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.
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.
This can only be checked with a tachometer. There are various kinds commercially available.
These are the two common propeller measurements. If a propeller is specified as 10 x 12 size, this indicates and 10" diameter by 12" pitch. Dimensions are always given in this order. Diameter is determined by doubling the distance between blade tip and center of hub. Pitch refers to blade angle. In this example, 12" pitch indicates that with each prop revolution the boat theoretically would advance 12". Due to slip loss, actual advance is somewhat less.
Yes, in most cases. Original equipment propellers are pitched a little on the high side. Not knowing the boat the engine will be used with, the manufacturer pitches the prop a little high so the engine does not exceed the top rpm if placed on a light boat. However, on a heavier boat, or with water skiers, this propeller tends to overload the engine, resulting in poor speed, poor acceleration and sluggish performance, making it difficult to get a skier up. This is corrected with a lower pitched prop.
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.
On average boats, it is best to mount the engine so the cavitation plate is approximately 1" below 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.
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.
Depends on the material. Those made of bronze or sand-cast aluminum are repairable at about 1/3 to 1/2 the new propeller price. Die-cast propellers generally are not repairable. The material is very brittle, breaks off easily in the straightening process and will not weld satisfactorily. (Original equipment propellers are usually made of die-cast.) It is advisable to discard such propellers and replace with the more durable sand-cast aluminum or bronze.
Extensive service is offered by propeller manufacturers at the factory and through authorized propeller repair stations located across the country. These stations are operated by factory trained people employing the same methods and equipment in use at the factory.
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.
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.
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.