Reply
 
Thread Tools Display Modes
 
Old 01-01-2011, 01:10 PM   #1
Newbie
 
Join Date: Jan 2011
Posts: 1
Fuel Consumption with a 40' CHB

Hi All,

Quick question. With a 40' CHB at cruise speed (would it be ~8 knots?) with a single Lehman 120 horse, how many gph is one burning?

How about the same boat with twin lehmans? (would cruise speed be similar?)

And, how important does having the redundancy of the twins seem to you all? is it a big issue cruising between seattle and alaska, maybe jumping out to Haida Gwaii on the way?

Thanks for feedback!
__________________
Advertisement

kimbo is offline   Reply With Quote
Old 01-01-2011, 03:40 PM   #2
Guru
 
2bucks's Avatar
 
Join Date: Oct 2007
Posts: 698
RE: Fuel Consumption with a 40' CHB

My boat is a 1980 40' with a single Lehman. My boat cruises comfortably at 1750 rpm, 7.1 knots, 2.1 GPH. 1700 rpm, 6.9 knots, 1.7 gph.

These numbers are per the FloScan, GPS and actual use with just over 800 running hours.

Ken
__________________

2bucks is offline   Reply With Quote
Old 01-01-2011, 04:18 PM   #3
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,705
RE: Fuel Consumption with a 40' CHB

Twins are in this class of boat the way it should be in my opinion put if you care much for fuel consumption the single is the way to go. I've been shopping lately and fuel burn is very important to me so I haven't looked at any twins at all. 32' boats to 42' boats come w the 120 Lehman or the Perkins 6-354 in single engine configuration and will perform at the 2 gallon an hour level at 7 knots but should, of course, be run harder. One should run a diesel at 50% loading or more but there is way more than 101 opinions on this and to my knowledge nobody's engine has croaked for running slow but w the single you will be much more inclined to run at 60% loading. The flat submerged stern boat will burn twice as much fuel at disp speeds as a full disp boat. Displacement speeds are not "hull speed"
for a FD boat but SD boats can run there more gracefully. The twin has an extra ton or more aboard running twice as much engine and that's fine if you intend to run 8-10 knots. That's what the boats were designed to do but it will cost you 6 to 9 gallons an hour. So your choices range from 2 to 10 gph unless you can find a boat repowered w an 75 or 85hp John Deere then perhaps 1.5 gph. But if you were to buy a 40' Willard 1.5 gph would be in the bag.
David M,
I thought 16 hp per gph was for Detroit's and others were 20hp per gallon. Perhaps the 20hp number was meant for modern engines like John Deere's, Cummins and Yanmars.
Do older engines like the Lehmans actuall have the same burn rate as the DDs? That would mean a 4-53 DD would push a boat at the same speed and fuel consumption as a Lehman. Would be nice as I like DDs.
"1.5 hp per 1000 lbs to push a boat to hull speed." That's 3 hp per ton. 24hp for my 30' Willard. I'm not sure I can even reach 7 knots at WOT. WOT would be 42hp. I would guess it would take more like 65 hp to push a 40' Chb to hull speed but if one's concerned about economy of fuel burn hull speed is not the place to be. Real economy occurs w about 20% less speed.
David,
I know you know more about this stuff than I do and I intend to submit private e-mail to you in the near future with your permission.
Nomad Willy is offline   Reply With Quote
Old 01-01-2011, 05:17 PM   #4
Senior Member
 
Join Date: Nov 2007
Posts: 126
Fuel Consumption with a 40' CHB

The gentlemen above have given you good advice...figure somewhere around 2 gals/hour for a Lehman 120 pushing a 40' CHB. Twin vs. single is a discussion on this and other forums that has no right answer but will go on forever.

We have taken our single engine, 33' boat all the way to Glacier Bay and back a couple of times and have never had a second thought. There are many, many Krogens, Nordhavns, Nordic Tugs, American Tugs, North Pacifics, Willards, Mainships, Roughwaters and Taiwanese Trawlers that have single engines and are everywhere between Olympia and Skagway every summer. Engine service is available in Campbell River, Port McNeill, Port Hardy, Shearwater, Prince Rupert, Ketchikan, Juneau, Sitka, Wrangell, and Petersburg. With twins, if one fails you limp in on the other. With a single you would anchor and call for help from a commercial towing service.*

The key is how you maintain and monitor your engine. Treat a diesel engine right and she will take you anywhere for well over 10,000 hours with no problems. Clean fuel, good injectors, oil changes every 100 to 200 hours and a clean cooling system will keep her happy. Belts, hoses and impellers may fail but they are easy to change and easy to stock as spare parts.* It's cheap insurance to change all the belts, hoses and the impeller when you buy a new boat and then set up a scheduled maintenance program to inspect and replace them before they fail in service.

