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Old 01-10-2009, 13:43   #1
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Fuel Consumption Puzzle

I repowered my sailboat when I bought her 15 years ago. At that time I selected a low reduction ratio and a two-blade prop. I've never been satisfied with her performance when powering into head seas. So, two years ago I replaced my transmission with one with a higher reduction ratio so I could spin a three-blade prop and have the engine running higher on the power curve. The performance results both before and after the transmission and prop change are summarized below:

Vessel: Bowman 46 fin keel sloop, 30,000 lb. displacement, 32 ft waterline
Engine: Perkins 4-108M
First Transmission: HBW-150, 0 to 1,034 hours
Replacement transmission: ZF-12M transmission = 1,034 - 1,315 hours

The original setup with the lower reduction ratio and two-blade prop gave the following results:
  • Hurth HBW-150 with 1.88:1 reduction
  • 18X12 two-blade prop
  • Cruise: 6.8 knots @ 2,000 RPM
  • Max RPM = 2,250
  • Fuel consumption @ 6.8 knots = 0.85 gph
  • Poor performance into head seas

Here are the results obtained since changing to the higher reduction ratio and three blade prop:
  • ZF-12M, (HBW-125), 2.63:1 reduction
  • 18X11 three-blade prop
  • Cruise: 6.3 knots @ 2,700 rpm
  • Max RPM = 3,200
  • Fuel consumption @ 6.4 knots = 1.2 gph

In addition, I've been working through a number of issues that I believe are related to the higher engine speeds and loads since changing the reduction gear and prop:
  • significant increase in oil leaks, (front & rear crank seals, valve and lifter covers, etc)
  • Engine over heats if pushed beyond 2,700 RPM, (changed both pumps, cleaned heat exchanger, cleaned water injection nipple, replaced all flexible and exhaust hoses)
  • lots of noise and increased engine vibration
  • I'm frustrated about burning more fuel to go slower
The boat does, however, have a lot more punch when motoring through head seas. So, why do I burn more fuel to go slower? I figured that I would be producing the same horsepower to go 6.8 knots regardless of the engine speed, since the engine should be doing about the same amount of work.

Any Ideas?

Steve
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Old 01-10-2009, 14:04   #2
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Steve,
The diference is in the max RPM.
You did not mention what the max rated RPM for the engine is?
Your max RPM turning the prop should be near that.
My guess is that you are now under propped. The three blade prop has more area so it holds speed better but the engine is not loaded properly.
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Old 01-10-2009, 14:32   #3
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Wow, that's high fuel consumption for a 4-108. The one in my 44 footer (38 waterline) consistantly burned about .62 gal per hour at nearly 8 knots. 3 blade max prop with hurth gear.
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Old 01-10-2009, 14:53   #4
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4-108M engin ratings

The Brochures claim that the 4-108M is rated at 51HP at 4,000 RPM, but no one really believes these engines will hold together at this speed/load. The real number is more like 37 HP at 3,000 RPM. Based on how unhappy mine is at 2,700 RPM, I'm afraid to try 3,000.

Based on the power curve for the 4-108M and my max RPM, I suspect that I'm only getting between 30 and 35 HP at 2,700 RPM. This is a bit weak for a 30,000 lb boat. I'll probably repower again.

Thanks for your thoughts and comments.

Cheers,

Steve
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Old 01-10-2009, 15:17   #5
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Steve,
Sounds like you are jumping real fast into an expensive project.
I bet that a good diesel mechanic will get to the bottom of the problem for a lot less cash.
Leaks are a normal maint item that needs attention.
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Old 01-10-2009, 15:19   #6
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The sweet spot on my above mentioned 44 footer was about 2400 rpm. I believe I was propped to max out at 2800 rpm. It had no problem with my heavy 44 footer.... surprisingly...(Ta Chaio built CT-44/Tanton 44) Until joining this forum, I was under the impression the 4-108 maxed out at about 2800.... not sure where I got that impression from. I think it was my diesel mechanic at the time...maybe what he was really saying was that is where it should be run! The 4-108 should be plenty of engine for your 40 footer.... Why didnt you just change prop pitch rather than transmissions?
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Old 01-10-2009, 15:54   #7
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Did you ever reach you maximum governed RPM?

Did you ever measure your gallons per hour and then do a conversion, knowing distance traveled, to miles per gallon?
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Old 01-10-2009, 16:37   #8
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If memory serves (been 10 years) the engine maxed out at the 2800 rpm I stated above... due to prop sizing.... Even my 85 HP Mercedes diesel in my Passport 47 used no more than a gallon an hour.... always just went by gallons per hour calculated between fill ups...
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Old 01-10-2009, 18:35   #9
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The 4.108 has a MAX 51 hp. The hp of your motor is listed on the injection pump. The hp is the last two digits of the number on the tag. With this number you can calculate your max rpm and the cruising rpm to use for your pitch/dimension calculations. Everybody complains about not getting 51 hp not realizing they have a 30 hp with a rotating assembly rated for 51. I like my overbuilt 30 hp. Probably why it still holds 60 lbs oil pressure 33 years old.
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Old 01-10-2009, 20:28   #10
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A diesel is load sensitive if it is using more fuel it has a higher load. You have to much prop
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Old 01-10-2009, 20:54   #11
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It looks like his maximum rpm reached INCREASED with the new gear and prop. This indicates that he was over propped before the change. As to whether he is still over propped, that depends on what the max rpm rating for the engine is and whether his new max rpm reached is within a few hundred rpms of that.

Mark
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Old 03-10-2009, 07:18   #12
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Thanks for your input

Thanks, Folks, for your input.

I think I'll pull the prop and have the pitch increased from 11" to 13". This should drop my max RPM to about 2,800. Cruising RPM maybe 2,400. Let's see what happens.

Cheers,

Steve
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Old 03-10-2009, 08:25   #13
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Steve,

You don't want to govern the max rpm using the prop. The max rpm is fixed and determined by the engine design and you want to prop the boat so that you can get close to this rpm under load. Anything lower is overloading the engine, which is not good for the engine.

I am not familiar with the Perkins 4-108M but in general, I suspect you should be able to reach 3400 rpms under load - close to what you are getting now.

I think your increased fuel consumption may be due to you are running at higher rpms with the new prop. While this may not feel right to you (or your wallet), it is the right thing to do for the engine.

Mark
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Old 03-10-2009, 08:37   #14
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Mark is exactly right. If you re-prop the boat and it cannot reach its maximum governed RPM with a clean hull and a healthy engine, then you have over propped the boat and the results will be a shortened engine life and a buildup of carbon in the combustion chamber.

There is a lot of belief out there that overpitching an engine is more economical. That is not true.
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Old 03-10-2009, 09:32   #15
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If you can't reach WOT then you are over pitched. FWIW we burn 2.2 gal/hr at 2,000 rpm with a W108, 65k weight and 55 ft of waterline.
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