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Author Topic:   Optimum Engine Speed for Longevity

Matt F posted 08-13-2008 02:16 PM ET (US)
[Give ] opinions and advice on the optimum engine speed for cruising. My boat with the 1998 Johnson 115 seems to want to cruise at roughly 4,000-RPM. At wide open it seems to run at a notch under 5,500-RPM. Essentially, is working the motor too hard? Will it shave precious years of its life?
If helpful, the boat -- admittedly, not a whaler -- is 16’7” at the water, 6’8” on the beam and probably 1,700 pounds unloaded. Prop pitch is 18 (I think). Oil is OMC XD30 metered at whatever the VRO calls for.
Any thoughts are appreciated seeing as my mechanical proficiency begins and ends at more of the British Seagull end of the spectrum… Perhaps not as easy a question as I think?

Tohsgib posted 08-13-2008 02:30 PM ET (US)
PERFECT!
Matt F posted 08-13-2008 03:04 PM ET (US)
Tohsgib, thanks for the good word.
jimh posted 08-13-2008 11:33 PM ET (US)
From the sound of its exhaust, and coupled with the readings of instantaneous fuel economy I get from my fuel flow meter, my engine also likes to run at 4,000-RPM. It is 16-years old.
Smithsm posted 08-18-2008 02:30 PM ET (US)
I have read lots of boat and engine combination performance reports and the vast majority I have seen deliver optimal MPG at close to 4000 RPM
cooper1958nc posted 08-20-2008 03:14 PM ET (US)
There is no reason to assume that one RPM setting will be more efficient from a purely engine point of view, excluding idle and WOT.
Peaks in mpg seen on performance curves are due more to hull factors than to engine factors.
Planing hulls exhibit nonlinear characteristics, including reduction of wetted area (sometimes) with increasing speed in certain speed ranges. This, not some mysterious engine characteristic, is the origin of the "sweet spot" heard, felt, and observed.
This falls near 4000 RPM because 40 mph hulls whose engines peak at 5500 RPM, are running in the high 20's, where optimum mpg occurs for these hulls.
Notwithstanding the above, some engines do sometimes develop unpleasant vibrations or sounds at certain speeds. This should not be confused with fuel efficiency, but it is a factor in selecting the best speed.

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Author	Topic: Optimum Engine Speed for Longevity
Matt F	posted 08-13-2008 02:16 PM ET (US) [Give ] opinions and advice on the optimum engine speed for cruising. My boat with the 1998 Johnson 115 seems to want to cruise at roughly 4,000-RPM. At wide open it seems to run at a notch under 5,500-RPM. Essentially, is working the motor too hard? Will it shave precious years of its life? If helpful, the boat -- admittedly, not a whaler -- is 16’7” at the water, 6’8” on the beam and probably 1,700 pounds unloaded. Prop pitch is 18 (I think). Oil is OMC XD30 metered at whatever the VRO calls for. Any thoughts are appreciated seeing as my mechanical proficiency begins and ends at more of the British Seagull end of the spectrum… Perhaps not as easy a question as I think?
Tohsgib	posted 08-13-2008 02:30 PM ET (US) PERFECT!
Matt F	posted 08-13-2008 03:04 PM ET (US) Tohsgib, thanks for the good word.
jimh	posted 08-13-2008 11:33 PM ET (US) From the sound of its exhaust, and coupled with the readings of instantaneous fuel economy I get from my fuel flow meter, my engine also likes to run at 4,000-RPM. It is 16-years old.
Smithsm	posted 08-18-2008 02:30 PM ET (US) I have read lots of boat and engine combination performance reports and the vast majority I have seen deliver optimal MPG at close to 4000 RPM
cooper1958nc	posted 08-20-2008 03:14 PM ET (US) There is no reason to assume that one RPM setting will be more efficient from a purely engine point of view, excluding idle and WOT. Peaks in mpg seen on performance curves are due more to hull factors than to engine factors. Planing hulls exhibit nonlinear characteristics, including reduction of wetted area (sometimes) with increasing speed in certain speed ranges. This, not some mysterious engine characteristic, is the origin of the "sweet spot" heard, felt, and observed. This falls near 4000 RPM because 40 mph hulls whose engines peak at 5500 RPM, are running in the high 20's, where optimum mpg occurs for these hulls. Notwithstanding the above, some engines do sometimes develop unpleasant vibrations or sounds at certain speeds. This should not be confused with fuel efficiency, but it is a factor in selecting the best speed.