2 trawlers, same engine, different fuel consumption???
- Thread starter seattleboatguy
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When I retire....when I have a bit more time and don't have to go fast....but we will see...
Daddyo
Guru
If you have a 42 then that is much higher than your mpg sweet spot. Your going to be much closer to 6-6.5 kts. If your bow starts rising your past your sweet spot.
SomeSailor
Guru
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Not if you do the math. Max hull speed is 1.4 times the square root of the waterline in feet. At 42 feet it comes out to 8.75 kts.
That's why I feel best at 8-8.5kts. I'll have to try it at the 6.5 you're talking about and see how that feels some nice summer day.
That's why I feel best at 8-8.5kts. I'll have to try it at the 6.5 you're talking about and see how that feels some nice summer day.
ranger58sb
Guru
That should be 1.34x... and if your OAL is 42' then your LWL is what, maybe 40? A little less?
And of course "max" still isn't necessarily the most fuel efficient... even though there's something to be said for "time efficiency" and slight differences in burn rate might not always be significant enough to incite change.
-Chris
psneeld
Guru
that 1.34 is not truly a constant....it varies with hull shape....the more slippery the boat...the higher the number.
many of our hard chined, vertical transomed boats are probably closer to 1.1...
My Albin 40 has close to a 36 foot waterline and the 1.1 works pretty well with my "guessing" of all these cool hydrodynamic rules ....that's because I have no idea what they are because I didn't compare the line drawings with tank test results and the NAs ideas for my particular boat.
many of our hard chined, vertical transomed boats are probably closer to 1.1...
My Albin 40 has close to a 36 foot waterline and the 1.1 works pretty well with my "guessing" of all these cool hydrodynamic rules ....that's because I have no idea what they are because I didn't compare the line drawings with tank test results and the NAs ideas for my particular boat.
psneeld
Guru
Every boat has it's own number based on it's design, how it's built and whether it's sitting on it's designed lines or not...
here are some highlights of the same old stuff I have learned and used for 45 years+...many opinions on this forum are only tastes of the whole concept of boat design. I'm getting ready to sell my library of design books if anyone is interested...keep an eye in the classified section.
http://www.sponbergyachtdesign.com/THE%20DESIGN%20RATIOS.pdf
Nevertheless, what this tells you is that most displacement boats travel most of the time
at Speed/Length ratios of at least 1.0 and slightly above, so you need enough volume to
support the hull at those speeds. If volume is either too much or too low—that is if Cp is
too big or too small—your hull drag is going to go up. Either the boat is going to have to
push too much water out of the way (Cp too big) or it is going to sink into its own waves
(Cp too small).at Speed/Length ratios of at least 1.0 and slightly above, so you need enough volume to
support the hull at those speeds. If volume is either too much or too low—that is if Cp is
too big or too small—your hull drag is going to go up. Either the boat is going to have to
push too much water out of the way (Cp too big) or it is going to sink into its own waves
Speed/Length ratio Cp
1.0 0.52
1.1 0.54
1.2 0.58
1.3 0.62
1.4 0.64
1.5 0.66
1.6 0.68
1.7 0.69
1.8 0.69
1.9 0.70
2.0 0.701.0 0.52
1.1 0.54
1.2 0.58
1.3 0.62
1.4 0.64
1.5 0.66
1.6 0.68
1.7 0.69
1.8 0.69
1.9 0.70
see how the s/l ratio is tied to the prismatic coefficient....the first 2 numbers are the s/l ratio...followed by the Prismatic coefficient
Definition: Speed-length ratio is the speed of the vessel in knots divided by the square
root of the vessel’s waterline length in feet = V/Lwl^0.5. At speed-length ratios less than
1.34, the vessel is in displacement-mode motion—that is, the hull is simply moving the
water out of the way as it moves forward. When speed-length ratio is between 1.34 and
2.5, the vessel is in the semi-displacement or semi-planing mode—that is, it is trying to
rise up over its own bow wave to get onto plane. Some boats are designed to operate
at these speeds. Above speed-length ratio of 2.0 to 2.5, the vessel is planing and relies
on dynamic lift to raise and hold it out of the water so that it can skim along the surface
of the sea.root of the vessel’s waterline length in feet = V/Lwl^0.5. At speed-length ratios less than
1.34, the vessel is in displacement-mode motion—that is, the hull is simply moving the
water out of the way as it moves forward. When speed-length ratio is between 1.34 and
2.5, the vessel is in the semi-displacement or semi-planing mode—that is, it is trying to
rise up over its own bow wave to get onto plane. Some boats are designed to operate
at these speeds. Above speed-length ratio of 2.0 to 2.5, the vessel is planing and relies
on dynamic lift to raise and hold it out of the water so that it can skim along the surface
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SomeSailor
Guru
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Nomad Willy
Guru
JB wrote;
"I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher. Today 10:14 AM"
Absolutely! Guys are still hung up on the "hull speed" thing.
