel for any foreign particles which could indicate contamination or metal parts failures.

Weekend boaters checking the oil before starting should be suspicious of oil levels that are too high or too low.

Too high a level might be a clue that water has found its way into the oil sump. You could crack the cylinder head, break a piston, or both, just by turning the engine over. The oil with water in it will also look "milky".

Too low a level could indicate an oil leak that could lead to engine seizure. Look in the bilge to see if there is any oil residue. Many marine engines sit very low in the bilge and water is consistently in contact with the oil pan. Over the years this can corrode and cause pinhole leaks in the pan.

Whenever there is a large deviation from normal, take that as an urgent warning. Start looking for more clues or seek the advice of an expert.

Marine Engines and Automotive Replacement Parts

Is this a good idea?

Most powerboaters know that their gasoline inboard or I/O engine is basically a GM, Ford or Chrysler block that has been marinized. Many engine parts are, indeed, interchangeable between boats and automobiles. There is one big exception, however. The engine in a boat is located in a closed area, where the engine compartment in a car is completely open at the bottom. Because of this, a boat has a problem with gasoline fumes that a car does not.


Gasoline fumes are heavier than air and, as a result, they sink downward. In a car they vent out the bottom of the engine compartment. In a boat, though, the fumes are trapped in the bottom of the hull (bilge). Because these fumes are highly explosive, all marine electrical components - starters, solenoids, alternators, etc. are sealed and cannot emit a spark or an arc into the engine compartment. The same components, for automobile use, are not sealed because the gasoline fumes have vented out the bottom of the engine compartment. So, will an automobile starter or alternator work on a boat engine? Yes it will - until an open spark from it ignites the fumes in the bilge!


The carburetor or throttle body on your car has an air filter on the top. On your boat, that same carburetor or throttle body must have a device called a flame arrestor on the top. A flame arrestor is a series of baffles that keeps an open flame caused by a backfire from entering your engine compartment. In the United States, this device must be U.S. Coast Guard approved and kept free of dirt and oily deposits.So when the time comes, and you have a choice between a marine replacement part and an inexpensive automotive part, bear in mind that there is a major safety factor to consider. If you are already using automobile replacement starters or alternators, you are really pressing your luck! If a law enforcement agency discovers you are using automobile alternators or starters, or you do not have an approved flame arrestor installed, they will terminate your voyage right then and there.


Remember, use the correct parts and always run your blower for four or five minutes before starting your engine.


Engine Maintenance

An internal combustion engine needs four basic ingredients to run efficiently and last a long time. They are: clean Oil (lubrication), clean Fuel, clean Air and a Cooling system to remove excess heat.Oil: Let's start with the oil. You should check your oil level with the dipstick on a regular basis, say every time you get fuel. If, however, you are on a trip, you should check it at least once a day. You must change both the oil and the filter at regular intervals recommended by the engine manufacturer. If that information is not available, then every 100-engine hours is the next best thing. Engine hours, on most boats, are recorded on the tachometer. Good oil does not wear out, but it does get dirty, so use the right oil for your engine and change it often.Fuel: There are two things that can mess up the fuel supply on a boat: Water and Dirt. Invest in a good filter/water separator and make sure that you always carry a spare replacement cartridge. On a diesel engine, problems are almost always fuel related. You can't have a filter that's too big or have too many of them. If you store your fuel for an extended period of time (3 months or more), you should consider adding some type of fuel stabilizer to it. Air: Your engine requires lots of air. Check your flame arrester located on top of the throttle body or carburetor, if you have a gasoline engine, for oil grease or dirt. Remove the flame arrestor if it is dirty. Clean it with a non-explosive cleaning solution. DO NOT run your gasoline engine without the flame arrestor! On a diesel engine, check your air intake filter periodically.Cooling: Engines use the water that they are sitting in to remove excess heat created by the burning of fuel during operation. You should find out what type of cooling system your engine uses (Direct or Closed). However, no matter which system you have, both use the water around them for cooling. Each engine has a water pump that pulls water from the lake, river or sea and sends it through the cooling system. Knowing where the pump is located and how to install a new impeller is a necessity. The pump is usually installed on the outside of the engine and is belt driven. Carrying a spare impeller can save much time and money when you really need it.While your engine is doing its job by pushing you through the water, check your engine monitoring gages regularly and learn what they read when everything is normal. If your oil pressure starts to fluctuate or go down, SHUT your engine down, check the oil level and look for signs of oil where it doesn't belong. If your temperature gauge starts to climb, STOP your engine Immediately and look for the problem, most probably a broken hose or the water pump. DO NOT try to coax an engine, that has little or no oil pressure, or one that is overheated, back to the dock or the launch ramp. If you do, you can plan on probably buying a new engine! Listen for a change in the sound of your engine. This is generally a sign of engine trouble.
The days when you could tune-up your own engine are mostly gone. Today's engines with their black box computers require a trained technician, with his own computer, to fix them.My experience shows that when boats that break down they are just getting even for the neglect that they've received. Take care of your boat and it will return the favor.Fresh Water Cooling – Questions and Answers

