Cruisers Forum
 


Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 14-10-2018, 08:43   #196
Moderator Emeritus
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
Everything that flies, rolls or floats is subject to this effect. Speed costs power (as A64 said) and therefore fuel. The only vehicle for which this is not true is a space ship traveling in a vacuum.


Even there I think the faster you go, the more power it takes.
While I do not try to understand Einstein, It was explained to me decades ago that the reason the speed of light is an absolute, is that if you could achieve 100% efficiency of mass to energy conversion, you would consume 100% of the vehicles mass, converting it into energy to accelerate the moment you reached light speed.

I know major thread drift.

But to get back onto track, I have graphed three vessels of speed vs fuel flow, all equipped with fuel flow devices. One Diesel, one gas inboard, and one gas outboard, and unfortunately there was no “sweet spot”. I was sure there would be, that was the point of the exercise, but it was fruitless.
They were planing vessels, but even off plane, the faster you go, the more fuel you burn, on plane or not.
It’s simply a matter of how fast do you want to get there, vs available fuel supply.
If you get yourself in a position where range is more important than speed, the slower you go, the further you will go.

Now the graph did have steep portions where going just a little faster cost a lot more fuel, like approaching hull speed, and hull speed is not an absolute of course, enough power and you can quite easily exceed it, just the power required to do so gets ever increasing more for each small gain in speed.

A ridiculous example is a non planning pontoon boat, put more than 100 HP on one and you can pull someone on a tube, your not on plane, but way past hull speed. We have some nut where I am now that does, and it rolls a bigger wave than a big powerboat.
a64pilot is offline   Reply With Quote
Old 14-10-2018, 09:30   #197
Registered User
 
Delfin's Avatar

Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
Re: Low RPM cruise

Quote:
Originally Posted by Delfin View Post
Sounds like you motored around 54 hours to travel 300 miles at 5.5 knots, burning 40 gallons. That would indicate fuel consumption per hour of 3/4 gal/hour. The fuel consumption graph for a 100 hp Yanmar indicates that you would have to motor at 1400 rpm to have that fuel burn, and at 2200 you'd burn around 2 gph. At 2600 rpm it's 3 gph. Not trying to be argumentative or doubt your numbers, but you are getting pretty amazing fuel economy that isn't really explainable based on what Yanmar publishes for that engine.

https://www.yanmarmarine.com/theme/y...eet_4JH110.pdf
Put another way, with that fuel consumption you're moving a 53' boat through the water at 5.5 knots against a current using around 9 hp. Can that be right?
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
Delfin is offline   Reply With Quote
Old 14-10-2018, 10:30   #198
Registered User
 
Exile's Avatar

Join Date: Sep 2010
Location: Land of Disenchantment
Boat: Bristol 47.7
Posts: 5,609
Re: Low RPM cruise

Quote:
Originally Posted by a64pilot View Post
* * * I have graphed three vessels of speed vs fuel flow, all equipped with fuel flow devices. One Diesel, one gas inboard, and one gas outboard, and unfortunately there was no “sweet spot”. I was sure there would be, that was the point of the exercise, but it was fruitless.
They were planing vessels, but even off plane, the faster you go, the more fuel you burn, on plane or not.
It’s simply a matter of how fast do you want to get there, vs available fuel supply.
If you get yourself in a position where range is more important than speed, the slower you go, the further you will go.

* * *
This discussion suggests that, to the extent there's a "sweet spot," it's less about the rate of consumption at any given speed and more about what you wrote, namely how fast you want or need to get somewhere and how much fuel you have available for the passage. What may be an acceptable 10-20% increase in consumption between 5 & 6 kts. for most of us may be unacceptable to someone like Dale who's next available fuel while transiting the Madagascar Straights is 1400nm away.
Exile is offline   Reply With Quote
Old 14-10-2018, 13:13   #199
cruiser

