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Old 07-08-2013, 16:32   #16
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Re: Fuel Question?

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Originally Posted by ranger42c View Post
Yes, GG, the Floscan will tell you instantaneous fuel consumptions rates. See Floscan Instrument Co. Inc.

You would extend that basic data with environmental context: speed at a given RPM level, modified by effects of tide/current and wind to the extent you can try to do that within the period of the sea trial. (A given RPM setting can render different speeds if you're heading into the tide/wind versus running with tide/wind.) A more accurate picture would present itself over time, where you'd be able to do some averaging... but probably the figures captured during sea trial will be useful enough.

You'll hear most diesels like to do most of their work within a specific RPM range. Various manufacturers -- and their individual products -- can be slightly different, but generally guidance shakes out to run at 75% of wide open throttle (WOT), 80% of WOT, 85% of WOT, or even "200 off the top" (i.e., 200 RPMs below WOT). Essentially, the motor should be operating at the right temperature, and these percentages are often easy ways to know you're doing that.

You determine WOT in two ways. One is to get the manufacturer's operators manual (often available on line, or in hardcopy from the current owner during sea trial) and read the rating (e.g., 300 HP at 2500 RPMs, or whatever). The second method is during sea trial; make sure the engine will achieve that rated WOT -- 2500 RPMs, in this example. (If it doesn't... long discussion about boat weight, bottom condition, tankage, props, etc. side-stepped here.... but it's not the end of the world.)

So you take GPH readings from the Floscan at those various RPM settings (WOT and varous percentages of WOT) and you take speed over ground (SOG) readings from either the Floscan (if so networked) or from an onboard GPS.

All that said... The engine manufacturer will also have "performance curves" available. These are sorta generic, but they will also show nominal fuel consumption under load at given RPMs. That doesn't quite give you speed info, but if you look at a fuel flow curve, you usually see a big increase as RPMs increase. (Duh!) The Floscan will tell you similar info; GPH increases but in a displacement hull boat speed will NOT increase at the same rate. The manufacturer's nominal info will get you in the ballpark, and the Floscan readings will show you how nominal translates to "in that boat, with given load, in specific sea states, at specific RPMs" and so forth.

So in your sea trial, as you approach hull speed (compute that: 1.34 times the square root of the boat AT THEWATERLINE (not overall length)) you'll see RPMs increasing, fuel consumption increase, speed... not so much. (Example computation: imagine a 70 foot OAL boat with guess-timate 60' length at the waterline (LWL); square root of that * 1.34 renders approx. theoretical hull speed of 10.38 kts. [Note: That may be why your broker is telling you info in the 10 Knot range.] More horsepower above that hull speed spends fuel with little speed return. Although you can probably fly a building, given enough horsepower... usually theoretical hull speed for a displacement hull gives you a good idea of a reasonable maximum target.)

Comparison of fuel flow (from Floscan reading) at that hull speed gives you what you're after, which is essentially NMPG at a given RPM setting/travel speed. And the rest of your notes can help you work out various NMPGs to see whether running at less than hull speed is sometimes more economical ENOUGH to be worth the loss in speed.

Pardon if not clear; I just typed it off the cuff... so I maybe didn't describe the simplest way to understand... Ask if you need clarification.

-Chris
That was a lot information Chris. Thank you!!! I had to read it a couple of times, but I think I understand. I basically have to figure out hull speed and determine what the burn rate is that speed, then slow down and figure out what the burn rate is at a slower speed, then determine what speed is most economical? Why do I have to do math to get the hull speed? I have to do math to get the hull speed.

Where do I get the waterline length? From the manufacturer's book? If for some reason there is no book, can I just measure with a measuring tape (I hope that wasn't a very stupid question)
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Old 07-08-2013, 18:15   #17
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Or you could just say that you are looking at a such and such boat with a particular powerplant and someone on here may know the needed information.
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Old 08-08-2013, 13:46   #18
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Re: Fuel Question?

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Originally Posted by GalaxyGirl View Post
That was a lot information Chris. Thank you!!! I had to read it a couple of times, but I think I understand. I basically have to figure out hull speed and determine what the burn rate is that speed, then slow down and figure out what the burn rate is at a slower speed, then determine what speed is most economical? Why do I have to do math to get the hull speed? I have to do math to get the hull speed.

Where do I get the waterline length? From the manufacturer's book? If for some reason there is no book, can I just measure with a measuring tape (I hope that wasn't a very stupid question)

Yeah, sorry, I probably could have written all that slightly more concisely, given more editing time and more user-friendly keyboard...

The math is for a displacement hull, and tells you the theoretical point at which there's no sense adding more RPMs. Once you've reached theoretical hull speed, addition RPMs usually just waste fuel. Usually the engine is sized proportionally by the boat manufacturer, i.e., with that theoretical hull speed in mind. Extra horsepower -- over what's required to meet hull speed in all conditions -- doesn't usually buy much.

Yes, determine theoretical hull speed and then check fuel burn at the resulting RPMs. Yes, determine WOT, so you can make sure the engine is making rated horsepower, and so you can afterwards check fuel burn at various normal lower (cruising) RPMs.

Sometimes LWL is a sort of guess. Or you can often eyeball it and be close enough, or maybe the manufacturer can offer specs like that.

-Chris
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Old 20-08-2013, 15:59   #19
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Re: Fuel Question?

The hull speed does not change very much if the LWL is resonable. As its is the square root of the LWL time a max of 1.3. On a 65 ft boat the low is about 60 so 8 knots. the Eagle is 51 ft so about 7 knots. As long as it reasonable. Then throw in the type, beam and how clean of the hull, which can significantly effect the actaul hull speed.


Plus most boats have what is call a sweet spot where the boat seem to run smoother. For the Eagel is 1400 rpm which is what I run so the speed varies from 6 to 10 knots depending on current and wind..
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Old 21-08-2013, 10:14   #20
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Re: Fuel Question?

Can you share some info about the make/model/engine configuration?

We used to have a 31' flybridge with twin V8's that got 1.5 mpg at 25kts (planing obviously), so 1mpg on the surface looks really bad but it may simply be trying to push a displacement boat beyond hull speed or it may be a drastically larger boat.

With displacement small reductions in speed can really boost your mpg. We cruise at around 6kts and get 6mpg but if we drop back to 5kts it jumps up to 10-12mpg. Prior to changing from 2stroke 40hp outboard to 25hp EFI outboard we only got 3mpg, so we really need details to tell you much.
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