"I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher. Today 10:14 AM"
Absolutely! Guys are still hung up on the "hull speed" thing.
SomeSailor
Guru
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Not "hung up" really. Just using it as a starting point to get a feel for best rate. That doesn't mean lowest GPH or hull speed. It's a lot like trying to get your arms around Best Rate versus Best Angle as a pilot.
For me... best rate is about 8-8.5kts. I have two plants running, pushing the hull just up to a speed where diminished returns start showing. That gets me there at a rate which optimizes full economy and time and wear and tear.
That number truly is different for every boat. I have the fuel curves for my engines and I have speeds versus RPMs so it's an easy calculation and model to build.
For me... best rate is about 8-8.5kts. I have two plants running, pushing the hull just up to a speed where diminished returns start showing. That gets me there at a rate which optimizes full economy and time and wear and tear.
That number truly is different for every boat. I have the fuel curves for my engines and I have speeds versus RPMs so it's an easy calculation and model to build.
ranger58sb
Guru
Yep, starting point.
Coincidentally I just made up a similar model of an imaginary trip from here to Norfolk, 140 NM, and using the published engine prop curves for fuel consumption info:
2400rpm 22kts 35gph 223gals 0.63nmpg $890($4/gal)
2200rpm 20kts 27gph 190gals 0.74nmpg $762($4/gal)
1200rpm 08kts 06gph 105gals 1.33nmpg $420($4/gal)
1000rpm 07kts 04gph 080gals 1.75nmpg $320($4/gal)
0800rpm 06kts 02gph 056gals 2.50nmpg $224($4/gal)
Our LWL is approx. 38' so theoretical max displacement hull speed is approx. 8.26 kts. Idle is 600rpm. From the table, slower is still "better" in terms of fuel cost.
Time efficiency? perhaps that's another thing... since fuel savings might be eaten up by marina costs unless we anchor out
And our hull shape isn't always comfortable on the Chesapeake in all sea states... so we sometimes couldn't travel at 6 kts without getting beaten up...
-Chris
Nomad Willy
Guru
Ranger I thought you really had something for a moment but what ever happened to 9,10, 11 knots ect ect?
But your numbers sure dramatically showed one thing. Slower = better (less) fuel burn and more mpg. Graphic evidence literally.
But your numbers sure dramatically showed one thing. Slower = better (less) fuel burn and more mpg. Graphic evidence literally.
ranger58sb
Guru
Those were the only RPM/speed combinations I could remember off-hand, my latest RPM/speed test table is on the boat, and I'm not (yet, today).
There's also a place in there around 1400-1600 RPMs (I think) where the boat is simply plowing, so it's neither fuel- nor speed-efficient in that range anyway.
In retrospect, I should have mentioned the engine setup is twin Cummins 6CTAs (450s), and speed/fuel consumption in the table is the total with both engines running.
-Chris
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