Low RPM cruise

[Dockhead]

Originally Posted by Delfin

Since diesels have essentially flat torque curves above a fairly low rpm, and since the torque required to move a boat through the water at less than hull speed is minimal, I think there is a bit more to it than that. As the speed through the water increases, torque demand increases until max torque is reached, and fuel consumption increases accordingly. Below that, the boat can move through the water at 85% of hull speed and you'll never see max torque, fuel efficiency in terms of range increases and wear on the engine decreases.

That's right. For displacement hulls, fuel efficiency in terms of liters/mile must needs improve at lower speeds, and that's not boat dependent. The "sweet spot" is somewhere around Froude number 0.22, which is a couple of knots below hull speed, but fuel efficiency in terms of liters/mile ALWAYS increases, the lower the speed is, assuming no odd wind or propping effects. An excellent analysis: https://www.awelina.co.uk/hull_drag/...nsumption2.pdf

[Exile]

Two knots below hull speed sounds like a good rule of thumb or at least a helpful starting point. It also happens to be about right (I think) for my own boat (~6.5 kts).

[sailorboy1]

Originally Posted by Exile

Two knots below hull speed sounds like a good rule of thumb or at least a helpful starting point. It also happens to be about right (I think) for my own boat (~6.5 kts).

From my experience acturally tracking my fuel usage I've determined that the mpg between 5-6.5 knots is basically the same. And that 6.5 knots is almost exactly 2 knots below my hull speed.

It's funny that before it took 2 years of fuel tracking for me to decide that it made no sense to go 6.5 knots instead of 5 knots because I was only looking at gal/hr and would motor at 5 knots for that. Back then I thought 6.5 knots was motoring HARD I still sometimes only motor at 5 knots were not in a hurry just because it's quiet and/or that I what to charge batteries longer.

[Kenomac]

Originally Posted by sailorboy1

No I don't feel it's a "general consensus" as much as it's one of those internet "everyone knows" things. It probably only makes a difference at the extreme end of the idle/hard scales. I.E. you don't idle for days and you don't run at max for days. In between is no man land where changing the oil probably makes a bigger difference.

This is just IMO and I'm not going to battle over it.

Well then, I’ll just have to agree with you.

[Dockhead]

Originally Posted by sailorboy1

From my experience acturally tracking my fuel usage I've determined that the mpg between 5-6.5 knots is basically the same. And that 6.5 knots is almost exactly 2 knots below my hull speed.

It's funny that before it took 2 years of fuel tracking for me to decide that it made no sense to go 6.5 knots instead of 5 knots because I was only looking at gal/hr and would motor at 5 knots for that. Back then I thought 6.5 knots was motoring HARD I still sometimes only motor at 5 knots were not in a hurry just because it's quiet and/or that I what to charge batteries longer.

Fuel consumption in miles per gallon for a displacement hull will go up rather steeply with increasing speed, almost from 0. That's a matter of physics -- hull resistance goes up exponentially even before you get to hull speed effects.


So your experience is bizarre -- could only be some mistake in measuring or calculating, or some oddity of the propeller or engine.

[sailorboy1]

It's because I'm a liar and/or stupid. It's been established already.

[eyschulman]

The problem with many of the posts on the site is that you have to be very knowledgeable to be able to pick the better answers out of all the fog. When it comes to marine engine issues the simplest path is to go to the manufactures position on the issue. Most engine manufacturers give detailed information on how to run and maintain the product. Yes there are many who will claim to know better and I and you are probably not in a position to definitively prove them wrong ,but on the other hand why believe them over the manufacturers advise?

[Kenomac]

Originally Posted by sailorboy1

It's because I'm a liar and/or stupid. It's been established already.

No, it’s because you actually included distance covered into the equation and not just gallons per hour and calculated the correct way. The boat will travel farther in an hour at the faster speed.

