Propeller basics

Propeller: Basics

By Kobus Kotze / Indian Ocean Skippers Training.

Here are just a few basics.

There are a variety of different propellers. There are propellers with two, three or four blades. There are also propellers that turn clockwise or anti-clockwise when moving forward.

A propeller works by converting torque (a force that causes it to rotate) into thrust (forward movement). A turning propeller moves water downwards and behind the blades, in other words an action which produces a thrust of water from the blades.

Each blade has a distinctive curved shape which as it turns helps to move the water down and behind it, then pumping the water out behind. The angle of the blade creates a pushing effect on the underside and a pulling effect on the top side. This accounts for why a propeller blade is twisted.

Propeller pitch

When a propeller moves through water, the distance it would move forward in one revolution is equivalent to the propeller’s pitch, so for example 15 inches (38cm). The diameter or width of the propeller might be 13.5 inches (34cm).

The diameter and pitch are the two key dimensions to describe a propeller and are usually marked on the hub of a propeller, for example 13.5 x 15. A propeller described as 14.5 x 19 has a diameter of 14.5 inches and a pitch of 19 inches.

Higher pitch for higher top speeds. Pick a propeller with a higher pitch to increase the top speed of your boat. Keep in mind that a higher pitch will also decrease your boat’s acceleration.

Lower the pitch for better acceleration. Propellers with lower pitches have higher RPMs, thus resulting in greater acceleration for your boat. Choose a lower pitch if you need to increase the acceleration.

Propeller diameter

It is simply the diameter of a circle scribed by the blade tips of the prop.

Increase propeller diameter for more control and power. Propellers with a larger diameter give your boat more control and power but create more drag and decrease your acceleration. Choose a propeller with a greater diameter if you have a large boat that requires more control and power.

Decrease propeller diameter for more acceleration. Propellers with smaller diameters rotate faster, thus creating more acceleration. Choose a propeller with a smaller diameter if you have a small boat that you want to get up to fast speeds.

Number of blades

Why the different number of blades? The quick answer is as follows:

• Two blade propellers are the best option for sailing boats – it gives adequate performance under power and has less drag through the water when boat is under sail.
• Three blade propellers give greater thrust than two blades but has increased drag. Three blade props are more commonly used for power boats. Three blades usually give a slightly better top speed performance than four blades. Propellers that have three blades provide the best all-around performance.
• Four blade propellers tend to be quieter and vibration free. They produce more lift at the stern which can help with acceleration.

Boat type, engine power, displacement and desired boat speed are all factors to consider when choosing the type and specification of a propeller.

Be safe and safe boating!

Contact us at 0828913652 or info@watersportdirect.co.za