Raven Yacht Owners Association

Most Ravens initially used a folding propeller. The reason for this was that by folding they provided less drag when sailing. They had however some limitations especially in reverse. As the owners of the Ravens ,in particular Raven 31s have been getting older many have fitted fitted three bladed props to give better speed under power and a more effective reverse.The sizing and pitch of the propeller is critical to get optimum performance. it is difficult to find the information required and a boat owner has to  often rely on the propeller provider to get it right.

Briski , http://www.bri-skipropellers.co.nz/ an Auckland manufacturer of propellers, has an extensive database of NZ built yachts and their propeller requirements and in the case of the Ravens with original installed motors can tell you which size prop and pitch is suitable. Sometimes  with other prop suppliers it is trail and error but this can be an expensive exercise with the boat having to be hauled out to change the pitch. Note that the propeller size can be different for the same motor depending on whether it is shaft driven or has a saildrive. Here are Yanmars recommendations http://www.yanmarhelp.com/i_proptbl.htm

 Here is some prop info from Volvo. Read especially the section at the end which allows you to work out whether the prop you have is the most efficient

3 OR 4 BLADES A 3-blade propeller usually offers top speed performance while a 4-blade propeller provides maximum thrust and smooth cruising operation.

DIAMETER Diameter is the distance across a circle encompassing all of the propeller blades. Measure from the centre of the propeller hub to the tip of the blades and multiply this by two.

PITCH Pitch is the theoretical distance a propeller will move through the water for each revolution (i.e. a 21” pitch propeller should move a boat forward 21” for each revolution, assuming there is no slippage).

RIGHT HAND OR LEFT HAND ROTATION In most cases, right hand propellers are used for single engine applications and one right hand and one left hand propellers are used for twin engine applications to achieve better steering control.

CUPPED BLADES This is a technique used to enhance the performance of propellers under certain conditions; a cupped propeller will exhibit very high efficiency at high speeds as well as minimize cavitation under heavy loads.

RAKE Rake is a special design feature. Positive rake benefits from normal bow lift to permit the use of a larger diameter propeller, while negative rake can provide extra blade strength for use in very high speed applications. Cupped Un-Cupped 0° Rake 20° Rake

W.O.T. Wide open throttle. With the boat fully loaded  the boat should run within 200RPM’s of the maximum rated RPM of the engine.

RPM: Revolutions per minute . Prop size differences have a big impact on boat performance. For example: • If at W.O.T. the RPM is too high, you will need to try a prop with a higher pitch or higher diameter. • If at W.O.T. the RPM is too low, you will need to try a prop with a lower pitch or a lower diameter.