This item came out of OMC a few years back and presents a very simplified approach to prop
selection. The concepts they cover can be used with any manufacturers product.
Understanding this approach and combining it with proper lake testing, you should be
able to reduce your time in selection and testing and insure a more accurate result in the
A successful boating dealer sells more than power. He sells performance -- performance for
anything from a high-transom racer to a heavy-load cruiser; from a utility runabout to a
water-skiing rig. And the all-important link between engine power and boat performance is
Today, with the broad range of hull, engine. and customer-use combinations possible,
choosing the right prop may seem a difficult and frustrating job. That's why the R.P.M.
System was developed.
The R.P.M. System can ...
... simplify initial prop selection.
. . make water testing easier and more accurate.
.. . provide a sure method for trouble-shooting.
The R.P.M. System can save time and money, and it will improve both sales and customer
relations. This booklet will provide a working knowledge of the R.P.M. System and make
most propping problems a thing of the past.
The R.P.M. System prevents many of the most persistent service problems and customer
complaints a dealer faces - problems that are often related to prop selection.
Based on personal experience, how frequently do the following service problems occur,
costing valuable man-hours and customer good will? Rate each by placing a check in
the appropriate space.
1. Poor performance at full throttle. ------- --------
2. Slow getaway and pick-up.
3. Poor fuel consumption.
4. Laboring engine.
5. Engine overheating.
6. Frequent plug fouling.
7. High-revving engine.
8. Excessive vibration.
9. Poor water ski performance.
10. Short motor life.
If there are more than five checks in the two left-hand columns. R.P.M. can produce
important savings in time and money. and strengthen customer relations.
If there are more than five checks in the "Never" column, you may know
intuitively what the R.P.M. System does systematically. In this case. the System's Value
lies in converting a "seat of the pants" skill to a sure method that works for
everyone in your Dealership. Either way, the R.P.M. System can save time and money for you
and eliminate some headaches as well - by routinely preventing these and other customer
difficulties associated with incorrect prop selection.
The R.P.M. System is a simple, two-step process that can be used by anyone. The only
requirement is a basic knowledge of propellers and how they work. Even if you're already
familiar with the meaning of words like "pitch" and "thrust cone."
this short review should be helpful. Look over the diagrams. read the description, then
take the self-tests which follow. filling in the missing words. Once you can answer these
few simple questions. you'll be better able to understand how the R.P.M. System works.
HOW DOES A PROPELLER WORK?
A propeller converts the torque of the drive shaft into thrust -- that is, it changes
"spin" into "go."
The prop works much like a wood screw. When you turn a screw out of wood. you can feel the
screw pushing against you with every twist of your hand as the threads grip wood. In much
the same way. propeller blades grip water, pushing against the boat and moving it forward.
But since water is liquid. there's a great deal of slippage. And as the prop bites, the
blades push water to the rear, creating what is called a thrust cone. It is actually this
thrust cone, pushing against the undisturbed water around it. which propels the boat
WHAT IS PITCH?
Prop design greatly affects the force of the thrust cone, and three propeller
characteristics are especially important in this regard. One of these is pitch: the twist
or angle of the prop blades measured against the direction of travel. Pitch is expressed
in terms of distance traveled. A 15-inch pitch means that, theoretically, a prop would
move 15 inches forward with each complete revolution. Slippage, turbulence. and other
factors make the actual distance less, but the higher the pitch, the greater the forward
At a given rate of revolution, the prop with higher pitch produces more speed.
WHAT IS PROP DIAMETER?
A propeller's diameter is the width of the circle described by the tips of the rotating
blades. As with pitch. changing the prop diameter changes performance. The greater the
diameter, thrust cone and blade area the more thrust a propeller will exert.
TWO-BLADE OR THREE-BLADE?
Increasing the number of blades is another way to increase total blade area and the
load-moving capacity of the prop. A three bladed prop of given pitch and diameter has
greater thrust than a two-bladed prop. For special applications, four or even five-bladed
props are used to gain greater thrust.
The objective of the R.P.M. System is to achieve "rated performance mates" -- a
prop/motor combination just right for the customer's boat and intended use.
The R.P.M. System provides a simple. fast method to correlate all the factors that assure
the correct initial prop selection. It's really very easy - just let your customer lead
Ask the customer these questions:
· What type of boat do you have?
· How do you plan to use the boat?
Suppose a customer has a 17-foot medium runabout which he intends to use for
general-purpose boating with average loads. Look in the OMC Accessories Catalog for the
Propeller Chart and locate the boat type that best describes the customer's rig .
The first column lists engine horsepower. Lets suppose your customer has a 1971
50-horsepower motor. All you need to do is read across to boat size and recommendation for
boat type and intended use. You'll find the propeller to choose for the initial propping,
with the right diameter, pitch. and number of blades for that particular customer
Notice that each propeller is described as follows: diameter x pitch number of blades. In
this case, for example - a 17-foot medium runabout with 50-horsepower motor - you find a
13 3/4 x 15 - 3 prop recommended.
The following three case examples describe a variety of customers and rigs. Read them
carefully. Then, using the propeller chart, find the right prop for initial selection in
each case. Write your specifications in the spaces provided.
EXAMPLE NO. 1: Jack is purchasing a 1971 50-horsepower outboard to power his boat: a fast.
17-foot runabout. He's primarily interested in performance with light loads -- only one or
two people aboard.
Part No. -------
A propeller is provided with each motor, but this propeller cannot possibly be correct for
every boat/motor combination. Example 1, presumes an ideal condition where the prop
provided is correct. If your initial selection is not the standard propeller that came
with the motor, it should be exchanged for an optional propeller at the time of purchase.
EXAMPLE NO. 2: Mike has become interested in water skiing and this season is trading up to
a brand new rig: a 19-foot runabout with a 1971 155 horsepower Stern Drive engine. He's
primarily interested in the quick getaway he needs to bring skiers up fast.
Part No. -------