I would not hesitate in taking my single engine boat to the Queen Charlottes (Haida Gwaii) if the weather was cooperating.* Weather is a bigger factor the farther north you go.* Engine reliability you can control...weather conditions are in Mother Nature's hands.

-- Edited by Steppen on Saturday 1st of January 2011 06:19:55 PM

-- Edited by Steppen on Saturday 1st of January 2011 06:24:05 PM
Steppen is offline   Reply With Quote
Old 01-01-2011, 07:58 PM   #5
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,630
RE: Fuel Consumption with a 40' CHB

I was about to say, you aren't gonna be cruising at 8kts in a 40 footer with a single Lehman unless you are close to WOT.
Baker is offline   Reply With Quote
Old 01-01-2011, 11:29 PM   #6
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,705
RE: Fuel Consumption with a 40' CHB

David M,
By that rule the Nordhavn 46 would have 72hp. It has 101hp (4 hp per ton).* According to what was presented in Voyaging Under Power the N46 made 8.7 knots on 101hp and her hull speed is 8.25. The Willard 40 has a hull speed of 8 knots and uses 53hp to achieve it.
This is about exactly 3hp ton or 1.5hp per 1000lbs. The Krogen 42 w a hull speed of 8.36 requires about 53hp as well to achieve the hull speed which means it only required 1.4hp per 1000lbs to get HS. The rule says my Willy should require 25hp and I've got 42 but I'm not sure I can get up to hull speed (7 knots). I knew it was'nt the most efficient fish in the sea but those numbers make Willy look quite bad indeed.
Nomad Willy is offline   Reply With Quote
Old 01-02-2011, 04:50 AM   #7
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,515
RE: Fuel Consumption with a 40' CHB

The numbers are "Rules of thumb" they are generally accurate for "normal" boats , doing normal things.

Run a plaining boat dragging its transom , wont work, run a canoe stern in an attempt to excede its "hull speed" it wont work.

Outsized engine ? It wont work.

Lousey underbody? , poor prop? sad eghine , It wont work.

Ballparks id fine , bit attempting to guesstimate to two decimal places? IT WONT WORK!
FF is offline   Reply With Quote
Old 01-02-2011, 06:40 AM   #8
Guru
 
O C Diver's Avatar
 
City: Fort Myers, FL... Summers in Crisfield, MD
Country: USA
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 4,803
How much HP would you choose?

Ok, so lets say your looking at a 38' trawler with a tired engine and a hull speed around 8 knots. We will say it draws 3'6" and displaces 24,000 pounds. We don't plan to drive the boat above hull speed (will likely cruise 7 knots). 24,000 pounds multiplied by 1.5 HP per thousand pounds, theoretically requires 36 HP.

So the question is:
"What amount of HP would you choose for this application and what's your reason for your choice"? I'm interested to see what multiple above the theoretical amount of HP people would choose.

Ted

-- Edited by O C Diver on Sunday 2nd of January 2011 07:41:51 AM

-- Edited by O C Diver on Sunday 2nd of January 2011 07:46:35 AM
O C Diver is offline   Reply With Quote
Old 01-02-2011, 07:32 AM   #9
TF Site Team
 
Larry M's Avatar
 
City: JAX, FL
Country: USA
Vessel Name: Hobo
Vessel Model: Krogen 42-120
Join Date: Nov 2008
Posts: 5,724
RE: Fuel Consumption with a 40' CHB

Quote:
nomadwilly wrote:

.... The Krogen 42 w a hull speed of 8.36 requires about 53hp as well to achieve the hull speed which means it only required 1.4hp per 1000lbs to get HS....
Eric:***Where are you getting your numbers?**We do have a KK42.* We displace 44,000lbs and have a water line of 39'2".*

