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This chart plots one motor's output against RPM. It's easy to see from the climbing line
that the faster the engine runs. the more horsepower it produces - up to a point. This
point is called the peak, and it is at this R.P.M. (in this case. 5000 RPM) that this
motor reaches its full rated output. After peaking, horsepower falls off again. Look at
the above chart once more, then complete the following statements.
1. At 3000 RPM, this engine produces only ----- % of rated HP.
2. At 6000 RPM, this engine produces only ----- % of rated HP.
It's clear that a motor must run in its operating RPM range for maximum performance. And
the R.P.M. System helps assure that it does.
Water Testing for R.P.M.
There are three propeller variables: Diameter, pitch and numbers of blades. In the water
test, the final "fine-tuning" adjustments can be made by changing the propeller
to one with a different pitch. Reducing or increasing pitch can raise or lower RPM.
That's because a low-pitch propeller takes a smaller bite than a high-pitch propeller. In
other words, low-pitch offers less resistance. With high-pitch. it's just the opposite --
there's more resistance.
Remember:
Lower-pitch = higher RPM
High-pitch = lower RPM.
That's really all there is to the second half of the RPM system.
With the motor properly installed, make trial runs at full throttle, checking the RPM on
the tachometer. Compare this with the full throttle RPM recommendation in the Owners
Manual listed under "Prop Selection".
Only three results are possible:
1. RPM correct range. Deliver the rig -- it's propped correctly.
2. RPM too low.
Reduce the pitch.
3. RPM too high.
Increase the pitch.
Trouble shooting with the RP.M. system
The R.P.M. System is designed primarily as preventive medicine. But if it should become
necessary to trouble-shoot a customer problem, the R.P.M. System can help. Basically, it's
the same two step process:
Step One: Rated Performance Mates
Use the OMC Propeller Chart to
double-check the initial prop selection.
Step Two: Revolutions Per Minute
Water test with a tachometer.
Adjust pitch to adjust RPM.
Naturally, most trouble-shooting situations will invoke only Step Two, the water test. But
it may be a good idea to include Step One as well, to be certain of the initial prop
choice.
And remember that the right prop will work as it should only when everything else is
right. As part of the regular trouble-shooting routine:
Check the transom height of your boat.
For outboard motors proper transom height is essential for maximum forward thrust from
your propeller. If the transom is too high, the propeller will ventilate (take in too much
air around the propeller) and may cause damage to your motor from excessive slippage. If
the transom is too low, the motor will cause excessive drag, resulting in loss of speed
and power and undesirable spray. To achieve the ideal transom length, it may be necessary
to cut out or add to the transom. This step, or course, is unnecessary when selecting a
stern drive propeller.
On a family-type boat the anti-cavitation plate of the outboard should run level to
1/2" below the boat bottom with the proper transom height. On large boats proper
level is 1/2" to 1" below boat bottom. On light. fast boats the engine can be
raised on the transom so that the anti-cavitation plate is level or above the bottom of
the boat. Cupped propellers may be necessary if slippage of the propeller occurs.
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