4108 RPM

[BigAl.NZ]

I think engine runs a little slow and interested to hear from other 4108 owners.

6 knots @2500 RPM she sounds happy
6.5 knots @2700 RPM she still sounds ok but starts to overheat

I am not sure what the prop is.

The overheat issue we are working on. But IIRC I saw a post a while back saying some people run 4108's @3000 all day without issue?

This one is a 1986 with 5100 hours on a 13 ton wet Hood 38.

Cheers

Al

[Lepke]

There are many reasons why an engine overheats at high rpm. Common is over propped, bad impeller, restricted strainer, foul bottom/prop, salt water side needs cleaning/flush, coolant side needs cleaning/flush. Less common restricted plumbing, restricted fresh air, restricted exhaust, bad trans/shaft bearing, thermostat not opening all the way, etc.
Some people use a bigger heat exchanger on 4108s.

[Wainui]

I would run mine at 2500 which would give me about 6 to 6.5 knots depending on conditions. I could get up to 2900 in gear WOT, but would overheat pretty soon. At 2600 I could get just over 7 knt.

[BigAl.NZ]

Originally Posted by Wainui

I would run mine at 2500 which would give me about 6 to 6.5 knots depending on conditions. I could get up to 2900 in gear WOT, but would overheat pretty soon. At 2600 I could get just over 7 knt.

Ok so it sounds like 2500 would be normal cruise RPM. Like you we overheat at full power (although IIRC some people said they can run all day at full power).

What size / tonnage you drive?

[Sojourner]

We have a Perkins 4108 in our Super Maramu, and the sweetspot for cruising 6+ knots is 2200 rpm....

[Wainui]

Cal 48 26,000lbs
Part of the problem with the 4108 is that it is a 3 bolt main ie the crankshaft is supported in only 3 places. At the higher rpms the crank can overtime whip around causing the rear main seal journal to wear or score. That happen to mine but it did take 52 years.
The other problem is that it generates 49hp at 4,000 rpm but that is a bench tested result so once you bolt on the exhaust manifold and transmission you are down to 3,000 rpms and about 38 hp. Running at 2,500 rpm is about 34 or less hp.
That’s why I went with a BM 50 which develops 50 hp at 2,800 rpm. It like a Yanmar, is a 5 bolt main.
Good luck
M

[a64pilot]

A two bolt main, refers to there being two bolts to hold a crankshaft cap in place, usually always in reference to a US Built V8 gasoline engine.
A four bolt main uses four bolts, two on each side and is stiffer.
The number of caps is a different but related thing, the more bolts and main bearings, usually is a better high RPM motor.

[svHyLyte]

FYI we have a 1986 era 4-108 with about 3500 hours turning a Gori 10x20 Prop on our 22K lb, 42 ft, boat. At 2500 RPM we average about 6.5 kn with the engine temp in the 180º F range. At 2750-2800, our speed is up to around 7.2 kn with no change in operating temp. We have run at 3000 RPM on occasion at slightly over 8 kn with temp rising to 190º-200º F but will rarely do so as we're not in that much of a hurry and "Perky" is happier at 2500 to 2750 RPM. Regarding the foregoing, note that our heat exchanger is "free and clear" of debris/scale given periodic flushing with fresh water and doses of Barnacle Buster recirculated through the system.

FWIW...

[BozSail]

We have a 4-108 (vintage 1980) in our 22,000 lb. Endeavour 37. Hurth 15MA manual transmission. We have a Max-prop (17 in. diameter, 15.7 in. pitch) and cruise at about 1,850 rpm making 5.5 knots. And, at that rate, we use about a half gallon of diesel per hour. I'm happy with that. At 2,000 rpm, we make a little better than 6 knots. The motor has at least 3,200 hours (I'm not sure when the prior owner, now deceased, installed the hour meter). Motor starts immediately every time and runs flawlessly. I love my old Perkins 4-108.

[BozSail]

Forgot to add... our 4-108 runs at about 175-180 degrees all day long.