Join Date: Jan 2012
Location: Pangaea
Posts: 10,856
Re: Low RPM cruise

Quote:
Originally Posted by Delfin View Post
Put another way, with that fuel consumption you're moving a 53' boat through the water at 5.5 knots against a current using around 9 hp. Can that be right?
You’re using the wrong engine specs for a completely different motor, basically... you have it all messed up.
Kenomac is offline   Reply With Quote
Old 14-10-2018, 13:42   #200
Moderator Emeritus
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
Low RPM cruise

Ken every motor has an efficiency “sweet spot”if you will, a point at which it does more work, for less fuel. It’s actually a spot that is both RPM and load dependent, not just RPM.
However the increase of hull drag, is orders of magnitude greater than any efficiency increase of the engine.
Now if you were generating power to sell or something, then you would design your generator to operate at that sweet spot.
Think about bicycling as I know your very, very adept at that and have ridden a track bike I think a lot and are very good at it.
Well on a bike if you measure force there is no point where increasing speed reduces work load, and you get to a point where just 1 more MPH separates the Men from the Boys
a64pilot is offline   Reply With Quote
Old 14-10-2018, 15:27   #201
Registered User

Join Date: May 2018
Location: Us: Australia, Boat: Caribbean
Boat: 50' Ligure power cat
Posts: 119
Re: Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
No, per liter. Caveat -- that's with a squeaky clean hull. The numbers are much worse with fouling. Second caveat -- absolutely glassy calm sea and no headwind.
I'm impressed!
bluenomads is offline   Reply With Quote
Old 14-10-2018, 18:21   #202
Registered User
 
Delfin's Avatar

Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
Re: Low RPM cruise

Quote:
Originally Posted by Kenomac View Post
You’re using the wrong engine specs for a completely different motor, basically... you have it all messed up.
Well no. Any diesel of any hp burns the same narrow range of fuel per hp produced per hour. The range is around 172 to 181 grams/hp/hr, so the idea that looking at the specific fuel consumption of a 110 hp motor rather than a 100 hp motor gives a wrong answer is a non sequitur. Be that as it may, I did mess up the calculation of hp by eye balling the graph. The listed fuel consumption of a 100 hp Yanmar is 175 gr/hp/hr, so given your stated total fuel consumed, distanced travelled and average speed, you're saying your 53' displacement hull needs 13.45 hp to move it at 5.5 knots into a 10 - 15 knot headwind and .5 knot contrary current. That is quite remarkable.
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
Delfin is offline   Reply With Quote
Old 15-10-2018, 01:14   #203
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 34,522
Re: Low RPM cruise

Quote:
Originally Posted by Delfin View Post
Well no. Any diesel of any hp burns the same narrow range of fuel per hp produced per hour. The range is around 172 to 181 grams/hp/hr, so the idea that looking at the specific fuel consumption of a 110 hp motor rather than a 100 hp motor gives a wrong answer is a non sequitur. Be that as it may, I did mess up the calculation of hp by eye balling the graph. The listed fuel consumption of a 100 hp Yanmar is 175 gr/hp/hr, so given your stated total fuel consumed, distanced travelled and average speed, you're saying your 53' displacement hull needs 13.45 hp to move it at 5.5 knots into a 10 - 15 knot headwind and .5 knot contrary current. That is quite remarkable.

I don't know about the headwind and the current, but for calm conditions Ken's number would sound right to me.


My boat's a little lighter, but I motored 430 miles on 350 liters of fuel in calm condition in the Arctic Ocean once last summer, at mostly at 6 to 7 knots. The fuel flow meter shows less than 3 liters/hour at 6 knots but spikes up rapidly as you increase speed from there, or if there is any resistance from head wind or sea.


People are surprised at this economy, but I think while it is NOT true that there is any displacement hull that can go faster without burning more fuel (per mile!), it IS true that a bigger boat with a long water line might use less fuel per mile than a smaller one at the same speed. That's because the hull resistance curves go up steeply already quite some ways before hull speed. In other words, you're burning fuel making waves already a few knots before hull speed. On my boat, there's a big difference in fuel consumption between 6 and 7 knots, although hull speed is 9.4. At 7 knots, my 54' 20 tonne boat for sure uses a lot less fuel than my previous 37' 10 tonne boat did.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Old 15-10-2018, 03:24   #204
cruiser

Join Date: Jan 2012
Location: Pangaea
Posts: 10,856
Re: Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
I don't know about the headwind and the current, but for calm conditions Ken's number would sound right to me.


