Proper Prop Use

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1967 Sportsman LDFV  L.Shuster
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Last Updated:

22 December 2006

The OMC Stern Drive Service Manual has the following information and recommendations on selecting the proper propeller as a guide to selecting the "best" match to particular applications. Separate propeller application charts from the service manuals are provided for the electric stringer V-8V-6; and I-4 engine families, that use the 14 and 14.25-inch diameter props. (V-4, 2-strokes, with the "bullet" gearcase require a 10-inch diameter prop.)

Propeller Selection

The selection of the correct OMC Accessory propeller is the most critical single factor in achieving the satisfactory performance of the boat and motor as a unit. 

  1. It should be custom-selected to match the engine, the boat, the load, and the job you want it to do.
  2. It must permit the engine to turn in the recommended RPM range. THE SELECTION OF THE INCORRECT PROPELLER CAN CAUSE SERIOUS DAMAGE TO THE ENGINE. Note: 1964 - 1966 models: 120, 150, & 200 hp must ONLY use the 14.25" Diameter ("tapered-hub") series to clear the exhaust outlet. 

An improved alloy now being used in OMC accessory precision die cast propellers resists chipping, cracking, or bending. The malleability of the new material aids reconditioning of damaged propellers.

Selecting the Correct Propeller

When operating the boat at full throttle, under normal conditions, the engine rated RPM limits, and the boat speed are the controlling factors in deterring correct propeller blade pitch.

To obtain peak performance, the power unit should be "propped" out to allow the engine to turn in the upper half of the recommended RPM range. (DO NOT EXCEED THE MAXIMUM (redline)). It may be necessary to reduce the pitch of the propeller to gain RPM or increase the pitch to decrease RPM.

REMEMBER: If the engine operates below the recommended RPM range (at full throttle), you are losing power and efficiency. If you operate above rated RPM, serious damage to the engine may result. For your (particular) recommended operation (RPM range), refer to the specification section of your OMC service manual.

Condition of Boat Bottom

The condition of the boat (hull) bottom effects the boat's performance considerably. Marine growth, which will accumulate in fresh or salt water, will cut down on performance and speed. A boat bottom with evidence of slime, barnacles, or other foreign matter will cause a great reduction of speed and performance. Boat bottoms should be cleaned periodically in accordance with manufacturer's recommendations. In some areas, it is advisable to coat the boat bottom with anti-fouling paint. Some metallic-type anti-fouling can cause corrosion of the outdrive. We recommend use of OMC Accessory Ant-Corrosion Anode Kit, part number 171637 as further corrosion protection for the outdrive assembly.

Boat Trailers

It is extremely important that a boat fits the trailer properly. Loss in performance and speed in many cases, is due to improper trailer support and excessive tie-down pressure which has caused the boat bottom to "hook." The boat should rest firmly on the trailer with maximum tie-down pressure applied at the bow and transom only.

Boat Trim

If engine has been removed from boat or engine mounts disturbed, be sure the engine is re-installed in the manufacturer's originally specified position. Adjusting front engine mounts allow for trimming vertical sterndrive angle, in or out, as required, to compensate for a major change in boat loading and distribution of weight. Such a change may require selection of a different propeller pitch. Re-check engine performance and if necessary select a propeller that will permit engine to operate within its full throttle RPM range.