To move our boat at 7 knots in calm flat water it takes ~51 hp according to Boatdiesel's power calculator program.* At 8 knots we need 76 hp.**<table align="center" cellpadding="2" cellspacing="0" border="0" style="width:100%;border:#000000 1px solid;"><tbody><tr><td colspan="2"><table cellpadding="0" cellspacing="0" border="0" style="width:100%;"><tbody><tr><td align="left" colspan="2" bgcolor="#ffffff">**DISPLACEMENT HULL**********</td></tr><tr><td align="right">Hull Speed: </td><td align="left">8.4 kts. *</td></tr><tr><td align="right">Power Required:</td><td align="left">87.3 shp.</td></tr></tbody></table></td></tr><tr><td align="left" colspan="2">**Speed********** </td></tr><tr><td align="center" colspan="2">To Achieve a Cruising Speed of 7 kts </td></tr><tr><td align="left" colspan="2">**Required Power********** </td></tr><tr><td align="center" colspan="2"><table cellpadding="2" cellspacing="0" border="0" style="width:98%;"><tbody><tr><td align="right">At Prop: </td><td align="center">50.9 SAE hp </td><td align="center">37.9 kW</td></tr><tr><td align="right">At Flywheel: </td><td align="center">52.5 SAE hp</td><td align="center">39.1 kW</td></tr></tbody></table></td></tr><tr><td align="left" colspan="2">** Select Engine: **********</td></tr><tr><td align="center" colspan="2"><table cellpadding="1" cellspacing="1" border="0" style="width:98%;"><tbody><tr><td align="center" width="20%" bgcolor="#444444">Rating </td><td align="center" width="20%" bgcolor="#444444">SAE HP </td><td align="center" width="20%" bgcolor="#444444">Metric HP </td><td align="center" width="20%" bgcolor="#444444">kW </td><td align="center" width="20%" bgcolor="#444444">Max Kts* </td></tr><tr><td align="center" width="20%" bgcolor="#aaaaaa">Pleasure</td><td align="center" width="20%">74.9</td><td align="center" width="20%">76.0</td><td align="center" width="20%">55.9</td><td align="center" width="20%">8.0</td></tr><tr><td align="center" width="20%" bgcolor="#aaaaaa">Light </td><td align="center" width="20%">65.6</td><td align="center" width="20%">66.5</td><td align="center" width="20%">48.9</td><td align="center" width="20%">7.6</td></tr><tr><td align="center" width="20%" bgcolor="#aaaaaa">Medium </td><td align="center" width="20%">58.3</td><td align="center" width="20%">59.1</td><td align="center" width="20%">43.5</td><td align="center" width="20%">7.3</td></tr><tr><td align="center" width="20%" bgcolor="#aaaaaa">Heavy</td><td align="center" width="20%">53.5</td><td align="center" width="20%">54.3</td><td align="center" width="20%">39.9</td><td align="center" width="20%">7.1</td></tr></tbody></table></td></tr></tbody></table>
Larry/Lena
Hobo KK42
Zihuatanejo, MX

The program calculates the theoretical power required to drive the selected hull form through calm water at the required speed. It is important to note the formula does not take into account wind or wave resistance...
*
Larry M is offline   Reply With Quote
Old 01-02-2011, 11:52 AM   #10
Guru
 
Egregious's Avatar
 
City: Sunset Beach, NC
Country: USA
Vessel Name: Polly P.
Vessel Model: Monk 36
Join Date: Feb 2010
Posts: 554
RE: Fuel Consumption with a 40' CHB

If you're calculating power (HP) required to drive a boat at a certain speed, are we talking at the prop?* The reason I ask is that the tranny takes away some HP depending on what kind it is.* So to say it takes X HP at the prop to drive the boat to hull speed, and the engine makes Y HP / GPH, you'd be missing whatever the tranny takes out of the equation, right?* Unless the X HP is measured at the engine, in which case these rules of thumb are only accurate assuming all trannies are the same...* Maybe the amount of loss is so small that it doesn't matter to a rule of thumb, but my understanding is that old Borg Warner velvet drive type gears do cost some of your HP between the engine and the prop.

That make sense?
Egregious is offline   Reply With Quote
Old 01-02-2011, 01:37 PM   #11
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,705
RE: Fuel Consumption with a 40' CHB

Woody,
At the flywheel. Yes the BW takes up to 5hp just to pump the hydraulic fluid and then the gears take another 5 or so. This is one place where synthetic oil may possibly save you a dollar or two since you can use a lower viscosity. Prolly fly stuff though.
Larry M,
Got the numbers from Voyaging Under Power by Robert Beebe. Your K42 is listed at 39000lbs disp. 7 knots on the graph in the book shows about 27hp required. Hull speed of about 8.3 knots requires about 53hp. What kind of hp rating they used is anyone's guess. I don't know where they get THEIR numbers from but I'm fairly certain it's the designers. There was a lot of interchange between the authors and the designers. 7 knots is definitely under 30hp on the graph. But whatever the reality is I can think of no better trawler hull than the K42. Not perfect in my opinion but what is?
Nomad Willy is offline   Reply With Quote
Old 01-02-2011, 07:04 PM   #12
Guru
 
