How to choose an outboard engine
Theory and practice

How to choose an outboard engine

Two or four tact? What kind of power should it be? Which screw is better? Answers to the most frequently asked questions about outboard engines
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For most boats an outboard engine is the only possible propulsion system, but its purchase raises a number of questions. The first thing you think about when buying an outboard engine is whether it is better to choose: two or four stroke? Boating magazine has asked the most popular questions to experts and manufacturers on this topic, and here are the answers.

Is four tact always better than two?

Is a Ford always better than a Chevy? Is paper better than plastic?

If you believe everything you hear, it's obvious that a four-stroke engine is better.

Dig deeper, and you'll realize that it's not that simple.

A four-stroke engine is similar to the one under the hood of your car: clean fuel is burned in the cylinders and oil is circulated in a separate system to lubricate the cylinder bushing. In this way, if everything is in order, fuel and oil are not mixed. The two-stroke engines burn the fuel and oil mixture.

Traditional two-stroke engines supply the fuel-oil mixture through a carburetor or injector into a cylinder. As part of the injection cycle, the exhaust valve is also open and up to 30% of the fuel goes with the exhaust gases.

Two-stroke engines with direct (direct) fuel injection feed the engine into the cylinder at a certain period of time when the piston overlaps the exhaust valve. In this way, fuel losses are minimised.

In principle, four-stroke engines cannot do this because fuel injection and exhaust emissions occur at different times thanks to four cycles.

The four-stroke and two-stroke engines equipped with Direct Fuel Injection (DFI) provide significant fuel savings compared to traditional two-stroke engines, also because they are computer-controlled and theoretically burn all fuel that is injected into the combustion chamber.

Evinrude two-stroke engines of E-TEC series carry out fuel injection twice as fast as conventional engines with SNWT, and also regulate the amount of fuel-oil mixture supplied depending on the required power. When we tested the E-TEC series engines, the noise and smooth running characteristics were at the same level as the four-stroke engines and still produced more power than the conventional two-stroke engines.

On the other hand, the four-stroke engines have now moved away from the limitations of what was once thought to be more powerful two-stroke engines. Previously, the weight of a four-stroke motor increased in proportion to its power growth, which forced manufacturers to limit the maximum power of four-stroke engines to 100 hp. Now you can find four-stroke engines with power over 300 hp on the market.

«The most common myth about two-strokes is that they are noisy, have a strong smell of exhaust and are not very economical," says Eric Pope of Mercury. To date, this is not the case.

The choice between two-stroke and four-stroke engines now depends more on the owner's preferences than on the technical differences in the engines. Both options at the moment are complete solutions without flaws.

So is it still a two-stroke or a four-stroke?

- Two-stroke engines with SNFT are lighter than four-stroke engines with equal power; -
Four-stroke engines are quieter than two-stroke engines; -
Two-stroke engine produces more power at start


Taking these differences into account, it is necessary to make a correction for the fact that the difference between the engines is rapidly reduced.

One or two motors on board?

Suppose the maximum engine power for your boat is 300 hp. Which is better - one engine at 300 hp or two at 150 hp?

In most cases, on offshore fishing boats you will see two or even three motors in a row. It is safe to say that duplicating the main engine is necessary to ensure safety in case one fails somewhere in the middle of the sea. Also, two motors make it easier to moor, because you can moor each separately in different directions.

However, such reinsurance will cost you a round sum.

It is not always justified to do so. Placing two motors on board also means duplicate control panels and more complex equipment installation.

«In fact, you won't see any major advantages," says Suzuki representative Rick Hauser. - Three hundred horses is always three hundred horses, whether it's on one engine or two.

It all depends on your finances. If you don't care much about the price of the question, then you can install two motors. One motor would be a simpler solution.

So should we duplicate the engines?
- Duplication of the engines does indeed ensure safety when one of them fails. But only if they are actually autonomous, including the power supply and fuel supply.
- Two engines will only give more power if their total power is greater than that of each engine separately.
- The alternative is to have a single-shaft unit and a satellite phone in the locker room in case of emergency. And, of course, it is a good idea to have a spare low-power engine on board.

The more octane gasoline, the better?

When high-compression engines appeared, the increase in cylinder pressure caused the fuel to ignite too early. This premature ignition (called detonation) was accompanied by an annoying sound, but it was not just that. Premature detonation had a detrimental effect on the engine.

In this case, impurities were added to the fuel that slowed the detonation so that it would occur when the piston was ready to absorb the energy of the explosion, thus negating the damage to the engine and effectively using the highest degree of compression. Thus, the higher the octane number, the stronger the fuel can be compressed.

