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Old 01-10-2009, 15:57   #1
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Does a 4-108 Have Enough Power?

Hi,
I have been prompted to join this forum to solve a dilemma I have. My 38ft, 13 ton steel ketch is powered by a Perkins 4-108m. The basic problem is that this engine only reaches 1700 rpm under load (reaches the rated 3000 rpm in neutral), yet still pushes the boat at 7knots in a flat calm. Push the throttle any further and black smoke appears. As I see it, the reasons for this could be: underpowered engine (possible), incorrect gearbox ratio (hurth 150 - unlikely), prop too big (17x12 - I wouldn't want to go smaller on this size of boat in the region I sail in), engine old and tired (maybe, but probably not to that extent) or incorrect fuel pump timing (quite possible).
If anyone out there uses a 4-108 in a boat of a similar size to mine, I'd be really interested to know gearbox type, prop size, cruising and maximum revs (under load), and what sort of speed you get out the boat. Hopefully the comparison will let me decide if I need to overhaul the engine, adjust the timing or just get a bigger engine.
Thanks a lot.

L. Poncet
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Old 01-10-2009, 16:43   #2
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Overpitched?

I'm looking at a similar problem on Boracay. I only get 2250 rpm max (of 2500) and the engine does not sound happy.

There's a nice looking propeller calculator at The Alberg 30 Site. You'll need to find the propeller part of the maintenance section and scroll about half way down. There's a lot of interesting information on the page that I'd suggest you read.

The propeller page is located here.

I ran Boracay's vital statistics through and it's telling me that I need a 18"x11" (dia x pitch) propeller with a possible 17"x13". Their suggestion is that a 1" reduction in diameter is equal to a 2.5" reduction in pitch so their results do not square up with my observations for my boat.

My current propeller is about 22"x12.5" and has already been depitched by about 4". My propeller shop is suggesting taking 1" off the diameter of my propeller and I'll probably get that done next time I come out of the water.

I'd suggest running your boats vital statistics through the calculations and then talking to your propeller shop.
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Old 01-10-2009, 16:53   #3
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See the "fuel usage" post below for info.... also... one nice thing about a Max Prop is you can change the pitch to get it right! Personally I would shooot for a prop that got me 2800 rpm max witha 4-108
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Old 01-10-2009, 17:18   #4
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Same engine in my 36ft boat, but I have seen it in much heavier boats. A 45 ft ferro cement boat in my former marina did well with it.

I am having power trouble with mine at the moment as well. Knowing what it' s capable of helps me to know that I simply have not found the problem yet. Don't repower.
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Old 01-10-2009, 17:50   #5
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You should prop your boat so that it can attain near rated max RPM. The 4-108 should attain near 3,000 RPM underway, and cruise between 2200 and 2500 RPM.

What size prop (diameter and pitch) it will take to do that depends on your boat's characteristics and, especially, the reduction gear.

I turn a 17" 3-blade MaxProp on my 42' 28,000lb sloop. Not sure of the pitch but, as a previous poster noted, it's adjustable :-)

BTW, the 4-108 is overrated at it's generally quoted power, i.e., 42hp, 47hp, 50hp. It's more like a 30hp diesel.

How much does it take? Well, about 1 shaft HP per ton displacement will push a displacement hull to its hull speed in calm water. This assumes that it's properly propped and the bottom is clean. It's probably a good idea to at least double this figure, though, to take account of the several factors which influence speed thru the water.

Bill
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Old 01-10-2009, 18:40   #6
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Overpitched is the first thing that I hear from most people when I first experienced my loss of power. While it certainly might be, was it always? It might also be as simple as a fouled prop. Or it could be timing, restricted exhaust, low compression ect.

The point I'm trying to make is that if you go down that road you might get higher RPM and still not have all that your engine is capable of.

Like I said, I am experiencing the same problem and I've thrown a lot of money at it. But my prop is the same prop that I've always had.

