The propeller is what “connects” your outboard motor to the water. The proper propeller, in good condition, can get you the best performance out of your outboard; a damaged propeller, or one of incorrect size, can not only hurt performance, it can also damage the outboard (Figure Prop-01). Propeller size is expressed by two dimensions; diameter and pitch (Figure Prop-02). The diameter of a two-blade propeller is the measurement, in inches, from the outer tip of one blade to the outer tip of the other. On a propeller with (3) or more blades, the diameter of the prop is the diameter of a circle that touches the outer tips of all of the blades. The pitch is the “angle” of the blades. The pitch is the measurement in inches of how far forward the prop would “screw” itself if it was rotated in a solid material.

Although there is usually little choice when it comes to the diameter of a proper for a particular outboard motor, often there is a much wider variety of pitches available. The pitch of a prop can be compared to the gearshift of a auto with a manual transmission. A low pitch or “flat”pitch prop is comparable to low gear in the auto; good for hauling heavy loads at low speed, while a high pitch prop is the equivalent of high gear in the auto; it allow the auto to travel fast without over speeding the engine. Just as one would not want to drive 70 mph on the highway in first gear and risk over speeding the engine, one does not want to run a low pitch prop on a light boat with plenty of horsepower. Conversely, running a high pitch prop on a heavy boat with a relatively small outboard can “lug” the engine just as trying to drive the auto at low speeds in high gear will lug the engine. Lugging an engine can cause excessive wear and overheating.

PROP-01 - What this old Johnson Service Promotion Bulletin lacks in political correctness it makes up in effectiveness: a damaged propeller or a propeller of improper size can cause excessive fuel consumption and even damage the outboard motor. (click images for larger views)

People who deal with modern outboards, especially the larger engines, with discuss such things as blade area, blade rake, and other qualities of modern propellers but we need not concern ourselves with such discussions when it comes to the older and smaller outboards. Just to add to your maritime vernacular, however, a propeller is often referred to as a “wheel,” and a wheel with a pitch of the same measurement as it’s diameter is called a “square” wheel, while a wheel with a pitch measurement that exceeds it’s diameter is an “over-square” wheel.

PROP-02 - You will find little choice in the diameter of propellers for your outboard, but there may be a fairly wide selection of pitch available.

New propellers are available from Johnson and Evinrude dealers and also from Michigan Wheel Corp. for just about all of the OMC-built outboards which are my favorite subjects of this column. The choices of new propellers may be limited to only one or two for the smaller engines, but there is a wide selection of propellers available for the larger engines, say over about 10 hp. A special note concerning the 10 hp outboards: the propellers used on 1957 and earlier models will not fit 1958 or later 10's due to a lower unit change. Conversely, the style of prop that will only fit 10's made earlier than 1958 will fit all OMC 12's, 15's, 18's & 20's from about 1953 until about 1970, and will also fit late-60's 25's due to the similarities of the lower units on all of these models. Used props are a common swap meet item but identifying exactly which props will fit your particular outboard can be problematic, as will be determining the diameter and pitch of a particular prop. Unlike props for inboard boats, which are usually stamped with the diameter and pitch, most outboard props are marked only with a part number. Jim Michalak posted a simple and cheap method of determining the pitch of a propeller on his website a while back, along with some interesting technical theory on propellers in general.

PROP-03 - The fish line cutter was intended to protect the propeller shaft seal which is easily damaged by fish line wrapped around the propeller shaft; the cushioned propeller hub was intended to protect the propeller and drive train from the shock of an impact with temporarily disabling the engine as a broken shear pin wood. Outboards using the cushioned hub propellers used a much stronger “drive pin” rather than a shear pin.

Lacking any other resources, try mounting the prop in question to your outboard to see if it actually fits, and make sure the rotation (clockwise=”right hand”: counter-clockwise=”left-hand”, while standing behind the boat looking forward) is correct for your engine. Used props may also have damage that is difficult to detect, such as an out-of-pitch blade or a “slipping” cushion hub. Another concern is that in 1956 the propellers and lower unit housings for almost all OMC models where modified to incorporate a fish line cutter to protect the propeller shaft seal from being damaged by fishing line wrapped around it (Figure prop-03). Although in most cases propellers for particular models can be swapped across this production change (i.e.; a “line cutter” propeller can be used on a “non-line cutter” outboard, and also visa-versa), this is not always the case, and in any event the fish line cutter system will not work unless both the lower unit and the propeller have the cutter features.

PROP-04 - For whatever reason, the method of mounting the propeller varied from model to model; #1 Refers to outboards of 35 hp and over #2 Refers to 10's and 18's #3 Refers to 5 1/2's and 7 1/2's #4 refers to 3 hp models. This is for 1958; be aware that the horsepower for these models may vary for other years.. And before you take the propeller nut tightening procedure to heart, note the apparently contradictory advise given one year later in Figure prop-05.

