Lehman rpm
- Thread starter bjensen
- Start date
The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
If you have answers, please help by responding to the unanswered posts.
JC53
Veteran Member
Seems like the “sweet spot” for Lehman 120 is all over the place. My single seems to run best at 7 knots and 1750 rpms. I’ve never had it up to 2,500. Every time this comes up there seems to be as many sweet spots as posts. I find it all fascinating.
RT Firefly
Enigma
Greetings,
Yep. 1750. Sweet...
Yep. 1750. Sweet...
Tator
Guru
- Joined
- Feb 16, 2015
- Messages
- 516
- Location
- Bainbridge Island WA
- Vessel Name
- Oz
- Vessel Make
- Formerly Bluewater 40' RPH 1979
My Lehman's sweet spot on 40' FD hull is 1475 at 7.5 knots. 2.3-2.5gph w/ occasional heater and genset use.
Tator
Tator
SteveK
Guru
- Joined
- Jul 5, 2019
- Messages
- 5,903
- Location
- Gulf Islands, BC Canada
- Vessel Name
- Sea Sanctuary
- Vessel Make
- Bayliner 4588
In the GB36 with twins
At 1800-1900 I am doing 9-10 knots
At 1500-1600 I am doing 7-7.5
But it is over propped
At 1800-1900 I am doing 9-10 knots
At 1500-1600 I am doing 7-7.5
But it is over propped
SteveK
Guru
- Joined
- Jul 5, 2019
- Messages
- 5,903
- Location
- Gulf Islands, BC Canada
- Vessel Name
- Sea Sanctuary
- Vessel Make
- Bayliner 4588
Not sure about bolded. Low load causes soot, low rpm maybe not.
SteveK
Guru
- Joined
- Jul 5, 2019
- Messages
- 5,903
- Location
- Gulf Islands, BC Canada
- Vessel Name
- Sea Sanctuary
- Vessel Make
- Bayliner 4588
Did you understand the low load part. My over propped is still loaded at 1600, yours, maybe not and running to easily. See the difference?
SteveK
Guru
- Joined
- Jul 5, 2019
- Messages
- 5,903
- Location
- Gulf Islands, BC Canada
- Vessel Name
- Sea Sanctuary
- Vessel Make
- Bayliner 4588
I never said 2500, that was another poster. Yes we all have our opinions.
motion30
Guru
- Joined
- Jan 14, 2010
- Messages
- 1,216
Bob adamerican diesel once told me 1900 RPM . He was adamant about that
psneeld
Guru
I ran a Lehman 120 at 1600-1700 rpm for about 3700 hours/20,000 miles.
About 1/2 way I did a head gasket(job) and there was some but not much carbon present.
I agree that there is more to carbon build than just rpm. Using engine rpm is a rule of thumb, not a definitive guide.
About 1/2 way I did a head gasket(job) and there was some but not much carbon present.
I agree that there is more to carbon build than just rpm. Using engine rpm is a rule of thumb, not a definitive guide.
Yes, manual states 2500rpm WOT. Average rpm is variable depending upon current, sea state, tolerance for noise. I Typically set the rpm's at 1700,Maybe 1800, and the speed is what it is.
- Joined
- Nov 8, 2012
- Messages
- 2,323
- Location
- Port Orchard, WA
- Vessel Name
- Sandpiper
- Vessel Make
- Bluewater 40 Pilothouse Trawler
Cruise RPM is dependant on single or twin, transmission gear ratio, pitch of prop, shape of hull - FD or SD, total boat weight, condition of hull bottom, sea condition wind direction and many other factors.
My 39' full displacement trawler, 40,000# with single Lehman 120 cruises at 1720 RPM @ 8 knots burning around 2 gph for 4 kmpg. It has a 2.5 to 1 transmission and is overpropped.
With slime on bottom, speed drops .1 to .3 knots.
Sandpiper has been cruising at 1720 RPM for over 22 years, 5,000+ hours without issue. Injectors are only lightly sooted when I remove them for periodic injector service.
When I install the prop shop recommended "correct prop" - not overpropped, 8 knots is at 1850 RPM and consumption increases to 2.4 gph for 3.3 kmpg. Took that prop off and had the pitch increased.
My 39' full displacement trawler, 40,000# with single Lehman 120 cruises at 1720 RPM @ 8 knots burning around 2 gph for 4 kmpg. It has a 2.5 to 1 transmission and is overpropped.
With slime on bottom, speed drops .1 to .3 knots.
Sandpiper has been cruising at 1720 RPM for over 22 years, 5,000+ hours without issue. Injectors are only lightly sooted when I remove them for periodic injector service.
