More power = higher speed ?

Our customers often ask us:

“By changing the engine for a more powerful version, my boat is going to sail faster, correct?”

One would be tempted to answer in the affirmative: more power = higher speed. The answer is not so straight. Customer is king but experts have to give good advice. We are going to talk about a few key points of boat propulsion.

Your boat has been designed to meet your requirements: sailing long journeys in all-weather condition, going fast (even very fast) or finding a compromise, for example, between speed and stability. It is the hull of the boat that will define these characteristics.

We can mention three main families of hulls that cover a wide spectrum of designs:

Displacement hull: it displaces the water to allow the ship to move forward. It is a very interesting solution for sailing at “low” speeds and optimizing consumption. The keel is long and deep, it allows you to navigate with peace of mind in many conditions. However, the speed of the ship is quickly limited, generally around 10 knots for a “Trawler”.

Planing hulls: they generally have a large flat surface towards the rear of the ship which generates lift. These ships plane "easily" and can reach very high speeds. On the other hand, the flat bottom is not optimal for navigation comfort. The engine must be powerful enough to reach planning condition.

Semi-displacement hulls: compromise between displacement and planning hulls. These hulls generally have a “V” shape on the bow which softens towards the rear of the boat to generate lift. With powerful engines, these ships can partially plane and reach high speed.

For each of these hulls, the boats “push” against several phenomena to move forward. The notion of power comes in. There are two main phenomena:

• Water friction on the hull: importance of a clean and smooth hull.
• Wave resistance: generation of waves during sailing.

The resistance to sail varies according to the speed. Here is an idealized graph for the three hull types:

For a displacement hull, from a certain speed the additional power to move the boat faster becomes unrealistic. The engines would not fit in the engine room. The consumption would become enormous and would go against the philosophy of the hull.

For semi-displacement and planing hulls, extra power could help to gain speed.

However, the relationship more power = higher speed is still not so direct.

It is the propeller that generates thrust against the resistances to sail. An engine drives a marine transmission which turns the shaft on which the propeller is mounted.

This propeller has been selected by a dedicated professional:

• It is selected in relation to the initial thrust to be provided
• It has a diameter limited by the space available between shaft and hull
• It has many other parameters including the number of blades and the pitch.
• It has a maximum speed of rotation not to be exceeded.

There is a “wedding” between a hull, an engine with transmission and a propeller:

• The propeller will not necessarily use the extra power of a bigger engine.
• A higher rotation speed can lead to problems such as cavitation.

A naval architect, an engine manufacturer and a propeller expert must work together to guarantee a gain in speed in good conditions.

More power doesn’t mean more speed