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Old 07-17-2013, 01:12 PM   #21
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Originally Posted by Delfin View Post
Regardless of the current, the water you travel through will be the water you travel through, even if you don't get anywhere because of current.
All true, but let's try an extreme example.

Suppose my most efficient speed is 3 knots through the water, and I decide I'm going to save money by never exceeding that.

I head upriver and encounter a 3 knot foul current. My speed over ground drops to zero, my NMPG drops to zero and my ETA becomes infinite. Obviously I'm not saving any money, even if I'm in no hurry to get anywhere.

In fact, the faster I go, the less time I'll spend fighting the current before I get there.

Somewhere, there's a "sweet spot"; a trade-off between minimizing the time spent fighting a foul current and burning more fuel.
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Old 07-17-2013, 04:28 PM   #22
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All true, but let's try an extreme example.

Suppose my most efficient speed is 3 knots through the water, and I decide I'm going to save money by never exceeding that.

I head upriver and encounter a 3 knot foul current. My speed over ground drops to zero, my NMPG drops to zero and my ETA becomes infinite. Obviously I'm not saving any money, even if I'm in no hurry to get anywhere.

In fact, the faster I go, the less time I'll spend fighting the current before I get there.

Somewhere, there's a "sweet spot"; a trade-off between minimizing the time spent fighting a foul current and burning more fuel.
And that is where a flo scan or similar fuel monitor hooked to a gps and spitting out a NMPG number would be the use I would most benefit from.

Hand crunching the sweet spot would probably give me a headache if I had to do it all the time.
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Old 07-17-2013, 04:42 PM   #23
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And that is where a flo scan or similar fuel monitor hooked to a gps and spitting out a NMPG number would be the use I would most benefit from.
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Old 07-17-2013, 04:42 PM   #24
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This may just be about terminology, but I feel compelled to jump in here. When we talk about "efficiency", the scientific meaning is work done per unit of time. In a car, it's measured in MPG. Similarly, in a boat, it's measured in nMPG. GPH, by itself, is a meaningless number when looking at efficiency.

A point of maximum efficiency would be some sort of local maximum on the nMPG curve. It would be a point where going either faster or slower yields worse nMPG. Such a point doesn't exist for a displacement boat.

When one talks about an "efficiency sweet spot", I start looking for a point where there is some maximum nMPG value, but for a boat that exists only at the boat's slowest speed. For every increase in speed, the nMPG number drops. For a displacement boat, it never even levels off. It's just all down hill the faster you go.

But there is also a subjective definition for a "sweet spot" which would be the point where the operator is happiest with the tradeoff between speed and nMPG. Going slower will always yield better nMPG, but most of us want to eventually get where we are going. Going faster will always yield worse nMPG, but we are willing to accept some degree of that in the interest of time. We all have that spot where we are operating at a happy tradeoff between nMPG and speed. Calling that a "sweet spot" is as good a term as any, but technically speaking, it's not a point of maximum efficiency. Not even close.
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Old 07-17-2013, 06:41 PM   #25
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I think "sweet spot" is now confusing in this thread...some I think were referring to where the engine and boat "sound" or "feel" in harmony...but little to do with efficiency...then some started using the term to mean where the boat is "relatively" efficient...but we all know that's a fleeting concept because of environmentals.

CaptTom made the only real point that I can see as far as looking for the most efficient rpm/hull speed translated to SOG...but I never have used the term sweet spot when referring to anything but when the engine sounds purdy .
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Old 07-17-2013, 07:37 PM   #26
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I think "sweet spot" is now confusing in this thread...some I think were referring to where the engine and boat "sound" or "feel" in harmony...but little to do with efficiency...then some started using the term to mean where the boat is "relatively" efficient...but we all know that's a fleeting concept because of environmentals.
I started it! I am guilty! on Sweet Spot. My intention for my own selfish purposes was to name the general (you must pick what works for you) convergence of speed, fuel use, and RPMs so that the boat moved through the water at best speed with least fuel use.

As for the current, that is out of anyone's control.

