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Old 26-07-2009, 13:25   #1
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Perkins 4-108 RPMs

Hey all. I have a perkins 4-108 on an Ericson 39b sailboat, Disp 19,000 lbs, with a 2 blade prop. Underway at 2000 rpm I am going 4 knots, I expected to be making 6 knots or so at 2000 rpm. Is it alright to run the boat at 2500 rpm to maintain hull speed. Is this bad? I have very little if any vibration and I just cleaned my prop. What should be the rpm that I should be running the boat at to maintain hull speed?
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Old 26-07-2009, 13:38   #2
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As I recall, the Perkins 4-108 manual specifies the MAX RPM for a pleasure boat at 3600 RPM, which would suggest a maximum continuous/cruise engine speed (80% of max.) of 2880 RPM.

The manual also rates her at 3000 RPM Max for commercial use, which would suggest a continuous speed of 2400 RMP cruise, in that application.

Shop Manual ➥ http://www.fujiyachts.net/manuals/Pe...p%20Manual.pdf
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Old 26-07-2009, 13:45   #3
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You might be better off with a three blade prop. Do the research, but if you are keeping the boat for a long time, you might get a good return on a new prop.
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Old 26-07-2009, 16:11   #4
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No question a 3-blade prop would do better, but it's more drag unless you have a folding or feathering prop. I have a 3-blade MaxProp, 17" diam, on my 42' 27K pound sloop.

Shoot for no more than 2500RPM cruising. 2100-2200 RPM is better; considerably less fuel use. With the proper prop, you should get 6-6.5 knots at cruising speed. Also, the 3-blade will give you much better control in reverse.

BTW, the 4-108 is WAY overrated in terms of HP. It's not a 50HP, nor a 42HP, nor even a 36HP engine...all very common ratings for the 4-108. It should be rated more like 28-30HP.

That said, I love my 28-year old 4-108. Going strong and very dependable.

Bill
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Old 26-07-2009, 16:28   #5
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wow, I dont think my old 4-108 was rated above 2800 rpm... Did they change later on? 3600 would be up at Yanmar rpm...my sweet spot and max hull speed was about 2450.....? I havent looked at the curves, but would be surprised if a 28 hp engine would push my 34000 # 44 footer through rough chop and everything else at hull speed at 2600 rpm or less
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Old 26-07-2009, 17:27   #6
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The general design rule, I believe, is that you should be able to drive the boat at 95 to 100% of hull speed at 80% of max rpm. If not, then either the prop is not right, or the engine is too small (or you have a compression or other engine problem).
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Old 26-07-2009, 18:05   #7
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50 HP 4-108

Our 1986 era British issue Perkins 4-108 is rated at 50 HP at 4000 RPM's. On our boat it is driving an approximately 20,000 lb 43+ foot hull through a Gori 20" dia folding 2-blade prop. At 2000 RPM's in calm water and light winds we will run at 5.5-6.0 knots. At 2500 we average 7 and at 3,000 rpm over 8 knots without difficulty. With the peddle to the fire-wall we get close to 9 knots. Our operators manual suggests one should operate the engine at 75% power for optimum fuel economy and life-span. We generally run at around 2800 RPM on extended trips when we loose the wind and find that we average about .9 gph without difficulty. If we can't maintain 4.5 knts or better (unless day sailing) we'll motor, but with any wind we average 6+ knots easily and base out trip estimates on 7.5.

FWIW...
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Old 31-07-2009, 09:59   #8
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Just wanted to say thanks for the info. Had a family emergency so have not been online in a few days.
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Old 31-07-2009, 10:36   #9
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I did a little research several days ago. Evidently the Perkins is continuous duty rated at something like 2800 rpm and Recreational rated at 3000 rpm.... If I'm remembering right... Evidently it produces about 42 hp in this range? and that is the sweet spot in the power curve. It will do higher rpm, but most of the info I found says it's not what's recommmended for fuel economy and longevity. Sounds like a different philosophy than what Yanmar mechanics recommend, which is usually running near peak rpm.... That has always been my rub with Yanmar. I contend they are selling you an engine rated at peak HP at peak RPM and claiming it's a lighter engine, realistically, maybe it's just an over rated engine...
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Old 31-07-2009, 22:26   #10
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Aloha Seamaid,
Welcome to the forum of many opinions. My opinion is that you can increase your RPMs to 2500 with no problems. Do not change the prop until you really are certain of what you want. Larger prop size or a 3 blade with the same pitch will certainly decrease RPMs but may overheat your engine at its best rated RPM.
FWIW 50hp at 4000 is much less power at 2000 RPM.
In addition to knowing displacement, waterline length, HP etc. You need to also calculate gear ratio of your transmission, diameter and pitch of your propeller. Some folks with the same engine and boat may have a different size prop because their transmission is a different ratio.
Kind regards,
JohnL
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Old 23-07-2012, 11:24   #11
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Re: Perkins 4-108 RPMs