The high-octane fuel usually ranges between 92-93 octane and, despite all its advantages, is a waste of money for most outboard engines. They are mainly designed to work with fuel, the octane number of which is 87 at best. Some manufacturers, such as Tohatsu, recommend fuel with octane number 89. You can find recommendations for your engine in the manufacturer's instructions. If you buy the fuel specified by your engine manufacturer, you will probably start to save some money, especially if you have bought high octane fuel before.

Facts about fuel

- only high-performance motors require high-octane fuel;

- high octane fuel, which is not designed for this, does not give additional power, but only increases the fuel bill;

- most engines are designed for average octane fuel with an octane number of 89.

Is it worth putting the maximum permissible engine power on the boat?

A few years ago, when buying a new boat, many people were skeptical about the ready-made versions equipped with weak motors. The sellers, before entering into the contract, persuaded to buy another more powerful engine Those who agreed, they carried on their hands, and those who stayed with the stock power unit were usually disappointed in their choice.

The situation is different now. On the market, there are usually several options for one boat with different engine power. This is the case when the higher the power, the better.

The scariest thing you can do is to cut the engine power. Then you'll never know what your boat is capable of.

You may not have enough speed to get away from an impending storm. If you make the right choice, on the contrary, you will not lack the admiring looks sent after your boat.

So don't be afraid to put the most powerful motor on the label of your boat - it's safe to do so. If in doubt, talk to the manufacturer about the chosen boat and the results of its running-in with motors of different power.

Make your choice by listening to what you want.

- Ride on a boat that has a smaller engine than the maximum allowable. When running on some boats, especially pontoon catamarans, we made sure that the smaller engines fit more.

- in the open sea and in harsh conditions, you will need all the power you can hang on your transom.

- Have you ever heard anyone complain that they bought an engine that's too powerful?

What about the screw?

Motor manufacturers have probably already equipped the motor with a suitable averaging screw. If not, the salesman will do it. But you'll be very lucky if you end up happy. After all, the screw must be selected, given how you will load the motor and for what purpose to use the boat.

A little bit about screws. The main characteristics of a screw are its diameter and pitch. If the diameter is clear from school, what is a propeller pitch? This is the theoretical distance that a screw will travel in one revolution if it rotates in a solid body without slipping.

Let's take our eyes off the screws for a minute. Let's imagine that the manufacturer indicates the power (HP) at a certain value of rpm by setting the limit revolutions. This value will only be valid in one mode, and a suitable screw will be matched to it. If the screw is lightweight (i.e. underloading the engine) and allows more speed to be gained than allowed, you will get excessive wear and overheating of the engine. If the screw is heavy (the engine does not reach its rated speed), you will lose your way and risk damaging the engine as well.

Therefore, to protect your motor and yourself, take care of its operation by selecting the correct screw pitch.

You can check the screw by walking on a boat with the throttle fully open. If the engine is gaining more speed than the manufacturer's specification, the flap must be closed until the speed drops to the required level.

While you're on shore, check the propeller pitch. Note that with each increase in the screw pitch, turns will decrease by 200 units. Using a concave propeller will also reduce turns by 200 units. Sometimes sellers can provide a screw for the duration of the check, provided that you return it intact immediately after use.

When the screw is picked up and the engine is running within its rated limits, note that the lower the rpm, the higher the tractive force, and the higher the maximum speed.

Briefly about the propellers:

- the condition of a propeller is just as important as its type. Damaged screw can destroy the motor due to unbalance.

- Stainless steel screws last longer and are five times stronger than aluminum screws. However, the latter are cheaper and can serve as a weak link to prevent the entire submarine part from being destroyed (e.g., an aluminium screw can be cut off while a stainless steel screw would tear off the shaft).

- The four-blade screws have an advantage over the three-blade screws in hydroplaning mode, at medium and low speeds.

Isn't it true that outboard engines need no maintenance?

To maintain modern outboard engines, you don't need much, but this little must be perfectly executed. You can easily burn the engine if you forget about the cooling system. The fuel system can become clogged if you ignore possible fuel quality problems and do not carry out checks. If you do not notice the seals on your boot, it can easily be torn in winter due to the presence of water inside. Failure to replace the anode protection in time will result in corrosion on the parts. Damaged screw may loosen bearings and other moving parts.

The maintenance part is rarely fully read in the operating instructions. But this does not mean that maintenance is irrelevant.

Service Precepts

- fully fill the fuel tank to minimize condensation when the engine is not in use;

- add a fuel stabiliser every time you add fuel - this is especially important when using ethanol fuel;

- check the screw regularly. Any damage to the screw, be it kinks or chips, will cause vibrations on the shaft, which can lead to serious consequences.

- check the fittings and the hoses. The water should circulate freely in the cooling system to prevent the engine from overheating.

The original article was printed August 15, 2017 in Boating magazine.

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