Q: What is fresh water cooling?

A: It is the marine version of the cooling system that you have in your car.

Most marine engines start out as a non-marine engine and were designed to have a clean non-corrosive antifreeze coolant circulating between the engine and a "radiator". In the marine version of this cooling system (fresh water cooling), the radiator is replaced with a "liquid to liquid" heat exchanger. Sea water, instead of air, passes through the heat exchanger and absorbs the heat from the engine coolant and is then discharged overboard.

Many marine engines, for cost reasons, utilize the initially less expensive, but in the long run much more expensive, raw water cooling system. In this case polluted corrosive sea water, pumped directly into the engine, eats at the very base of a marine engine causing irreparable damage.

Q: What benefits will I get from fresh water cooling?

A: The important advantages: Longer engine life; Simplified winterizing; Easy galley water & cabin heat.

Longer engine life due to:

No Corrosion - Most marine engines "rust out" rather than "wear out". By replacing corrosive raw water with antifreeze solution, you will prevent corrosion damage.

No Scale - Complete elimination of scale build up inside the water passages of the engine. The salts that exist in sea water, as well as "hard" fresh water, create scale that will restrict proper coolant flow and heat transfer.

Proper engine temperature - For best engine life and performance your car operates at a temperature much higher than a raw water cooled marine engine. By converting to FWC you can bring the engine temperature up to proper level using a higher temperature thermostat. This, is turn, will give you improved fuel economy, less engine wear, no crankcase condensation, no oil dilution, and no sludge.

Simplified winterizing - By keeping an antifreeze solution all year round, winterizing is simplified.

Easy galley water & cabin heat - Galley water heaters and cabin heaters utilizing excess engine heat are easily added to the system.

Q: Are there different types of fresh water cooling?

A: There are two basic systems.

Block and manifold, full system. 

In these systems not only the engine block, but the exhaust manifolds as well, are included in the antifreeze system. The exhaust elbows, where the raw water enters the exhaust system, always remain on the raw water side. Full systems may not be possible on all engines due to lack of proper water connections on the exhaust manifolds and a lack of raw water pump capacity. This is often a problem with sterndrive engines. These problems can sometimes be solved but will get expensive. Typically, full systems are more expensive in terms of hardware and, especially, installation.

Block only, half system.

In "block only" systems, the most expensive part of the system, the engine block itself is on the antifreeze system. Exhaust manifolds remain on the raw water side. Half systems are less expensive to buy and much easier to install. When replacement exhaust manifolds are available at a reasonable cost, this half system is often the most cost effective. Carefully make an overall cost comparison between full and half systems before making a decision.

Q: Should I have FWC if I keep my boat in fresh water?

A: Yes, to simplify winterizing, eliminate all corrosion and debris damage.

Although lake and river water is not as harmful as sea water, it often contains scale forming salts, pollutants, mud, sand and other marine organisms which often can be quite damaging.

