Fuel Mileage

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FlyWright said:
If that's the case, shouldn't the GPH be doubled and the NM/Gal be halved? The sweetspot of 1800 RPM remains, but the data plots shift.
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FlyWright the load on each engine if it's a twin is about half of the load that a single would have. Same work being done to push the boat. So the fuel burn on a single engine is more, much more and that probably makes it more efficient. Most engines in trawlers are most efficient about 1800 rpm and under relatively heavy loads.

This only applies to boats that have the same engine (or power) in the single and the twin. A stupid arrangement for trawlers in my opinion. A twin engine trawler should have about the same amount of power as the single engined version of the same boat. The "go fast go slow" marketing ploy is not a good thing.
 
I have done well over 20,000 miles in the last ten years on our FL120s. 1400rpm (7,000+ miles on this boat with twins). 1500 rpm on the single on the prior and have never had an internal engine problem and have always over-propped to compensate for the low rpm. Never have burned over 2gph.
 
manyboats said:
A twin engine trawler should have about the same amount of power as the single engined version of the same boat. The "go fast go slow" marketing ploy is not a good thing.
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I don't agree Eric. My hypothesis is that the primary reason for two engines is to have more power than a single engine so the boat can go faster.

Boats my size and smaller with two engines have more than twice the horsepower of my single so they can break out of their bow wave. For example, the Californian 34 LRC weighing 18000 pounds might have 200 HP (varies among individual boats) with its two engines (100 HP each) compared to the Coot's 80 HP engine pushing 28000 pounds.
 
Al I think I get Scott's point in the confusion. He didn't say HE was confused. Some ppl are using the phrase to refer to the spot (RPMs) where the engines SOUND good. Others are using it to mean RPMs that are maximizing efficiency, minimizing noise, actually getting you somewhere against the current... That could mean running at a higher RPM than the other "sweet spot" where they SOUND so happy.

I have no idea what the most fuel efficient RPMs are for my Perkins, but I imagine it's below where the turbo whine kicks in. :). They've got a pretty good range where they "sound" happy.
 
Pineapple Girl said:
I have no idea what the most fuel efficient RPMs are for my Perkins, but I imagine it's below where the turbo whine kicks in. :).
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:rofl::rofl:

I'm with you Jennifer, no idea either when it comes to efficiency. Sure is efficient at making fuel disappear though. Once the boat is on plane it puts a smile on my face and that's my boats sweet spot :socool:

If ever the decision is made to go long distance cruising I'll buy a Flo-Scan. By then they should be able to interface with our fuel card balance and help adjust the throttle accordingly based upon desired travel distance.:)
 
manyboats said:
FlyWright the load on each engine if it's a twin is about half of the load that a single would have. Same work being done to push the boat. So the fuel burn on a single engine is more, much more and that probably makes it more efficient. Most engines in trawlers are most efficient about 1800 rpm and under relatively heavy loads.

This only applies to boats that have the same engine (or power) in the single and the twin. A stupid arrangement for trawlers in my opinion. A twin engine trawler should have about the same amount of power as the single engined version of the same boat. The "go fast go slow" marketing ploy is not a good thing.
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Eric,

Maybe I misunderstood and failed to put it clearly. I thought Ben plotted the data Blue Heron provided on another thread for EACH of his engines. I thought the data on the plot should show the data for BOTH of the engines. If that's accurate, the plot for GPH would double and the plot for NM/Gal (listed as Kt/gal) would be reduced by half.
 
FlyWright said:
I thought Ben plotted the data Blue Heron provided on another thread for EACH of his engines. I thought the data on the plot should show the data for BOTH of the engines.
Click to expand...

Well, Blue Herons data, my interest, and it was confirmed by BH to be each. Since I drive a single screw with a similar engine as BH's twins, it's of interest.

So my disclaimer is to not use this as an exact measure, only a reference in determining optimal points for diesels of the 120-135 hp variety. Prop size will also figure in, load on engine will figure in.

These data won't be applicable to a 435hp Caterpillar, but there are a lot of trawlers out the sporting Lehmans, Perkins, and possibly some Volvos where it might be useful. Use only as a benchmark as I will for your own optimal point and understanding of your boat's performance characteristics.

Good grief. I sound like an engine manual now.
 
Daddyo said:
I have done well over 20,000 miles in the last ten years on our FL120s. 1400rpm (7,000+ miles on this boat with twins). 1500 rpm on the single on the prior and have never had an internal engine problem and have always over-propped to compensate for the low rpm. Never have burned over 2gph.
Click to expand...