My boat's a little lighter, but I motored 430 miles on 350 liters of fuel in calm condition in the Arctic Ocean once last summer, at mostly at 6 to 7 knots. The fuel flow meter shows less than 3 liters/hour at 6 knots but spikes up rapidly as you increase speed from there, or if there is any resistance from head wind or sea.


People are surprised at this economy, but I think while it is NOT true that there is any displacement hull that can go faster without burning more fuel (per mile!), it IS true that a bigger boat with a long water line might use less fuel per mile than a smaller one at the same speed. That's because the hull resistance curves go up steeply already quite some ways before hull speed. In other words, you're burning fuel making waves already a few knots before hull speed. On my boat, there's a big difference in fuel consumption between 6 and 7 knots, although hull speed is 9.4. At 7 knots, my 54' 20 tonne boat for sure uses a lot less fuel than my previous 37' 10 tonne boat did.
Our boat is now considerably lighter than yours. It’s for sale, stripped of all personal items.
Kenomac is offline   Reply With Quote
Old 15-10-2018, 03:42   #205
Registered User
 
malbert73's Avatar

Join Date: Jun 2008
Boat: Tartan 40
Posts: 2,481
Re: Low RPM cruise

Quote:
Originally Posted by Kenomac View Post
Our boat is now considerably lighter than yours. It’s for sale, stripped of all personal items.


Why? Are you done or downsizing?
malbert73 is offline   Reply With Quote
Old 15-10-2018, 03:50   #206
cruiser

Join Date: Jan 2012
Location: Pangaea
Posts: 10,856
Re: Low RPM cruise

Quote:
Originally Posted by malbert73 View Post
Why? Are you done or downsizing?
We upsized to an Oyster 62 for improved mileage at low rpms..
Kenomac is offline   Reply With Quote
Old 15-10-2018, 06:54   #207
Moderator Emeritus
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
Re: Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
I don't know about the headwind and the current, but for calm conditions Ken's number would sound right to me.


My boat's a little lighter, but I motored 430 miles on 350 liters of fuel in calm condition in the Arctic Ocean once last summer, at mostly at 6 to 7 knots. The fuel flow meter shows less than 3 liters/hour at 6 knots but spikes up rapidly as you increase speed from there, or if there is any resistance from head wind or sea.


People are surprised at this economy, but I think while it is NOT true that there is any displacement hull that can go faster without burning more fuel (per mile!), it IS true that a bigger boat with a long water line might use less fuel per mile than a smaller one at the same speed. That's because the hull resistance curves go up steeply already quite some ways before hull speed. In other words, you're burning fuel making waves already a few knots before hull speed. On my boat, there's a big difference in fuel consumption between 6 and 7 knots, although hull speed is 9.4. At 7 knots, my 54' 20 tonne boat for sure uses a lot less fuel than my previous 37' 10 tonne boat did.


The Autoprop is responsible for a large part of your part throttle economy due to it self pitching to keep the engine under a load, if you were a fixed pitch prop, you would be slower at the same part throttle RPM.
People that overpitch a prop really do achieve better fuel economy at partial throttle.
Of course they do overstress the engine if they try to up the RPM, which can cause damage, so it’s not recommended.

People don’t want to hear this, but a Maxprop is probably the least efficient prop for motoring, but among the best for sailing
a64pilot is offline   Reply With Quote
Old 15-10-2018, 07:22   #208
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 34,522
Re: Low RPM cruise

Quote:
Originally Posted by a64pilot View Post
The Autoprop is responsible for a large part of your part throttle economy due to it self pitching to keep the engine under a load, if you were a fixed pitch prop, you would be slower at the same part throttle RPM.
People that overpitch a prop really do achieve better fuel economy at partial throttle.
Of course they do overstress the engine if they try to up the RPM, which can cause damage, so it’s not recommended.