Q: How long will my raw water cooled engine last?

A: Impossible to say.

Water conditions and, therefore, corrosion rates vary from one location to the next. The main problem id that since the damage is internal and completely hidden, there is no accurate way to measure the damage and predict when the engine is going to fail.

Q: How long will my engine last with FWC?

A: It will wear out in a normal fashion.

Most marine engines "rust out" rather than "wear out". With an average of no more than 50 hours annually on pleasure boats the wearing out for an engine that is FWC and well maintained should be measured in decades rather than years.

Q: My engine is several years old. Does it make sense to convert to FWC?

A: Definitely.

Ideally a conversion to FWC should be made when the engine is new. That way you can prevent any damage to your engine. But by installing FWC on an older engine, you will prevent further damage and you will enjoy all the other benefits from FWC.

Q: I thought that FWC was primarily for larger boats?

A: It may have been, but should not be.

Factory fresh water cooled engines are usually installed in larger, more expensive boats. The additional cost is fairly low in proportion to the total cost of a larger boat. However, in smaller boats the engine represents a far larger proportion of the boats total value. The owner should, therefore, be more concerned with the condition and value of his engine, or he will pay for it in the long run in the form of premature re-powering or lack of resale value.

Q: Some boat dealers tell me that FWC is not necessary.

A: It depends on what you mean by necessary.

FWC is not necessary in the sense that your engine will stop functioning tomorrow if you don’t have it. Nor is changing your oil and filter and many of the other things that you do in order that your engine will give you maximum life and performance. The benefits of fresh water cooling are more of the long term type. In order to keep the cost of a new boat to an acceptable level, some dealers hesitate to recommend options that are not absolutely necessary when the boat is purchased.

Q: If fresh water cooling is any good, why isn’t it standard from the engine manufacturer?

A: It is simply a matter of cost.

The boat building business is very cost competitive. Most boat builders do not want to spend any more money than is absolutely necessary on the power package and, therefore, specify the less expensive raw water cooled engine. Improvements such as FWC are usually left to the owner’s option.

Q: I will probably sell or trade my boat before I have any problems due to lack of FWC, why should I convert?

A: The trade-in value of a boat with FWC is much higher.

A FWC system on your marine engine is the best way to defuse the time-bomb that a raw water cooled engine represents to a potential buyer. A marine engine is not like the tires on a car, where it is possible to judge how much life is left. Corrosion damage is completely hidden and impossible to measure. A FWC system shows that there are no unpleasant surprises waiting. It also shows that the previous owner was knowledgeable and probably in every other respect took good care of the engine.

Q: How can I get galley water heat and/or cabin heat through FWC?

A: It is a simple addition.

Your marine engine can provide this heat in the same way as your car engine provides car heat. A full range of galley water heaters and cabin air heaters are available. Installation is an easy do it yourself plumbing job.

Q: Will FWC affect my basic engine warranty?

A: There is no reason why it should.

Drive SystemsInboard and Vee Drives

An inboard engine is generally heavier and less compact than an outboard engine of comparable horsepower (HP). Its size and power are not limited by a need to have everything in one unit. The engine is placed near the center of the hull, very close to the lowest point, creating a low center of gravity. This is a very good thing! On the back of the engine is a transmission (gear box), which is attached to a propeller shaft. The shaft passes through the bottom of the boat and is supported by one or more struts, located on the bottom of the hull. Attached to the end of the shaft is the propeller. A rudder, placed just aft of the propeller, controls the direction of the boat.

Some advantages of an inboard are:

The engine is positioned to give the boat better balance fore and aft, and create a low center of gravity.

Although open to some debate, the straight inboard system is generally considered the simplest and most efficient method of transferring torque (power) from the engine to the propeller.

Some of the disadvantages are the inboard system takes up a lot of space inside the hull and, because the propeller is in a fixed position, the inboard is not as maneuverable at a low speed or while in reverse.

