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Propeller Installation
, Determining RPM, Propeller Sizing, Propeller Terminology

Q. What prop should I use with my boat and motor?
A. First, determine how the boat will be used or what the normal load will be. If this boat normally operates with one specific passenger load, propeller size is relatively easy. If it has multiple uses ranging from light to heavy loads, the selection of one or two propellers may be necessary.
Q. Why change propellers?
A. The stock outboard with which most outboards are equipped is a compromise. Since it has fixed diameter and pitch, it is limited in its use and may not provide satisfactory performance for every combination load that will be encountered. One important fact to note is that the propeller moves the boat through the water at a specific engine R.P.M., and H.P. is directly related to that R.P.M. The engine cover is marked with a certain H.P. rating, but in most instances the full benefit of the possible H.P. is not realized. Along with the H.P. rating, equal emphasis should be placed on the R.P.M. at which the related H.P. is developed. This is where the propeller comes into the picture. Outboard engines are designed to run at peak R.P.M for full efficiency. Excessive R.P.M. with its increased friction and wear is harmful. It is equally harmfull to run the engine overloaded to the point that it can not achieve its rated R.P.M. This results in excessive carbon build up with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.
Q. How can I be sure my motor is operating within the recommended rpm range?
A. This can be checked with a tachometer. There are various types commercially available.
Q. What are diameter and pitch?
A. If a propeller is specified as 10x12 in size, this indicates it is a 10" diameter by 12" pitch. Propeller Dimensions are always given in this order. Diameter is determined by doubling the distance between the blade tip and the center of the hub. Pitch refers to blade angle. In this example, the 12" pitch indicates that with each prop revolution the boat theoretically would advance 12". Due to slippage, actual advance is somewhat less.
Q. Why do outboards of the same power sometimes take different props?
A. This is due to differences in lower unit gear ratios. Stock outboards are geared so that the propeller shaft turns at a slower speed than the R.P.M at the powerhead. This is usually expressed as a ratio such as 12:21 or 14:28, referring to the number of teeth in the drive gears. In the first example, the crankshaft gear has 12 and the propeller shaft the gear has 21. This means the propeller shaft turns only 57% as fast as the indicated R.P.M at the powe head. The lower the gear ratio, the larger the propeller that can be used. In other instances, engines of different makes may develop their maximum horsepower at different levels. Check your owner's manual to determine the correct R.P.M for your engine.
Q. What is the correct transom height for my outboard engine?
A. On average boats, it is best to mount the engine so the cavitation plate is approximately 1" below the bottom of the keel, or 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. The best transom height can only be determined by experimenting. The best performance will be obtained by mounting the engine as high as possible, or to the point just before propeller cavitates excessively.
Q. Will a different prop correct bad torque action (listing and hard steering).
A.Usually not. Most likely it is the result of any of several irregularities in the hull, the steering hook-up, or the engine mounting. If an engine has a right rotating propeller, steering wheel should be on the right or starboard side. Modern outboards have built-in features in the lower units to compensate for torque. Engine tilt should be such that the prop is horizontal when underway. If it is up or down, the propeller can have a definite pull to one side. See that the engine is at the exact center of the transom and is sitting level. Check boat bottom for warping., distortion, which could cause difficulty.