Low RPM cruise

[907Juice]

Originally Posted by jimbunyard

.2 gph seems rather significant to me...with a 50 gallon tank 5 knots gets you 10 extra gallons (83 miles) at the end of the tank, as compared to 6.5 knots.

Perhaps you mean it's matters little financially?

50G X 5KPH / 0.6 = 416miles traveled
50G X 6.5KPH / 0.8 = 406miles traveled

Basically the same

The fuel per hour rate is only part of a bigger equation. Try using fuel x quantity x speed to get the full picture.

[jimbunyard]

Unless there is a serious need to calculate the actual mileage, at this point, given the comparatively tiny consumption of sail diesels, this is all pretty much 'fun with math' or 'see how many different answers can be had from the same information'.

At 5 kts and .6 gph he's getting 8.333 kts/gal, at 6.5 knots and .8 gph he's getting 8.125 kts/gal, for a difference per 51 gal tank of 424.983 - 414.375 = 10.608 nm…

Perhaps more interesting is the almost doubling of fuel consumption used to not-even-reach theoretical hull speed of 8.24 kts, unless of course the consumption somehow stays the same at any speed over 7 knots...

[Kenomac]

Originally Posted by jimbunyard

Unless there is a serious need to calculate the actual mileage, at this point, given comparatively tiny consumption of sail diesels, this is all pretty much 'fun with math' or 'see how many different answers can be had from the same information'.

At 5 kts and .6 gph he's getting 8.333 kts/gal, at 6.5 knots and .8 gph he's getting 8.125 kts/gal, for a difference per 51 gal tank of 424.983 - 414.375 = 10.608 nm…

Perhaps more interesting is the almost doubling of fuel consumption used to not-even-reach theoretical hull speed of 8.24 kts, unless of course the consumption somehow stays the same at any speed over 7 knots...

We’re not discussing 6.5-8.5 knots when one begins digging a larger whole in the ocean and the engine begins actually working harder. We’re discussing low rpms when the engine is just cruising along on Easy Street. Basically, we’re debunking the myth that fuel consumption starts at 0 and only increases exponentially all the way up to hull speed, and the myth that diesel engines need to be run at 80% of max in order to be happy and live a long life.

[Kenomac]

Originally Posted by a64pilot

Yes, they were planing hulls, although I graphed from idle to full RPM.
I don’t think it matters, the result is the same.
Of course a displacement hull would have used less fuel at displacement speeds.
Bottom line, to go faster requires more energy, more energy means more fuel burned, it really is that simple.
If you ever get yourself in a situation that absolute fuel range is important, slow down, the more you slow down the longer it’s going to take, but the less fuel it will take too.
It just like riding a bicycle, anybody can average 10 mph, but 20 mph is tough, 5mph is even easier.

It does matter and it’s not “that simple.”

Yes it takes more energy to increase from a stand still up to 5 knots, and to increase from 5 knots to 6 knots during acceleration; BUT after a steady speed of 5 knots or 6 knots has been attained and stabilized, there really isn’t any difference in fuel consumption, so you might as well travel at the faster speed and get where you want to be quicker.

Physics: After the boat is in motion, it doesn’t take much energy for a big diesel to keep it in motion.

Your bike example is not appropriate. Regarding diesels and boats, it would be more akin to an 800 pound gorilla pedaling a bike at 5mph vs 6mph on a flat road at a steady speed, virtually no difference in energy consumption.... until he hits the hills.

[Dockhead]

Originally Posted by Kenomac

We’re not discussing 6.5-8.5 knots when one begins digging a larger whole in the ocean and the engine begins actually working harder. We’re discussing low rpms when the engine is just cruising along on Easy Street. Basically, we’re debunking the myth that fuel consumption starts at 0 and only increases exponentially all the way up to hull speed, and the myth that diesel engines need to be run at 80% of max in order to be happy and live a long life.

The first thing is not a myth. Hull resistance of a displacement hull increases exponentially almost from zero. This is hydrodynamics 101. The hull resistance curve becomes much steeper near hull speed, but it is never flat. There is a "hump" and a "dip" in some hull resistance curves for big ships, but this takes place way down on the curve, compared to what we are talking about.


Hull resistance consists of three components: (a) viscous resistance (skin friction); (b) wave-making resistance (the power needed to move water out of the way; and (c) wind resistance.



Viscous resistance dominates at low speeds (fraction of hull speed), and this rises at roughly the square of the speed. Wave making resistance is more chaotic, and is more dependent on the shape of the hull and beam length ratio, but generally rises more steeply than viscous resistance. It is this component of resistance which gives us "hull speed".


If you would measure fuel consumption accurately (I use a Maretron fuel flow system https://www.maretron.com/products/ffm100.php), you would see clearly that miles per liter decreases all the way from near zero to top speed, with a much steeper decrease starting about 1.5 to 2 knots before hull speed.


To have constant miles per liter at different speeds, hull resistance would have to go up as a linear function of speed. But it is not a linear function of speed; it's exponential. Hydrodynamics 101. If you're actually interested in the subject, here is a really good explanation of all of the principles, together with some accurate measurements:


https://www.awelina.co.uk/hull_drag/...nsumption2.pdf

[Dockhead]

Originally Posted by a64pilot

. . . Bottom line, to go faster requires more energy, more energy means more fuel burned, it really is that simple.
. . .

This is a true statement.


There are a very few exceptions with boats, and ALL of these exceptions deal with interaction of bow and stern waves. One exception is what happens when a planing hull gets up on plane. Another exception occurs with ships at a fraction of hull speed, when bow and stern waves get exactly out of phase with each other.


