TECHnic Journey into the fabulous world of propellers

Rule number one when it comes to propellers: there are no rules. Each boat should have a custom one depending on the characteristics of the hull: if you have just bought a boat, and the performance while motoring does not match what is stated on the data sheet, it is a case of checking, first and foremost, the propeller. Various models are commercially available: fixed-blade, folding-blade or swivel-blade. Here are our tips for a good choice.

WITH FIXED, LOWERABLE OR STEERABLE BLADES?
As a rule, shipyards deliver cruising boats with fixed blade propellers, which, if properly sized, are considered the most efficient in forward motion, somewhat less so in reverse. Their great disadvantage, however, is their great hydrodynamic resistance during sailing.

To cope with this problem, propellers with retractable and steerable blades were devised. The former during motor sailing are in every way similar to those with fixed paddles, but when sailing they take on a “duck-billed” shape, reducing incidence with the water to a minimum. The latter are equipped with a mechanism that can make the blades orient so as not to create friction on the water.

When the engine is turned off, the reverse gear is engaged, and the blades rotate on their axis and position themselves horizontally.These are usually models with modifiable pitch, allowing the incidence between the blade axis and the propeller axis to be adjusted with a screw on the hub. The big advantage of these models is their adaptability to different types of hulls and to engines with different reduction ratios (i.e., the ratio of propeller revolutions to engine revolutions: for heavy boats with low-power propulsion, such as cruising boats, high ratios are advisable because the slower the propeller turns, the better the engine torque is used). Then there are the variable pitch propellers, also designed for sailboats: the price goes up, but the pitch adjusts dynamically and allows for better performance while motoring.

PITCH AND PROFILE
If you want to change the pitch (i.e., the distance ideally traveled by a propeller after a 360-degree rotation), it is advisable to do so when the boat is dry. An important datum, then, is the blade profile, although it turns out to be affected by the thickness of the blade. Mostly oval or asymmetric profiles are used: they do not have any particular differences in performance, except that the asymmetric profile tends to develop the leading edge of the propeller more so that the blade’s entry into the water is more progressive and effective.

NEW PROPELLER: HOW TO MOUNT IT?
When fitting a new propeller, check its distance from the hull, because a measurement error can cause a significant loss of efficiency. Care must be taken that the flow of water generated by the rotation of the blades does not generate turbulence with that produced by the movement of the boat. If the propeller is too close to the hull, the water flow, colliding with the hull, can produce annoying noises. Before choosing the propeller that is right for you, consider well the intended use of the boat. For leisurely cruising the fixed-blade model will not disappoint, while if you own a performance racing boat you will have to enter the realm of more expensive folding or steerable propellers.

FIND OUT WHAT THE MAXIMUM SPEED OF YOUR BOAT IS
The formula “2.43 x square root of waterline length” determines the maximum theoretical motor speed of a boat when sailing at displacement, i.e., not in gliding conditions. This universally recognized formula (which we also use for our tests) is a reliable parameter, especially for medium displacement cruising boats. The maximum speed obtained is, in theory, that which you will have to obtain by sailing with calm seas at maximum engine speed. If the speed of your boat is significantly lower than that obtained with the formula, then something is wrong. There can be many reasons for this: the type of engine, dirty hull or stern foot, undersized propeller, of the incorrect model or mounted incorrectly, the boat being too heavy, or the instruments not calibrated. Compare the speed of your boat with those in the table below, considering the waterline length of your boat.