What is your strategy for saving fuel?

[Rambler241]

Kip336 stated:

That actually does not save fuel, as your diesel engines need to be on a higher speed setting to get the batteries charged.

No they don't - Sailor Steve got it right - the starboard engine is disconnected from the screw, clutched to the generator, and run at flank speed. The batteries will charge at the same rate whatever speed the boat is moving at, even stationary. Check out the rev counters in the control room, to the left of the planesmen while charging.

Charging batteries uses a lot of fuel - the charging engine runs at flank speed, and the other has to be run at higher revs to maintain ordered speed. There's also the energy lost in the energy conversion - diesel to generator current loses energy, conversion of that current to chemical changes in the battery loses energy, converting the chemical energy to current loses energy, current conversion to electric motor rotation loses energy.

In general, "ahead slow" gives the best fuel economy for a simple engine speed order, though it isn't necessarily the most efficient speed setting. On my current mission, we were ordered from Brest to a grid square WNW of Capetown. Shortest plotted route was 11,595 km to NW corner of GR51. Leaving harbour, I did some range tests. Boat is a type IXC with MAN engine upgrade. Once speed settled down for each ordered speed, The NO reported a range of

1. 23631 for ahead slow, shown as almost exactly 8 knots on the knot meter
2. 26450 for an ordered 10 knots (clicked on the top on the zero of the 10 on the knot meter), settled down to 9.5 on the meter)
3. 24872 for an ordered 9 knots, settled down to about 8.7 on the meter
4. 17,286 for 1/3 ahead, shown as 12 knots but actually 11.5 knots on the knot meter
5. 9276 for ahead standard, 17 knots shown, 16.5 on the meter

Even if I follow the shortest route, using speed setting #2, it leaves little room for bad weather (slower speed or use more fuel?), excursions to intercept targets, running away from nasty inquisitive warships, recharging batteries, and actually patrolling the target grid and getting back home. The Bahamas are nice this time of year.....