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09-13-2022, 01:02 PM
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#1
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Veteran Member
City: Portland
Vessel Name: JOYSEA
Vessel Model: Grand Banks 36 Classic
Join Date: Sep 2013
Posts: 37
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Lehman rpm
1970 GB 36 twin Lehman 120. What should RPM be at 7 it’s? And what should top RPM be?
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09-13-2022, 01:26 PM
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#2
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Guru
City: Baltimore
Vessel Name: Sea life
Vessel Model: Krogen 42 #61
Join Date: Nov 2013
Posts: 686
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WOT 2500. 7kts is boat dependent. But 1800-2100 seems to be sweet spot. At 1600 over long period of time (1000 hrs) engine/injectors sooted up a bit.
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09-13-2022, 02:07 PM
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#3
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Veteran Member
City: Olympia, WA.
Vessel Name: Sea Witch
Vessel Model: Kadey Krogen 42 #47
Join Date: Jan 2015
Posts: 38
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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.
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09-13-2022, 02:16 PM
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#4
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Senior Member
City: Richmond
Join Date: Mar 2017
Posts: 310
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Maybe call Brian at American diesel and ask him what WOT should be as well as a good cruising RPM.
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09-13-2022, 02:33 PM
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#5
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Senior Member
City: Essex, Ct.
Vessel Name: Harmony
Vessel Model: 1982 41' President
Join Date: May 2013
Posts: 442
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My twins run nice at 1750 RPM's which turn the props at 875 RPM's thru the tranny. That yields 8 kts
__________________
Tom
"Harmony"
1982 41' President
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09-13-2022, 04:33 PM
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#6
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Enigma
City: Slicker?
Join Date: Oct 2007
Posts: 16,566
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Greetings,
Yep. 1750. Sweet...
__________________
RTF
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09-13-2022, 04:56 PM
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#7
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Guru
City: Bainbridge Island/Petersburg Alaska
Vessel Name: Oz
Vessel Model: Bluewater 40' RPH 1979
Join Date: Feb 2015
Posts: 514
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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
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09-13-2022, 05:16 PM
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#8
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Guru
City: Gulf Islands, BC Canada
Vessel Name: Sea Sanctuary
Vessel Model: Bayliner 4588
Join Date: Jul 2019
Posts: 5,017
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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
__________________
SteveK
You only need one working engine. That is why I have two.
Sea Sanctuary-new to me 1992 Bayliner 4588
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09-13-2022, 05:22 PM
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#9
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Guru
City: Gulf Islands, BC Canada
Vessel Name: Sea Sanctuary
Vessel Model: Bayliner 4588
Join Date: Jul 2019
Posts: 5,017
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Quote:
Originally Posted by Sealife
WOT 2500. 7kts is boat dependent. But 1800-2100 seems to be sweet spot. At 1600 over long period of time (1000 hrs) engine/injectors sooted up a bit.
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Not sure about bolded. Low load causes soot, low rpm maybe not.
__________________
SteveK
You only need one working engine. That is why I have two.
Sea Sanctuary-new to me 1992 Bayliner 4588
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09-13-2022, 05:23 PM
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#10
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Guru
City: Baltimore
Vessel Name: Sea life
Vessel Model: Krogen 42 #61
Join Date: Nov 2013
Posts: 686
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FYI, 1800 is 80% aka the Golden rule of load. I ran 1600, for a 1000ish hrs due to range/fuel requirements (long story) it definitely sooted up head.
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09-13-2022, 05:25 PM
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#11
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Guru
City: Gulf Islands, BC Canada
Vessel Name: Sea Sanctuary
Vessel Model: Bayliner 4588
Join Date: Jul 2019
Posts: 5,017
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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
You only need one working engine. That is why I have two.
Sea Sanctuary-new to me 1992 Bayliner 4588
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09-13-2022, 05:34 PM
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#12
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Guru
City: Baltimore
Vessel Name: Sea life
Vessel Model: Krogen 42 #61
Join Date: Nov 2013
Posts: 686
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I do, but not a fan of over prop, and most boats aren't set up that way (hence your inability to get to 2500 and other issues). That is a different discussion getting into governors etc.
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09-13-2022, 05:39 PM
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#13
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Guru
City: Gulf Islands, BC Canada
Vessel Name: Sea Sanctuary
Vessel Model: Bayliner 4588
Join Date: Jul 2019
Posts: 5,017
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Quote:
Originally Posted by Sealife
I do, but not a fan of over prop, and most boats aren't set up that way (hence your inability to get to 2500 and other issues). That is a different discussion getting into governors etc.
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I never said 2500, that was another poster. Yes we all have our opinions.
__________________
SteveK
You only need one working engine. That is why I have two.
Sea Sanctuary-new to me 1992 Bayliner 4588
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09-13-2022, 07:39 PM
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#14
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Guru
Join Date: Jan 2010
Posts: 1,181
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Bob adamerican diesel once told me 1900 RPM . He was adamant about that
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09-14-2022, 05:27 AM
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#15
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,154
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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.
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09-14-2022, 09:53 AM
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#16
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Guru
City: Boston
Vessel Name: Adelante
Vessel Model: IG 30
Join Date: Nov 2016
Posts: 1,611
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Quote:
Originally Posted by Sealife
WOT 2500. 7kts is boat dependent. But 1800-2100 seems to be sweet spot. At 1600 over long period of time (1000 hrs) engine/injectors sooted up a bit.
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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.
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09-14-2022, 10:24 AM
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#17
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Guru
City: Gig Harbor
Vessel Name: Sandpiper
Vessel Model: Bluewater 40 Pilothouse Trawler
Join Date: Nov 2012
Posts: 2,315
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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 worst day working on the boat is still better than my best day at a job"
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09-14-2022, 11:46 AM
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#18
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Veteran Member
City: Portland
Vessel Name: JOYSEA
Vessel Model: Grand Banks 36 Classic
Join Date: Sep 2013
Posts: 37
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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?
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09-14-2022, 05:10 PM
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#19
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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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.
__________________
Eric
North Western Washington State USA
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09-26-2022, 09:51 PM
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#20
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Senior Member
City: Miami
Vessel Name: Hyperion
Vessel Model: Grand Banks Heritage Classic 42
Join Date: Sep 2022
Posts: 157
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My 73 Glass 36 with twin 120s runs at 7 knots with both engines at 1500 RPM.
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