Fuel Economy Question

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Tony B

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Jul 18, 2011
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Cruising/Live-Aboard USA
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Serenity
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Mainship 36 Dual Cabin -1986
I am not sure if this is the proper forum for this question. If not, just shoot me.
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I posted this question on a different site and now I will try it here.

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I am looking at several different trawlers at the moment. The Marine Trader 34 typically has a 120 HP engine with a cruising speed of 6K. A 36' Morgan West Indian has a 160HP engine with a cruising speed of 8K. Being retarded when it comes to motors, I made some simple calculations:

160HP divided but 120 HP = 1.333 larger.
8 Knots divided by 6 Knots = 1.333 Faster.
Looking at the above it would appear that the consumption ratio is the same exact as the Speed/Distance ratio.

Does this mean that if I am burning 1.333 times as much fuel and traveling 1.333 times further I am getting the same MPG from both vessels?
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I do know that the boat with the longer waterline wil have a higher theoretical hull speed. What I dont understand is that I have had several Marine Trader owners write and pretty much all claim the same speed and fuel consumption all using the same 120HP engine on the 32' - 18000 lbs* up to the 40' @ 30,000 lbs. Surely this cant be so.
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Anyway, I am in the seriously looking stage and whatever I buy will be a used boat probably around 30 years old. We intend to do the Great Loop and fuel economy will be a big factor. We have pretty much settled in our minds that a 34' to 38' would be good for us depending on cost and scondition.
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Please educate me on fuel consumption.
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Thanks in advance
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Tony B
 
TBH... I think your splitting hairs. There are a lot of things that affect your burn rate. And while over the entire distance of The Great Circle even a small difference in fuel burn can mean hundreds of dollars in your total fuel bill, you won't find better fuel economy than a single-screw trawler. Either of those motors will run for a lifetime with proper care and only sip fuel (my 130hp Perkins only burns 1.666g/hr.). The difference of just 100 rpm's over that distance could yield just about the same results between the two motors. You would be better served to spend your time learning what you need to know to pick the best cared for diesel out there. (and it I can inject my own personal opinion... a normally aspirated motor would be a better idea. It's simple, has fewer things to break, and turbo motors have had a little bit more stress and heat applied over their life. And you know the saying that a bulb that burn twice as bright... But that is just my own opinion.)

Tom-
 
I am looking at several different trawlers at the moment. The Marine Trader 34 typically has a 120 HP engine with a cruising speed of 6K. A 36' Morgan West Indian has a 160HP engine with a cruising speed of 8K. Being retarded when it comes to motors, I made some simple calculations:


This is advert garbage , useless for real world comparisons.

A real speed / fuelflow test of both would be required , not addvert blah blah.

Usually the displaced weight will determine the fuel use at every speed ,
however the speed desired will change the fuel burn by the greatest percentage , 300% extra to go fast.

Simply find the square root of the boats LWL , and that number in K is the cheap speed.

Hull speed? , that 200%-300% extra will apply.
 
Lots of posts have been made on this forum and elsewhere and it gets very technical and boring (to me anyway). Bottom line is pretty much what Gonzo said: a single screw trawler is the most economical boat you can get. How you run it will make a (relatively speaking) big difference (1800 rpm vs 2200 let's say). But it's still cheap compared to anything else. I know it cost me between 1.5 and 2 gallon an hour (around $10 an hour, give or take $2-$3). That's all I need to know. The theory and calculations behing those figures is just mumbo jumbo to me. I'd rather be sailing than calculating.
 
If fuel economy is of great concern the best boat (not trawler) is usually a sail boat. But having two expensive propulsion systems, a draft over twice that of a power boat and a cabin layout that requires one to stand out in the weather while operating their boat and once in the cabin it's hard to see outside you may want to consider a trawler. Ninety five + % of trawlers are semi-displacement and burn (generally speaking) twice as much fuel as a full disp trawler so serious economy is available only w the full disp boats but there are VERY few having been built 30 yrs ago. So the first thing to do if low fuel burn is of great concern is to look at full displacement trawlers. Even they are mostly over powered. The Krogen 42 and Willard 40 are excellent examples. Then if you come up empty handed look at semi-disp trawlers. Almost all trawlers are SD. Look for the narrowest, lightest boats with the smallest engine. Pass on 99% of twin screws.
 