"Sweet Spot" Ah ha ha ha ha ha! Now you must go crazy.

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Old 07-17-2013, 08:15 PM   #27
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Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.

There's no way running the engine for an hour at 3.1 knots burns less fuel than running it for 6 minutes at 4 knots.

Hence the made-up and un-scientific concept of a "sweet spot".
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Old 07-17-2013, 08:36 PM   #28
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Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.
0.1 knots is not sweet in anyone's book. Not sure I'd want to cruise the ICW doing 3.1 knots in a good current in any circumstance. That puts me at about 1050 RPM. Nice to pull in the slip with, but not much else. I'll run about 1750-1950 RPMs and do 6.7-7.5 most days. I'm not picky, but I do want to know how best to treat my engine and fuel burn.

The knowledge of the engine performance is the value. Now I just got to degrease, change pipes, and clean my heat exchanger! (I digress)
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Old 07-17-2013, 08:41 PM   #29
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This may just be about terminology, but I feel compelled to jump in here. When we talk about "efficiency", the scientific meaning is work done per unit of time. In a car, it's measured in MPG. Similarly, in a boat, it's measured in nMPG. GPH, by itself, is a meaningless number when looking at efficiency.

A point of maximum efficiency would be some sort of local maximum on the nMPG curve. It would be a point where going either faster or slower yields worse nMPG. Such a point doesn't exist for a displacement boat.

When one talks about an "efficiency sweet spot", I start looking for a point where there is some maximum nMPG value, but for a boat that exists only at the boat's slowest speed. For every increase in speed, the nMPG number drops. For a displacement boat, it never even levels off. It's just all down hill the faster you go.

But there is also a subjective definition for a "sweet spot" which would be the point where the operator is happiest with the tradeoff between speed and nMPG. Going slower will always yield better nMPG, but most of us want to eventually get where we are going. Going faster will always yield worse nMPG, but we are willing to accept some degree of that in the interest of time. We all have that spot where we are operating at a happy tradeoff between nMPG and speed. Calling that a "sweet spot" is as good a term as any, but technically speaking, it's not a point of maximum efficiency. Not even close.
Well said, twistedtree. No problem with the term sweet spot from my perspective. I suspect most know we're talking about a subjective, personally-accepted compromise between noise, engine health, speed and fuel consumption. That was the point of the discussion here and seems to have carried through on this thread as well.

Nothing says that sweet spot can't change with the conditions of the day, i.e. working against a strong current.

What's the confusing part?
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Old 07-17-2013, 09:08 PM   #30
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Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.

There's no way running the engine for an hour at 3.1 knots burns less fuel than running it for 6 minutes at 4 knots.

Hence the made-up and un-scientific concept of a "sweet spot".
I understand completely...you are giving great examples.

Unfortunately it is a lot of tables or graphs or hand calculations..or just a good fuel monitoring system that includes SOG for an accurate, up to the minute KMPG for day to day use...

In general...confusion over sweet spot?...not in my mind...as I've been around boats in general for over 50 years and marine engineers pretty much for the last 35 years.
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Old 07-17-2013, 10:25 PM   #31
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I think "sweet spot" is now confusing in this thread...
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What's the confusing part?
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Originally Posted by psneeld View Post
In general...confusion over sweet spot?...not in my mind...as I've been around boats in general for over 50 years and marine engineers pretty much for the last 35 years.
Now I'm confused.
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Old 07-17-2013, 11:25 PM   #32
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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.
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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Old 07-18-2013, 12:15 AM   #33
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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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Old 07-18-2013, 12:37 AM   #34
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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 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.
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Old 07-18-2013, 12:46 AM   #35
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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.
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Old 07-18-2013, 01:02 AM   #36
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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. .


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

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.
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Old 07-18-2013, 01:55 AM   #37
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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.
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.
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Old 07-18-2013, 06:59 AM   #38
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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.
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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Old 07-18-2013, 07:03 AM   #39
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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.
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.
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Old 07-18-2013, 08:00 AM   #40
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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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