The Vee drive is a variation of the inboard system. This system is sometimes called an Angle drive. In this configuration, the engine is installed over the propeller and rudder, with the drive shaft facing forward instead of aft. The shaft then enters another gearbox that changes its direction by 180º, sending the propeller shaft back through the hull to the propeller under the engine.

The main advantage of the Vee drive is that it places everything in the stern of the boat, leaving much more interior space for cabins, etc.

A main disadvantage is that the entire weight of the engine and propulsion gear is located at the very stern of the boat (that old balance thing again). Another drawback is that, by passing through a change of direction gearbox, you are losing some efficiency and increasing your chances of a problem with the drive system.

 Inboard / Outboard and Jet DrivesI/O (Inboard/Outboard) drive. As the name implies, this drive system is, indeed, a combination of the straight inboard and outboard systems. The engine is located inboard, in the stern of the boat. Power from the engine is sent through the transom into an outboard drive unit, which is very similar to the intermediate housing and lower unit on an outboard engine. The steering system on the boat turns the entire outboard unit, changing the direction of thrust from the propeller, just as it does on a true outboard.Some advantages of the stern drive are:
  • You have the both the size and low center of gravity of the inboard, along with the maneuverability of the outboard.
  • You have more usable interior volume than the straight inboard.
On the other hand, there are some major disadvantages to the stern drive system. Power from the engine thrust must pass through two 90° gears to reach the propeller, causing a considerable loss of efficiency. Also, because they are inherently more complicated than the straight inboard system, I/O drives are more prone to failure. The jet drive is a powerful, engine-driven water pump that sucks in great quantities of water through the bottom of the boat and shoots it out through a nozzle at the stern. Operating on the principle of physics that, "for every action there is an equal and opposite reaction", the boat is propelled forward. Steering the boat is done by changing the direction of the discharge nozzle.A few of the advantages in using a jet drive system are:
  • There is a complete absence of all underwater appendages. There is no rudder, no propeller, no strut, and no shaft - no nothing!
  • Jet boats are safer around people in the water because of the lack of all underwater appendages, especially the propeller.
  • Because the system is so simple, there is less likelihood of failure.

Among the disadvantages of jet drives are that, even though great strides have been made in improving jet pumps, they are still not as efficient as other systems. Also, debris in the water (weeds, trash, etc.) can be sucked into the pump and put you out of the powerboat business for some time.

OutboardsIf you've read about hull types and engine types, then you probably have some idea about the kind of boat and engine you'd like to have. We will now explore the various methods of transferring the power generated by the engine to the water.First, you need four basic things to make your powerboat move:
  • A power source (your engine).
  • A propeller or pump to provide push.
  • A connecting device of some kind, to connect the engine to the propeller/pump.
  • Devices to control the amount and direction of the thrust.
Outboard motors range all the way from lightweight 2 horsepower (HP) portable models, to 350 HP monsters bolted directly to the transom (stern of the boat) or to a bracket that is then bolted to the stern.The outboard is basically a self-contained unit, consisting of a power head (the engine), the intermediate housing (the drive shaft) and the lower unit (gear, housing and propeller). The throttle, gearshift and steering are attached to the power head. To change the direction of a boat using an outboard motor, the entire unit - power head, intermediate housing and lower unit - swivels, thus changing the direction of the propeller thrust. Some advantages of the outboard motor are that they're very lightweight for the HP that they produce, and because they are located "outboard", they take up little or none of the boat's interior volume. Conversely, a disadvantage is that there is generally a cut-out in the transom for the outboard, leaving the vessel more vulnerable to water breaking over the stern and coming into the boat. Also, by the nature of where it's located, the majority of the engine's weight rides outside of the boat itself. This tends to adversely affect the trim of the boat, causing it to ride down at the stern.Outboard motors range all the way from lightweight 2 horsepower (HP) portable models, to 350 HP monsters bolted directly to the transom (stern of the boat) or to a bracket that is then bolted to the stern.