Fuel Burn Stuff

Economical cruising – an interesting topic for the tekkies! You have the conflict of the optimum (=most fuel efficient) speed for the boat itself and the optimum (=most efficient operating speed and load) for the engine. Guess what, on many boats they don’t always meet up too well.  

With a full displacement boat like our Nordhavn (40 tons when full of fuel and water and cruising stores) it is pretty simple. The slower you go, the more efficient she is. However, if you travelled at around  2 knots all the time it would be pretty slow, lots of going backwards against adverse spring tides etc. You would also need a very small low powered engine to try and keep it loaded properly as diesels hate running without a proper load for long periods. Of course then you wouldn’t have enough power to push tides / wind when needed or to manoeuvre the boat either. So, like most things in life, it is a compromise.

The matching of the hull, engine and propeller is a complex science with some artistry in it too. If you want to read about it and become a real long distance motorboat design expert, read “Voyaging under power” by Robert Beebe. Only for the seriously committed boat people though.

As we have a fixed hull, propeller and engine combination we will oversimplify it to what speed the hull likes and what speed the engine likes:

The engine bit:  I was always taught that for a diesel to be reasonably efficient and for longevity, it has to be operated under at least 30% of the maximum designed load as a rule of thumb. Otherwise the engine just isn’t running efficiently – lots of “losses” from friction and ancillary components mean that the power actually produced compared to the fuel burnt is poor at lower loads. Of course, any engine also needs good bursts of full load to keep the cylinder bores clean and in our case to stop the dry exhaust stack from getting heavy soot deposits inside it. These are then sprayed around as nice wet soot blobs when she is started from cold. That really upsets neighbours in their shiny white go faster boats… So we aim to run at something over 35% load all the time and have at least 10 minutes each day at wide open throttle.

Some people have asked about running their planing boats at slow speeds and if the same still applies. Well, bad news. The typical high speed aftercooled engines hate "ticking over" for long periods even more. There are some interesting technical reasons for this (overcooling at low power outputs, exhaust gas temperature etc) but as we don't have an aftercooler they dont worry us too much!

The hull bit: If you read the books they can explain why a displacement boat has a so called “hull speed”. Any faster than that, you just burn diesel to make bigger waves and don’t go much faster. It is all to do with the square root of the waterline length of the boat. ie “longer is better / can go faster”. No comment.

Of course, you also want to travel at a speed when the boat rides comfortably. Not enough power / speed allows her to wallow about more and so you also have a trade off in ride comfort versus outright fuel economy.

Here is how our fuel burn varies with speed when the boat is full of fuel / water in calm seas:

RPM      lph         speed   MPG

                                               

1216       4.6          5.3          5.24

1410       7.3          5.8          3.61

1510       9.6          6.2          2.94

1575       9.9          6.4          2.94

1610       10.4        6.7          2.93

1703       12.5        7.1          2.58

1810       15           7.6          2.30

We tend to run at 1450rpm or so on longer trips as it keeps the engine reasonably loaded (35%)  and gives us sensible passage times (about 6 knots) and fuel burn (about 3.5 nautical miles per UK gallon).  Naturally this improves as you burn off fuel and the boat gets lighter. Wide open throttle gives us about 2350rpm and a wild 34 litres an hour burn at only 8.5 knots. See how it works?

Because we go slowly, we don’t need much power. On long runs the main engine is only running at about 65hp to give us 6 knots. That is why the little emergency “get you home” wing engine only needs to be 40hp. It can still push 40 tons of boat along in an emergency.

As a comparison, our old boat was a planning speed Broom 415. She weighed about 17 tons. At 20 knots, she managed 1.1 mpg. She needed 880hp to get to 31 knots when the consumption was 0.7mpg. That is why you have to go long distance cruising slowly!

Some more information about the boat and equipment - for trainspotters or people with time on their hands!

