how to work out fuel consumption ?

grumpyoldman

grumpyoldman

Well-Known Member
Joined
7 Apr 2007
Messages
77
Location
Fenit, Ireland
Visit site
Obviously senility is starting to kick in - long overdue some would say!

Cannot get a sensible consumption figure - can you help please?

I'm looking at buying a second-hand 65 foot long range displacement steel motor cruiser (trawler type 75 tons) which is six years old the facts:-

10500 litre fuel capacity 8000 plus 2500 day tanks
Engines 2 x John Deere 300 PK 6081 AFM 75

Readings show:-

1200 RPM 17 Ltr Per engine per hour giving 6.5 knts
1750 RPM 21 Ltr per engine per hour giving 8.5 knts

When i try to work out the range with and without 20% safety margin it dos'nt seem to make sense for the quantity of fuel, can you tell me what it should be for the figures above.

Thanks in advance
 
grumpyoldman said:
Obviously senility is starting to kick in - long overdue some would say!

Cannot get a sensible consumption figure - can you help please?

I'm looking at buying a second-hand 65 foot long range displacement steel motor cruiser (trawler type 75 tons) which is six years old the facts:-

10500 litre fuel capacity 8000 plus 2500 day tanks
Engines 2 x John Deere 300 PK 6081 AFM 75

Readings show:-

1200 RPM 17 Ltr Per engine per hour giving 6.5 knts
1750 RPM 21 Ltr per engine per hour giving 8.5 knts

When i try to work out the range with and without 20% safety margin it dos'nt seem to make sense for the quantity of fuel, can you tell me what it should be for the figures above.

Thanks in advance
Click to expand...

Calcs are:

@ 6.5kts 10500/35(lts per hour) = 300 hours X 6.5 = 1950nm range without reserve

@ 8.5kts 10500/42 = 250 hours X 8.5 = 2125nm range

8.5kts looks like the sweet spot.
 
The odd one is that it looks like your range is just about 2,000 NM @ 6.5 Knts and just above 2,100 @ 8.5 Knts ... without the 20% safety margin ...

Suggests that engine/hull speed/prop combination is better at 8.5 knts ...... or given figures are wrong...
 
Simple.

Capacity/consumption = hours* av speed = range

so 10500/34 = 309*6.5 = 2007. *.8 = 1605 with 20% safety margin

or 10500/42 = 250*8.5 = 2125. *.8 = 1700 with 20% safety margin
 
jfm said:
Howe sure are you?! They're implausible.
Click to expand...

hence my mental problem - never seen such a pronounced "sweet spot"

however figures provided by well know European naval architect so on the face of it accept them in good faith

We are very interested in making an offer for same - in most ways it would suit us perfectly - but - its a lot of cash - so need to do homework carefully - the piggy bank would not recover from such an expensive error.

Even i (impossible as it might sound) can make a mistake - perhaps he could too.

can i pm you his provided pdf chart compiled on recent sea trial ?

Thanks grumpy
 
Spi D said:
This one by any chance?

http://www.elburgyachting.nl

Stunning..
Click to expand...
Has to be this
http://www.elburgyachting.com/sp/s4900en.htm
and what a machine!

Well naval architects don't come much more highly regarded than Pieter Beeldsnijder -I've been on his Hyperion and Gliss (he did the interior only on Gliss) so of course I'm not about to start arguing with him but the concept of 8kts being sweeter than 6 on a 65 foot/75 ton displacement boat just doesn't figure. On my boat which is D at those speed the fuel per mile increase significantly at 8 vs 6 knots and there aint THAT much difference in the hydrodynamics. MapisM's boat is similar, just scaled down a bit c/f Bonker, and I think he will tell you his range at say 7 knots is less than 5. Feel free to send spr/sheet in case anything jumps out of that

We are rather splitting hairs though. What a beautiful machine. Where is she lying? If you buy her you MUST keep the name :D
 
I just looked in more detail. What an amazing machine. Apart from the saloon table cloth, everything looks specified to complete perfection

I do not know if those Deeres have electronic fuel metering. If they have, the data should be good. Alternatively if they are using floscan there is risk of calibration error so generally I wouldn't automatically trust them.

This is quite an expensive toy to maintain. They re-did the Awlgrip at 4 years as recommended by Akzo, which means it could be due again in 2 years. Not much change there out of £100k. Makes search for fuel sweet spot or whatever seem small beer!


EDIT - bit of a guess but I think she is lying in Club De Mar, Palma. Deleted User could confirm but he is at sea thisvweek
 
Last edited:
Spot on - would not like to try to keep anything from you lot - very nice bit of sleuthing by yourself and Spi D.

Has features that would be useful for when i end up needing a wheel chair - probably the last boat (the next one ) i will buy - our current boats we have for 12 and 7 years respectively.

Would have every intention of keeping the name my young fellow (40) thinks it reflects my mental state and is an apt description of his favourite pastime.

We would on average do about 500 hours on the engines per year on my current boat (why have it if you don't use it) and if fortunate enough to be able to deal would see perhaps 50% more per year, so fuel is a cost that needs to be calculated properly. We will get over buying (bless the pension fund) but running costs need to be known as best one can before dealing.

i will pm you the pdf later
 
jfm said:
OK. Do you boat in NI or just live there? Do you by any chance remember Silver Dee at Coleraine?
Click to expand...