People don’t want to hear this, but a Maxprop is probably the least efficient prop for motoring, but among the best for sailing

This is true, but I'm not sure it makes a huge difference. Not having the Autoprop just means you have to use more RPM. Diesel engines are remarkably consistent in fuel burned per horsepower/hour -- all efficiency improvements, even with turbochargers, are relatively small. I think the only really game-changing difference in diesels is common rail.


One should not confuse diesels with spark-ignition engines in this regard. Spark-ignition engines have very significant efficiency losses at partial load because of pumping losses around the throttle plate. So you can save a lot of fuel by getting them into "overdrive" and getting the throttle plate open more and reducing manifold vacuum. There is no such effect with diesels, which have no throttle plates and don't pull a vacuum to modulate their power output.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Old 15-10-2018, 07:47   #209
Registered User
 
Delfin's Avatar

Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
Re: Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
I don't know about the headwind and the current, but for calm conditions Ken's number would sound right to me.


My boat's a little lighter, but I motored 430 miles on 350 liters of fuel in calm condition in the Arctic Ocean once last summer, at mostly at 6 to 7 knots. The fuel flow meter shows less than 3 liters/hour at 6 knots but spikes up rapidly as you increase speed from there, or if there is any resistance from head wind or sea.


People are surprised at this economy, but I think while it is NOT true that there is any displacement hull that can go faster without burning more fuel (per mile!), it IS true that a bigger boat with a long water line might use less fuel per mile than a smaller one at the same speed. That's because the hull resistance curves go up steeply already quite some ways before hull speed. In other words, you're burning fuel making waves already a few knots before hull speed. On my boat, there's a big difference in fuel consumption between 6 and 7 knots, although hull speed is 9.4. At 7 knots, my 54' 20 tonne boat for sure uses a lot less fuel than my previous 37' 10 tonne boat did.
I wasn't really questioning his data, I just found it remarkable given the head wind and current. That said, even my boat, which weighs 65 tons but is essentially the same length as yours has dramatically increased fuel economy at very low speeds. At 1,000 rpm, she only uses a gph or so, and at that power level I think she's doing around 5 knots. That translates to only about 18 hp. Which, now that I think about it, was the size engine in my 12 ton Cape George, and motoring she would do around 6.5 knots in a flat sea. I guess the point is that when you are at well under hull speed, the amount of energy to move a displacement hull through the water is astonishing low.
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
Delfin is offline   Reply With Quote
Old 15-10-2018, 08:04   #210
Registered User
 
Delfin's Avatar

Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
Re: Low RPM cruise

Quote:
Originally Posted by Dockhead View Post
I think the only really game-changing difference in diesels is common rail.

Surprisingly, that isn't true, at least in terms of fuel efficiency at less than full power. The Cummins QSB7 common rail engine uses 181 grams of fuel per hour to generate 1 hp at about 1/4 power, which is right in the range of a mechanically injected engine. They shine in terms of emission reduction and are more efficient at max power, which boaters in displacement hulls use very rarely.
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
Delfin is offline   Reply With Quote
Reply

Tags
cruise, rpm


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Penta D1-30 with 130S saildrive low RPM Kemp Engines and Propulsion Systems 18 19-09-2018 10:04
Perkins dies at low RPM pfammi Engines and Propulsion Systems 18 31-05-2016 18:19
Johnson 4 HP Outboard Low RPM butch Engines and Propulsion Systems 9 17-04-2016 17:33
Yanmar 2gm white smoke low rpm Captryan23 Engines and Propulsion Systems 8 25-01-2016 11:09
Fouled Injectors > Low RPM & Smoke? Northeaster Engines and Propulsion Systems 30 11-05-2009 14:26

Advertise Here


All times are GMT -7. The time now is 02:11.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.