My thoughts on the (near) perfect drive train for your boat

I am a member of the Nordhavn Dreamers group and the Nordhavn owners group. There is a lot of good information there from a lot of knowledgable people including industry experts like Bob Senter and Steve D’Antonio who frequently contribute. I loved my Nordhavn 55 and might someday own another Nordhavn and like to keep in touch. There has been a long discussion recently in the group regarding engine and prop configurations. Most Nordhavns have too large of an engine. On long voyages they are using a small amount of the rated horsepower of the engine going at a slow speed mainly for fuel efficiency purposes. For example, on my Nordhavn 55 and its 330hp John Deere engine the best long distance cruising speed was at 7.5 knots burning around 4 gallons per hour. At this speed I was using only about 25% of the rated horsepower of the engine. The problem with this is that diesel engines like to be properly loaded keeping the oil, internal components’ and exhaust temperatures in the proper operating range for optimum combustion. On my Diesel Duck I tried to avoid this by sizing the engine correctly, trying to find the optimal engine size and prop configuration to maximize fuel economy, cruise at a reasonable speed and load the engine properly. My Duck has the John Deere 4045TFM75 Tier II M2 version that puts out 121hp. It is electronically controlled. It is paired with a 5 blade Max prop that is fully feathering for when I am in sail only operation to eliminate any drag with my Ketch rig. In addition, it has a feature where the pitch is adjustable in the water with a special tool. The poor man’s Hundested prop. When I left China for the USA I knew I was going to be going against the prevailing currents, wind and swell. I needed maximum fuel efficiency for the 10,000 mile voyage but at the same time needed maximum torque for bashing into those conditions for months at a time. In addition, I knew I was going to put a lot of hours on my engine and wanted to provide the optimum operating conditions like engine load that would contribute to engine life longevity. With the 121hp engine and 45 ton displacement I never felt underpowered even while encountering those horrible conditions going west to east 8,000 miles across the middle of the Pacific.

The first step was choosing the right engine size with regards to horsepower and displacement. My 121hp engine is less than half the horsepower you will find in a Nordhavn 52 which displaces almost the same amount as my boat. It has a 266hp John Deere six cylinder. After having spoken to owners and reviewed various blogs I get DOUBLE TO TRIPLE the fuel efficiency that they do. At 6 knots I burn 1.5 to 2 gallons an hour depending on sea conditions. They are able to throttle down to burn only 1.5 knots but the engine is so severely underloaded at less than 20% of horsepower and that is not practical for long distance cruising. Preliminary reports regarding the new Nordhavn 475 say it will have the 160hp version of the John Deere 4045. Based on what I experienced in my N55, the numbers from the 52s and my DD this will be too much. This N475’s JD 4045 is also rated M2 like mine but is a tier III engine. This means additional pollution controls. The fuel consumption curves in the literature show it burning considerably more fuel than mine. The fact that they are getting an additional 40 horsepower now out of the same engine concerns me. In my experience that additional horsepower comes from not only higher fuel consumption but higher temperatures and pressures with result in more efficient combustion but which might effect the reliability and overall life of the engine. Also, this means additional engine room heat.

The second step was choosing the right type of engine. Many Diesel Ducks have an Iveco engine that has about the same horsepower as my JD. It is a six cylinder engine non electrically controlled engine versus my four cylinder electronically controlled. The good news is that it is a simple bullet proof engine that is easy to work on and is more tolerant of lower fuel quality. Getting the same horsepower out of the higher displacement in the 6 cylinder versus my 4 means the engine will be less stressed. The bad news is that at the larger displacement and the lower efficiency from not having the electronic controls it burns much more fuel. The electronic controls mean not only is my engine more fuel efficient but it also runs smoother and the exhaust is cleaner. Also, the 6 cylinder puts out much more heat than the 4 cylinder into the engine room which means more power needed to cool it and a higher temperature air entering the engine resulting in less efficiency. One more advantage of electronic controls is that they can report back to an electronic display real time information exactly how the engine is performing. Things like oil temp, coolant temp, rpm, engine load percentage and fuel consumption numbers are instantaneous.

The third step would be installing the correct prop. My boat has a five blade versus other Duck’s three blade. The five blades make it burn less fuel and provides more torque at the same rpm versus the three blade Ducks with a similar engine. It seems to run smoother than the other Ducks I have been on. My Max prop is pitch adjustable in the water. Before a long passage I adjust the prop to overprop the boat by increasing the pitch. This means at a given rpm the engine will have to work harder. In an analogy to a car, it is like cruising in 5th gear instead of 4th. You are going farther at the same rpm and the engine is working harder but fuel consumption is less. Everything is so positive i.e. proper engine load and great fuel efficiency at the same time. Seems simple so you are probably thinking “why doesn’t everyone do this”. Diesel engine manufacturers feel that for a boat to have a proper prop configuration it must reach 100% horsepower at full throttle. No more and no less. The way my boat is configured is that the boat will reach full horsepower before full throttle. In the rare case I accidentally applied full throttle I could damage the engine by overloading it. This is why the engine manufacturers insist for the warranty to be valid the boat must be propped “properly”. The advantage of my Max prop with the in water adjustability is that when I have finished a long passage I can easily change the pitch back to what will satisfy the manufacturers warranty.

If you are knowledgable about diesel engines you might have noticed something in my narrative that does not seem correct. The general rule of thumb is 1 gallon of diesel equals 20 horsepower. On my DD I have said I can burn 1.5 gallons per hour at cruising speed with my 121hp engine and keep it properly loaded at about 40% for long distance cruising. Based on the general rule of thumb 1.5 gallons is equal to 30 horsepower or 25% of my engine’s horsepower. My Murphy electronic gauge provides the 40% number along with the fuel consumption number which is usually highly accurate. So the question comes down to “how can my boat be that fuel efficient at that load using that much horsepower?”. My answer is that with a properly pitched prop matched to the torque curve of a highly efficient electronically controlled engine operated at the proper speed for the sea conditions with the highest spec fuel available you can exceed the 20hp to gallon of fuel number. The majority of my fuel has always been automotive spec dosed with a liberal amount of Stanadyne Performance fuel additive. I think my boat’s super efficient hull form also helps. If you look at the most recent addition of “Voyaging Under Power” you will note that Diesel Ducks seem to be the most fuel efficient trawlers out there in the comparison chart in the book. I should also add that you can monitor your engine’s various operating temperatures to determine proper loading. My engine oil temperature is always above 180 degrees and the EGTs are between 500-600 degrees which are really good numbers.