Cost per hour of running engine

[newhaul]

Originally Posted by kev_rm

I figured USD$4/gallon.. which I think is an OK average but I am still a little surprised by the low gph figures. Your boats get 10mpg + ??

my Spencer burned .4gph at 7 knots so about 16 miles per gallon .
My columbia burns .25 at 5 knots. So even better.
Fuel price Tuesday here was $2.68 per gallon USD. So that's $.67 per hour fuel cost on the defender .

[a64pilot]

Diesel what 60 miles away in the Bahamas is over $5 a gl. So you can play with the numbers as they will change by location and even from one boat to the next, but I think his methodology is sound.
My 44 HP Yanmar running at 1800 RPM gives me 6.5 kts and about 2/3, maybe 3/4 gl an hour.
I “flight plan” 1 GPH, cause you May have a head wind or current

[Jim Cate]

I “flight plan” 1 GPH, cause you May have a head wind or current

What do wind and current have to do with GPH? Miles per gallon, sure, but the engine at 1800 rpm burns about the same fuel/unit time with or without extra load from externals... you just go slower.

Jim

[Stu Jackson]

I've never understood that nonsensical thought process either, Jim.


Look, I''ve thought about this since it was posted. Since the entire concept rests on assumed longevity, it's all meaningless.


Find a way to do it yearly with reasonable costs, then it starts to make sense.


This from a guy who just changed his fuel filter after 900 hours!


I just forgot, honest...

[Perna]

Not sure why you would need to change the oil and filter after 50 hours. Most car engine specify around 20000 miles, say 600 hours conservatively. Little difference in the duty cycle.

[a64pilot]

Originally Posted by Jim Cate

What do wind and current have to do with GPH? Miles per gallon, sure, but the engine at 1800 rpm burns about the same fuel/unit time with or without extra load from externals... you just go slower.

Jim

It just gives you a fudge factor of 25% is all, by increasing burn rate.
This is just initial planning is all, if we decide to go then it gets more detailed.
Thankfully fuel has never been an issue, I always arrive with more than I planned.
My background of course meant that running out of fuel would be a very bad thing so I just by nature am very conservative.
Only thing that should throw a wrench into the works is a fuel leak.

To try to explain better, you First start in planing with a burn rate, you then calculate speed, and the two of course ends up with fuel used over distance.
We used an E6B circular slide rule, the wind face side to get ground speed based on forecasted winds aloft. It also could easily calculate a heading to hold a course, or drift. You could even work it backward and determine winds based on heading vs track and ground speed. Its the same on a boat, they both move through a fluid.

https://en.m.wikipedia.org/wiki/E6B

[valhalla360]

Originally Posted by Jim Cate

What do wind and current have to do with GPH? Miles per gallon, sure, but the engine at 1800 rpm burns about the same fuel/unit time with or without extra load from externals... you just go slower.

Jim

I've never got the GPH as a measure of range. Maybe for being becalmed mid ocean where you just want to keep moving even if at a slow pace where you don't really care about the actual speed/distance, it might have some use.

But coastal cruising, yes, there is a relationship to hours but I need to know how far I can travel with the given fuel supply. The number of hours is only useful in the sense that it allows calculation of a distance traveled per gallon.

[a64pilot]

Originally Posted by valhalla360

I've never got the GPH as a measure of range. Maybe for being becalmed mid ocean where you just want to keep moving even if at a slow pace where you don't really care about the actual speed/distance, it might have some use.



But coastal cruising, yes, there is a relationship to hours but I need to know how far I can travel with the given fuel supply. The number of hours is only useful in the sense that it allows calculation of a distance traveled per gallon.

Without knowing both your burn rate and your ground speed, you can’t determine range, that is what GPH has to do with it.
The third piece is of course how much fuel you have on board.

[valhalla360]

Originally Posted by Perna

Not sure why you would need to change the oil and filter after 50 hours. Most car engine specify around 20000 miles, say 600 hours conservatively. Little difference in the duty cycle.

While I agree 50hr is really low.

It's a drastically different duty cycle with boats.

Cars, particularly city driving, typically start and hit the gas within seconds, then it's stop and go with heavy power demand followed by light demand in city driving. Often trips are short never getting the engine up to temperature. It's also common to operate in dusty conditions.

