Outboard engines are a special type of propulsion system and engine specially designed for boats. They are fixed externally at the rear of the boat. They are compact engines that use a gearbox combined with combustion to generate power to a propeller that allows forward navigation.
An outstanding advantage of outboard motors is their easy installation and removal. They can be transported to repair shops without the need for the technician to go to the port. And for the same reason they can be stored in a safe and secure place when there is no need to sail.
As it is an exterior engine we have the whole interior of the boat. For example, on a boat of about 8 meters with this type of outboard motor we have the possibility of having a cabin inside that would otherwise be occupied by the engine room.
Outboard engine characteristics
With outboards, power efficiency increases and consumption decreases. The power-to-weight ratio is significantly improved as the engine is totally compact. A lot of power is achieved in a very small volume. Shaft friction, hull appendage friction to support the propeller shaft, rudder resistance and hull design are all eliminated. We can say that the power and range of this type of engine are exceptional, managing to scratch several knots of speed, with the same power and the same consumption. This is achieved by adjusting the working angle of the propeller thanks to the trim and by adjusting the appropriate thrust angle for each speed of the boat.
The outboard is usually small in size and for this reason it is a very revolutionary engine but also its small parts give it a precise character and great fit. Today, the technological advances that have been made on these engines make them perfect engineering jewels. Nowadays we find electronic multipoint injection, variable resonance chambers in the exhaust systems, camshafts that advance or reverse the valve opening crossing, etc.
Details that we must take into account in our boats
It must be taken into account that the outboard motors are cooled by water, and if you do not take the precaution of submerging the suction pipe (next to the propeller) you can damage the motor by a heat.
We need to be careful that the engine does not touch bottom, because as we said before, being placed at greater depth of the boat, we can produce damage to the tail and propeller. We also need to be careful when bathing, as the propeller is very sharp and very exposed to the bather. For safety reasons it is essential to make sure that the engine is stopped.
The outboard is usually small in size and for this reason it is a very revolutionary engine but also its small parts give it a precise and tight fitting character. Today, the technological advances that have been made on these engines make them perfect engineering jewels. Nowadays we find electronic multipoint injection, variable resonance chambers in the exhaust systems, camshafts that advance or reverse the valve opening crossing, etc,
What would be the correct size outboard motor for your boat?
2hp – 4hp outboard engines (less than 19 kg in weight)
Suitable for portable boats such as canoes, folding or inflatable boats, dinghies, small sailing boats and any other lightweight boat.
They are normally two-stroke engines, although four-stroke engines start from 1.5 upwards. Relatively light (two-strokes more than four-strokes) and with a carrying handle.
They sail between 8 to 15 km/h depending on the power, weight and design of the boat, the wind and the current.
5hp – 6hp Outboard engines (between 18-25 kg)
They allow small inflatable boats up to 3.5m in length, portable folding boats, 2.5-3.5m inflatable boats. This type of boat weighing about 115kg requires a short shaft outboard motor, i.e. 15″ in length. For towing yachts and hulls less than 6mtrs. in length the configuration of 20″ axle length is required.
They propel the ships at about 16-28 km/h if the cargo is light. For heavy loads these engines will not work properly.
They are light, compact and easy to transport, and many of them allow you to reverse.
8hp – 9.8 hp (Weight 25-45 kg)
Small and powerful, it is used with 3m – 4.5m inflatable boats, 3m – 4.5m aluminium boats, larger portable folding boats, and lighter fibreglass boats up to 4.5m.
The weight of these boats is usually up to 136kg with a short shaft length of 15″ outboard. Also available are the long axles of 20″ and 25″, making them the best option for towing yachts up to 7 or 8 meters in length.
This type of engine is used as an emergency auxiliary engine. The four-stroke outboards in this range have increased features: audible and visual warning indicators for low oil pressure and overheating, electric start and 12v load, etc.
It sails at about 15-28 km/h with light to moderate load. They are a good choice for trolling engines, yacht engines and auxiliary outboards.
15hp – 25 hp (Weight approx. 36 kg-52 kg)
Although they are much heavier to handle they are still considered “small” outboard engines.
They are used with 10-14 foot inflatable boats, aluminum fishing boats and 12-16 foot flat-bottomed, fiberglass boats of the same length as the above.
These boats require a short shaft length of 15″ and weigh up to 250 kg.
Ideal engines for trolling rivers, tenders and heavier towing yachts.
Like the previous four-stroke model they are equipped with audible and visual warning indicators for low oil pressure and overheating, electric start and 12v charge.
They’re sailing at about 25 miles an hour with a moderate to heavy load.
Finally, our Invictus boats start working from 115hp. In a next article we will talk about the large outboard engines.