We bought our Nordhavn 47 early in 2009. She was a couple of years old, having been built for a man who kept her in Denmark.  That is relevant when you look at the equipment he fitted – the boat is unusual in having Thinsulate insulation in the roof lining area and the windows in the saloon and pilothouse are double glazed. Great news for liveaboards, avoids all the condensation that you get when the heating is running in cold, wet weather and makes the boat easy to keep warm in the depths of winter (or summer of course!)

She was fitted out ready for serious cruising with all the necessary kit based on good advice from the Nordhavn Europe team. Hydraulic stabilisers, redundant autopilot heads and computers, water maker, SSB radio etc. She was completed just as Furuno were switching to Navnet 3.  Luckily (I think), the boat had a suite of Navnet II equipment which has was  rock solid reliable.  We decided to upgarde it during the winter of 2008/9 not because it was causing trouble but because we feared that spare parts supply might become an issue in future and the old kit had good resale potential too. There was  a built in PC that ran Maxsea software which interfaced to the Furuno equipment. The delights of Microsoft – I thought I had escaped from all the compatibility issues and problems when I gave up working in IT! Her is the old style office:

for informationm on the upgrade to Furuno TZT2 kit look at http://rocklandlog.blogspot.com/2019/02/trashing-boat-electronically.html

and to see the final result, try http://rocklandlog.blogspot.com/2019/03/all-re-electroniced-and-broke.html

The boat has the larger pilothouse berth that can be used as an extra double for short term visitors if the forecabin is already occupied. This cuts down the seating area a little but there is plenty of space for 4 people in the pilothouse when underway.  If we were having another 47 built, I would go for the standard smaller berth though as we only use it for the “off watch” person on passages.

Luckily, she also had a full height Stidd helm seat fitted – most boats have a half height seat and when underway in the “rougher stuff” we really appreciate the extra support (age and bad back thing I suppose). The Stidd seats are crazy prices but just so comfortable on longer passages. We would smile and fit one to a new boat despite the bill.

Domestic equipment – again built for living on board. She was fitted with Miele appliances from new, including a slimline dishwasher that makes having guests on board so much easier.. The best part is having a separate washer and drier, no need to go to a marina launderette:

 

Entertainment is taken care of by a B&O system – remember, she was built for a Danish guy!

The best part (OK, in the Captain’s opinion the best part) of a Nordhavn is the engineroom. For anyone who has battled trying to access engines or fuel filters buried under floor panels in a boat that needs a slim contortionist to work on them, this is heaven. In fact, you can be ham-fisted and overweight yet still get to all the important parts of the engine, wing engine and genset. The access is great and encourages proper maintenance and regular checks. Our boat was built with an internal phone system so the captain can phone the galley for tea whilst working down in the engineroom. Getting the phone answered isn’t guaranteed of course…. There are two other great resources. The Nordhavn owners groups on Yahoo are a mine of information – many very experienced owners who have circumnavigated / crossed oceans in their boats. The second is the support from the Lugger (Alaska Diesel) factory training man. Known as “Lugger Bob” he is also a keen boater and offers advice and technical hints willingly – most unlike the approach from people like Volvo who don’t talk to customers and just  say “call a dealer out”. That isn’t too practical when you are mid ocean and linked only by sat phone!

The main engine (see link in the “useful stuff” section) is a marinised John Deere unit. For the tractor trainspotting types, it is a Deere 6068. For the rest of us, it is a big capacity (6.8 litres) block to make a lazy 170HP or so, slow revving and gives the impression of being bullet proof. Underway, it runs at about 40% load to push the boat along at her optimum hull speed for efficiency. So far, no Nordhavn owner has reported a major mechanical failure with one of these. Naturally as a prime mover, you need that. The engine is relatively low tech. No common rail high pressure injection, no fancy control systems, no supercharging, just a turbo to clean up the exhaust at higher rpm. The KISS approach!

The other big plus is that standard service parts and any replacement components are available via John Deere agricultural dealers. Sensible prices (sorry Volvo, Yanmar) and great logistics as they are geared up to supply parts for commercial users worldwide.