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What boat builders need to know about marine gas engines. Why are they different from engines in cars?

I was participating in a forum on-line recently and someone asked, “why can’t I simply drop an auto engine into my boat, and use that?” In response I put together the following which I called Marine Engines 101.

1. Marine engines are generally based on commonly used automotive blocks. Up until the late 80's it could have been a Chrysler, Ford, or GM block. Newer engines are almost all based on GM blocks. A good example is the GM 8.1 Liter engine that is in my motorhome, in a lot of pickup trucks, and a lot of other vehicles. But that is basically where the resemblance (other than outward appearance) ends.

2. Marine engines usually run at much higher continuous duty loads than automotive engines. Auto engines run up to speed, then the transmission shifts and the RPM drops to save fuel and because all that power is not needed to keep the car moving at the same speed on level ground. A car cruising on a level highway at 60 mph is running at about 2500 rpm (rpm depends on the gear ratio of the differential), much lower than it's max torque, and using about 20% of it's rated hp. Cruising speed on most motor boats is around ¾ throttle. A marine engine at 3/4 throttle is running anywhere from 3500-4000 rpm or more (depends on the engine, propeller size and pitch, and the reduction gear in the transmission) and near it's max hp. It is generally running at or near max torque. So the loads placed on marine engines are much higher and they must be built to handle heavier loads.

3. The cam shaft in a marine engine is designed for heavier loads. Whereas in a car the cam is selected for general use over a wide range of rpm, the cam in marine engine is selected for max rpm and torque.

4. Because of the heavier continuous torque loads, rods, pistons, etc are stronger.

5. Because they run under heavier load they need more cooling water. So the water passages are larger in a marine engine and a more powerful water pump is needed.

6. Valves are generally heavier duty like in trucks.

7. The head is usually heavier with larger water passages.

8. A higher capacity water pump is required.

9. A higher capacity oil pump is required.

10. Carburetor: The carburetor on a car and on a boat may look the same but internally they are not. If the float sticks in the bowl and fuel flows out the vent on a car it goes out of the carburetor. On a marine carburetor it goes down the throat. Marine carburetors are not allowed to spill any fuel into the boat. Plus that, marine carburetors have different jets and setup because of the reasons above in #2. Carburetors also have to be fitted with a flame arrestor. The flame arrestor stops any flame from a backfire from exiting the air intake and igniting any fumes in the engine compartment. There are flame arrestors available that both prevent back fires from igniting fumes, but also act as a filter. But most only act as a flame arrestor (also called a backfire flame arrestor).

11. Marine engines use electrical wiring and equipment specifically designed to operate in a moist environment and prevent sparks. Starters and alternators are sealed, or have flame screens to prevent sparks from igniting any vapors that may be in the engine compartment. So, even the spark plug wires and caps are different.

12. Cooling can be raw water or closed cooling. Raw water is picked up by a through hull routed through the engine and then dumped into the exhaust generally where the riser exits to the exhaust line. Closed cooling uses a radiator much like an auto.

13. Some marine engines use a dry exhaust. This is not seen much anymore but is becoming more popular on large houseboats due to carbon monoxide problems. However, most use water cooled exhaust.

14. Most marine engines have an exhaust manifold that has a riser on it that lifts the exhaust up through essentially a p-trap arrangement. This is to keep water out of the engine. When decelerating, water will back flow through a marine exhaust system. The riser keeps it out of the engine. The height of the riser depends on the engine and where the water line is (How far below the waterline is your exhaust?).

15. Fuel pumps on a marine engine must be designed to not leak fuel into the boat. Many are double diaphragm. Today most engines have electric fuel pumps. The pump must be mounted on the engine or within 12 inches of the engine. This minimizes the amount of fuel line that is under pressure. It also means that unlike newer cars the line from the tank to the engine is under negative pressure. In other words, the fuel is sucked to the pump rather than pushed to the pump. That way if there is a leak the engine just starves for fuel and stops. The fuel pump must be fire resistant.

16. Fuel lines from the pump to the carburetor must be metal or USCG Type A marine fuel hose. This hose is fire resistant. The hose from the tank to the pump can be USCG Type A or B. Type B is not fire resistant and is hard to find.

17. Fuel filters can not leak into the boat either and must be fire resistant.

18. Fuel tanks must also be fire resistant and pass a 3 PSI pressure test. Today most fuel tanks on gasoline powered boats are aluminum or plastic (Polyethylene). Both must pass all the requirements of the Coast Guard. Other materials are not prohibited but need to be built to specific standards. The tanks must not be integral with the hull.