New props for these older OMC outboards will always be made of aluminum, while most used props you will run across will also be of aluminum. Occasionally you will run across bronze (not brass) props. There was a debate in the “old days” about whether running a bronze prop was harmful to the engine. The thought was that the weight and inertia of the heavier bronze propellers would damage the dog clutch in the lower unit. My opinion is that most dog clutch damage is caused by improper shifting technique; the engine should be “snapped” into gear, but not forced; “easing” the engine into gear leads to excessive wear on the clutch dog and gear engagement faces, although the increased inertia of the heavier bronze prop probably does not help the situation.

PROP-05 - Improperly tightening the threaded propeller nuts some models used can lead to damage to the hub of the propeller. Be sure to check the hubs of any used propellers that you consider buying for enlarged and worn drive pin slots.

OMC used several different methods of attaching the prop to the propeller shaft (Figure prop-04). The little 3 hp models had a threaded prop nut covered with a “snap-on” rubber nose cone, although often the nose cone is missing. The 5 1/2's and 7 1/2"s had the snap-on rubber nose cone but no threaded prop nut; the prop was held on by the shear pin while the nose cone kept the shear pin in place. The 10's through 20's had threaded nose cones secured with a cotter pin, as did the 25's and 30's from the early and mid 1950's. The 35 hp model introduced in 1957 featured a plastic “slip-on” nose cone secured with a cotter pin. Many of these plastic nose cones have been broken by people who put a wrench on them thinking that they were threaded.

The shear pin is located at the aft (prop nut) end of the propeller on some motors and at the forward (lower unit) end of other models. So just what is a “shear pin?” It is a metal pin that locks the prop to the spinning propeller shaft and which is intended to “shear” or break should the prop come into contact with a hard object such as a log or rock, hopefully sparing the prop or other engine components from damage. The shear pin is intended to be a sacrificial “weak link.”

PROP-06 - The function of the rubber shock-absorbing hub on some models can be affected by how tightly the propeller nut is tightened.

In the mid 1950's, however, “cushioned hub” propellers were introduced (Figure prop-03). These props had a rubber-mounted hub which was intended to absorb the impact of a hit. The props continued to be locked to the prop shaft with a pin, but now it was a much stronger pin and the factory referred to it as a “drive pin” and not a “shear pin.” Although with some searching a genuine shear pin or drive pin for a particular model can be found, few people go to the trouble, and often pieces of nails or other material will be found in place of the proper pin. My own habit is to purchase brass rod of the proper diameter at a hardware store or hobby shop and to use a hacksaw to cut pins of the correct length. The brass is somewhat more prone to breakage than a proper shear pin (and definitely weaker than a steel drive pin) I just consider this as extra protection for the prop and engine.

Keep in mind, however, that a broken shear pin in the wrong situation could put your boat and it’s passengers in danger, so the weakness of a substitute brass pin could at times be a safety issue. I always make it a point to carry spare pins and I usually carry a spare prop as well. I have had the rubber hub of a cushioned hub propeller fail, allowing the body of the prop to slide on the hub and reducing the top speed of the boat to about one mile per hour. It would have been a long 6 mile trip back to the launch ramp if I had not had a spare prop in the boat.

PROP-07 - Greasing the propeller shaft before installing the propeller is always a good idea on these old outboards, especially if you intend to run in salt water.

If you are going to carry a spare, consider having one of a different size than your regular propeller; That way you can have a “speed” prop for light loads and a “power” prop for heavy loads.

Improper tightening of the threaded propeller nuts can lead to shear pin/ drive pin breakage and can also cause damage to the propeller or even the propeller shaft. Those models that use the threaded propeller nuts have a slot machined into the forward face of the propeller hub that the shear pin/drive pin engages. If the propeller nut is too loose, there will be excessive slack between the pin and the slot which will cause the slot to wear and enlarge, permanently damaging the hub of the propeller (Figure prop-05). Improper tightening of a threaded propeller nut can also cause damage to the rubber cushioned hub itself (Figure prop-06). When installing a propeller on an outboard, it is usually a good idea to grease the shaft to guard against corrosion and wear (Figure prop-07).

PROP-08 - Test wheels are something you are not likely to run across, and are really something you can live without, but this is their function.

Occasionally someone will bring up the subject of “test wheels.” These are special propellers used by mechanics to run outboards in test tanks (Figure prop-08). Since the typical test tank provides very poor flow of water to the propeller, a engine run at high rpm’s in a test tank will often surge faster and slower as the propeller alternately “bites” the water and then loses it’ bite. This surging can make testing and adjusting an outboard difficult. Test wheels are designed to properly load the engine in test tank conditions but to produce no forward thrust, eliminating the surging. A test wheel can not be used to propel a boat, and is an object that you will probably never run across. You should be aware, however, that running your outboard in gear with it’s standard propeller in a 55 gal drum of water, for example, will cause this same surging and it is not necessarily indicative of a problem with the engine. Also be aware that an outboard run for long periods of time in a test tank can easily overheat.