When I install the prop shop recommended "correct prop" - not overpropped, 8 knots is at 1850 RPM and consumption increases to 2.4 gph for 3.3 kmpg. Took that prop off and had the pitch increased.
OP
OP
bjensen
Veteran Member
Thanks for all the input! 1300 gives me about 7kts. One engine runs at a low temp 140 and doesn’t like to start in cool weather. 1800 rpm gets pretty Smokey.
Maybe the old Italian tuneup is the answer?
Maybe the old Italian tuneup is the answer?
Nomad Willy
Guru
Cruising rpm has little to do w power applied unless you’re propped to maximum power. That is 2500rpm on the Ford 120hp industrial/marine engine.
So if you get something lower than 2500rpm at WOT (w your chosen prop) you may be over propped and comparing your rpm w others may be misleading. Ideal rpm is (IMO) 50-100 rpm over WOT rated number if your governor is working properly. An engine in good condition is hard to damage if you’ve propped it 50-100rpm over the rated number. Most engine manuals state how long one can run safely at numerous specific high rpms. The range to watch most closely is 500 (or so) rpm below rated rpm.
But cruising engine speeds called out buy others is almost meaningless unless WOT rpm is also given. It’s the engine LOAD and resultant heat that is the danger. That’s why under-loading is good for your engine. It’s very hard to cause damage to an engine under-loaded 100rpm … that is a WOT engine speed of 2600rom on the Lehman. You could probably even run very near rated rpm for five to ten minutes normally.
Look up the FL’s continuous rpm rating for the Lehman. You will find how long you can safely run at various high engine speeds (assuming proper WOT rpm).
Some use a rule of thumb that at 500rpm below WOT rpm you’re “good to go” but that definitely not be the case if the engine was over-propped. But if you’re propped correctly 500 under should or could be good max continuous rpm for most common engines. Cruising rpm could be a bit less that that. My trawler engine was a 3000rpm engine at WOT and I considered 2500 to be OK for continuous running. Never ran it that hard for over 10 minutes tho.
Please call me out if I got something wrong. This type of post is hard to do w/o mistakes.
syjos wrote on post #17;
“My 39' full displacement trawler, 40,000# with single Lehman 120 cruises at 1720 RPM @ 8 knots burning around 2 gph”.
Since WOT fuel burn is 6gph 50% load would be 3gph. And 2gph would be well below that I’d say syjos is cruising very light. I’d be think’in 1900rpm. But maybe that’s not good vibration wise. Sometimes a “sweet spot” falls in the wrong place.
But he may just like less noise.
So if you get something lower than 2500rpm at WOT (w your chosen prop) you may be over propped and comparing your rpm w others may be misleading. Ideal rpm is (IMO) 50-100 rpm over WOT rated number if your governor is working properly. An engine in good condition is hard to damage if you’ve propped it 50-100rpm over the rated number. Most engine manuals state how long one can run safely at numerous specific high rpms. The range to watch most closely is 500 (or so) rpm below rated rpm.
But cruising engine speeds called out buy others is almost meaningless unless WOT rpm is also given. It’s the engine LOAD and resultant heat that is the danger. That’s why under-loading is good for your engine. It’s very hard to cause damage to an engine under-loaded 100rpm … that is a WOT engine speed of 2600rom on the Lehman. You could probably even run very near rated rpm for five to ten minutes normally.
Look up the FL’s continuous rpm rating for the Lehman. You will find how long you can safely run at various high engine speeds (assuming proper WOT rpm).
Some use a rule of thumb that at 500rpm below WOT rpm you’re “good to go” but that definitely not be the case if the engine was over-propped. But if you’re propped correctly 500 under should or could be good max continuous rpm for most common engines. Cruising rpm could be a bit less that that. My trawler engine was a 3000rpm engine at WOT and I considered 2500 to be OK for continuous running. Never ran it that hard for over 10 minutes tho.
Please call me out if I got something wrong. This type of post is hard to do w/o mistakes.
syjos wrote on post #17;
“My 39' full displacement trawler, 40,000# with single Lehman 120 cruises at 1720 RPM @ 8 knots burning around 2 gph”.
Since WOT fuel burn is 6gph 50% load would be 3gph. And 2gph would be well below that I’d say syjos is cruising very light. I’d be think’in 1900rpm. But maybe that’s not good vibration wise. Sometimes a “sweet spot” falls in the wrong place.
But he may just like less noise.
Hyperion
Senior Member
- Joined
- Sep 26, 2022
- Messages
- 157
- Vessel Name
- Hyperion
- Vessel Make
- Grand Banks Heritage Classic 42
My 73 Glass 36 with twin 120s runs at 7 knots with both engines at 1500 RPM.