We are almost as far south west as possible to go on the map of the republic close to Dingle between Cork and Limerick,hence the interest in trawler type craft we get interesting Atlantic seas in the South West and West of the island.

Silver Dee rings a bell, cannot think why, i used to race and rally quite a bit in the north particularly at Kirkistown perhaps half a century ago now.
 
grumpyoldman said:
We are almost as far south west as possible to go on the map of the republic close to Dingle between Cork and Limerick,hence the interest in trawler type craft we get interesting Atlantic seas in the South West and West of the island.

Silver Dee rings a bell, cannot think why, i used to race and rally quite a bit in the north particularly at Kirkistown perhaps half a century ago now.
Click to expand...

Ah ok. Silver Dee is a 52 ft trawler yacht and lived in Coleraine for many years till my brother bought her 6 or 7 years ago. She has been on a swinging mooring in Union Hall/Glandore for the last 2 summers but is now in Scotland OOW prepping for a move to the Med, and is as I type having a paid of new JDs fitted. I used to sail a bit further anticlockwise than your patch, namely Cork-Kinsale-Glandore-Baltimore stretch a lot. My father still has a house there with a day boat. Yes you get interesting seas where you are! Bonker would be quite the most special machine there - people will drive for miles just to get a glimpse!
 
We are just back from a weekend around Schull, Baltimore and had a lazy afternoon watching a very busy mini regatta at Glandore - weather brilliant - small world - would visit that area on the way to Kinsale and Crosshaven a few times a year especially while en route to Dunmore East and Waterford. nice spot for Grub, although hotel is currently up for sale. normal summer today, wet windy and raining!

Perhaps thats why Silver Dee name is familiar perhaps seen at Glandore in the past.
 
jfm said:
MapisM's boat is similar, just scaled down a bit c/f Bonker, and I think he will tell you his range at say 7 knots is less than 5.
Click to expand...
Yep. Not just less, but MUCH less.

@GOM:
with all due respect for PB, the numbers he gave you sound seriously wrong.
Just a thought, are you sure that the fuel burn at 1200 rpm isn't 7 Lph, rather than 17? That would be much more plausible.
In fact, according to the JD prop demand curves, the 6081 AFM 75 engine burns (Lph @ rpm):
10.5 @ 1200
28.3 @ 1700
33.2 @ 1800
Now, of course the prop demand figures can vary, depending on the hull where the engine is installed.
But you can easily see why I wouldn't trust the numbers you mentioned in your OP.
I can send you the PDF of JD specs sheet if you wish, just pm me your mailbox.

Anyway, she's a great boat regardless of actual fuel burn..
TBH, for a 66' steel trawler, my preference would be for a layout with a raised p/house plus portuguese bridge, and with a single rather than twin engines.
But that's me of course, and personal preferences aside, nobody can deny that the boat looks stunning.
And surely built with a superyacht approach, at least to some extent: just look at the size of those teak planks, and the 3ph genset!

Btw, she was debated also towards the end of this YF thread:
http://www.yachtforums.com/forums/g...ssion/23110-sailboat-trawler-but-trawler.html
Someone said that she's the personal boat of the designer, which might well be true.
Also, they debated the sale price, and both 1M$ and 3M$ were mentioned...?!?
 
GOM, I have to say I agree MapisM's capitalisation of "MUCH"

I just looked at the fuel graph you sent. Prepared with great precision, in PB's own handwriting. But the only conclusion you can rationally make is that he has just made a mistake where he wrote 17 LPH. The whole graph and data he has recorded hangs together beautifully, EXCEPT the only two data points for LPH (ie the figures you give) are simply unbelievable. They just cannot both be right. My best guess is he meant 7 not 17, exactly as MapisM said.

(I'd then have a hunch that if the engines are doing 7@1200 and 21LPH at 1750rpm, both numbers being 3/4 of the JD full power curve values, the thing might be under propped for 8knot running and you'd be better adding an inch to the pitch, but that is another subject and anyway is upside for you in the fuel burn stakes)

7@1200 and 21LPH at 1750rpm implies 4735 and 2125nm range (at 100%, no reserve) and the relative size of those numbers is very much what you'd expect on a 75t D boat going from 6.5 to 8.5kts. I get pretty much those numbers on my dashboard at those speeds

So I'll bet you a coat of Awlgrip that it was a slip of the pen and he meant 7 LPH, exactly as MapisM said above. In which case, happy days!

One small point is that if those figures are with empty tanks expect the fuel burn to be 5-10% worse when you add 12 tonnes of fluids. On a long passage with such a huge watermaker, you could dump 2.5 of the 3 tonnes of freshwater and make it again when you arrive. Indeed with only 6 beds you never need more than a thousand litres in the tank. I like my water fresh not sitting in a tank for a week! And obviously make sure the 1.5 tonnes of grey and black are empty!
 
Mapism

thanks for your thoughts and the benefit of your experience, we've gone back to the vendors requesting clarification of the figures and whether a slip of the pen was possible - depending on their answer we might take you up on your kind offer.
just under the latter figure
 
You must log in or register to reply here.

Share this page

Top