With a semi-displacement boat, I can't imagine much better economy that 1.666/hr. Are you saying my usage would decrease with a full-displacement hull?
 
I think I finally figured it out.

With a given size semi-displacement hull, it does not matter whether you have a120HP or a 220HP engine. What matters is the RPM's. If for example, I found my most efficient speed to be 7 knots, I should be gettting pretty much the same consumption rate with either engine because the larger engine will be going much slower (RPM's).

Am I even close?
 
Nope , the Power required for your 7K is a given.lets say its 40 hp.

40 HP can be best (most efficiently ) produced with a 50 HP motor geared properly.

The problem is sometimes you might want to run faster , or harder (headwind and 15 ft waves) or charge the batts , make AC , or run a freezer compressor.

So the question then becomes how much will you be willing to loose by having a larger less loaded (less efficient) engine?

For most that get 16 hp from a gallon of fuel lowering the efficiency to only 12 hp from a gallon is usually OK , esp for 200 hours a year.

Efficiency is a key IF you are creating a new build , but usually not controllable with a used boat.
 
Tony B wrote:
I think I finally figured it out.

With a given size semi-displacement hull, it does not matter whether you have a120HP or a 220HP engine. What matters is the RPM's. If for example, I found my most efficient speed to be 7 knots, I should be gettting pretty much the same consumption rate with either engine because the larger engine will be going much slower (RPM's).

Am I even close?
*Yes, of course you are close.* The output HP of any motor is a function of the fuel consumed.* Larger engines can burn more fuel, releasing more energy, but the most efficient engine will be one that is operating at 75% or so of its rated capacity delivering the HP you need for the specific boat, prop, trim, displacement, hull form, at the desired speed.

For example, a Cat 3408 displaces 18 liters, and burns*11.7 gph to produce*216 hp, max HP 536.

A Cat 3306 displaces 10.5 liters and burns 12.1 gph to produce 215 hp, max HP 270.

*

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For example, a Cat 3408 displaces 18 liters, and burns 11.7 gph to produce 216 hp, max HP 536.

Comes to 18.46 hp/gal

A Cat 3306 displaces 10.5 liters and burns 12.1 gph to produce 215 hp, max HP 270.

About 17.7 gal/hp

The problem is this is the full , max power output , where the engine IS efficient.

Reduce the power output at the prop to 30- 40hp and 8 to 12 HP/Gal may be the result.

That is why (proper loading) CPP props are on the commercials and cruising props on the distance cruisers.
 
A simple equation. Speed = Money2 (money squared) So, you need to ask yourself, "How fast do I need to go?"

Some may tell you that you need some reserve speed to outrun a storm. With todays doppler radar, wind modelling, 24 hour constant news updates, weather fax, Sirius weather, and a pair of binoculars..... which storm is going to sneak up on you that 12 knots will outrun, but 7 won't?

If you believe that a 36' boat cruises at 8 knots, for the same price as a 34' boat cruises at 6 knots, then I have a bridge to sell in New York.

There is a reason that the old guys in trawler style boats cruise at 6.5 knots or so. It isn't because they get scared at 8 knots, well, most of them anyways, and it isn't because they're afraid of getting somewhere too quickly. It simply is more efficient and less costly. They've been there, bought the fuel and know the numbers. This stuff ain't rocket surgery. Look around, find a boat type/style you like and then go to the marina's on weekends and talk to the guys that run them and aren't trying to sell you anything. You just might find a couple that will tell you the things you need to know.

Ken
 
My 35.75-foot trawler consumes fuel at least twice the rate at*8 knots (full throttle on calm water) versus 6.5 knots.
 
