I forgot to add Vigilant, our engines do run at much lower RPM. However the difference is the reduction in our transmission. You can appreciate, in a truck application, you are up and down on the throttle, and you have your gears to work with. We are in gear, that's it. Then you bring your RPM usually to you "economy notch". Example, my Mitsubishi will turn 2000 rpm on the governor, but I run it at 1650. That is where I pull the best for the amount of fuel I am burning. You have probably looked at the power curve graph compared to fuel consumption. For me to give another 200 RPM, I might gain 1 knot, but burn about 30% more fuel. This also cuts my travel range for the fuel I carry down considerably.
Many years ago, I was on the "Island King" which I spoke of. The old Chief engineer, Bill Shields was educating me on fuel consumption. That ship has one of those V20 Jimmys in it. They are actually locomotive engines. When towing 24 hours a day on the governor, they burn a gallon per HP per day. I found it hard to believe him at the time, but it almost always works out. Now that big "John Ross" my uncle captained with the two engines in there they don't go by gallons per hour. Its tons of fuel per day.
If you keep yourself at a constant towing RPM, as most of us do, it is always best as I have said to keep some power in the bank. Which brings me to a story I get a real laugh out of. In 1970 when that "Island King" was built and was new. They tried to take the 4000HP out of that engine like in a locomotive, when they were actually supposed to only be taking 3600. Well we loaded the log barge in Quatsino Sound at the camp where I worked. (My job was to keep bringing the bags of logs alongside the barge with the small tug I ran). A friend of mine was captain on the King. When we were heaving anchors for them to leave, I hollered up at Joe the skipper, "You get right back here Joe, we're really plugged with wood". I'll be right back he says.
Well Joe don't showup. A few days later they finally get back. I say ok, what 'er you doing?..heavy date?. He says you'll never believe this. They are in Howe Sound, about 4 miles from where they dump. (In the above pictures I put up where the barges are having the loads blasted off, you can see the Port Mellon pulp mill. Well Joe is just coming by the Langdale ferry dock, and that old Chief engineer calls him from the engine room and says Joe, we just blew a liner (cylinder liner). I've got to shut 'er down. Joe says ok....shut 'er down Bill. Now Joe says to me, I'm doing close to 8 knots, so now I know just how long it takes to stop that barge. He's got 4 miles to go, and he is headed right for the deep sea dock where the freighters load pulp. He says, I was able to steer, cause the steering nozzle on that ship is powered by the auxilliary. The barge went by him, and he says I just kept letting a little tow line out and then kept snubbing it. Kind of like playing a big spring salmon. But he says, that barge would have wiped out that deep sea dock. One of the tugs a little smaller was just hooking up his chip barges to go back to Vancouver. He let his barges go and ran over and pushed the log barge around and got it tied up so all was well. The log barge has a load on it of 26,000 tons. Now we know, it takes more than 4 miles to stop it.