Or I’m stupid too, which has already been determined by concensus. Unless the engine is pushing the boat up towards hull speed or there’s lots of wind resistance, I don’t think there’s much difference in mileage between 5-6.5 knots and agree with Sailorboy. I guess we’re both idiots.

[Exile]

Originally Posted by Kenomac

No, it’s because you actually included distance covered into the equation and not just gallons per hour and calculated the correct way. The boat will travel farther in an hour at the faster speed.

Or I’m stupid too, which has already been determined by concensus. Unless the engine is pushing the boat up towards hull speed or there’s lots of wind resistance, I don’t think there’s much difference in mileage between 5-6.5 knots and agree with Sailorboy. I guess we’re both idiots.

No, but you all could very well be right. It's not a linear equation so it probably takes little extra effort (i.e. fuel) to push your boats from 5 to 6.5 kts., but a disproportionately larger amount from 6.5 up to hull speed. I think that's the deal with mine anyway.

SB -- what's your waterline length? My 47'-er has a lot of overhang and is only 37' on the waterline. I'm thinking your (more modern) 41' boat may have a similar waterline length, although it's most certainly a lot lighter. Interesting that our 3 very different boats appear to have similar fuel burn efficiencies when it comes to cruising speeds.

[Kenomac]

It has more to do with the fact that our diesels really aren’t working that hard up until they try to dig a bigger hole in the ocean.

What are we now, the Three Stooges?

Think of it like the Jimmy Carter years when the knucklehead decreased the speed limit from 65mph down to 55mph to supposedly safe fuel, but all he ended up doing is slowing down commerce and pissing everyone off. After the engine gets up to speed pushing a car or truck to 65mph, it doesn’t take much to keep it there, it’s the accelleration or climbing hills that burn the fuel. It’s the same with displacement hulls.

[Exile]

Originally Posted by Kenomac

What are we now, the Three Stooges?

You can play Curly. Me and Sailorboy will have to fight over the Moe & Larry roles.

Btw, did you manage to cross the pond this season with your new 62?

[sailorboy1]

My boat will use 0.6 gph to go 5 knots. It will about 0.8 gph to do 6.5 knots. But near as I can determine is that it will use 1.1 gph at 7+ knots.

Those as facts in my logs, it matters little whether I motor at 5 knots or 6.5 knots far as fuel economy goes.

Of course I might be lying.

[Kenomac]

Originally Posted by Exile

You can play Curly. Me and Sailorboy will have to fight over the Moe & Larry roles.

Btw, did you manage to cross the pond this season with your new 62?

Nyuk, Nyuk, Nyuk!: http://www.cruisersforum.com/forums/...ew-208259.html

[Kenomac]

Originally Posted by sailorboy1

My boat will use 0.6 gph to go 5 knots. It will about 0.8 gph to do 6.5 knots. But near as I can determine is that it will use 1.1 gph at 7+ knots.

Those as facts in my logs, it matters little whether I motor at 5 knots or 6.5 knots far as fuel economy goes.

Of course I might be lying.

And your numbers are good for our 53 and our 62, right about the same. The diesels are hardly working and the burn rate comes out about even when distance covered is factored in.

[Delfin]

Originally Posted by StuM

Strange, everyone that I've seen giving actual data gets much better mpg at lower speed.



In my case it makes a big difference. Running on one engine:
5 knots - 2 lph = 2.5 nm /l = 9.5 nmpg
6 knots - 3.3 lph = 1.8 nm /l = 6.8 nmpg

6.5 knots - 5 lph = 1.3 nm /l = 4.9 nmpg


i.e Almost double the range or half the cost per mile at the slower speed.

It depends entirely on the boat and motor and prop and prop pitch. For my boat, 8 knots burns 4 gph for 2 nmpg. At 7 knots it burns 2.9 gph or 2.4 nmpg. Slow down to 6.25 knots and she only burns 2.1 gph or almost 3 nmpg. I think your experience is not very typical, primarily because a twin running on one engine is hardly operating as designed. Apples and oranges, methinks.