American Tug 395 for non-owners (and some other makes), post 3: Engine Size

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Nov 2, 2021
In previous posts, I described how this series is intended to give info to potential AT customers who do not yet qualify for the official owners' forum. I discuss why we chose an American Tug, and why an AT 395 specifically and some of the considerations.

In this post, I discuss the AT 395 engine choice between the 380hp and 550hp options. This discussion largely applies to other boats in the AT lineup, and applies to other makes such as Nordic Tug or Helmsman, when there are similar choices between engine sizes (perhaps larger or smaller but similar in relative terms).

Let's get some basics out of the way. All American Tug boats are single engine diesel with no wing engine. If that bothers you, then you'll want something else. (Historically they delivered maybe 1 or 2 with twin engines, but those were exceptions and not offered for many years.) The current boats are all offered with Cummins QSB engines (some older ones may have Volvo engines). For the AT 395, the options are the Cummins QSB 6.7 with either the 380hp or 550hp rating.

In some ways, the Cummins 380hp and 550hp are "the same". In particular, they are the same size engine block where the 550hp has been up-powered through various modifications that I don't completely know (I'm guessing the computer, fuel system, turbo charger, etc.) So you do not have to worry that the 380hp will fit better in the engine room.

What do you get with the 550hp? Roughly speaking, you get 4-6kts of potential additional speed at the high end, thanks mostly to a higher maximum RPM rating. To handle that, there are a few other changes such as a 5-blade propeller and a larger drive shaft.

What is the speed difference? At WOT, the factory sea trials reached 18.1 knots with the 380hp at 3000 RPM; and reached 23.6 knots with the 550hp at 3300 RPM.

What is the fuel performance? Using the factory's published data, I have attached a chart of the performance, expressed as the fuel burn (NMPG vs. the tested speed).


As you can see in the chart, the 380 appears to be more efficient (more miles per gallon) at displacement speeds below 8 knots. I asked the factory and they said they thought that was incorrect and that the measured difference at slow speed probably reflects other things such as the conditions on those days and the boat loading. They don't believe it reflects the engines as such. They expect the engines to have roughly identical performance at all speeds up to the maximum for the 380hp. You can see that in the curves for performance at and above 9 knots.

As you can see, fuel performance will be optimal if the boat is kept within displacement speed and ideally around 6 knots. You pay a steep penalty (2x fuel consumption) to go 50% faster (9 knots) and even more (4x consumption) to double the speed to 12 knots.

On the positive side, once the boat enters the semi-displacement area at 11 knots or higher, fuel consumption is roughly the same up to the maximum speed. If you're going to go fast then you might as well go faster (assuming you can watch for logs and don't mind the noise!)

Question: but I don't want to go fast ... is sustained low speed OK? Quick answer: Yes it's OK. For many years, I have heard that diesels should be run hard and that going at low speed for long times is a bad idea. As far as I can tell, that is NOT true. It appears that running at 30% power (like 1000 or 1200 RPM) is basically fine forever -- IF you occasionally vary that and "blow it out" and also assuming that you maintain the turbo per recommendations. For more, see the engine guru Tony Athens's discussion at Low Speed Running & “Break-In” of Marine Diesels

BTW, the 550HP is Cummins's engine rating for recreational usage, meaning that it is used at high power (more than 80% of load) no more than about 10% of the time (1 hour out of 8). That's not a problem on a trawler!

So ... the 380 and 550 engine options are roughly comparable in efficiency and expected longevity and ability to handle low speeds. That leaves the big question: is the extra 4kts or so of reserve speed worth the extra cost? When we signed, the cost difference was $25,000. I assume it would cost more today. Most AT owners travel at displacement speeds, and the 380hp engine handles that just fine. So the base case of the 380hp engine is extremely reasonable.

Yet in the end, we went with the 550hp option and here's why. The extra reserve speed is a good thing to have, and there's no evidence that the power will hurt the engine. Fuel efficiency and engine room space are the same. The 550 option has some upgrades with the shaft and propeller. The 550hp can achieve the same speeds at slightly lower RPM, which is a good thing in my book. The cost difference might contribute to higher resale value (TBH, I'm not sure, because ATs sell quite readily regardless of such details). Bottom line, it seems like the only tradeoff is cost.

Hopefully this is helpful to someone else, whether AT or a similar choice on another tug design. Cheers!
BTW, I mentioned that the 550hp can achieve higher speed at a given RPM. That seems to be true mostly in the semi-displacement range. I say "mostly" because some of the data are not necessarily exactly comparable, as noted in the post above.

Here's a chart for speed vs RPM, using the factory sea trial data. Up to 1500 RPM (displacement), the achieved speed appears to be the same. After that, the upgrade prop installed with the 550hp engine achieves more speed at a given RPM.

at-spped vs rpm.jpeg
Thanks for posting that article from Tony. Interesting how that flies in the face of a lot of what you read. I have a 480 on a Helmsman which is more power than I “need”. Your comment on achieving the same speed at slightly lower RPM’s makes sense to me.
Agreed, the old takes were contrary to Tony's but I think (maybe) they can be squared under the view that occasional WOT can be good for the turbo and exhaust, rather than the engine as such.

I'm certainly not an engine guru but I'd worry much more about running high RPM too much of the time -- with more and more HP coming from the same sized engine blocks, that is a awful lot of power density exploding inside a block of metal.

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