Otherwise, what A64 said is absolutely true -- the faster you go, and this starts from zero, the more fuel PER MILE it takes to get where you are going.

[Kenomac]

Well, me and Sailorboy with our real world numbers and experience are just going to have to disagree with you two and your physics classes. Modern diesel engines and displacement hulls apparently didn’t take your class, so they don’t know how to respond correctly to mid-range constant hull speeds. Plus, over and over again, you keep forgetting to include distance covered into your equations in calculating fuel economy.

And BTW placing print in bold or underlining it in your post doesn’t make a statement more factual or more correct. ‘Just looks like you’re shouting.

[daletournier]

After approx 40k nms while using two yanmars and coming from a motor mechanics background I have to agree with A64. I'm not convinced with the low rpm glazing theory and I'm not convinced that you need to run your diesel at 80% load. Fuel consumption definitely goes up dramatically after approx 1800rpm with minimum gain.

My fuel consumption beliefs come from measuring my usage. My engine longevity beliefs come from what I've witnessed that conflict with what I used to believe (what I read).

I've seen yanmars with 14,000 hrs on them from owners that idle them morning and night to charge batteries and fridges etc, they get used to motor very regularly but very rarely over 2000 rpm. I ask other cruisers regular as it interest me and nearly 100% cruise under 2000rpm for fuel economy. Few I know regularly put their engines under load. Many don't read cruisers forum this don't know they should

Get your engine to operating temperature and I believe it will be fine.

Just my opinion.

[Dockhead]

Originally Posted by Kenomac

Well, me and Sailorboy with our real world numbers and experience are just going to have to disagree with you two and your physics classes. Modern diesel engines and displacement hulls apparently didn’t take your class, so they don’t know how to respond correctly to mid-range constant hull speeds. Plus, over and over again, you keep forgetting to include distance covered into your equations in calculating fuel economy.

And BTW placing print in bold or underlining it in your post doesn’t make a statement more factual or more correct.

No one participating in this thread forgot distance traveled or confused liters per hour with miles per liter. That's a red herring. I never wrote anything about liters per hour, only about miles per liter or miles per tank, and nor did anyone else.



I have pointed you to good information about both the hydrodynamic principles of this, and also to some accurate measurements. You are welcome to believe what you like. If you think you have measured something that defies the laws of physics, then either you are not measuring it accurately, or you have a magic boat. If you don't have a quality fuel flow gauge, then my theory is the latter.



Cheers.

[daletournier]

This conversation is relevant to me at the moment.

Boats are piling up waiting for a window to cross to south Africa. Its looking like there's some really light days coming up and we maybe motoring across the Mozambique channel, the channel isn't a place to sit around waiting for wind.

Several skippers including myself have been discussing motoring range and fuel consumption. All these guys have crossed oceans. The 100% consensus is low rpms gets you the greatest range. Yes there's a loss of SOG but a substantial gain in range. The loss of 1 knot can result in 100's of miles gained.

[Kenomac]

Originally Posted by Dockhead

No one participating in this thread forgot distance traveled or confused liters per hour with miles per liter. That's a red herring. I never wrote anything about liters per hour, only about miles per liter or miles per tank, and nor did anyone else.



I have pointed you to good information about both the hydrodynamic principles of this, and also to some accurate measurements. You are welcome to believe what you like. If you think you have measured something that defies the laws of physics, then either you are not measuring it accurately, or you have a magic boat. If you don't have a quality fuel flow gauge, then my theory is the latter.



Cheers.

So Sailorboy, I and anyone else who disagrees with you, have now been officially declared wrong! I can live with that. Seems this is the way many discussions on CF have been going lately.

Most of the coments on this thread have compared gallons per hour, not miles per gallon, or have not specified how fuel economy was calculated.

[Kenomac]

Originally Posted by daletournier

This conversation is relevant to me at the moment.

Boats are piling up waiting for a window to cross to south Africa. Its looking like there's some really light days coming up and we maybe motoring across the Mozambique channel, the channel isn't a place to sit around waiting for wind.

Several skippers including myself have been discussing motoring range and fuel consumption. All these guys have crossed oceans. The 100% consensus is low rpms gets you the greatest range. Yes there's a loss of SOG but a substantial gain in range. The loss of 1 knot can result in 100's of miles gained.

That’s true Dale, but over the course of 24 hours of motoring, you’re now 24nm behind where you would’ve been having traveled at the faster speed, and now you’ll need to burn five additional hours worth of fuel to catch up. Did you really save anything? No.

[daletournier]

Yes Ken, range is range. At 3.5 liters per hour I'm getting approx 800nm. When I motor at an rpm of 1800 I burn approx 2.5l per hour. This gets me up over 1000nm, yes it takes longer but ultimately I can go further.

From here the next fuel stop is 1400nm...

[Kenomac]

Originally Posted by daletournier

Yes Ken, range is range. At 3.5 liters per hour I'm getting approx 800nm. When I motor at an rpm of 1800 I burn approx 2.5l per hour. This gets me up over 1000nm, yes it takes longer but ultimately I can go further.

From here the next fuel stop is 1400nm...

I’d strap on a few jerry cans of diesel, not worry about it and get out of harms way as fast as possible. The slightly faster pace might actually allow your boat to take off and motor-sail, which will then lower your overall fuel consumption.

Note: This forum really needs a good thread on motor sailing.

[Wotname]

Yep, in calms and light airs, just put a few more revs on and when you hit the magic number you can motor sail but don't forget to pop the gearbox back into neutral as soon you are sailing - don't want the locked prop drag to slow you down and then have to motor again....