Re: Mfg's. recommend props allow designed rpm range

From: Robert Skinner (robert@140.com)
Date: Wed Mar 08 2000 - 17:17:49 PST


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Bill Blohm wrote:
...
> Oh, one other question. You talk about the motors purring along
> at 3/4 throttle. Ok. Tell me, how can I tell if the motor is
> running smoothly and neither being pushed hard nor overrun?
> I am completely deaf, so I can't tell by ear and I've yet to
> figure out how to tell when the engine is in the groove. I can
> tell if it's overrevved beyond a point by the sudden increase
> in high vibrations. I can tell when it's running too hard by
> the sudden balking, but there's a wide range in between those
> two endpoints.

Tough one. If I remember some conversations with another
deaf person correctly, he said that he could feel whether an
engine was running properly by placing his hand on top of
it.

If the engine was "in the groove", the vibrations were
"smooth". If it was straining, the vibrations got a little
"sharper". It may have something to do with the vibration
wave shape - sinusoid when the engine is happy, saw-tooth or
square (with a lot of harmonics) if the engine was not.

However, until your hand can learn what a happy engine feels
like, there are some RPM meters that will attach to the case
or block of an engine and give you some measure of shaft
speed. A oscilloscope might also serve as a temporary
bio-feedback device to help you correlate vibrations with
engine speed.

Another less complicated alternative would be to have a
_good_ mechanic go out with you and set the motor at optimum
speed, below optimum, and faster than optimum so that you
could feel what that was like.

BTW - the balking may be due to a bad carburetor setting,
spark problem, or incipient engine seisure, among other
causes. You are right to be concerned about it.

So do whatever it takes to make the cross-modal connection.
Good luck, sailor -- it can be done.

-- 
Robert Skinner, Rockville, Maryland
'87 Potter 15 HMS #1618 "Little Dipper"



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