Outboard motors have a lot of benefits among which a higher top speed. Of course, this is a fact that only superficially interests us, for the simple fact that the time we spend on the boat at the maximum possible speed is actually marginal, especially if we look at cruises…

Yet the increase in power of outboard engines in recent years has been such that their supremacy over their sterndrive colleagues in terms of performance is quite clear. Yes, because alongside the increased speed there is also the greater acceleration, which in some more driven outboards is really burning.

Certainly we cannot forget the fact that the sterndrive engine requires a greater number of interventions, and yes, in general it also undergoes faster wear.

The outboard motor, on its side, has in fact the possibility of being raised whenever it is not used, so as to get out of the water even when the boat is soaking. Those who have to maintain the outboard would like to thank.

The lower unit of the sterndrive engine, on the other hand, always remains underwater, requiring antifouling and facing greater corrosion – and more. Furthermore, the outboard motor gives the possibility to fully exploit the space inside the boat, without having to sacrifice lockers for the engine. Therefore, there are several reasons to install an outboard motor.

However, the most important thing is to install it correctly.

And this is where doubts usually begin: what aspects must be considered to properly install an outboard motor? Each manufacturer obviously accompanies their engines with a detailed guide on how to mount and the parameters to follow; each boat, moreover, has particularities that redefine the operations to be completed from time to time.

Regardless of all this, however, browsing through the forums, we realized that there are two main questions that are the most popular when it comes to installing the outboard motor: a great doubt is that relating to the correct height of the outboard moto; the second question – extremely widespread – concerns the number of engines to be fitted.

So let’s begin: How high does an engine have to be to be really effective, and above all not to do any damage?


Let’s start with topic number one, which lenght should i get?

To understand at what height to install the engine, it is necessary to start from the assumption that not all engines are the same: not only the type of power supply, the displacement and the power change, but also the height of the shaft.

Before seeing when an outboard motor has a short shaft and when, instead, it has a long or extra-long shaft, it is necessary to understand how to measure its actual height, so as to solve the problem at the origin.

Let’s start with the anatomy of the outboard: looking at it from the outside, we have the grille in the upper part, inside which is the actual engine, and therefore ignition, pistons, cylinder heads and so on.

Today, however, we are not interested in all this: what we are interested in is immediately below, under the transport socket and under the fixing bracket. In fact, there begins the stem of the outboard motor, which ends where the anti-cavitation plate begins, placed above the propeller.

Now that we have identified these basic elements, we can understand how to measure the height of an outboard motor stem: to do this you have to start from the support on the transom and go down, up to the lower edge of the anti-cavitation fin.

In case you do not know how to “catalog” the shaft of your outboard motor, then, all you have to do is measure the distance between these two elements.

There is no mention, it must be said, of absolutely precise measurements: there is a certain variability between one manufacturer and another, as between the different models. However, there is talk of outboard motors with a short shaft when this, following what has been seen above, measures between 38 and 42 centimeters.

The long shaft, on the other hand, measures between 48 and 55 centimeters, with on average a good ten centimeters more than the short-shaft colleague.

Then there is the extra-long shaft: here it reaches about 60 centimeters.

  • SHORT SHAFT 15″ 38/42cm
  • LONG SHAFT 20″ 48/55cm
  • X LONG SHAFT 25″ 58/64cm
  • XX LONG SHAFT 30″ 72/75cm

Now we already know what the next question will be in your head: when to choose a short stem, and when a long stem instead?

In fact, there is no answer.

In fact, it is not the boat owner who chooses the height of the shaft, but the boat builder, who designed the boat in a certain way, with different transom heights and suggests it accordingly to buoyancy and number of engines installed.

As we will see shortly, getting the height wrong is a mistake that can lead to important negative consequences.

The correct height installation

Before understanding the correct height of the outboard motor, it is good to understand why it is important to mount the propeller in the exact position.

There are two main reasons:

  1. the propeller must be properly submerged below the surface of the water, without being neither too deep nor too high
  2. the motor shaft must be wet for a correct section, in order to better manage the cooling process

That said, we can proceed. The right height of the outboard motor is found – in a standard configuration – when the anti-cavitation plate is in line with the hull of the boat, when the propeller shaft is in parallel aligned.

This is why there are engines with more or less long shaft, as we have seen above: when the transom varies, which goes from 38cm to over 60cm, motors with short, long or extra-long shafts will be used. So far, in fact, there are no major problems.

Sometimes, however, there is a tendency to slightly vary the installation height of the motor.

There are models of small displacement outboard motors, for example, which by their very nature – on the advice of the manufacturer, therefore – must be installed slightly below the hull of the boat, moving downwards by 2-4 centimeters.

What happens if an engine is too low?

First of all, as you can guess, the stem is more likely to touch the bottom where the draft is low, or to collide with objects of another nature.