2bucks's Avatar
 
Join Date: Oct 2007
Posts: 698
Fuel Consumption with a 40' CHB

Quote:
O C Diver wrote:

Ok, so lets say your looking at a 38' trawler with a tired engine and a hull speed around 8 knots. We will say it draws 3'6" and displaces 24,000 pounds. We don't plan to drive the boat above hull speed (will likely cruise 7 knots). 24,000 pounds multiplied by 1.5 HP per thousand pounds, theoretically requires 36 HP.

So the question is:
"What amount of HP would you choose for this application and what's your reason for your choice"? I'm interested to see what multiple above the theoretical amount of HP people would choose.

Ted

-- Edited by O C Diver on Sunday 2nd of January 2011 07:41:51 AM

-- Edited by O C Diver on Sunday 2nd of January 2011 07:46:35 AM
I'd put a 120 Lehman in it and run about 7 knots burning about 2 GPH. This runs the engine at it's correct temperature, and according to the engines official HP curve chart is about 90 HP, which is about 75% load,*and it just purrs across the water.

I know that doesn't jive with the theoretical hp/GPH/LWL/hull speed calculations in the books that we see from the other posters, but I*own a 40 CHB with a single 120 Lehman that you can come take a cruise in and see for yourself how it works.

Ken*


-- Edited by 2bucks on Sunday 2nd of January 2011 08:05:57 PM
2bucks is offline   Reply With Quote
Old 01-02-2011, 09:20 PM   #13
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,705
Fuel Consumption with a 40' CHB

2bucks,
Nice that you're around again. I think he's going to need well under 120hp to get the job done. And I would rather have twin engines. Lets go w 2 Mitsubishi 42hp or 2 Yanmar 40hp engines. Both modern engines and more fuel efficient. The little engines being so small will be easy to work around and most likely be quieter.
If you wanted a single engine installation Go w one of the best** ..John Deere. They make an 85hp w a naturally aspirated 4 cyl w a balance shaft that should make it as smooth as a six** ..or better. I'd say the boat will require at least 45hp to maintain 7 knots. So that means you'll be at cruise speed w almost 50% power left to go. One could go w a bit less** ..say 75hp and still have plenty of reserve. If you can't afford a Deere or a lugger go with an Isuzu or any other good engine of 75 to 85hp.
Ken,
75% load on a Lehman is 4.5gph. Fuel consumption is directly proportional to fuel burn and the Lehman will burn 6gph at WOT. Two gph is about the 37hp level. 90hp on the chart is probably 75% of rated rpm. Not load or percentage of throttle. At 50% load you should be burning three gph and making 60hp. I'm glad you mentioned it though as I've been shopping for a 2gph boat so I need to focus on just that. Let me know if you know of a 32 Nordic Tug under 100K. Glad your'e back.

-- Edited by nomadwilly on Sunday 2nd of January 2011 10:20:43 PM
Nomad Willy is offline   Reply With Quote
Old 01-02-2011, 09:44 PM   #14
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,251
RE: Fuel Consumption with a 40' CHB

Quote:
nomadwilly wrote:

I'm glad you mentioned it though as I've been shopping for a 2gph boat so I need to focus on just that. Let me know if you know of a 32 Nordic Tug under 100K. Glad your'e back.

Gee, the 8-ton*Nordic Tug 32 with its 280 h.p. Volvo is supposedly capable of speeds exceeding 20 knots.* With a 2 g.p.h consumption rate, the engine will be working at a very small fraction (about one-seventh?) of*available power.

*
markpierce is offline   Reply With Quote
Old 01-02-2011, 09:46 PM   #15
Guru
 
Carey's Avatar
 
City: Bellingham, WA
Country: USA
Vessel Name: Happy Destiny
Vessel Model: Custom Lobster Yacht
Join Date: Oct 2007
Posts: 1,101
RE: Fuel Consumption with a 40' CHB

Quote:
markpierce wrote:

*
nomadwilly wrote:

I'm glad you mentioned it though as I've been shopping for a 2gph boat so I need to focus on just that. Let me know if you know of a 32 Nordic Tug under 100K. Glad your'e back.
Gee, the 8-ton*Nordic Tug 32 with its 280 h.p. Volvo is supposedly capable of speeds exceeding 20 knots.* With a 2 g.p.h consumption rate, the engine will be working at a very small fraction (about one-seventh?) of*available power.Those are totally bogus numbers. It just isn't possible!!!

