Fuel efficiency and engine size

In trying to understand important concepts like fuel efficiency and engine sizing, there are advantages to not having experience with a particular boat. People often try and generalise from what they know, so if they know a particular boat or two they will believe that most boats will be the same. In my case, ignorance could be a benefit…

So here is what I have found regarding power cats for longer range (and not) cruising. I haven’t tried to be exhaustive as no one paying me and I’m not getting academic benefits. So the choices I’ve made are my own (so there’s no Leopards in there for example). They are also based on either published or personal data on forums. ie. don’t trust anything I’ve written and do your own research, but see if this helps.

I looked at the following boats and tried to find information online:

  • Fountaine Pajot
    • Greenland 34
    • Highland 35
    • Maryland 37
    • MY37
    • Summerland 40
    • Cumberland 44
    • MY44
    • Cumberland 46
  • PDQ
    • MV 34
    • MV 41
  • Aspen 120
  • Lagoon 43
  • Malcolm Tennant
    • PH8 (54′)
    • Domino (66′)
  • Roger Hill
    • Tenacity (66′)
    • Lola (66′)
  • Ligure 50  (the boat we’re looking at purchasing)

For each of these, I tried to find the fuel capacity, dry displacement, engine size, and fuel economy at speed at various RPM’s (one or both engines). I have separated out long range (LR) cruisers (1200nm+ without extra tanks) from more shorter range boats (usually 1000nm or less).

The results are:

LR power cats.powertoweight

Power-to-weight is maximum rated BHP (usually added for two engines) divided by the weight in tonnes.

Some points I read in the data:

  • There usual power-to-weight band is about 25-40. Some boats offer a low-end and high-end option: for example the Maryland 37 with 75hp engines and 7.4 tonnes is rated at 20, whereas with twin 150hp engines is rated at 41 – for the same boat.
  • The outliers on the high side are the Fountaine Pajot Greenland 34 with 135hp motors, and the Lagoon 43 with 300hp motors, and the new Fountaine Pajot MY44 with 435hp.
  • The outliers on the low side are Fountaine Pajot Highland 35 with 75hp and the Ligure 50 with 135hp.

Another graph is of fuel economy:

LR power cats.fuelecon

Again, some of the problem is that these are sometimes reported figures by owners, and sometimes by the manufacturer or even by a boat tester. We have no way of knowing the state of the boat and hull and even age of the engines when the owner reports, are the manufacturers guilding the lilly, and are the boat testers truly independent.  So the actual details need to be taken with quite a degree of scepticism. However, the wide range of input means that the overall picture may be reasonably good.

Some points:

  • The smaller boats have better fuel economy (not surprisingly!).
  • The long range cruisers are least efficient – but that’s almost certainly because they are carrying lots of weight of fuel and water and living space.
  • Particular outliers on the efficient (lower) end are the Fountaine Pajot Greenland 34 – personally reported 16 knots at 1.06L/nm with 135hp motors.
  • Particular outliers on the less efficient (higher) end are the Fountaine Pajot MY44 and Ligure 50.
  • We need more points to be able to get a sensibly fit to the data – it’s too messy to explain much.

Of particular note for providing good reliable information, at opposite ends of the cruising spectrum, are SnoDog (PDQ 34) and PH8 (Tennant 54′ LRC).

And lastly, because I could, I looked at the relationship between boat length, engine size and displacement:

LR power cats.lengthmotor

Note that the engine size is not total for the boat. For example, the Fountaine Pajot MY44 may run two 435hp engines, but the Aspen 120 has only one such sized engine. Because there are a number of outliers, it’s interesting to remove these and see if there is a reasonable fit:

LR power cats.lengthmotorwithoutoutliers

Removing the single-engine Aspen 120, the underpowered Ligure 50, and the overpowered MY44, it turns out that the fit is very good with 0.8 for a linear match against engine and 0.85 for a linear match against displacement (even with displacement being quite suspect due to entirely different measures from different manufacturers). I have put in the formulas for those who may like to see whether there boat matches the data.

So the biggest point I found from this analysis is that I quite like analysing things. I believe there’s a red pill to solve that…

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