GUIDE: Which outboard motor to choose for a boat?

GUIDE: Which outboard motor to choose for a boat?

Today almost all boats are equipped with engines. From the small tender, to the zodiac through sailboats of all sizes. But each boat needs a different mode of propulsion. Indeed, the same engine will not be installed on a fishing boat and a large pleasure yacht... This is why there are dozens of different boat engines: gasoline outboard engines, electric boat engines with short shaft or boat engines from 6hp to 250hp! The question that therefore arises is: which engine is best suited to my boat? There are so many different choices and criteria that you can quickly get lost. To make it easier for you, we have written this article that will guide you in choosing the best engine for your boat. Follow our instructions to find out which outboard motor to choose for a boat.

ATTENTION: This article describes in detail all the parameters to take into account to choose the best outboard motor for your boat. You can also consult our synthetic articles answering the same question but with more targeted answers for different boat lengths:

The criteria to know when choosing a boat engine.

There are many options to consider before buying a boat engine. You will have to choose between:

And once you know the answers to all these questions, you will also have to decide:

In the rest of this article, we explain to you point by point which criteria to choose in order to install the most suitable outboard motor for your boat.

Which type of engine is best suited for your boat?

The gasoline outboard motor is the most common on boats up to 7m. This motor is favored over inboard motors or pod drives which are often reserved for larger boats.

 In addition, the gasoline outboard motor adapts to any What kind of boat : whether it is an open hull, a speedboat, a semi-rigid inflatable boat, a sailboat tender or a fishing boat.

Finally, its versatility makes it the ideal choice for all types of navigation. It will be suitable for outings at sea as well as on freshwater areas such as lakes or rivers. It will only be enough to adapt the power of the engine to the currents of your navigation area.

Be aware, however, that there are two exceptions. Indeed, some boat owners prefer to install a stern drive or an electric boat motor. (we explain everything to you in the next chapter)

Finally, if your boat is over 7m long, then we recommend installing an inboard diesel engine. As the boat is no longer transportable beyond 7m, the advantages of the outboard motor disappear. Suddenly, inboard diesel engines become the most economical, the safest and above all they have better seakeeping.

Under what circumstances is a sterndrive engine recommended for a boat?

Boats under 7m in length that do not choose an outboard motor install almost all a semi-outboard engine. We do not recommend this choice because the advantages of the outboard are always greater than the gains generated by the stern drive. The main strength of sterndrives comes from their propeller which is perfectly aligned with the boat. This is due to the combination of an inboard motor and a gear case like on an outboard.

Why choose an electric motor on a boat?

Before buying a gasoline outboard motor for your boat, find out about the legislation in force in your boating area. Indeed, some areas such as natural parks are protected and it is forbidden to use gasoline or diesel boat engines. Other reserves prohibit heat engines for more or less long periods that coincide with the breeding cycles of the animals so as not to disturb the fauna.

Under these circumstances, the choice of an electric boat motor is worth considering. However, you should know that electric outboard motors are not recommended for boats over 4m. They are even generally used for boats of less than 3 meters...

Indeed, despite all their advantages (silent, vibration-free, environmentally friendly, light, etc.), electric outboard motors today are still not powerful enough to meet the needs of a boat over 4m long. .

In addition, electric boat motors are much more expensive and have very limited battery life, which limits boating time.

It should therefore be noted that a electric motor on a boat should not be used if:

  • your boat is over 3-4m long
  • you are looking for speed,
  • you navigate in areas with strong currents,
  • you have to travel long distance.

In summary, if your navigation area does not prohibit internal combustion engines, choose a gasoline outboard motor for your boat instead!

Which outboard motor power to choose for a boat?

A gasoline outboard motor can have a power between 0,5 HP and 450 horsepower! There are 6hp boat motors, 15hp, 25hp, 30hp, 50hp, 60hp, 75hp, 100hp, 150hp, 250hp, etc. Because of this wide choice, many boaters do not choose the power of their outboard motor well. Indeed, there are two categories of buyers. Either the power of the outboard motor is chosen according to the budget and the boat is often underpowered. Either the power of the outboard motor is chosen to go as fast as possible and the boat is often overpowered.

Both situations are not ideal. Underpowered, the boat will lack speed and you risk becoming disenchanted quickly. Especially if you want to do water activities like wakeboarding, tubing or water skiing. In addition, you put yourself in danger because without the appropriate power, you lose handling and seakeeping – especially in rough seas. Finally, even if it's counter-intuitive, the more power you lack, the more fuel you consume. Thinking about it, that makes sense! You are asking a small engine to move a boat that is heavier than expected. Inevitably, it will need more energy and therefore more fuel!

