Because the Nordhavn 52 is a new boat and because we chose to go with a more powerful engine than standard, we receive many questions on how we like it, whether more power is a good idea, and the fuel burn at different speeds. Now that we have over 600 hours on the engine and have used it on both short trip and multi-day 24×7 runs (when displacement beats planing), we’re in a pretty good position to discuss how its working out.

 

Dirona is powered by a John Deere 6068AFM75. There were two primary drivers influencing our choice of the Deere engine. The most important was horsepower. The 52, like the 47, uses a 165 HP intermittent duty engine but our 52 is 110,000 lbs whereas the 47 is advertised to be 85,000 lbs. With Nordhavns, the delivered boat will almost certainly be heavier than the published spec and, we’ve learned over the years, they get heavier with use.  Our old boat went up 4” in the water when we moved off of it (Down to one boat). Partly this is driven by us being full time liveaboards but mostly by having enough spares on board to be able to do long range cruising. Speed and fuel consumption on displacement boats with like hull designs, is just about 100% driven by displacement. Generally, if it weighs more or you want to get faster, then more power is needed. Understanding this, I like to look at hp per thousand lbs when thinking through when enough is enough. From an earlier blog entry, Engine Brand Choice, we compared different Nordhavns on the basis of hp per thousand lbs:

 

·         N40: 3.30     (50,000 lb @ 165 HP)

·         N43: 2.75     (60,000 lb @ 165 HP)

·         N43: 1.75     (60,000 lb @ 105 HP original engine)

·         N46: 1.75     (60,000 lb FD @ 105 HP)

·         N47: 1.94     (85,000 lb FD @ 165 HP)

·         N50: 3.75     (80,000 lb FD @ 300 HP)

·         N55: 2.66     (124,500 lb FD @ 330 HP)

·         N57: 2.66     (122,000 lb FD @ 325 HP)

·         N62: 2.19     (155,000 lb FD @ 340 HP)

 

I find this chart useful in that I’ve never heard a N50, N55, or N57 owner say they wish they had more power but I have heard a few argue that less would make more sense. From some 47 owners I respect, I’ve heard the power as delivered is perfect 90% of time but “I sure would love to have a bit more power when needed”. Or, “I would love to have the option of running faster when running coastal.” None really are unhappy with the choice but the trend seemed to be that those with less than 2.0 hp per thousand lbs often thought they could use more power whereas those with more than 2.5 hp per thousand lbs often thought they would prefer less. However, it’s very clear that there is no right answer with these decisions. Some I spoke with before purchase argued they wish they would have less horsepower, so they could run there engine at closer to rated load during normal operation.

 

In the diesel engine world, there is considerable concern about engine under-load and this is part of the reason, why many folks argue it’s better to size an engine on the lower end of the spectrum of what works in a boat. If an engine is run at very low load factors for long periods of time, it can cause cylinder glazing and increased wear due to excessively cool operation. This is a problem that does show up in generator applications so it is a potential concern. The key is to ensure the load is sufficient to maintain adequate engine operating temperatures, both water and oil. Modern electronically-controlled, common rail fuel-injected engines have very wide operating ranges so this problem is less of a concern than it was with less precise mechanically injected engines. The key to avoid problems at lower load levels is to keep the engine at or near the manufacturers specified operating temperature and give it a good wide open throttle run periodically. Many recommend daily short, wide open throttle runs.

 

Another thing to keep in mind is that, for all the industry concern with under-loading, over-loading remains a larger problem and destroys more engines. See Diesel Engine Overload for more detail. These problems show up most frequently in planning hull boats but, if you can’t achieve more than the rated RPM at wide open throttle on your boat, you are over-loaded and need to reduce pitch in your prop or change operating conditions (e.g. excessive bottom growth).

 

Another factor where I have seen some confusion is the belief that a larger engine consumes more power at a given output. A 265HP engine operating at 150HP will consume very nearly the same fuel as a similarly designed engine rated at 165HP and operating at 150HP output. Fuel consumption is driven by the HP being consumed not by the rated hp. The rated horsepower is the capability to produce the horsepower if needed. But with all other factors equal, a higher rating does not increase the fuel consumption.

 

We went with a 265 hp engine which places Dirona at 2.4 hp per thousand pounds. Because it is fairly close to the center of the Nordhavn fleet by that measure, we were pretty confident that it would work out. After 600 hours, we are glad we went with more power and, even at incredibly light loading, the water temperature doesn’t drop below 170F. Our usage patterns vary greatly and we use the boat over a very broad operating range. We use our boat most weekends and when only out for two days, we love the extra speed. On these trips we tend to run around at 2200 RPM, which on our boat is just over 210 hp. When coastal cruising we usually run in the 1800 to 2000 RPM range, which on our boat is 135hp to 165hp. When long range running we range widely between 1200RPM and 1800RPM, which is 50 hp to 135 depending upon fuel levels and conditions. On one trip, we needed to get to Anacortes, Washington and I couldn’t leave work in Seattle until later in the day. It was wonderful to be able to run over 9 kts the entire trip which is over 250hp. We burned a lot of fuel but it was nice to be able to that day. Looking at this data in tabular form:

·         Rarely (2400):                  250 hp   (9.2kts)

·         Weekends (2200):           210 hp (8.7 kts)

·         Coastal (1800-2000):       135 to 165 hp (8.2 kts 8.5 kts)

·         Passage (1200-1800):      50 to 135 hp (5.9 to 8.2 kts)

 

All the data above comes from a two way instrumented sea trial done by John Deere some months back. Since then the boat has since gotten heavier, 1” of prop pitch has been taken out, and those instrumented runs were done with a light fuel load, I’ve adjusted the speeds down by 0.3 kts in the table above to be closer to current reality. For those interested, the original raw data is at: N5263 Fuel Burn. In the spread sheet, you’ll find the two sea trial runs in opposite direction and for each run, the RPM, the fuel consumption, and speed.  I average both runs to get average RPM, speed, and range and then compute the range at 90% tanks. I then fit the fuel range curve to get speed to range data:

 

As I mentioned above, Dirona is now somewhat heavier than when these data were produced so, when making planning decisions, we conservatively assume roughly 0.3 kts less. And, having had the experience of one multi-day 24×7 passage, one of the things we learned is that waves and rough conditions have a much larger impact on speed than we originally guessed. Big swell will slow the boat down and increase the fuel burn to speed considerably. If you want to use different assumptions or play with the data, the spread sheet is at: N5263 Fuel Burn.

 

Looking at the data above, you can see that Dirona spends much of its life under the output of the standard 165 hp engine. This is true and its clear the standard engine will serve well in most operating modes. Personally, I prefer not running intermittent duty engines at max output so, we wouldn’t be comfortable asking 165 hp from the standard engine for more than very short periods of time, whereas the Deere is continuous rated at 221 HP. We just about never exceed that point.

 

Overall, we’re loving the engine and its working out really well across a wide spectrum of usage patterns. My summary, looking across many different dimension: 1) we like the fuel efficiency of a modern high pressure common rail (roughly 15% better than our previous mechanical engines), 2) we love all the instrumentation available from an electronic engine including real time fuel burn and engine load levels, 3) instant cold start is great, 4) the overall engine sociability (no smoke on cold start, reasonably quiet, low soot) is nice, and 5) it is really nice to have lots of power at a continuous duty cycle. After 600 hours, we continue to really like our Deere 6068.

 

                                        –jrh

 

Update: Dirona Prop Curve: