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.

How's this?

Wow, great! I might have to do the cosine function now.

So based on the above info what is the best RPM to run?

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).

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.

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

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.

TDunn said:

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.

Click to expand...

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.

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.

I’m a little dense and working from a phone. Could you write out the calculation for that last column?

=(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.