*
Carey is offline   Reply With Quote
Old 01-02-2011, 09:50 PM   #16
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,251
Fuel Consumption with a 40' CHB

Quote:
Carey wrote:

*
Those are totally bogus numbers. It just isn't possible!!!
Then the marketing data is bogus:

http://www.boattest.com/boats/boat_video.aspx?id=2095

"... That said, with her standard 280-hp Volvo Penta turning a 24 four bladed prop, she is capable of much more, but it just seems out of place to me. Of course, I realize Im in the minority on that page, so having a comfortable cruise in the early 20s serves a much broader audience ..."


-- Edited by markpierce on Sunday 2nd of January 2011 10:53:41 PM
markpierce is offline   Reply With Quote
Old 01-02-2011, 09:51 PM   #17
Guru
 
Carey's Avatar
 
City: Bellingham, WA
Country: USA
Vessel Name: Happy Destiny
Vessel Model: Custom Lobster Yacht
Join Date: Oct 2007
Posts: 1,101
RE: Fuel Consumption with a 40' CHB

Quote:
markpierce wrote:

*
Carey wrote:

*
markpierce wrote:
Those are totally bogus numbers. It just isn't possible!!!
*
Then the marketing data is bogus:

http://www.boattest.com/boats/boat_video.aspx?id=2095Exactly!!!

*
Carey is offline   Reply With Quote
Old 01-02-2011, 10:08 PM   #18
Guru
 
Carey's Avatar
 
City: Bellingham, WA
Country: USA
Vessel Name: Happy Destiny
Vessel Model: Custom Lobster Yacht
Join Date: Oct 2007
Posts: 1,101
Fuel Consumption with a 40' CHB

Quote:

-- Edited by Carey on Sunday 2nd of January 2011 11:10:35 PM

OK, 2 gph is possible, and so is 2o knots (although questionable with only 280hp), but certainly not 20 knots at 2 gph. All the supplied numbers mean nothing, as they are not relative.


-- Edited by Carey on Sunday 2nd of January 2011 11:20:56 PM
Carey is offline   Reply With Quote
Old 01-02-2011, 10:19 PM   #19
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,251
RE: Fuel Consumption with a 40' CHB

Carey, the first sentence is based on the quoted statement I added/edited to the post linking the marketing data.

The second sentence is not based on the marketing information, but rather on the principle that modern diesels generate horsepower based on the rate of fuel consumption.
markpierce is offline   Reply With Quote
Old 01-03-2011, 06:53 AM   #20
Guru
 
O C Diver's Avatar
 
City: Fort Myers, FL... Summers in Crisfield, MD
Country: USA
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 4,803
Fuel Consumption with a 40' CHB

Quote:
2bucks wrote:I'd put a 120 Lehman in it and run about 7 knots burning about 2 GPH. This runs the engine at it's correct temperature, and according to the engines official HP curve chart is about 90 HP, which is about 75% load, and it just purrs across the water.

I know that doesn't jive with the theoretical hp/GPH/LWL/hull speed calculations in the books that we see from the other posters, but I own a 40 CHB with a single 120 Lehman that you can come take a cruise in and see for yourself how it works.

Ken


-- Edited by 2bucks on Sunday 2nd of January 2011 08:05:57 PM
*
Hi Ken, engine HP curve charts Give you the maximum HP and GPH at a given RPM. If you load your boat with an extra 2,000 pounds of weight you can still turn the same RPM, but you will burn more fuel. The more weight you add, the more HP will be required to push it (for the same speed), the more fuel you will burn.

The chart indicates the motor can produce a maximum of 90HP (5.6 GPH) at that RPM, but in reality you are only producing 32 HP at 2 GPH.

Thanks for the information as many of the boats I have seen in that size were twins.

Ted

*


-- Edited by O C Diver on Monday 3rd of January 2011 07:53:52 AM
__________________

O C Diver is offline   Reply With Quote
Reply

Thread Tools
Display Modes

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
rpm and fuel consumption Plumber General Discussion 15 05-20-2011 04:43 PM
LRC 34/3208 fuel consumption Fighterpilot Californian 4 04-21-2011 06:07 PM
Fuel Consumption..3406 Cat floorme1 Power Systems 6 03-31-2011 04:56 AM
Fuel consumption floorme1 General Discussion 4 09-12-2010 09:56 PM




All times are GMT -5. The time now is 06:10 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012