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!

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

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.

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

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.

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.

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!

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

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....

Click to expand...

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...
*

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?

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?

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

Click to expand...

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,
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

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.

Click to expand...

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 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.

Click to expand...

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 wrote:

*
Those are totally bogus numbers. It just isn't possible!!!

Click to expand...

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 wrote:

*
Carey wrote:

*
markpierce wrote:
Those are totally bogus numbers. It just isn't possible!!!

Click to expand...

*
Then the marketing data is bogus:

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

*

Click to expand...

-- 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, 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.

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
*

Click to expand...

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

markpierce wrote: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.

Click to expand...

Modern diesels? What else could they use to produce horsepower?

Rudolph's first engine and every steam engine before that had its efficiency, value, utility, or economy based on and measured*on the same principle ... BTUs in vs*HP or*kW*out.

*

I thought Nordic 32s had at first Yanmars and the rest were Cummins except special orders (Nordic was/is good for that right?) Never heard of one w a Volvo. Did they change the std engine from Cummins to Volvo? What happened to "made in America"?
OC***** I should have explained it like that "Give you the maximum HP and GPH at a given RPM." Right*** ..that's max power (WOT).
If they changed the shape of the stern all these square assed boats would would require half their power (lots and lots of them much less than even that) to cruise at 7 knots and max at 8.5 or so. That's what most of square stern boaters do isn't it? If the shape of the N32 was changed to disp 50hp would be just fine. 140hp is the least powerfull engine I;ve seen in a N32.
Rick,*** ...."Modern diesels? What else could they use to produce horsepower?" Lehman's.


-- Edited by nomadwilly on Monday 3rd of January 2011 12:00:58 PM

nomadwilly wrote:*What else could they use to produce horsepower?" Lehman's.

Click to expand...

That was good ...*

*

Doh!** <table cellpadding="0" cellspacing="0" class="genmed"><tbody><tr><td width="100"></td><td width="40"></td></tr></tbody></table>

The thought was that diesels have become somewhat more fuel efficient.

-- Edited by markpierce on Monday 3rd of January 2011 03:36:35 PM

Actually the new engine I bought for Willy is hardly any different that the 35 yr old Perkins that I pulled out and sold. The only modern features that I know of is thin wall casting of the block and glow plugs. And I know glow plugs were readily available when the Perkins was made. With 10 seconds of "glow" the engine starts instantly. Didn't care about the thin wall but since we were moving to Alaska I really wanted the glow plugs. The new engine had the same type of combustion chamber, the exact same bore and stroke, same style injection pump ect. And it burns a gallon an hour just like the Perky but it's quieter and smoother. I think I gained about 50 rpm after break-in. Used the same BW gear and the same prop. Interesting though that I gained 200rpm when the engine was fully broken in. (400hrs) So I can say there wasn't much new and modern about my "new" engine. When I'm out in Dixon Entrance or other big waters the "new" seems to be a very nice thing though.

nomadwilly wrote:

Never heard of one w a Volvo. Did they change the std engine from Cummins to Volvo? What happened to "made in America"?

Click to expand...

People we know bought a new NT34 a couple of years ago.* They were told it would have a Cummins in it but when they took delivery of the boat it had a Volvo in it.* I don't know if NT has used Volvo regularly in their boats over the years but it apparently is a standard powerplant in some of their current models.

"Made in America" went out the window the first day somebody realized that the weird littleToyotas and Datsuns didn't crap out after a year or two like American cars did.* So sometime in the early 1970s I think.

-- Edited by Marin on Tuesday 4th of January 2011 06:07:38 PM

Marin wrote:

"Made in America" went out the window the first day somebody realized that the weird littleToyotas and Datsuns didn't crap out after a year or two like American cars did.* So sometime in the early 1970s I think.

-- Edited by Marin on Tuesday 4th of January 2011 06:07:38 PM

Click to expand...

As a body and fender repairman at a GM dealership in 1970 I had a set of metric wrenches in my rollaway.

*

Can anyone advise about the rate of fuel consumed by a pair of 150 hp Cummins diesels? The boat is a 40' Kha Shing -- perhaps going 8 knots? Any advice would be great.

Kevinbrian wrote:

Can anyone advise about the rate of fuel consumed by a pair of 150 hp Cummins diesels? The boat is a 40' Kha Shing -- perhaps going 8 knots? Any advice would be great.

Click to expand...

Little more information on the motors would be helpful. Are they 4BT3.9 (4 cylinder turbo), or 6B5.9 (6 cylinder natural), or something else? Other information on the boat would also be helpful such as the displacement and draft to determine the load on the motors.

LOA 40'
Beam
Draft
Displacement
Cruising speed 8 knots

Ted

*

8K is probably above "hull speed" so it wont be as cheap as say 6K , where the fuel bill will be painless.