We have been asked from time to time for recommendations concerning the plastic or non-metallic propellers. As of late these requests have become more frequent warranting the issuance of this bulletin.
On the plastic propellers we have checked to date we have found the efficiency to be lower than metallic propellers. The lack of efficiency is due primarily to the increased blade thickness particularly at the leading edge, necessitated by lower material strength.
In many applications where propellers are subject to abuse such as being run thru sand bars, gravel and oyster beds, etc., it was found that plastic propellers were not too durable. Under these conditions the plastic propellers will be worn down very rapidly, reducing the blade area further effecting their efficiency. The reduction of blade area in some cases will allow the engine to turn up above the recommended operating range, which could result in extensive damage.
Also, we have found that if the rubber shock absorber hub is installed directly in the plastic material, the propeller will slip very easily on the rubber causing early failure.
Make certain that the above facts are brought to the attention of all Sales and Shop personnel. It is our intention to further advise our dealer organization on future developments and information as it becomes available.
JOHNSON MOTORS
Jack Penland
Service Department
JP/eb