Choosing a Prop

Home Up Choosing a Prop Props OMC Models: 62-72 OMC Models: 73-77

 

1967 Sportsman LDFV  L.Shuster
Proper Prop Use



Last Updated:

22 December 2006

Lets face it, selecting the right prop for your boat can be a daunting task. Props come in many sizes, designs and materials. This page will attempt to demystify prop selection for your boat and your intended marine application.

Prop Diameter by Prop Pitch:  The diameter of a prop is measured from the outer blade tip and is usually expressed first in prop specifications.

Prop Diameter

Prop Pitch


The pitch of a prop is defined as the theoretical distance the boat will travel with one complete revolution of the prop without slippage. Typically, there is 15 to 20 percent prop slippage at speed in water. For example, a  14"- diameter by 18"- pitch sized prop (or 14" x 18"), simply means that the blades are 14" in diameter and the blades are set at an angle that will theoretically move the boat 18" in one complete revolution of the prop. An automotive analogy to prop pitch is your final drive ratio in your car's differential or drive axle.

But you ask, who the heck cares how far the boat moves, only how well it performs it's intended tasks?  Start by checking the engine RPM on the tachometer at Wide Open Throttle (WOT). Most outboards redline at 5,500 RPM wide open; inboards generally red line at 4,500 RPM. Check your owners manual.

If your boat does not run at or very near its recommended redline RPM at WOT, several things may be contributing to lower RPM.  A mechanical problem (such as poorly tuned ignition timing or incorrect air/fuel ratio) can exist. Altitude above sea level and ambient air temperature can also greatly affect the performance of a motor. If it is an extremely hot day, performance will drop. Also, how your boat is loaded or it's displacement weight can have an drastic effect on it's performance. Is the hull freshly waxed and free of speed-robbing marine growth? Your boat simply may not have the correctly sized prop for your intended application and locale.

If your motor is mechanically sound, and it is in "average" climate conditions, but your WOT RPM is too slow or BELOW redline, you may need to decrease pitch an inch or two in pitch, assuming you maintain a constant prop diameter. Typically, a one-inch pitch decrease will increase WOT redline engine speed by 250 RPM. 

If you engine runs above the specified redline (too fast) at WOT, you may need to increase pitch. A one-inch pitch increase will typically drop engine speed by 250 RPM at WOT. Most times, pitch can be altered by a good prop repair shop, without the need for purchasing a new prop.

Use the software link below to calculate you prop pitch. Or view the prop charts listed on the Proper Prop Use page.

Prop Calculator by Surfbaud Freeware (requires Excel Spreadsheet)
< calculates your speed and pitch required >

Designs and materials: Props typically come in three materials, and have many designs. Typically, props are made of aluminum, stainless steel, and brass. Historically, props were made of brass and had only two blades for most applications. This was fine back then, due to the lightweight designs of boats. Modern applications require more power to carry the load.

Three blade props are now most common, and the aluminum prop is probably the most popular on Sportsmans, but if you want higher performance with your boat, consider upgrading to a stainless prop. Stainless steel props are two times stronger than bronze (but weigh less) and five time stronger than aluminum (but weigh about double aluminum), resulting in greater durability. A stainless steel prop will exhibit less slippage than aluminum, resulting in better fuel economy and improved performance.

OMC and MWC offered a 14.00" or 14.25" (see note below regarding 1964 - 1966 models) diameter prop in pitches ranging from 12" to 22" for the four-cycle I/O Sportsman. (The V-4, 2-stroke and 110 hp models used a 10-inch diameter prop and are not covered here). Typically, a Sportsman will operate best with a 16" to 20" pitch prop, but your mileage may vary. Stainless steel props were offered in odd-size pitches and are often painted black or were Teflon-S coated. Aluminum props were offered in even size patches and were originally painted white.

Available Propellers for the OMC Electric Stringer (Type 14):

Note: The 1964 - 1966 120, 150 and 200 hp models must ONLY
use the
14.25" Diameter ("taper-hub") series to clear the exhaust outlet.

Part No Diameter Pitch Material Mfg Blades
381296 14.25 10 AL OMC 3
382676 14.00 11 AL OMC 3
012089/PR119 14.00 11 AL MWC 3
382017 14.00 12 AL OMC 3
380187 14.25 12 AL OMC 3
012090/PR120 14.00 12 AL MWC 3
172490 14.00 13 SST OMC 3
381185 14.00 14 AL OMC 3
380074 14.25 14 AL OMC 3
012091/PR121 14.00 14 AL MWC 3
172491 14.00 15 SST OMC 3
381186 14.00 16 AL OMC 3
380188 14.25 16 AL OMC 3
012092/PR122 14.00 16 AL MWC 3
172492 14.00 17 SST OMC 3
381187 14.00 18 AL OMC 3
380189 14.25 18 AL OMC 3
012093/PR123 14.00 18 AL MWC 3
172493 14.00 19 SST OMC 3
383287 14.00 20 AL OMC 3
379652 14.25 20 AL OMC 3
012094/PR124 14.00 20 AL MWC 3
380190 14.25 22 AL OMC 3
(bold indicates prop within inventory)

MWC = Michigan Wheel Corporation