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!