I have a 4-108 on a 40' 24,000 LB sailboat with a small (12" 3 blade prop) and a one speed transmission. I think I used to get 2400 (or maybe more) rpms at top cruising speed before my Lucas CAV fuel injector main shaft broke after running it full tilt through rough water while up-wind sailing for 10 hours with a dirty fuel tank in the Georgia Straight. After replacing the fuel injector pump with a rebuilt one, installing a new lift pump, rebuilding the fuel injectors, and cleaning the fuel tank and lines, I haven't been able to get the rpm's under power to go above 2,000 rpm and my top cruising speed seems less (barely 6 knots on calm water and 3.5 knots against the wind in rough water). Though out of gear, I can easily rev the motor up to 3600 rpm or more (which I dare not do).
I have come up with two possible solutions that may help increase my on-load rpm's and would like your opinions.
First: could the governor on the rebuilt fuel injector simply be adjusted too low? And if so, can adjusting this governor give more rpm's under load or will it just increase my idle speed?
Second: what I.D. size flexible fuel lines are recommended for this motor coming from the fuel tank to my new mechanical lift pump? I notice the original fuel line coming from the tank to the shut off valve (one foot away) is huge (aprox. 3/8" to 1/2" I.D.). But then the fuel line changes to 1/4" I.D. for three feet to the primary filter and 1/4" I.D. from the pri. filter to the lift pump. Even though this is the same arrangement I had originally, I'm wondering if changing these lines to the maximum size (maybe 5/16", 3/8", or even 1/2" I.D.) might help increase rpm's under load.
Do any of you guys who are getting these high (2400 - 3000) rpm readings under power know what size fuel lines they have pre lift pump? And could it hurt anything to increase my pre lift pump lines to maximum size?
Also, my motor used to start easily and almost instantly without needing to heat the intake with the flame thrower. But after the rebuilt fuel injector pump install, I needed to install the flame thrower and use it for 30 seconds before it will start unless it's already warmed up.
Extra info: My engine did not over heat before the F.I. pump broke, though it may be possible that my valves have gotten dirty from that first 10 hour lug against the wind at angle. Since the F.I.P. rebuild, I have put at least 100 working hours at 2000 rpms, which may have helped clean the valves a little bit. I do get a little blue smoke on start up sometimes, but not enough to worry about it. My engine temp never exceeds 160 degrees.
Thanks for any comments.
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Old 23-07-2012, 12:06   #12
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Re: Perkins 4-108 RPMs

My Perkins 4-108 is 30 years old and still going strong...Can you see me on my 30'(35' LOA) making motion at 2500rpm? Make way baby!

@dancamp009 - I cannot speak to the fuel line sizings ...I will take a closer look at mine this evening, but with slow consideration you may be onto something if the governor on the rebuilt fuel injector is adjusted too low. hmmm, I would be interested in what feedback you get.
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Old 23-07-2012, 12:21   #13
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Re: Perkins 4-108 RPMs

I'm not certain I know the answer to your question but have a couple thoughts. When you changed the fuel injection pump and did all the work on your fuel system were you in the water or out? Did you change the prop?
Have you cleaned the bottom or inspected it lately and how clean is your prop? A dirty prop will slow rpms a bit and a dirty prop and bottom will slow the boat considerably.
Exhaust systems have a habit of clogging things up a bit and effecting your rpms at load.
It's always best to point at the things you "fixed" that changed your engine's performance. The worst would be another fuel injection pump. A governor problem would not allow you to get 3600 rpms. A throttle problem too would not allow you to get to 3600 in neutral.
A retrace of all the work you did on the fuel system and the adjustments you made might just allow you to find a flaw but your symptoms point to over sized prop or clogged exhaust system except you say that you are not overheating. This is a mystery.
kind regards,
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Old 23-07-2012, 12:32   #14
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Re: Perkins 4-108 RPMs

Another problem could be transmission or shaft. Put your transmission in neutral and see if you can turn your drive shaft by hand. It should turn pretty easily. If it is binding then you know something is causing friction in your driveline or something has fouled your prop.
Good luck in your troubleshooting.
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Old 23-07-2012, 12:52   #15
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Re: Perkins 4-108 RPMs

Fuel lines should not be an issue, just not burning that much fuel. My 4-108 (in a much heavier application) burned an average of .65 gal per hour over the life I owned it. That's probably a fast drip!
Unless it's a prop covered with barnacles or seized shaft as noted, I'd say it must be something in the rebuild. Someone correct me if I'm wrong, but, couldnt the engine be getting enough fuel, unloaded, to get to 3600 rpm, but not getting enough under load?
Have you had the mechanic who rebuilt the injection pump look at it?
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