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Old 27-12-2010, 04:50   #1
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Perkins 4-108 Power Curve

Hi all - We're looking at purchasing a boat with a 1989 Perkins 4-108 and I've been trying to figure out if she'll have enough umph to move her along. The formulas in Nigel Calder's cruising book tell me I need 39 SHP to do the trick. Based on that I thought that the 50 HP Perkins would be fine, but I've been warned that the Perkins HP ratings are a bit optimistic. Anyone have a power curve for a 4-108 or know where I can find one online? Haven't had any luck with Google...

Thanks, mike
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Old 27-12-2010, 05:47   #2
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What's the boat?
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Old 27-12-2010, 07:08   #3
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4.108 power

The attached is a copy of the original Perking sales brochure. The claimed 50 HP is at 4,000 RPM. The engine does however have a lot of low speed torque which is what makes it a workhorse.
A more realistic claim would be 35 to 36 HP at 2,500 RPM which is a typical run RPM.

Have fun, Stanley
Attached Files
File Type: pdf Perkins 4.108.pdf (467.6 KB, 2690 views)
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Old 27-12-2010, 07:45   #4
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This is obvious--but I'll ask anyway. How does the boat perform in it's present set-up, in your opinion.
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Old 27-12-2010, 10:41   #5
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4108 in a 37ft gulfstar - 20,000 pounds - works fine - many boats were set up like mine, large prop - can't really run the motor over 2000 rpm or you overload it...using not more than 30 hp, more like 15-17 hp at 6.5 knots - about 3/4 gallon of fuel burn per hour - rule of thumb, 20 horsepower run for 1 hour takes 1 gallon of diesel.

They will last forever if not overloaded, but they are not a modern diesel, I agree that 2000-2500 rpm should be considered max continuious cruise rpm if any load at all is on the engine. A Perkins in a sailboat that is overloaded usually lets you know it by having an overheat problem...slowly rising coolant temps- most had a fairly marginal system on them, so when you pull too much power, they get hot....

Search under "Perkins 4108" on this forum and you will find a lot of info.

fair winds
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Old 27-12-2010, 13:28   #6
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Thanks all for the info.... the boat we're looking at is a 45' one-off, advertised light displacement is 28,500#. I added a ton when crunching the numbers in Calder's book, but of course that's much less than a full cruising load. 35-36 HP at cruising revs will leave us undergunned, but there are compromises in every boat, hey? Just trying to evaluate how big a tradeoff this one will be. The hull is quite slippery for a cruising boat. I haven't seen the boat, as she's on the other side of the ocean from us. Broker tells me that she is either over-propped or has an issue with sticking throttle cable, as she currently tops out at 1900 RPM. He claims 6 knots at those revs, but of course that's with an empty boat and flat water!
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Old 27-12-2010, 15:30   #7
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Our 1988 41' Passport is loaded big time to 36,000lbs. Our 4108 with a Maxprop pushes her at 2000rpm cruising speed of 6.5 kts while burning .8 gal/hr. We have the prop set up for max rpm of 2800 at a speed of 7.4. We are very satisfied with 6500 hrs and close to 50,000 miles under the keel.
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Old 27-12-2010, 16:42   #8
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Thanks for that, Alan... I suppose we'll know more when we get this boat in the slings and see what she really weighs. I suspect our cruising displacement would be more than yours, but even if we are a ton or so heavier it sounds like we might be ok with the 4-108... Mike
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Old 27-12-2010, 17:58   #9
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Mike, Here is what I have learned from Perkins Engineers after threats of lawsuits and a bunch of haggling. We had a 4-108 in our Mariner 40 and almost lost the boat once because we did not have the power to get out of our own way in anything above flat calm. Well, maybe that is an exaggeration. The 4-108 looses almost 20 percent of the rated horsepower because it needs a larger exhaust, however it is not possible to put a larger exhaust on that engine. In addition the rated 50 horsepower is on a bench at 4,000 RPMs with no load. Another impossibility with an install on a boat. The transmission and prop also make a big difference in the equation. Without all of the gory details, our Mariner 40 was 28 horsepower at the prop, way too little for a 30,000 lb. boat and no way to improve it. So we re-powered with a 56 horsepower Yanmar with the same transmission and prop and low and behold we had 56 HP at the prop with maximum RPMs. Chuck
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Old 28-12-2010, 01:58   #10
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I'm wondering if I am under propped...

I have a 4-108 in my CSY 33. I often run at 3000rpm for cruise, 1-3 hrs at a time before dropping back a tad for a bit. I have run at 3500 when powering off a bar. Have a 16 R10 3 blade prop.

Wonder if I should add some pitch?

Btw, no overheating problems after taking apart the cooling system and degunkifying the coolant passageways of slime, and cleaning the heat exchanger. The slimy, waxish buildup was apparently from the PO using different coolants that were incompatible with each other. (I documented my cleaning in posts and pictures here).
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Old 28-12-2010, 07:08   #11
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I have never met a 4108 boater that revs full throttle above 3000 RPM. I would say you are definitely under propped.
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Old 28-12-2010, 07:20   #12
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We are powering our 20,000+ pound yacht with a 1986 4-108 spinning a 10x20 Gori 2-blade folding prop. We typically cruise under power at 2800 RPM which gives us a bit over 7 knts at about 3/4's gal per hour. At 3000 RPM we do 8+ knots but burn .9 gals per hour which pretty much matches the chart shown on the 4-108 brochure. At WOT we get a bit over 3700 RPM but the stern (of our IOR era boat) "squats" so our speed increase is modest. At this point we have 3300+ hours on the engine and she continues to perform well. Something to be said for tractor engines, eh?
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Old 28-12-2010, 09:38   #13
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Some thoughts on the discussion above and Perkins 4108

If you really want to get into the mysteries of propping a boat, you could go to www.boatdiesel.com and look at all the discussion and articles on propping from our powerboat friends. If nothing else, reading about the discussions of fuel burns in the range of 30-40 gals per hour makes me feel lucky.