Boats rarely are put in gear unless they have been running for at least a couple of minutes (usually, you start the engine and then untie the dock lines or pull the anchor). Then there is usually at least a few minutes in gear at near idle speed (to get out of marina or anchorage) before throttling up to cruise speed...where the motor is commonly left for a few hours at constant speed before throttling back to go into port with the engine idling for a few minutes while you get tied up/anchored.

(if you are a purist sailor only using the bare minimum of engine time to get to open water, 600hr might be several years and other issues come into play).

[Stu Jackson]

Originally Posted by valhalla360

I've never got the GPH as a measure of range. Maybe for being becalmed mid ocean where you just want to keep moving even if at a slow pace where you don't really care about the actual speed/distance, it might have some use.

But coastal cruising, yes, there is a relationship to hours but I need to know how far I can travel with the given fuel supply. The number of hours is only useful in the sense that it allows calculation of a distance traveled per gallon.

That's kind of like the chicken & the egg dilemma.


It's either one or the other and both are right.


Only issue I have is that unless you are going UP a river against a stiff current, or dealing with swift currents on a regular basis regardless of direction of said current, one determines range by volume and consumption.


This is more true for boats because fuel gauges are notorious for being non-linear on boats for many reasons.

[valhalla360]

Originally Posted by a64pilot

Without knowing both your burn rate and your ground speed, you can’t determine range, that is what GPH has to do with it.
The third piece is of course how much fuel you have on board.

If I'm getting 6 MPG and I have 20 gal, I've got 120mile range 6*20 = 120. Since this type of data is typically used for planning purposes, you rarely know your actual ground speed ahead of time.

If you are adjusting range on the fly, I can just as easily adjust MPG for the effects of current, if you want to get into STW vs SOG. Example: if normal cruise is 6MPG at 6 MPH and I'm fighting a 2 MPH current, I've got 80miles of range (6-2)/6 *6 = 4 MPG or 80 mile range.

Feel free to use KtPG if you want to be more nautical.

Of course, if you are looking at operating costs, you want to maximize MPG not minimize GPH. In calm conditions with no current, the math works out that you are doing both but in heavy current, there are times, when a higher GPH produces better MPG.

[valhalla360]

Originally Posted by Stu Jackson

That's kind of like the chicken & the egg dilemma.

It's either one or the other and both are right.

Only issue I have is that unless you are going UP a river against a stiff current, or dealing with swift currents on a regular basis regardless of direction of said current, one determines range by volume and consumption.

This is more true for boats because fuel gauges are notorious for being non-linear on boats for many reasons.

Certainly you can calculate based on either but MPG is more direct in terms of efficiency rating.

Maybe it's because I've spent most of my boating around significant currents, I do find it easier to work with MPG. Open water with minimal current, it's pretty much 6 to one, half dozen to the other.

Not knowing how much fuel you really have is never a good thing no matter how you calculate your range. It doesn't play into the discussion.

[Stu Jackson]

Originally Posted by valhalla360

If I'm getting 6 MPG and I have 20 gal, I've got 120mile range 6*20 = 120. Since this type of data is typically used for planning purposes, you rarely know your actual ground speed ahead of time.

Sure you do. If you've owned your boat for more than a week, you should know what your speed is THRU WATER at various rpms.


Except in heavy current infested water and rivers, currents will be known (English Channel, SF Gate, you name it).


Simple.

[a64pilot]

Originally Posted by valhalla360

If I'm getting 6 MPG and I have 20 gal, I've got 120mile range 6*20 = 120. Since this type of data is typically used for planning purposes, you rarely know your actual ground speed ahead of time.

You do sometimes, in the gulfstream for example.
However to determine what your MPG is, you had to know burn rate, you didn’t get there any other way.
That is why burn rate or consumption is important to know.
However as has been stated MPG is a variable, but burn rate is a constant, that is why you use burn rate for planning , whether you realize it or not you are.
You just can’t figure MPG without knowing consumption rate.

[a64pilot]

We are all discussing the same thing, just derived at by different means I guess. I maintain it’s important to know burn rate at varying RPM, cause that way you can compute range.
It may be nice to know that if I go from 1800 to 2000 RPM I go from 3/4 gl to 1 GPH, an increase in 25% in fuel consumption, but if I only got a 10% increase in speed, then I will have a reduction of 15% in range.
Then you can make an informed decision to speed up or not