FF wrote:
For example, a Cat 3408 displaces 18 liters, and burns 11.7 gph to produce 216 hp, max HP 536.

Comes to 18.46 hp/gal

A Cat 3306 displaces 10.5 liters and burns 12.1 gph to produce 215 hp, max HP 270.

About 17.7 gal/hp

The problem is this is the full , max power output , where the engine IS efficient.

Reduce the power output at the prop to 30- 40hp and 8 to 12 HP/Gal may be the result.

That is why (proper loading) CPP props are on the commercials and cruising props on the distance cruisers.
*Emm, not so much.* I am right now moving along at 1300 rpm, prop demand hp of 56, burning 3.4 gph or 16.47 HP/Gal.* Some drop off in efficiency, but certainly not enough to worry about.
 
Wait... 35 POINT 75 feet? You know how long you boat is down the the 100th of a foot? Pardon me sir, but...... GEEK! :-D
 
GonzoF1 wrote:
Pardon me sir, but...... GEEK! :-D
What makes it really hilarious is that the length overall has nothing at all to do with the reasons this topic even exists ...*
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GonzoF1 wrote:
Wait... 35 POINT 75 feet? You know how long you boat is down the the 100th of a foot? Pardon me sir, but...... GEEK! :-D
*OK.* Should I say "35 feet and*9 inches," "35 feet," "36 feet," "something in the mid-thirties." "between 11*than 12 meters," etcetera?

Waterline length is 31 feet and 8 inches, etcetera.* Empty or loaded?* I don't know.* Who here is the real geek?

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"Take a look at the fuel consumption curves of some modern engines."

No doubt , but most "trawlers" have 30-40 year old farm implement or light truck motors.

My own has a 1936 designed DD 6-71.

What a "modern" 30,000 psi rail electronic injected turboed engine has little today to do with most folks.

This WILL change as we see quite fancy auto engines being marinized.

The Yanmar BMW , or Toyota conversions will do quite well at low speed fuel burn , weather they will get 5000 hours at much over half throttle is the question.
 
FF wrote:
"Take a look at the fuel consumption curves of some modern engines."

No doubt , but most "trawlers" have 30-40 year old farm implement or light truck motors.

My own has a 1936 designed DD 6-71.

What a "modern" 30,000 psi rail electronic injected turboed engine has little today to do with most folks.

This WILL change as we see quite fancy auto engines being marinized.

The Yanmar BMW , or Toyota conversions will do quite well at low speed fuel burn , weather they will get 5000 hours at much over half throttle is the question.
*The idea that fuel consumption is much lower on older diesels running at slower speeds is simply incorrect, and not even true about Detroit 671s.* My 3306 is a 50 year old design and at half throttle produces about 90% of the hp per gallon of fuel consumed as it does at 80% throttle - just like the Detroit.* Longevity is not materially affected by low speed use as long as the engine is handled properly per the manufacturer's guidance.

What effect low speed use has on modern engines*may well be more of an issue, but I suspect that even there exercising the engine properly will go a long way to ensuring long life.*
 
DavidM,

Good interesting post. But as to efficiency a long stroke engine has a heat sink advantage. The area of the cylinder and combustion chamber per cubic displacement is noticeably less so less heat is lost in this way. Heat loss in a heat engine is lost efficiency but there are other elements of the efficiency issue that may or probably swing toward the short stroke engine. So I think your implication is probably correct. Air cooling could conceivably swing it the other way. But variation in engine efficiency is small potatoes compared to variations in hull design. Good economy will not be achieved without a full disp hull in any boat that could be called a trawler. By the way how can a supercharged DD be referred to as a "naturally aspirated" engine. Won't even run without scavenging assistance.
 
nomadwilly wrote:
......*By the way how can a supercharged DD be referred to as a "naturally aspirated" ...........
*Can anyone answer this with a straight face?


-- Edited by Tony B on Sunday 24th of July 2011 01:22:44 PM
 
" .... By the way how can a supercharged DD be referred to as a "naturally aspirated" ...........