But it is better to be underpowered than overpowered! Indeed, overpowering is much more dangerous. It is extremely inadvisable to mount an outboard motor more powerful than the maximum power recommended by the manufacturer of your boat. Why ? Because you will exceed the maximum safe speed which will cause physical stress on the boat. Since the structure of your boat is not designed for such speeds, it risks being damaged and, in the worst case, breaking. Usually it's the boat transom that supports your outboard motor that breaks under pressure and vibration. All you have won is a burst of speed and a boat engine in the water.

It is therefore necessary to choose a boat engine with a power adapted to its boat. The advantages are many:

  • Safety: with the right power, you can get out of the wake of a cargo ship, maneuver easily at low speed, navigate easily in rough seas, etc.
  • Comfort: A boat motor with the correct power will be quieter than a small outboard motor that is always on full throttle or a large motor.
  • Fuel: we consume more with a boat engine that is too powerful or not powerful enough.
  • Wear: an underpowered boat engine that is always used at full speed wears out faster. And an overpowered outboard motor wears out the structure of the boat (it also gets dirty if it is underused). Whereas with the right power, you reduce maintenance costs on your boat.
  • Speed: once hull speed is reached, the boat will never go any faster. So beyond the adapted power, it is very likely that your boat will maintain the same top speed.

So how to determine the ideal power of an outboard motor for a boat ?

First of all, you should know that the length of the boat is not the only parameter to take into account. You will not choose the same engine power if you are sailing in very strong currents or on a lake. The currents in your sailing area are therefore an essential element. The stronger the current, the more power the outboard motor will need to be.

Finally, you also have to consider the weight! Indeed, a satisfactory outboard motor on an empty boat can prove to be underpowered once the guests are on board, the coolers with the refreshments on board and the nautical equipment attached – such as a towable buoy. To determine the power of an outboard motor for your boat, it is therefore necessary to calculate the maximum weight envisaged on the boat. Add the weight of the boat with the maximum number of people on board as well as the fuel and the equipment on board.

In conclusion, the theory would like to take into account the length, the maximum weight of the boat and the currents. But in practice the currents change according to the day and the hour. The weight varies enormously depending on the guests and the activities. In short, it is impossible to calculate.  

The best (and only) rule to consider is to choose a outboard motor power equal to or slightly less than the maximum power prescribed by the manufacturer of your boat. Thus, you will be able to face all situations without ever risking damaging the physical integrity of your boat.

The only exception being if you are always maximum two on board or if you use your boat exclusively on a river or a lake. So instead of equaling the maximum power, you can take a boat engine with the power recommended by the manufacturer (and save money on the purchase!).

NB: Be aware that a boat license may be required for outboard motors over 6VC (or 4,5 KW for electric outboard motors).

What electric outboard motor power to choose for a boat?

As we saw in the previous paragraph, you may have no choice but install an electric outboard motor on your boat . You will therefore need to invest in an electric motor with a suitable configuration in order to have suitable power and autonomy. So you have to ask yourself how far am I going to travel? On an electric boat motor, it is the capacity of the battery that influences your autonomy and your engine power. On thermal boat engines we speak of horsepower (HP), but for electric boat engines we measure the power in kilowatt/hour (kWh) or in pounds (lbs).

The formula to remember to find the right electric outboard motor power is:

Electric motor power = Power required to propel the boat to its maximum weight

What is the maximum weight of an outboard motor for a boat?

It is imperative to read the manual of your boat to know the maximum weight recommended by the manufacturer on the transom. Some outboard motors are heavier than others for the same horsepower. This is the case with 4-stroke boat engines. You can therefore change your two-stroke outboard motor to a 4-stroke outboard motor of the same horsepower and end up with damage to the transom due to the weight of the new motor. It is therefore necessary to be very careful with the weight of boat engines, especially for the oldest boats.

NB: Often the outboard motor weight becomes the first criterion in the choice of power. (the more powerful a boat engine, the bigger it is and therefore the heavier it is)

2-stroke direct injection engine or 4-stroke outboard motor for a boat?

Older two-stroke boat engines didn't consume all of the fuel-air mixture – which affected their performance. This is why most boat builders have turned to the 4-stroke boat engine. But the two major flaws of the 4-stroke outboard engine are its complex mechanics and greater weight. Finally, although 4-stroke engines are much more efficient and reliable, they are also more expensive and less powerful than traditional 2-stroke engines.

Fortunately, today there are modern 2-stroke engines known as direct injection (DFI). This DFI technology allows two-stroke boat engines to no longer waste gasoline that has not exploded. As a result, these 'new' two-stroke engines for boats only have advantages.

They are the simplest, lightest, cheapest, most powerful, least polluting, most fuel efficient and best performing outboard motors on the market today! We therefore recommend without hesitation a direct injection 2-stroke gasoline outboard motor for a boat up to 7m.

