Here’s how to find the ideal propeller for your engine

There is nothing better than putting the boat in the water and trying it out, whether it is an outboard, inboard or inboard propeller shaft.

The boat must be in optimum condition, the kind that every yachtsman should always aim to have: a clean hull, a small crew, enough fuel and a well-distributed load.

Another important element is the trim (and if necessary the trim tabs), which must be absolutely neutral and also the sea (or lake) conditions must be calm or only slightly rough.

It is important to know the maximum rotation speed of the engine being tested.

Desk calculations also help to skim the number of alternatives. For example, we know that each additional inch of pitch corresponds to approximately 200 engine revolutions, so a change from a 17″ propeller to a 19″ one loses an average of 300 to 400 revolutions.

We are concentrating on propellers and, therefore, we take for granted the correct execution of an equally important coupling: that between engine and hull and, in particular, the height and relative immersion of the foot in the water. Let’s consider that for every hole less than the support (therefore with the propeller more immersed, Ed) you lose an average of 100 engine revolutions. Be careful, however, not to mount the propeller too high, as the performance gains but the risk of cavitation is always present when turning.

The propeller for every need

There are other types of use that require specific propellers, such as water skiing, for example, where the skier has to accelerate hard to get out of the water. In this case, four-blade propellers with a smaller pitch are advisable and it doesn’t matter if at maximum speed you risk over-revving.

If, on the other hand, the objective is slow motion for trolling, a short pitch will be preferred, which allows, without intervening on the engine idle speed, to sail slowly, even if there is then the risk of going overspeed when looking for speed.

Are you just cruising quietly with the family, so a lot of weight and not much speed? You will be fitting a propeller that favours performance in the medium range. Do you like deep-sea fishing? Then you will opt for a more powerful model that maximises performance.

DIAMETER

It is the circumference of the “circle” at the tips of the blades obtained by the rotation of the propeller. The diameter of each propeller model is determined by its design and intended application.

A larger diameter pushes more water and the blades reach deeper, so a large diameter is typically used on large, heavy boats.

A smaller diameter is normally used on lighter boats, where the propeller operates less deep in the water or when an increase in engine speed is desired. Within a specific propeller range, as the diameter increases, the pitch generally decreases.

PITCH

This is the distance a propeller would travel in one complete revolution if it were travelling through a solid.

The right propeller allows the engine, under normal/heavy load, to touch the upper portion of the manufacturer’s maximum rpm range without exceeding it. A smaller pitch will have more acceleration and ‘boost power’ but a lower top speed. A larger pitch will offer less acceleration but more top speed potential.

ANGLE OF ATTACK

This is the angle of the blades relative to the propeller axis and is expressed in degrees.

A high angle is more suitable for high mount engines and reduces ventilation and increases stern lift, but if it is too high it can adversely affect performance, handling and engine effort.

A low angle results in less strain on the engine and better acceleration at higher rpm at maximum power.

Used on lighter boats where the propeller operates less deep in the water or when a higher engine speed is desired. Within a specific propeller range, as the diameter increases, the pitch generally decreases.