Basic Sailboat Propeller Theory
Understanding parts of the propeller and its basic theory can be useful when researching propeller upgrades or solving problems when your boat is not performing under power. Hopefully this article will provide you with some of the basics when it comes to understanding how your propeller works.
Parts of the Propeller
- Hub – The hub (or boss) will usually accommodate a taper or spline that allows you to fasten the propeller to the prop shaft.
- Keyway – The key is usually a long rectangular section of material that fits into a milled slot in your shaft. The propeller hub will also have a milled section to the correct width accommodating the top half of the key material. Its job is to transmit torque to the propeller from the shaft.
- Blades – The blades are foils connected to the hub that will generate lift/thrust.
- Blade Root – This is the point at which the blade attaches to the hub.
- Blade Tip – This is the outermost section of the blade.
- Leading edge – The leading edge is the driving edge of the blade that would cleave the water.
- Trailing edge – This is the edge that that is adjacent to the aft end of hub when driving forwards.
- Blade Face/Back – The Blade face is your high pressure side that faces aft when driving forwards. The back of the blade is your low pressure side that faces ahead when driving forwards.
Propeller Basic Characteristics
A propeller is normally described by :-
- diameter;
- pitch;
- rotation;
- and the number of blades
So, a 16×12 propeller will have a diameter of 16 inches and a pitch of 12 inches. A “16″ 3B Featherstream” will normally describe a 3-blade Featherstream propeller with a 16-inch diameter.
- Propeller Diameter – The diameter of a propeller is the circle which the tip of a single blade describes in one complete rotation. Diameter is determined by measuring the radius of one blade (from the centre of the propeller Hub to the tip) and then doubling the result. This is the most important factor that will govern the amount of power a propeller can abb/transmit. For most Sailboat installations (displacement hull forms) the bigger the diameter the greater the efficiency! Especially for those hull forms that have large wetted surface areas! If you own a long keel boat then that’s you!
- Propeller Pitch – The pitch is the theoretical/virtual distance travelled per one revolution of the shaft/propeller. In reality this is less due to propeller slip (rotation with no forward travel). The pitch converts torque into forward thrust determined by its angle of attack. Imagine driving a screw into a piece of wood – it is the distance that the screw moves into the wood through one rotation of your screwdriver (theoretical/virtual pitch).
- Propeller Revolutions per Minute (RPM) – Shaft rpm is determined by the Gearbox Reduction Ratio and in turn will govern the propeller size. Slower the shaft speed the bigger the propeller so be careful when replacing your installation, speak to a propeller expert who can check for correct engine and gearbox pairing!
- Number of Blades– A sailboat propeller is usually designed with two to five blades. The number of blades is generally determined by the weight of a vessel, hull form and installation. A propeller with more surface area will usually perform better in those moments where resistance on the boat increases due to conditions. For example a sailboat that is struggling to push through adverse conditions that has a 2B propeller will usually see an improvement with a 3 or 4 bladed propeller in this moment. More surface area means less slip and more bite! The problem with sailboat applications is that with an efficient propeller drag is increased reducing your sailing performance. That’s where low drag sailing props come in! The major benefit of a Feathering or folding (FeatherStream or Flexofold) propeller is that you are able to fit the most efficient propeller for your motoring performance with the knowledge that it will feather/fold when sailing thus reducing drag and increasing sailing performance by up to 15%! If you want to learn more about low drag propellers explore our website or contact us.
- Propeller Rotation – The rotation or “hand” is the direction in which the blades of a propeller are designed to rotate. Standing at the back of a boat and looking forward towards the bow – a right-handed propeller will rotate clockwise when going ahead (lever forwards); and a left-handed propeller will rotate counter-clockwise (lever forwards). The shaft rotation is governed by your engine/gearbox. It is not unusual to see old Volvo Penta installations with left hand propellers for example. If your boat is in the water and you are not sure as to the shaft rotation a LH propeller will always kick to Starboard when reversing!
If you need any guidance or advice on what to do about your Sailboat propeller or wish to explore the popular low drag propellers we offer contact us today where you can talk to one of our engineers.
Posted on