I am weak on prop understanding - is over propped increasing the diameter, pitch, or a combination of the two?

How do you determine and calculate the amount to do this and is vibration a concern from the difference? Can you say how much prop difference equals what RPM difference?

Very interesting point.
 
Daddyo said:
I have done well over 20,000 miles in the last ten years on our FL120s. 1400rpm (7,000+ miles on this boat with twins). 1500 rpm on the single on the prior and have never had an internal engine problem and have always over-propped to compensate for the low rpm. Never have burned over 2gph.
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Show-off! :hide:

:D:D:D:D:D

(j/k)
 
Ben said:
I am weak on prop understanding - is over propped increasing the diameter, pitch, or a combination of the two?


Proper propeller pitch just means that you are always running on or below your engine manufacturer's designed power curve. Because most boats don't have multi speed transmissions or variable pitch propellers, propeller size and pitch has become the go to tweak to accommodate some of the dynamics affecting our boats. Simply put a properly propped boat can handle the changes in loading without creating excess loads on the engine/engines. Some of the dynamics you might want to consider when propping are heavy loads, foul bottom, towing, running on one engine for the twin engine guys, and big seas. If you can run your engine's max rpm under all these scenarios then your good to go. If not then your making your engine work harder at all rpms, it's kind of like starting up a hill in 2nd gear.
Click to expand...
 
Ben said:
Well, Blue Herons data, my interest, and it was confirmed by BH to be each. Since I drive a single screw with a similar engine as BH's twins, it's of interest.

So my disclaimer is to not use this as an exact measure, only a reference in determining optimal points for diesels of the 120-135 hp variety. Prop size will also figure in, load on engine will figure in.

These data won't be applicable to a 435hp Caterpillar, but there are a lot of trawlers out the sporting Lehmans, Perkins, and possibly some Volvos where it might be useful. Use only as a benchmark as I will for your own optimal point and understanding of your boat's performance characteristics.

Good grief. I sound like an engine manual now.
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Concur. It's a reference. Your "mileage may vary "... Hehehe
 
Mark wrote;
"I don't agree Eric." Great ... that means we've got something to talk about but I'm short of time these days.
"to have more power"? Well your boat is powered correctly I believe. Something of a rarity around here it seems. So what if I bought a Coot and "wanted more power"? Can you imagine a Coot w 160 hp??? Or try 40. Stupid either way. Boats (FD or close to it like a DeFever) can only gracefully use X amount of power. In your case it's 50hp so an 80hp engine is perfect. But a 50hp engine would be shy for you and a 120hp engine would be something you'd leave in the boat let it run it's course but nothing a responsible NA would specify for a build.

Semi planing hulls that are closer (dynamically) to planing hulls have a wider speed range and some can effectively use more widely varying power. Observe the newer GBs and many others. The legitimate "go fast go slow" crowd. However that's what a planing boat is.

"More power" is stupid for boats close to FD design and can't be used except for marketing (how many people chose 6cyl Chevy's over V8s?). It clearly goes back to our automotive culture. And our overall culture leans heavily toward "more" and "bigger".
 
I'm pretty new to this boat, just got her this spring, but have been taking pretty detailed notes on fuel fills, sightglass readings, RPM, speed, etc..

I don't yet have a Floscan, but I'm finding my "happy spot" is at about 1600-1650 RPM doing about 7 knots, which burns about 1.5 GPH.

I hope to install a fuel metering system sometime this winter to give me more detail.
 
fuel economy President or Med Yacht

I am looking at a 45 Med Yacht with a pair of FL 275s and a 41 President with a pair of FL 225s both enjoined are turboed. What can I expect for fuel economy at 7 knots on either. Also I do not need the speed would I do better on fuel if I removed the turbos?
 
The hand on the throttle determines fuel consumption per NM. Expect about 1.5 to 2 gph per engine in either case at 7 knots. There is no fuel economy gain in removing a turbo. The remaining life of 30 year old engines will be at the whims and care the POs have bestowed.
 
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the fuel difference is not worth considering. Neither boat is an efficient way to travel. Buy what is best condition with best features. Larger is always better as they do shrink after owning them for a while.
 
The sweet spot for any boat (if low fuel burn is the goal) is about at the SQ RT of the LWL in knots,( SL.)

SL x .9 to 1,15 is the efficient range.

What ever RPM gives about that speed is the place to be in terms of MPG .

Pushing water aside costs money.

The faster you push it aside the faster the fuel tank empties.
 
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