In the case of high-powered engines, the performance can decrease, while the splashes can change and increase in volume.
But you certainly cannot “go wrong” only in depth. It is also possible to make a mistake in the opposite direction, by installing the outboard motor too high, a rather common mistake.

Example of engine TOO LOW: top plate not visible

But why do someone “take risks” by mounting the engine at an heigher standard? Simple: by bringing the propeller a few centimeters higher it is possible to acquire higher speed. Especially on two strokes.

Precisely for this reason, following the manufacturers’ instructions – but not always – the outboard is often installed 3 to 8 centimeters above the standard position. Doing so decreases the resistance of the water. Furthermore, the probability of hitting submerged obstacles is reduced, maneuverability is improved, and sometimes, in certain cases, even gliding can be improved.

However, even here, by exaggerating, other problems can occur.

The first and best known is obviously that of the cavitation of the propeller, which we will return to shortly. That’s not all, mind you: raising the engine too much on the transom runs the risk of not being able to ensure good circulation of fresh water inside the cooling system. This because the hull limits water access.

Engine overheating, therefore, is just around the corner. Raising the engine also risks nullifying the trim flap.

Going even further into detail, an excessive height of the propeller can in some cases negatively affect the glide and even increase vibrations (with all the negative consequences on the comfort and wear of the boat itself).

Example of engine at CORRECT height.

Propeller cavitation

We have seen that an excessive height of the outboard motor can lead, among other things, to cavitation of the propeller.

Cavitation is not actually a problem… it is what moves us ahead! It is like the movement of a screw entering the wood, n The issue which can occur, is called cavitation but is technically OVER-cavitation.

It is something that is often talked about, but in the end most people only know that it is a “bad thing”.

But what actually happens to the helix when it cavits? And for what reason?

The concept, in words, is simple.

A propeller is in cavitation when around it, instead of being water, there is a vacuum. When this happens, the resistance encountered by the blades is obviously reduced to a minimum: consequently the propeller starts spinning faster, better, too fast, exceeding the rpm threshold and putting the transmission under stress.

In the long run, cavitation ruins the propeller, with damage that can be confused with that of corrosion.

The problem, in this case, lies in the fact that the “air bubbles” that form around the sail collide violently on the blades, damaging them a little at a time, due to the increases in pressure and even in temperature.

In addition to all this, cavitation causes greater noises and greater vibrations (just think that, in the field of military boating, the search for propellers that cannot under any circumstances enter cavitation is continuous, starting from the assumption that a cavitation propeller is easier to detect by the “enemy”).

Number of outboards

We come to the second big doubt that often circulates when it comes to installing the outboard motor.

What changes when you decide to install more than one? And how should multiple outboards be chosen? And again, why should you opt for more engines?

Well, as standard rule we use more engines when we need more power than a single outboard can give us.

If my boat, due to its characteristics, needs 600 horsepower (assuming that the most powerful outboards on the market rarely and exceptionally touch 450 horsepower) I will be forced to mount 2 outboards of 300hp, or 3 outboards of 20hp.

Rhibs that mount two outboard motors should tend to install them 2/3 cm lower taking into account that, when turning, the engine that is outside it is automatically higher.

When choosing the outboard motors that will be mounted on the boat, it is essential to choose the same engines:

  • same horsepower
  • same displacement
  • same model

Why? Simple: combining a 30 horsepower engine with a 70 horsepower one does not mean having 100 horsepower in the boat. No sir, indeed, it will mean having the power of 70 horsepower, or maybe even less.

Yes, because the stronger engine will not be helped at all by the weaker one, and indeed, it could even be disturbed by it, in the face of greater expense and consumption.

IMPORTANT: on a 2 engines configuration the propellers MUST TURN OPPOSITE. One clockwise, the other anticlockwise, so the lower unit must be changed accordingly.

More than two engines configuration

These configurations must be evaluated by a technician, since the wrong heigh of the central engine could bring to very bad ventilation effects on the other props.

Generally speaking for multi-engines configurations the boat builder will give advice to follow.

Triple engine configuration on a rhib

Is it always better to install two, three or four motors instead of just one outboard motor?

Not really: if we can only buy one propeller that has enough power for our boat, we should.

Vibrations, noises and consumption will be lower. If not, it’s best to keep the number of outboards as low as possible: if two are enough, let’s stop at two.

It must also be said that those who try a boat with two outboard motors tend to fall in love with it, due to the maneuverability ensured by the pair of propellers which can be connected to an electronic management, so as to side movements.

Nor can we forget, it is true, that having two engines automatically means having a spare engine when the other one breaks down. But if this is the only question, we might as well have on board an auxiliary engine of reduced displacement and weight, the one to be used, so to speak, also for the tender.