NB:

  • There are other types of injection such as carburetor engines or electronic fuel injection (EFI) engines, but today DFI technology coupled with a 2-stroke boat engine is the best.
  • Engine technologies do not all consume the same amounts of gasoline or diesel. If your priority is the budget, we have classified them from the technology which consumes the most to the one which consumes the least: direct injection turbo-diesel, turbo-diesel, diesel, modern direct injection 2-stroke engines (designed from 2010), lean burn or lean burn 4-stroke engines, classic 4-stroke engines (before 2010), classic two-stroke engines (before 2010).

Electric or manual start outboard motor for a boat?

Boat engines equipped with a recoil starter with a small integrated tank are reserved for small boats. For a boat over 4m, we recommend an electric start with a charger for the starter battery. 

What shaft length should I choose for a boat's outboard motor?

Not all boats are alike so they won't need the same shaft length. The shaft length of the outboard motor is the distance between the anti-cavitation plate (the metal plate horizontally above the propeller) and the motor mount (the lowest point of the yoke of fixation). It must therefore be chosen according to the distance between the engine mount and the surface of the water (or according to the height of the transom).

This guide explains which outboard motor to choose for a boat: discover all the criteria for selecting the best boat motor.

Depending on the architecture of your boat, you will need more or less shaft length so that the propeller is submerged to the right depth. If the propeller is too low, it may bottom out. If the boat's propeller is too high, it will suck in air and lose performance. The choice between short shaft or long shaft is therefore essential.

NB: all outboard motors exist with short shafts and long shafts. Thus, they can adapt to all boat designs.

So how do you know which shaft length to choose for your boat's outboard motor ? Just measure the distance between the motor mount and the water surface. Then refer to this table:

Motor mount/water surface distanceRecommended shaft length
From 0 to 25 cm76 cm
From 25 to 40 cm92 cm
From 40 to 55 cm107 cm
Over 55 cmConsult a reseller
Shaft length of a boat motor

If the outboard motor manufacturer does not give the shaft length in centimeters, then simply measure the height of the transom:

  • Short shaft boat motor: 38 cm transom height.
  • Long shaft boat motor: 51cm transom height.

Once outboard motor installed, you can check whether its shaft length is suitable. To do this, just check if the anti-cavitation plate is aligned with the lowest point of the transom.

The ideal is to be perfectly aligned but a margin of error of 25mm maximum below is acceptable.

Generally you will not have to encounter any problems because the transoms have standard dimensions. If you are too high there is not much you can do except change the boat motor to a longer shaft length… However if you are more than 25mm below you can adjust the length shaft by installing wooden spacer blocks on the outboard motor mounting point (to raise it slightly).

Which battery to choose for the electric outboard motor of a boat?

We repeat, we do not recommend a electric outboard motor for a boat over 4m because they have a very low power and autonomy. They are ideal for 3m boats such as tenders, small sailboats or fishing boats.

If you opt for an electric boat motor, you will need to choose suitable marine batteries. There are 3 types:

  • The lithium battery: it is lighter, offers more charge cycles (+5000), has no self-discharge and can discharge to 100%. But it is also the most expensive...
  • The carbon lead battery: it takes up a lot of space, it is heavy but it resists extreme cold! Indeed the lithium battery does not work well below -5°C.
  • The lead/calcium battery: it is the least expensive and the most common but it self-discharges strongly, it only has a maximum of 300 charging cycles and it cannot be discharged more than 50%.

It is therefore advisable to equip yourself with lithium batteries. Even if the initial investment is higher, the lithium battery compensates with a much longer lifespan. It is the best choice for an electric outboard motor.

NB: it is important to have a means of recharging the batteries on board (such as solar panels or a 12 V power supply for example) and/or to have a replacement battery. This is extremely recommended for your safety but also to increase your autonomy if necessary.

Which engine propeller to choose for your boat?

The propeller is chosen according to the speed of your engine and not the length of the boat. When the boat reaches its maximum speed, the engine must be at full speed (between 5000 and 6000 rpm depending on the model). Otherwise, the outboard motor is damaged either by pushing it too hard or because it is overspeeding. So a speedboat will not need the same propeller as a sailing ship.

NB: Cupped propellers are not recommended if you do not plan to sail at more than 30 knots.

Choosing the diameter and number of blades of an engine propeller

As a general rule, to get the best performance out of your boat engine, you need a large diameter propeller and as few blades as possible. But the more powerful your engine, the more propeller area (and therefore blades or diameter) will be needed to avoid cavitation problems. In practice, it is therefore recommended to choose a three-blade propeller for the engine outboard of your boat.

