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Old 07-19-2011, 11:04 AM   #1
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Fuel Economy Question

I am not sure if this is the proper forum for this question. If not, just shoot me.

*

I posted this question on a different site and now I will try it here.

*
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?
*
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.
*
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.
*
Please educate me on fuel consumption.
*
Thanks in advance
*
Tony B
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Old 07-19-2011, 04:23 PM   #2
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RE: Fuel Economy Question

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.)

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Old 07-22-2011, 03:17 AM   #3
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RE: Fuel Economy Question

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.
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Old 07-22-2011, 08:31 AM   #4
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RE: Fuel Economy Question

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.
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Old 07-22-2011, 08:32 AM   #5
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RE: Fuel Economy Question

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.
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Old 07-22-2011, 09:22 AM   #6
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RE: Fuel Economy Question

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?
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Old 07-22-2011, 12:12 PM   #7
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RE: Fuel Economy Question

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?
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Old 07-22-2011, 12:54 PM   #8
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RE: Fuel Economy Question

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.
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Old 07-22-2011, 09:22 PM   #9
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RE: Fuel Economy Question

Quote:
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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Old 07-23-2011, 03:27 AM   #10
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RE: Fuel Economy Question

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.
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Old 07-23-2011, 08:37 AM   #11
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RE: Fuel Economy Question

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
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Old 07-23-2011, 09:22 AM   #12
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RE: Fuel Economy Question

My 35.75-foot trawler consumes fuel at least twice the rate at*8 knots (full throttle on calm water) versus 6.5 knots.
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Old 07-23-2011, 04:52 PM   #13
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RE: Fuel Economy Question

Quote:
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.
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Old 07-23-2011, 04:54 PM   #14
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RE: Fuel Economy Question

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
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Old 07-23-2011, 07:02 PM   #15
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RE: Fuel Economy Question

Quote:
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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Old 07-23-2011, 07:52 PM   #16
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RE: Fuel Economy Question

Quote:
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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Old 07-24-2011, 03:49 AM   #17
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RE: Fuel Economy Question

"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.
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Old 07-24-2011, 08:05 AM   #18
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RE: Fuel Economy Question

Quote:
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.*
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Old 07-24-2011, 09:04 AM   #19
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RE: Fuel Economy Question

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.
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Old 07-24-2011, 12:22 PM   #20
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Fuel Economy Question

Quote:
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
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