By Thomas P.

Certainly the choice of the right propeller for our boat is not something simple since it depends on a number of factors which in turn interact with each other to such an extent that it is not possible to know what exactly is going on back there in our stern.

choice of propeller

The purpose of this article is to offer some help in the choice of the right propeller for a given combination of hull, engine, mounting height and for certain operating conditions.

We must have in mind that our ultimate goal is to achieve the best possible match of the candidate propeller with our hull and engine, always in relation with the desired usage and the load carrying. It is an especially complex combination, with many inter-related factors.

Unfortunately, there is no formula that will automatically show us the desirable propeller. General knowledge of physics and hydrodynamics, rules and theories must be used for our calculations, all of which are more or less involved.

Trying to organize our thoughts, in the effort of selecting the best propeller, I would say that first of all we have to:

  • be able to «read» the characteristics of a propeller and to know what exactly each role of them is.

choice of propeller

It is a fact that no propeller is perfect and no propeller can do everything. Each propeller is unique, has been designed to behave very specifically and provide specific performance depending on its design characteristics.

First of all, therefore, we must be able to «read» a propeller, to observe its characteristics and have the ability to distinguish which features may on may not suit to our engine-boat combination so as not to run aimlessly for unnecessary tests without meaning.

And of course the most important characteristics of a propeller, which must take into account, are its diameter and pitch. So first we have to choose the proper diameter and pitch which match in our engine-boat combination and then to focus on the other features of the propeller.

If, for instance, a propeller with diameter of about 15 and with pitch of 21 inches is the one we want, we will see that, in the above dimensions, there are so many styles, designs and options of propellers available in the market.

Which of them therefore should be chosen?

Apart from the diameter (which is determined both by the combination of engine horsepower with the gearbox reduction and by the hull itself) and the pitch, we have to consider about the number of blades, their total surface area, their shape, their cup, their rake, their skew and so on.

So, to be able to choose the most suitable propeller we need to know what each one of the above characteristics does. Ie what is the role of the diameter and the pitch, of an extra blade, of the cup and the rake and how each of them affects the rpm of our engine?

Since we know the above mentioned things we can estimate the general behaviour of a specific propeller.

So are we now ready to proceed in the selection of the desired propeller?

Unfortunately, the answer is no. This is because, quite simply, a propeller doesn’t run alone but it must «co-operate» with the hull.

choice of propeller

But do not be dιdisappointed. Knowing the characteristics of a propeller and therefore what exactly this propeller can offer a big step has surely been taken in the right direction and we are ready to go to the next step:

  • to know the characteristics of our hull’s design , since this is one of the most important factors that determine the choice of the propeller

No hull is the same with another one, and each hull is designed for a particular use. As there is no the ideal propeller there is no the ideal hull too. Each hull has its own characteristics, its own requirements and needs some specific features of the propeller chosen which will improve its performance.

We will never be able to find the ideal propeller if we do not know the behaviour of our hull.

So before we proceed to the selection of the propeller we must spend several hours observing our hull’s attitude during riding and how it behaves both in good and in bad sea to find lacking, and where the hull needs to be helped to perform better.

choice of propeller

So the secret here is to select the propeller that has those characteristics that match with the characteristics of our hull and will help it reaching its full potential.

For example, if our hull design is such – like a stepped one- so as to hold down the bow of the boat it is reasonable not to prefer propellers that lift the stern too, because in this case the cruising speed will decrease and our top speed too, while the fuel consumption will increase and the riding can be even dangerous especially when we have following seas. In this case it is better to choose a propeller that raises less the stern and more our bow.

Since we will understand the characteristics and needs of our hull then we have made another one important step for choosing the best propeller. But it is not the last.

Things are complicated even more because the right combination of propeller with our hull should match with also other factors that are equally important, such as:

  • our engine’s horsepower and its reduction ratio, the mounting height on the transom, since the height of the engine is defined both by the type of the hull and the propeller, we will eventually choose.
  • the loads we are usually carrying, the usage our boat is intended for and the sea conditions in the areas we usually ride.
  • and of course we should never forget the most important of all: the propeller that we will eventually choose should allow our engine to reach, at least, at the middle of the maximum rpm range recommended by the manufacturer.

Choice of propeller

So, it is obvious that the choice of the most suitable propeller is a fairly complex process which requires knowledge, experience, patience and of course, many trials at sea.

...keep Ribbing!