I made the mistake of doing major work instead of pursuing the cheaper answers first. I am in Indonesia right now. My boat is in Mexico. My next plan of attack when I can get to it, is the exhaust system. A fellow cruiser in Baja with the same engine, bigger boat, was experiencing strange intermittent power power loss problems. After looking in a lot of wrong places (like me) he found the problem in the exhaust. That's the one place I have not looked, but I understand that it is a very common (and cheaper) fix.
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Old 01-10-2009, 19:56   #7
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As I stated in a different post. Different build numbers of the 4.108 were built with different hp. The hp of your engine is the last two digits of the number on the tag on the injecter pump.
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Old 02-10-2009, 09:43   #8
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The exhaust is a good place to start. It could be clogged up and restricting the engine. As I stated in the Fuel Consumption post, a 4-108 pushed my fairly heavy 44 footer at hull speed at about 2400 rpm, no problem with head seas and about .62 gal per hour... I would think even with a derated version it should push your boat no problem...
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Old 02-10-2009, 10:00   #9
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The first place to start is to scrape the barnacles off your hull and prop.

With a 45 ft, 15 ton boat, my 4108 was happiest with a 2.7:1 gear ratio, swinging a 20 inch 3-blade Maxprop adjusted to a pitch angle of 20 degrees. It used about 0.6-0.7 gallons per hour at 6 knots.
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Old 02-10-2009, 12:12   #10
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I'd guess you are overpropped. The Fuji 32 with a 4-108 had a 12" prop. I don't know the trans ratio but that information is critical to your calculations.
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Old 02-10-2009, 19:21   #11
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Thanks everyone for your examples. Quite a few of you have said that my prop is probably too large. I checked out the propeller calculator which confirms this (although I wouldn't place too much faith in it). However, there are plenty of examples posted here as well as others I've come across of the 4-108 doing fine with a much larger prop than mine.
Boracay, what reduction is your gearbox? Your 22" prop is huge compared to mine and yet your 4-108 still reaches over 500rpm more... My gearbox has a 2:1 reduction ratio. Hull and prop are clean.
My nearest propeller shop is probably somewhere in Brazil, but I have three perfectly good Westerbeke 58 engines sitting right here, which take the same gearbox and have the same mount dimensions as the 4-108, so I'm still thinking hard. Thanks again for your relplies.

L. Poncet
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Old 02-10-2009, 22:37   #12
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Easy math:

If you get 4000 RPM underway you are good.

3600 RPM static. (Tied to a dock) Also good. (90% 0f max)

Max continous on the 4-108 is 3000 RPM, Max emergency power is 4000.

Best RPM is 70% of max contious, 2100 = cruise power, should be 70% of hull-speed.

If your prop can't deliver the above, ya are overpropped.

(Never heard of a 4-108 being under-propped)

Come on guys, this is an 1.8 liter engine...Bulletproof and all but don't run it like it was a 65 HP..It never was.
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Old 02-10-2009, 23:03   #13
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Not the 4108...

lponcet
Mine's actually a John Deere 4045, 85hp continuous, ZF63 about 2:1.
Sorry for the confusion.
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Old 03-10-2009, 03:12   #14
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Quote:
Originally Posted by CSY Man View Post
Easy math:

If your prop can't deliver the above, ya are overpropped.
Unless any number of other things is wrong.
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Old 04-10-2009, 09:32   #15
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- - Size of propeller is a two part situation. If the your hull shape and prop aperature will determine the maximum (allowing for prop tip clearance) diameter of the prop. The "pitch" is what will determine the utilization of the engine power delivered to the prop.
- - Ideally in a propeller in free water with nothing within any distance from it - Diameter and pitch work together. A larger diameter with a smaller pitch equals a smaller diameter with a larger pitch. But when the actual location of the prop is restricted by hull shape then diameter get locked and you vary pitch to get best performance.
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