The choice of propeller pitch on a boat

Choosing a propeller with the right pitch is crucial because it determines the acceleration, the maximum speed and the fuel consumption of the boat. But what is propeller pitch? This is the distance traveled by the propeller on each revolution. Propeller pitch is very often expressed in inches.

If we take the analogy of a car:

  • A small step is like first gear. In the car, it is used to climb hills and to accelerate. On a boat, we use a small step when we are very loaded or when we are water skiing.
  • A medium pitch is like third gear. It is versatile in town for cars and on the sea it is the propeller pitch that we recommend for pleasure boats.
  • A big step is like sixth gear. The boat's engine becomes quieter and consumes less. This is ideal when maintaining cruising speed for a long time.

To know the exact pitch you need, you must first know the maximum speed of your boat in relation to the power of the outboard motor you have chosen. Once you know this speed, apply the formula below:

Calculation of the theoretical propeller speed in knots:

Vt = max rpm * 60 / gear ratio * propeller pitch in inches / 72 (inches per nautical mile)

Now you know:

  • the maximum speed of your boat
  • the theoretical speed of the propeller in knots

(always in relation to the power of the outboard motor you have chosen.)

The rule is to choose a propeller pitch that balances the equation:

Theoretical propeller speed = Maximum boat speed – 5 knots

Once the ideal propeller pitch has been found, you can:

  • Add an inch if you do a lot of cruising distance. In practice, it will reduce fuel consumption and make the boat's engine quieter. But you will lose speed under load.
  • Take an inch off if you prefer to use your boat for water sports such as wakeboarding, waterskiing or tubing. In practice, it will increase the speed under load. But you will lose idle speed while having more noise and consuming more.
  • Only opt for a propeller with a cup if you want to go faster than 30 knots.   

What is the best engine brand for a boat?

There is no classification of best boat motor brands. Quite simply because all the big brands are equal. They all offer excellent engines and warranties of at least 3 years.

NB: if the budget is your first criterion, you should know that all brands of nautical engine are equal in terms of fuel consumption (for an identical type of engine).

  • For two-stroke outboard motors, the brands Evinrude, Mercury, Selva, Tohatsu, Nissan and Yamaha. 
  • For four-stroke outboards, you can trust Johnson, Honda and Suzuki.
  • For inboard engines, you will find the brands Nanni, Volvo and Yanmar.
  • For electric boat motors, there is the leader Torqeedo but also Minn Kota and Motorguide.
  • For semi-outboard motors, the brands Ilmor, Mercruiser and Volvo Penta are references.

However, for a boat of 7m or less, we recommend Mercury two-stroke gasoline outboards. They have a 5-year warranty and are the most common. Their popularity is a strength because they not only have a very wide range but above all an after-sales service accessible almost anywhere in the world.

Indeed, as all established brands are the same, the number one criterion for choosing a boat engine is the ease of engine maintenance and quick access to spare parts.

This is all the more important as the marine engines must pass a technical inspection every year to maintain the manufacturer's warranty. Plus, you don't want to wait months for your mechanic to receive a part if your outboard motor fails...

So if you sail close to a workshop that does not service Mercury boat engines (it is very rare but possible), then favor the favorite brand of this marine mechanic (Evinrude, Selva, Tohatsu, Nissan or Yamaha for two-stroke gasoline outboard engines).

Depending on the country, some brands are more present than others. It is therefore necessary to inquire in your port to find out which brands of marine engines have a maintenance network nearby.

Conclusion: which outboard motor to choose for your boat?

Below is the summary of this long article that answers the question: which outboard motor to choose for a boat?

For a boat up to 7m in length, we recommend a Mercury two-stroke direct injection (DFI) gasoline outboard engine, rated close to the manufacturer's maximum recommended output.

Choose a short shaft outboard if your transom height is 38cm and a long shaft if it is 51cm.

Next, you need to choose a medium-pitch three-blade propeller.

NB:

  • Choose an outboard motor close to the maximum power recommended by the manufacturer of your boat to increase not only the speed but above all your safety and comfort.
  • It is essential to respect the maximum weight of the motor indicated by the manufacturer of your boat.

If your boat exceeds 7 meters in length, then prefer inboard diesel engines. 

Finally, if you are looking for cheap boat motors, you can look for used boat engines on sites like ebay. There are some real good deals from time to time! But it will absolutely be necessary to ask for the report of an independent marine mechanic to be sure that it is in good condition.

CAUTION: This conclusion is a summary that communicates general information about outboard motors. Read the entire article to understand all the nuances that may apply to your boat. Your boat may not be 'average'. You will then have to refer you to a professional who will be able to provide you with tailor-made advice.  

Learn more about boat motors:

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