Can anyone answer this with a straight face?"

SURE , The blower on most DD is for exhaust scavengine and simply blows the cylinder clean.

Since the DD fuel injected there is no efficiency loss that happens with a carb engine and fuel and air are pushed thru,

The DD usually has under 1 pound of boost , if anything left after the valves close.
However since the inlet air is much warmer (the density is lower) so "Supercharging" for added power does not exist.

1 pound might be called "Supercharging" but for most the Turboed DD are the ones that make use of above atmospheric pressure for added HP.

OK?
 
DavidM wrote:But I couldn't find any marine diesels that produce 12 hp per gph at low power loadings, much less 8.
*Why are you guys using hp per gph? It is a bogus home-made marketing expression. If you really want to compare fuel efficiency please use specific fuel consumption based on the propeller curve. That is what you will find on most engine spec sheets and are based on standard condtions and fuel density/heating value.
Brake specific fuel consumption or BSFC is the weight of fuel burned to produce one horspower for one hour. It is the industry and engineering standard and using something else is like using lily pads passed per fortnight as a way to describe speed.
If I am not mistaken, the basis of this thread is founded on the fantasy that there is such a huge difference in engine efficiency between the holy 75 percent and some lower number that that alone is reason not to run larger engines at low power. This has been debunked several times, and is again each time one of the gurus laments the lack of fuel maps as being the reason buyers are led so far astray.
Running the numbers on several old and new engines including a very modern turbocharged, high pressure common rail electronic marvel provides a glimpse at reality.
One common mechanically injected normally aspirated engine running at 75 percent power delivers 1 hp for every .39 lbs of fuel burned. At 25 percent it burns .43 lbs. This is .04 lbs per hp per hour difference between 150 hp and 50 hp for that 200 hp engine. Point zero four, is four one hundredths of a pound, 0.64 ounces per hp.**Another way to look at it is if that engine was equally inefficient at high*as it is at*low power, eveery hour at 150 hp it would burn an addtional 32 ounces*of fuel.* In real life what it means is that instead of burning 3 gph at low power, that engine would burn 2.88 gallons if the efficiency were the same as at high power. So, to*reduce fuel consumption from*8.3 gph at 75 percent, you slow down and burn 3 gph.*Does it really matter that you are theoretically leaving*0.12 gallons*per hour on the table? How long and how far do you have to go to even know there is a difference? I suggest not a single one of us could ever tell the difference since wind, currents, poor steering and just fooling around makes those numbers so small as to be statistically irrelevant.
The engine described above was the worst of the 4-stroke engines in the range normally installed in recreational trawlers. Other engines are very similar but with 0.37 or 0.38 at low power. The latest high tech HPCR engine with all the bells and whistles shows an increase in efficiency at low power with a BSFC (propeller curve) of .36 at 75 percent power and .35 at 25 percent power. These savings if extended over a weekend cruising will be invalidated by an extra few*moments idling at the dock before departure or by letting one of your children steer for 5 minutes.
Anyone with access to engine specs (if you are reading this you do) and a calculator can confirm this for themselves. Each boat is different and each owner operates it differently. The variation among operators and even location is far wider than the small*percentage points that separate the newest from the oldest and the best from the worst engines.
Those who claim that using a controllable pitch propeller will widen the gap are still ignoring the fact that a CPP wheel is less efficient to begin with, weighs more, costs more, and requires more maintenance than the savings of a few ounces of fuel over a weekend or*a few gallons*on a trip through the "loop" or "up the inside."*Those things are great for harbor tugs and icebreakers or other boats that require rapid and frequent maneuvering but are highly unlikely to save anything for a recreational trawler owner.

*
 
Once again, Rick's expert post has required me to take two aspirin and l lay down for a few minutes to prevent spontaneous brain combustion. :-D
 
DavidM wrote:

So it seems that the manufacturers are implicitly supporting hp per gph.

