First determine how the boat will be used, or what the normal load will be. If this boat usually operates with one specific passenger load, propeller size selection is relatively easy. If it has multiple uses ranging from light to heavy loads, the selection of one or two propellers for best results will be necessary.
FREQUENTLY ASKED QUESTIONS
WHAT PROP SHOULD I USE WITH MY BOAT AND MOTOR?
HOW CAN I BE SURE MY MOTOR IS OPERATING WITHIN THE RECOMMENDED RPM RANGE?
This can only be checked with a tachometer. There are various kinds commercially available.
WHY CHANGE PROPELLERS?
The stock propeller with which most outboards are equipped is a compromise. Since it has fixed diameter and pitch, it is really limited in its use and it does not provide satisfactory performance for all the combinations of hulls and loads that will be encountered once it is installed. One important fact to note is that the propeller moves the boat through the water at a specific engine rpm, and h.p. is directly related to the developed rpm. The engine cover is marked with a certain h.p. rating but in most instances the full benefit of the possible h.p. is never realized. Along with the h.p. rating equal emphasis should be placed on the rpm at which the rated h.p. is developed. This, of course, is where the propeller comes into the picture. Outboard engines are designed to be run at peak rpm for full efficiency. Excessive rpm with its increased friction and wear is obviously harmful. It is equally harmful to run the engine so overloaded that it cannot achieve its rated rpm, since this results in excessive carbon build up in the cylinder with subsequent problems of; poor fuel-economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.
CAN I TROLL BETTER WITH A DIFFERENT SIZED WHEEL?
Low pitched propellers are always best for trolling. The lower the pitch, the better. Standard propellers with relatively high pitch troll too fast and in throttling down to extremely low speed, they tend to overload the engine. A low pitched wheel relieves overloading, permitting the engine to idle faster while moving the boat slowly.
WHAT IS THE CORRECT TRANSOM HEIGHT FOR MY OUTBOARD ENGINE?
On average boats, it is best to mount the engine so the cavitation plate is even with the bottom of boats without keel. For racing boats, better speeds can be attained by raising the engine to reduce lower unit drag and exhaust back pressure. Best transom height can be only be determined by experimenting…get the engine as high as possible, or to the point just before propeller cavitates excessively.
CAN ONE PROPELLER GIVE TOP SPEED PLUS LOTS OF POWER?
Generally no. To gain high speeds involves sizes or pitch ratios unsuited to load carrying or working conditions. Decide what is most important and select the prop accordingly, or change props with boating conditions. We’d recommend looking into a shifting propeller, which we sell at Precision Propeller.
IS IT ADVISABLE TO HAVE OUTBOARD PROPS REPAIRED?
We can repair about any propeller at Precision Propeller. If you have a damaged prop, we’d encourage you to send us a photo of your damaged prop for a proper estimate. Upon looking at the damage, we’ll recommend repairs if the cost is below ½ of what purchasing a new prop would be.
THE RUBBER HUB IN MY PROPELLER SEEMS TO BE SLIPPING. IS THIS POSSIBLE?
It is a definite possibility, but it does not occur too frequently. Take a look at the propeller. If the blades are visibly bent or distorted, you very likely are experiencing cavitation – and cavitation is often mistaken for a slipping bushing. Have it checked by the prop manufacturer or a reliable prop service station. The bushing can be replaced if it needs it or the blades can be restored to proper accuracy to eliminate the cavitation.
WHAT ABOUT PLASTIC PROPELLERS?
Nothing to date has been developed that has all the qualities of propellers made of metals. A good propeller must be durable, repairable and above all, it must perform well. So far the available plastics fail in these important requirements. Plastics are too thick and not hydrodynamic.
WHAT DO THE STAMPINGS ON MY PROP MEAN?
Most props are stamped on forward end with diameter, pitch and shaft size. Diameter is always first, pitch second. Size is likely repeated on the side of the hub with the manufacturer’s name, type prop, hand and date of manufacture.