Average fuel burn rate on different size boats..

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wipe_out

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As you do I am looking at bigger boats.. Obviously one of the major factors is how much fuel it will burn but this also seems to be the hardest information to find.. Even finding "average" numbers for say 35ft, 40ft, 45ft, 50ft..... boats is proving problematic..

I know there are the calculated consumption methods of xx litres per HP but then you need to know how many HP are being used to maintain say 20kn..

So lets say you were looking at flybridge cruisers and wanted to get some idea of how much fuel they would burn at 20kn how would you go about finding the information so you could make a comparison and evaluation?
 
Didn't jfm say in his new Match build thread that he was hoping his new Sq 78 had better fuel consumption than the old one and would burn less than 20 gallons a minute at 20 knots? Or did I just make that up?
 
For a very rough rule of thumb, when i was looking, i worked on a 35 -40 ft average twin shaft driven flybridge boat burning 1 mpg at cruising speed of 20 knots, i.e 20 gph. If powered by twin outdrives then nearer 1.5 mpg. Though as i say this is only a rough guide, as there are many factors to be considered to give an accurate estimate.
As mentioned in the above posts, cruising at 20 knots ain't cheap these days, though the smile factor is still great :)
 
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I think more importantly you need to look at your planned use area etc., and determine how long the "average" journey will be. Then asess how long you will spend at 20 knot during these journeys....

Speed = £££ and size combined with speed = £££££££

Very few leisure boats will spend much above 2 hrs (on average) in the 20 knot area, but I know that from often running a heavy 45 foot boat for 4 hrs plus in this range, you will not get much spare for 90 - 110 litres per hour....

However, our average hourly consumption is significantly less than that as our average speed, to and from marina, in amongst other boats etc., means our speed and thus engine load drops big way.

So advise would be to look at how you would use the boat, area to cruise in and assess if you really need those 20 knot as cruise when you have a bigger boat to enjoy the journey in...
 
Ths is a thoroughly debated coconut with a similar answer to the length of a piece of string !

In very rough terms, and I now expect to be shot thoroughly, for budgeting and planing at 20 knots ...

30' 2-2.5 mpg
35' 1.5 - 2 mpg
40' 1 - 1.5 mpg

Most owners will likely and optimistically suggest their boat is far better than this, whilst wincing as they fill the thing up.

This is Loa excluding swim platforms, which most boats now count in the declared length, which is akin to including the flap in the end of a durex as a valid part of your manhood dimensions, but then I guess for many of us our boats are just penis extensions, that's what SWMBO thinks anyway.

Drive type and qty of shafts, engine size and type and driving style, not to mention persons aboard and tank levels, inventory / unnecessary rubbish levels will all have an affect. Then there's sea state, wind direction, tidal flows.

If you think about it too hard you will conclude in short order that the non obsessed would never own a boat, especially a planing power boat, but that's where common sense and boat ownership part company !
 
I think that the weight of the boat gives a good clue, my boats so far using weights from the boat lift gave the results below, on calm seas. Gets rough you use more
sealine 410, length 42 feet, weight @ 10 tonnes with 71Bs did @ 1.1mpg @ 20kns ish
targa 43 length 45 feet, weight @ 9 tonnes with 63Ps did @ 1.2mpg @ 20kns
sealine t50 length 50 feet, weight 18 tonnes with D9s does 0.65mpg at 22kns
so a crude rule of thumb is to compare weight and length to give an indication of what a future purchase might do to the wallet.
 
another method i have used is to take the estimates from MBYs tests and make them 10% worse. On the boats I have owned that Mr Marsh has tested this seems to work quite well too. I guess test boats are effectively stripped bare and light on fuel to keep the weight down and the mpg up.
 
Thanks.. All very useful information..

Plan at the moment, which admittedly is still in early formation, is to get something "comfortable" enough to spend a lot of time on (practically live on) but not so big that it never leaves the marina because the running costs are too scary.. Of course as the list of "features" grows so it seems does the LOA.. :)
 
There is, of course, an element of choice in cruising speed. We were on the NYA cruise in company last week (an excellent week but that is another story) and did several sections of the sea passages at 7-8 knots.

Our boat is very comfortable and whilst fuel consumption was a significant consideration when we bought her we concluded that our sea trips wouldn't always require 18-20 know cruising but it is good to have the choice (so displacement boats were not a consideration).
 
That's where I am as well.. Full displacement gives you no choice of speed where planing hull maybe a little less efficient at displacement speed it had the option of going faster..

I know I will run at different speeds depending on many factors like who is on board, the weather and what I feel like but just chose 20kn as the reference for side by side comparison..

Out of interest do bigger boats come onto a plane a lower speed or are they all about the same?
 
bigger boats plane properly at slightly higher speeds - but not much more. re- planing boat at displacement speeds, I think the economy is very similar to displacement boats at these speeds as planing boats weigh quite a bit less. another thing to think about is that the longer the waterline length of a boat the faster its displacement mode speed is.
 
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