However, my 4108 in the 20,000 # displacement Gulfstar 37 is deliberately overpropped (3 blade 15 diameter x 12 pitch) so as to hold down the rpm and noise needed during normal operation - at 1850 rpm and a clean bottom I get between six to six and 1/2 knots, burn less than 3/4 gal hr, and the noise is minimal...which is why I sail, but sometimes you need to get somewhere so you motorsail. My Perkins works great when you have 5-7 knots of wind but need to get somewhere...about 1200-1500 rpm while sailing makes a huge difference - run the engine up to 2000 rpm about once an hour for a few minutes keeps the carbon and cylinder glaze from building up and burns little fuel.

I know it is this way, and I am happy with it. Theoretically, a boat should be propped so it will run the majority of the time at about a 60-75% load factor for a diesel engine to optomize engine life and diesel burned to power produced ratio. In practice, I like the quiet and having owned other boats, found that running the diesel at high speed made more noise than I wanted to deal with long-term.

I can run the engine up to about 2800 rpm propped this way, and the boat will move accordingly, but it will slowly but surely overheat if run this way for more than 15 minutes or so.

So if one was offshore they would prop more conservatively since they might need high power levels for storms or such for long amounts of time, but for my sort of sailing - mostly daysailing or overnighters in good weather, this works fine.

A Perkins, because of its design, when run at high rpm and high power load factors, will probably not have the same life as most of us are expecting from our relatively low power factor applications in a sailboat. More power=shorter life..its all a function of the amount of fuel consumed during the operation of the engine. The 50 hp rating quoted was generally only available for machinery or automotive type applications, where the 50 hp is needed for only short periods of time, not continously as in a marine application. Most writers on the Perkins agree that 25-30 hp continuous is about what you should expect from a Perkins 4.108 for long life.

I had a 4 cylinder 1967 perkins installed in an agricultural application that literally ran all summer for years pumping water...rebuilt it once in 1978 or so, not because it needed it, but because I couldn't believe after running all those hours it didn't need it. I sold the farm the other day the pump is located on, the new owner changed the oil, put fresh diesel in the tank( it had not been run for 10 years or so) and fired it right up - after bleeding it of course, as any perkins owner can tell you is a PITA. I can't imaging a Yanmar or any other modern diesel doing the same thing after sitting for so many years.

If my 4.108 dies, I'm putting in a rebuilt one - assuming parts are available in the future, it is a great sailboat engine if cared for properly.
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Old 29-12-2010, 07:13   #14
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Quote:
Originally Posted by jlogan View Post
However, my 4108 in the 20,000 # displacement Gulfstar 37 is deliberately overpropped (3 blade 15 diameter x 12 pitch) so as to hold down the rpm and noise needed during normal operation - at 1850 rpm and a clean bottom I get between six to six and 1/2 knots, burn less than 3/4 gal hr, and the noise is minimal...

. Theoretically, a boat should be propped so it will run the majority of the time at about a 60-75% load factor for a diesel engine to optomize engine life and diesel burned to power produced ratio. .

I can run the engine up to about 2800 rpm propped this way, and the boat will move accordingly, but it will slowly but surely overheat if run this way for more than 15 minutes or so.

I have a Perkins 4-108 built in 1985 that is installed on a 22000lb displacement(actually more like 30K fully loaded) 37 foot Tayana, with a 17 inch 3 bladed max prop set at a pitch of around 13.5 inches, and a 2.1:1 borg warner velvet drive tranny.

After calibrating my tach several years ago I found that I was running at a lower rpm than orginally thought. My max rpm is 2300 (WOT) under load and I cruise at 6 knots at 1900rpm. I know that I'm overproped, but don't think that I'm harming the engine since I'm staying less than 80% of the WOT rpm's. Most other threads seem to think this is bad since I should be able to achieve the 3600 rpm max continuous rating of the engine and say this is lugging the engine resulting in poor engine health. As long as I'm operating at less than 80% of the WOT, I don't see this...
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Old 17-12-2016, 15:39   #15
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Re: Perkins 4-108 Power Curve

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Originally Posted by lancelot9898 View Post
I have a Perkins 4-108 built in 1985 that is installed on a 22000lb displacement(actually more like 30K fully loaded) 37 foot Tayana, with a 17 inch 3 bladed max prop set at a pitch of around 13.5 inches, and a 2.1:1 borg warner velvet drive tranny.

After calibrating my tach several years ago I found that I was running at a lower rpm than orginally thought. My max rpm is 2300 (WOT) under load and I cruise at 6 knots at 1900rpm. I know that I'm overproped, but don't think that I'm harming the engine since I'm staying less than 80% of the WOT rpm's. Most other threads seem to think this is bad since I should be able to achieve the 3600 rpm max continuous rating of the engine and say this is lugging the engine resulting in poor engine health. As long as I'm operating at less than 80% of the WOT, I don't see this...
i have a 1988 beneteau 430 i have always found that she is really slow.around 5kns in flat calm.i lifted her out and had the hull preasure washed ,it didnt look to bad no barnicles or growths. I noticed that the prop was well worn and rather thin having said that when i returned it back into the water and set sail she jumped up to just under 7.5kns.
i didnt measure the prop at the time when she was out on the hard but it looked like a 16" and now ive bought a 17"x12r what will my improvements be as to speed and consumption.running a 4108
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