GUIDE: Which engine to choose for a 7m boat?
On a boat, the engine is a crucial part. Choosing the wrong engine can cost you money but more importantly put your life in danger. It is therefore necessary to analyze many parameters in order to choose the most suitable boat engine for your boat. We will mention criteria such as the weight of the engine, its power in hp, the currents encountered in your navigation area or the usual number of passengers. Of course, the size of your vessel is also a major parameter in the choice of your engine. This article explains in detail which engine to choose for a 7m boat.
WARNING: This article is a summary in which we provide targeted information concerning the motors for 7 meter boats. But to fully understand how to choose a boat engine, we invite you to read the much more complete article “Which outboard engine to choose for a boat? ". Also we have written other synthetic articles answering the same question but with targeted answers for different boat lengths:
- Which engine to choose for a 4m boat?
- Which engine to choose for a 5m boat?
- Which engine to choose for a 6m boat?
Contents of the page
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:
- different power supplies (gasoline, diesel or electric boat engine),
- different setups (outboard, sterndrive or inboard motor),
- different types of engines (two-stroke or four-stroke boat engine),
- different injection modes (direct or electronic injection boat engine),
- different engine powers (6hp, 25hp, 50hp, 100hp or even 250hp engine)
And once you know the answers to all these questions, you will also have to decide:
- what is the correct shaft length for your boat,
- which is the battery and the propeller adapted to the chosen boat engine,
- et what brand of boat motor to choose depending on your budget.
In the rest of this article, we explain to you point by point which criteria to choose in order to to install the most suitable engine for your 7m boat.
Which type of engine is best suited for your 7m boat?
The gasoline outboard motor is the most common on boats up to 7m. This motor is preferred to onboard motors or pod drives which are often more expensive.
In addition, the gasoline outboard motor adapts to any what kind of 7m boat : whether it is an open hull, a speedboat, a semi-rigid inflatable boat or a fishing boat.
Then, 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.
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.
NB: Electric outboard motors are not recommended for 7m boats. They are generally used for boats of less than 3 meters... Indeed, despite all their advantages (silent, vibration-free, environmentally friendly, light, etc.), electric outboard motors are today still too little powerful to meet the needs of a 7m boat. In addition, electric boat motors are much more expensive and have very limited battery life, which limits boating time.
Which outboard motor power to choose for a 7m boat?
It is absolutely necessary to choose a boat engine with a power adapted to your 7m boat. The advantages are numerous:
- 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 will wear out the structure of your boat (it will also get dirty if underused). Whereas with the right power you reduce maintenance costs on your 7m craft.
- Speed: once hull speed is reached, a 7m 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 7m boat ?
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 7m 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!).
Builders often recommend a minimum engine power of 200hp for a 7m boat. This is the case on an OSTREA 700. But such a boat has a maximum recommended power of 2x 270 hp!
The difference is enormous in the piloting as well as in the wallet. This is why many owners have equipped their 7m boat with a single 270hp engine. It's a good compromise.
However, these indications may vary depending on your type of boat (sailboat, yacht, zodiac, etc.), the weight of the boat and the currents in your navigation area. It is therefore necessary to inquire with the manufacturer or with a local marine mechanic.
What is the maximum weight of an outboard motor for a 7m 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 7m 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 7m boat?
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 engine for a 7m boat.
What shaft length to choose for the outboard motor of a 7m boat?
Not all 7m boats are alike so they won't need the same shaft length. 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. To know what shaft length to choose for your 7m boat, just measure the height of the transom:
- Short shaft boat motor = 38cm transom height.
- Long shaft boat motor = 51cm transom height.
Which motor propeller to choose for your 7m 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 7 meter yacht will not need the same propeller as a 7 meter sailboat.
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 your 7m boat.
The choice of propeller pitch on a 7m 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's versatile in the city for cars and on the sea this is the recommended propeller pitch for 7m 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.
What is the best engine brand for a 7m boat?
There is no classification of best engine brands for 7m boats. 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 semi-outboard motors, the brands Ilmor, Mercruiser and Volvo Penta are references.
However, for a 7m boat, 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.
Conclusion: which engine to choose for a 7m boat?
Below is the summary of this article that answers the question: which engine to choose for a 7m boat?
For a 7m boat, we recommend a Mercury gasoline, two-stroke, direct injection (DFI) outboard engine, with power close to the maximum power recommended (usually 2x 270HP) by the manufacturer.
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: It is absolutely necessary 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 outboard motors 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 on outboard motors suitable for 7m boats. 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:
GUIDE: How to Fix a Smoking Boat Engine?
