Reply
 
Thread Tools Search this Thread Display Modes
 
Old 02-12-2018, 11:15 AM   #1
Guru
 
City: Fairport
Country: United States
Join Date: Jan 2017
Posts: 1,683
slow boats, fast currents

Thanks to a recent post by a KK42 owner, we have the fuel burn data below. I've added a new column on the right.


current on the nose:
0 kts 3 kts
RPMs Gal/hr Kts nm nm/g nm/g
1,800 2 7.8 2,730 3.90 2.4
1,700 1.75 7.2 2,880 4.11 2.1
1,600 1.55 6.6 2,981 4.26 1.8
1,500 1.45 6.2 2,993 4.28 1.6
1,400 1.15 5.7 3,470 4.96 1.35
1,300 1.1 5.5 3,500 5.00 1.25
1,200 0.8 4.8 4,200 6.00 0.9
1,100 0.7 4.6 4,600 6.57 0.8
900 0.5 3.8 5,320 7.60 0.4

What you have is the scenario of, for instance, living on the coast in N FL and going to the islands fighting the gulfstream for a good distance. Let's say that is by choice, because you don't want to go down the ICW or hug the shoreline. Maybe you want to dolphin fish.
Anyway, you can see now the mileage goes up with increasing power. Not sure where this limits out, but I'm thinking perhaps at 2000 rpm or so.

Taken one step further, if you choose to ANGLE into the islands, with the GS on your stb quarter, then you might find the angle at which the mileage is virtually a constant, no matter the engine rpm!

sorry, a perfect formatting job is destroyed once posted.
__________________
Advertisement

diver dave is offline   Reply With Quote
Old 02-12-2018, 11:38 AM   #2
Guru
 
boathealer's Avatar
 
City: North Carolina
Country: US
Vessel Name: SCOUT
Vessel Model: Great Harbour N37
Join Date: Aug 2017
Posts: 592
How's this?
Attached Thumbnails
Click image for larger version

Name:	chart.jpg
Views:	130
Size:	47.3 KB
ID:	72896   Click image for larger version

Name:	graph.jpg
Views:	132
Size:	46.5 KB
ID:	72897  
__________________

boathealer is offline   Reply With Quote
Old 02-12-2018, 11:55 AM   #3
Guru
 
City: Fairport
Country: United States
Join Date: Jan 2017
Posts: 1,683
Wow, great! I might have to do the cosine function now.
diver dave is offline   Reply With Quote
Old 02-12-2018, 12:01 PM   #4
Guru
 
Alaskan Sea-Duction's Avatar
 
City: Inside Passage Summer/Columbia River Winter
Country: USA
Vessel Name: Alaskan Sea-Duction
Vessel Model: 1988 M/Y Camargue YachtFisher
Join Date: Jul 2012
Posts: 4,811
So based on the above info what is the best RPM to run?
Alaskan Sea-Duction is offline   Reply With Quote
Old 02-12-2018, 12:10 PM   #5
Guru
 
City: Northport
Country: USA
Join Date: Aug 2015
Posts: 1,425
Makes sense to us as well. As one example sometimes when we head up the Hudson and do not want to travel at night (for good reason) the actual speed over ground vs fuel used favors going faster even though you may end up burning slightly more fuel for the journey (or parts thereof).
smitty477 is offline   Reply With Quote
Old 02-12-2018, 12:18 PM   #6
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 19,736
In most cases a 10% speed increase should pay for it self in an adverse current.

Similarly a 10% speed reduction in a favorable current can save fuel.
FF is offline   Reply With Quote
Old 02-12-2018, 12:42 PM   #7
Guru
 
TDunn's Avatar
 
City: Maine Coast
Country: USA
Vessel Name: Tortuga
Vessel Model: Nunes Brothers Raised Deck Cruiser
Join Date: Feb 2015
Posts: 864
You made a mistake in your spread sheet. Your last column divides speed made good against current by the 1800 rpm fuel consumption for all speeds. The final column should be:

2.4
2.4
2.32
2.21
2.35
2.09
2.25
2.29
1.6
TDunn is offline   Reply With Quote
Old 02-12-2018, 12:45 PM   #8
Guru
 
City: Fairport
Country: United States
Join Date: Jan 2017
Posts: 1,683
A bit more complete with some options for crossing the stream with different angles of attack. Fixed an error in the data too.

You can see now with a 30 degree angle on the gulfstream, the boat RPM makes only slight differences in MPG over ground.
Attached Thumbnails
Click image for larger version

Name:	mpg.jpg
Views:	99
Size:	50.7 KB
ID:	72902  
diver dave is offline   Reply With Quote
Old 02-12-2018, 12:46 PM   #9
Guru
 
City: Fairport
Country: United States
Join Date: Jan 2017
Posts: 1,683
Quote:
Originally Posted by TDunn View Post
So how did you calculate the last column? It seems to me your numbers are incorrect. It seems to me that for current on the nose, the formula for distance made good would be:

nm/gal = (no current speed - speed of current)/(gal/hr)

For example, at 900 rpm you are burning 0.5 gal/hr and making 3.8 knots with zero current. If the current is 180 degrees to your direction your speed over ground drops to 0.8 knots, but you are still burning 0.5 gal/hr. So 0.8 knots/0.5 gal/hr = 1.6 nm/gal.
yes, my bad. Fixed with more stuff added. Always good to have a proofreader.
I should also state that this is simply based on miles over ground, NOT miles made good to a particular destination. And, that should be the normal mission.
diver dave is offline   Reply With Quote
Old 02-12-2018, 12:47 PM   #10
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 17,438
Faster into current. Slower with current.

Going into the current, the faster you go, the lower the percentage of energy used to fight the current.

Going with the current, the slower you go, the greater percentage of gain you're getting from the current. Your greatest efficiency would actually be to cut the engine off and just go with the current. Of course then you'd want to hang some sails and use the wind too.

Now, both of these statements are true only within a certain range on each engine. For instance, WOT is not likely to become the most efficient. We cruise at 20 knots and above and current is of minimum impact.
BandB is offline   Reply With Quote
Old 02-12-2018, 12:49 PM   #11
Guru
 
High Wire's Avatar
 
City: Cape May, NJ and Englewood, FL
Country: USA
Vessel Name: Irish Lady
Join Date: Dec 2007
Posts: 2,809
I’m a little dense and working from a phone. Could you write out the calculation for that last column?
__________________
Archie
1984 Monk 36 Hull #46
Currently southbound in Georgia.
High Wire is offline   Reply With Quote
Old 02-12-2018, 12:58 PM   #12
Guru
 
City: Fairport
Country: United States
Join Date: Jan 2017
Posts: 1,683
=(F18-3*COS(0.52))/E18

F18 is the still water speed. E18 is the gal/hour at that rpm. .52 is the radians of 30 deg.

What's curious about this example of 30 degrees and 3 kt current, is that the mileage is about independent of engine RPM, for the KK42.
__________________

diver dave is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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





All times are GMT -5. The time now is 05:26 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012
×