10. The fill and vent hose can be USCG type B hose but again it's hard to find. You can use Type A. Also on cars fuel tanks and the whole fuel system are closed and under slight pressure. On boats this is unsafe. If a leak developed anywhere in the system it would literally empty the entire contents of the fuel tank into the bilge of the boat. So boat fuel systems are vented to the atmosphere to keep them at atmospheric pressure.

Since January 2011 the rules have changed somewhat due to environmental regulations.  Fuel systems are now closed but not allowed to exceed a maximum pressure of 1 psi.  This is achieved with a 1 psi pressure relief valve built into the vent line.  The system also has a carbon canister to collect and scrub vapors.  See Gasoline Fuel Systems and Fuel Tanks

NOTE: GM engines use some of the same parts on both the auto and marine versions. For instance, truck cams are very much the same as cams on boats. However, if it is a fuel injected engine there may be a problem. Most fuel injected auto engines have a pump in the fuel tank that then pressurizes the fuel lines. You don't want pressurized lines running through your boat, so you can't have the pump in the tank. On boats the fuel pump is near (within 12 inches) or on the engine and fuel is sucked to the engine rather than pushed. So how do you pressurize the fuel line so the engine will start quickly without having to crank it for several minutes? A time delay on the pump is allowed. By this I mean that on most marine fuel injected engines, when you turn on the ignition the electric pump comes on and pressurizes the short fuel line from the pump to the fuel injection system, it then shuts off until the engine starts, then the pump comes back on. Most marine fuel injected engines do not have a pressurized return line to the tank. They have a built in recirculating tank on the engine, and a gravity feed back to the tank.

More Thoughts on Marine Engines

Having said all that, there are exceptions. First, the engines described above are the typical engine found in most runabouts and cruisers. These are not the kind of engines you find in some types of boats. Take drag boats for instance. Racing engines are pretty much just like auto racing engines. If the engine is open to the atmosphere, not in an enclosed compartment, then some of the regulations don’t apply, such as ventilation, and flame arrestors. These engines are not intended to last longer than a race. Then they are stripped down and rebuilt. Some people build what are called hot boats or performance boats that are not intended for racing but look like they are. The engines are basically auto engines. They sit out in the open air, usually have a supercharger, ram air induction and open exhaust systems. You can see what I am talking about on the following web site at

How about engines on sailboats? Most sailboats with inboard engines use diesel, but there are gasoline engines available. The most famous is the Atomic 4 made by Universal. These engines are no longer in production but there are about 20,000 of them still in use. Universal designed and built these from the ground up. There are engine rebuilders and suppliers that specialize in the Atomic 4. As far as I know, no one in USA makes a new gasoline inboard engine for sailboats.

Many boats have auxiliary generators. Again, most are diesel, but there are some manufacturers that make gasoline powered generators for boats. There is a difference between a marine generator and a generator not intended for use on a boat. The differences are the same as for auto engines and marine engines. Do not use a non-marine generator on your boat. See

Another subject that seems to puzzle people not familiar with marine engines is engine rotation. I am not talking prop rotation here, but the actual direction that the engine turns. The reason for the confusion is, on older marine engines, when you go shopping for parts they ask you, what is the engine rotation? The direction it rotates determines the part you get.

Most marine engines are left hand rotation, also known as standard rotation. But, left hand from what view point? The usual way of looking at this is to use the flywheel to determine rotation. If you look at the engine from the end with the flywheel and the flywheel turns counterclockwise, or left at the top, it is a left hand rotation. If it turns right, or clockwise it is right hand, or reverse rotation.

But most people don’t look at the flywheel end of the engine. They see the end with all the pulleys and belts. So at that end you would reverse the above. If the engine appears to turn clockwise at the pulleys, it is left hand or standard rotation. If it appears to turn counterclockwise at the pulleys it is right hand or reverse rotation.

However, in this age of fuel injected engines, and the use of mostly truck blocks for marine engines, all of them are standard rotation. They are all left hand rotation.

At the prop it is another matter. If the boat has a single engine, then the prop usually turns the other way, to counter the torque of the engine. This is done with a transmission, or what in the marine world is called a reduction gear, that changes the rotation to right hand at the prop. If you have twin engines then counter-rotating props are used. That is, one prop turns to the right, the other turns to the left. Usually the starboard (right) prop is the right hand prop, and the port (left) prop is the left hand prop. This is done to improve maneuverability and make the boat go in a straight line. Occasionally a designer will reverse this, so check your boat to see which way they turn.

On sterndrive units all the engines are left hand, standard rotation engines. If you have twins the gears in the lower unit determine prop rotation.

Links to engine information.

Engine Rotation:

Start -In-Gear switches, Kill Switches, Neutral Safety Switches, and Shift Interrupter Switches Copyright 2006  Revised 11/15/2018