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The marketing people massage the data to make it more palatable to the consumer. They also tailor it to fit a very narrow audience as it should be obvious that the hp/gph thing is meaningless to the*majority of the world that does not use the US gallon.

*Then again, who knows, maybe they are using imperial gallons to make the numbers look better*
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I'll also note that hp per gph has become the parameter of choice in discussions over on boatdiesel. These are real world mechanics, shop owners and boaters. So are the members of this forum.

You can use either in these discussions but I would prefer to leave BSFC to the engineers (Uh oh, I are one!!).**

It may be the parameter of choice because the folks who ask the questions ask in the terms they hear and see "on the street."

Personally, I look at hp/gph as a form of technical ebonics. Call me a stodgy old traditionalist but BSFC is universal. What works on the corner should stay on the corner unless we want to limit our readership to the "hood."

You are right though, that if people are comfortable with it, there is no law saying they have to use legitimate metrics in forums like this, but I believe it is a form of slang that doesn't translate well.
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-- Edited by RickB on Monday 25th of July 2011 08:35:02 AM
 
Tony B

You say* "Can anyone answer this with a straight face?"

Looks like you are trying to be funny But I'm not amused. If you want to be rude at other people's expense go to OTDE and play games w others of like minds.

*

Eric Henning
 
RickB wrote:Call me a stodgy old traditionalist
*Ok. You're a stodgy old traditionalist.

(Can you tell I have a little free time today at work?)
 
RickB wrote:
The marketing people massage the data to make it more palatable to the consumer. They also tailor it to fit a very narrow audience as it should be obvious that the hp/gph thing is meaningless to the*majority of the world that does not use the US gallon.

One of the reasons folks use the GPH figure is because it is a more easily understood metric that is exactly equivalent to lb/hp-hr and hardly a lilypads passed per hour measurement.* Which is why these two metrics*are laid out side by side as equivalent in performance tables for almost every engine.

But it is certainly true that the idea that fuel consumption is materially adversely affected at low speeds is a canard.

*
 

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Delfin wrote:One of the reasons folks use the GPH figure is because it is a more easily understood metric**
*I have absolutely no qualms about showing gph at various power settings, it is quick and useful and easily understood by all in the terms that apply at the fuel dock. It is what I look at*for small engines. I never suggested that publishing consumption in gph on the graph was inappropriate.

I do suggest that using "horspower produced per gallon of fuel burned" is engine ebonics.*That figure*is not displayed on the spec sheet you posted. It is an abstraction used online in boating and some engine forums simply to avoid using an established metric for reasons that are best left to OTDE.
 
The*metric for the mods and Tony Athens on boat diesel is BSFC. Of course many posters will spout off*other numbers.

In the off road non marine diesel business even though BSFC and its derivatives are very much monitored, *it gets more *complicated with hydraulic pumps/servos consuming huge HP and electric traction motors with grid dissipating braking downhill. For the past decade or more on board computers and telemetry to fixed stations keep tabs on fuel burn, temps, RPM, EGT, EG O2, NOX etc*content and turbo pressures (as many as 4 per engine). A savings of 5% fuel burn can easily be $1 million/year at a big mine. Plus, erratic fuel burn leads to early engine demise.

GPH is my boat key though, it tells me how far* we can go and what is best combination of RPM and through the water speed for the prevailing conditions. A friend of mine with a big Westport prefers 10 knots because it "feels right" and*he can "go further."
 
"That is what you will find on most engine spec sheets and are based on standard condtions and fuel density/heating value."

The big problem with Rick B 's post is that while the PEAK hp ,fuel flow , hp produced and rpm are always accuratly given , the prop curve and the fuel consumption figures for it are not measured , they are mere math creations.

"Prop demand data bases on .......... a fictional GUESS.

That good old (and unaviliable) fuel map DOES use actual measurements of load vs fuel flow.

Weather its GPM or cc per min is meaningless as all can be converted accuratly to what is easy to visualize.

The folk with pumps generators etc need REAL numbers , not math creations.
 
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