Fuel Question?

[GalaxyGirl]

The broker doesn't have much information about the fuel consumption. He only said that the boat burns 10 gallons @ 10knots. I noticed the boat has a floscan installed. Will that tell me more about fuel burn at slower speeds if used during a sea trail? Or is the sea trial too short a trip to get an accurate measure?

[Teknav]

Hiya GG! How much does your boat approximately weigh? What is the rated horsepower for your engine? Any info about your engine will be helpful; someone out there might have the same, and can tell you.

Mauritz

[Sandero]

10 gal per hour at 10 knots... that sounds like not terribly much...

[boatpoker]

Sounds like a lot compared to my 2gph at 10knots but that's not a fair comparison without more info.

[Minggat]

Originally Posted by GalaxyGirl

The broker doesn't have much information ......He only said that the boat burns 10 gallons @ 10knots.

I wouldn't count on this broker for any information at all. Doesn't sound like he knows sailboats.

[capttman]

Originally Posted by GalaxyGirl

The broker doesn't have much information about the fuel consumption. He only said that the boat burns 10 gallons @ 10knots. I noticed the boat has a floscan installed. Will that tell me more about fuel burn at slower speeds if used during a sea trail? Or is the sea trial too short a trip to get an accurate measure?

There are fuel curves for each engine put out by the manufactures. You can adjust these by the weight of the boat and the hull type. But to answer your question, yes the sea trial may be too short to get an accurate reading. The reason for this is there is not a direct relationship between the burn and RPM, often times as the RPM grows fuel consumption increases rapidly.
What engines do you have

[rw58ph]

Must be a twin fosr 1gallon per mile. However 10 is probably high than hull speed. Some detail would help.

[RaymondR]

Unless it is a real dog of a design displacement and hull speed are probably the two decisive factors in fuel consumption on a sail boat, if the hull speed is seven knots and you push it up to ten and the displacement is sufficient to prevent planing you will burn a lot of fuel.

[Jim Cate]

HEy Boatpoker, she said "10 gal (per hour?)@10 knots. That seems to work out pretty darn close to one gal per mile!

And for all of you other guys, pay attention: she is specifically NOT looking at sailboats. Misguided IMO, but that's what she is doing!

Cheers,

Jim

Edit: the post that I was replying to has disappeared... guess he re-read the OP!

[GalaxyGirl]

Originally Posted by capttman

There are fuel curves for each engine put out by the manufactures. You can adjust these by the weight of the boat and the hull type. But to answer your question, yes the sea trial may be too short to get an accurate reading. The reason for this is there is not a direct relationship between the burn and RPM, often times as the RPM grows fuel consumption increases rapidly.
What engines do you have

Thanks for the answer.

[GalaxyGirl]

Originally Posted by Jim Cate

HEy Boatpoker, she said "10 gal (per hour?)@10 knots. That seems to work out pretty darn close to one gal per mile!

And for all of you other guys, pay attention: she is specifically NOT looking at sailboats. Misguided IMO, but that's what she is doing!

Cheers,

Jim

Edit: the post that I was replying to has disappeared... guess he re-read the OP!

Your right Jim. Not considering sailboats.

The boat has a single engine Cat.

[Sailmonkey]

If your sea trial allows for extended time at various speeds you can build up a rudimentary graph of consumption (based in the floor-scan) vs speed. There is usually an obvious departure from the trend line when you start burning vastly more fuel for every 1/4 knot gained.

[GalaxyGirl]

Originally Posted by Sailmonkey

If your sea trial allows for extended time at various speeds you can build up a rudimentary graph of consumption (based in the floor-scan) vs speed. There is usually an obvious departure from the trend line when you start burning vastly more fuel for every 1/4 knot gained.

I don't even need to know the consumption at every speed, I just wanted to get a good idea of what it would do at cruise. I figured most trawlers cruise at 6.5-7knots. The broker was telling me fuel burn at 10k, so I was thinking that it had to getter better than a gallon a mile if the boat was slowed.

[Sailmonkey]

Originally Posted by GalaxyGirl

I don't even need to know the consumption at every speed, I just wanted to get a good idea of what it would do at cruise. I figured most trawlers cruise at 6.5-7knots. The broker was telling me fuel burn at 10k, so I was thinking that it had to getter better than a gallon a mile if the boat was slowed.

Economy is based on scale in this aspect. A longer trawler may well cruise efficiently at 10kts if the hull shape allows for it. On the same thought there is a sweet spot that you'll need to find by checking consumption at various speeds. Your trawler may in fact be less efficient at 6 knots than at 7.3 knots. Or rather your engine may operate more efficiently on your installation at 1400 rpm rather than 1200 rpm.

[ranger58sb]

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