Cruisers Forum
 


Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 01-10-2009, 13:43   #1
Registered User
 
Bowman Sailor's Avatar

Join Date: Oct 2009
Location: Kilmarnock, VA
Boat: Bowman 46
Posts: 40
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
Bowman Sailor is offline   Reply With Quote
Old 01-10-2009, 14:04   #2
Registered User

Join Date: Mar 2008
Location: Easton, CT
Boat: MJM 50 Z
Posts: 343
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.
Highlander40 is offline   Reply With Quote
Old 01-10-2009, 14:32   #3
Registered User
 
Cheechako's Avatar

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,747
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.
Cheechako is offline   Reply With Quote
Old 01-10-2009, 14:53   #4
Registered User
 
Bowman Sailor's Avatar

Join Date: Oct 2009
Location: Kilmarnock, VA
Boat: Bowman 46
Posts: 40
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
Bowman Sailor is offline   Reply With Quote
Old 01-10-2009, 15:17   #5
Registered User

Join Date: Mar 2008
Location: Easton, CT
Boat: MJM 50 Z
Posts: 343
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.
Highlander40 is offline   Reply With Quote
Old 01-10-2009, 15:19   #6
Registered User
 
Cheechako's Avatar

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,747
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?
Cheechako is offline   Reply With Quote
Old 01-10-2009, 15:54   #7
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,405
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?
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 01-10-2009, 16:37   #8
Registered User
 
Cheechako's Avatar

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,747
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...
Cheechako is offline   Reply With Quote
Old 01-10-2009, 18:35   #9
cruiser

Join Date: Sep 2007
Location: Tampa to New York
Boat: Morgan 33 OutIsland, Magic and 33' offshore scott design "Cutting Edge"
Posts: 1,594
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.
forsailbyowner is offline   Reply With Quote
Old 01-10-2009, 20:28   #10
Registered User

Join Date: Feb 2009
Posts: 2,439
A diesel is load sensitive if it is using more fuel it has a higher load. You have to much prop
motion30 is offline   Reply With Quote
Old 01-10-2009, 20:54   #11
Registered User
 
colemj's Avatar

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,110
Images: 12
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
__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 03-10-2009, 07:18   #12
Registered User
 
Bowman Sailor's Avatar

Join Date: Oct 2009
Location: Kilmarnock, VA
Boat: Bowman 46
Posts: 40
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
Bowman Sailor is offline   Reply With Quote
Old 03-10-2009, 08:25   #13
Registered User
 
colemj's Avatar

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,110
Images: 12
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
__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 03-10-2009, 08:37   #14
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,405
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.
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 03-10-2009, 09:32   #15
Registered User

Join Date: Nov 2005
Location: Ohio
Posts: 2,939
Images: 4
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.
Joli is offline   Reply With Quote
Reply

Tags
consumption, fuel

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
fuel consumption xort Monohull Sailboats 54 19-05-2019 02:47
Yanmar 75 fuel consumption edsailing Engines and Propulsion Systems 5 05-05-2009 08:43
Which of two fuel consumption formulas? Buddy_Y Engines and Propulsion Systems 13 16-03-2009 16:54
fuel consumption linkavitch Engines and Propulsion Systems 7 14-02-2009 21:36
Mahe 36: Fuel Consumption jean1146 Fountaine Pajot 19 21-08-2008 07:20

Advertise Here


All times are GMT -7. The time now is 06:08.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.