Potential first boat- questions about fuel consumption and range

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porphyro

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I'm a total noob looking to make the unwise financial decision of buying my first boat and getting into boating, I'm hoping some more experienced boaters might be able to sanity check something for me.
The boat in question has two 165 horsepower Mercator 2.8L Diesel engines, is just under 10m LoA and has 2.75m beam, deep V hull. It has a 250L fuel tank. The current owners have used it primarily inland and weren't willing to hazard a guess as to the overall consumption/range at higher speeds, but I've been given a chat GPT guess/summary:

Speed
Fuel Use (L/hr)
Endurance (hrs)
Range (nm)
6 knots
8 L/hr
31 hrs
~186 nm
8 knots
15 L/hr
16.6 hrs
~133 nm
18 knots
33 L/hr
7.5 hrs
~135 nm
26 knots
55 L/hr
4.5 hrs

These numbers are all quite a lot higher (in the sense of higher range, higher fuel efficiency) than my calculations I've made based on what stats I've been able to crib together. Am I missing something or are these ChatGPT numbers somewhat generous?
 
I would say the those figures are optimistic , with a polished bottom you might get close but I would add atleast 25% to fuel usage at the higher speeds
 
My plan was to moor at Chatham, 20 minute drive from my wife's parents and reasonably easily accessible from where we are in London.
 
n.herring said:
I would say the those figures are optimistic , with a polished bottom you might get close but I would add atleast 25% to fuel usage at the higher speeds
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Glad to hear it's not something insane like 2x off- if I combine the fuel efficiency for this similar engine, that's 22.2l/h each so 44.4l/h total consumption at 3000rpm, and then this review of the model suggests that gets you 17.5 knots. Combine the two and that gets you a range of just about 100nm and a fuel efficiency of around 1.8 nm/gal. That's not far off your 25% estimate.
 
Sites like this one were scaring me with a claim that each engine would be over 33l/h at similar revs, which gives you a pretty dire range of around 50 nautical miles- barely enough to get to Ramsgate!
 
I am inclined to agree with other posters the fuel burn is optimistic.
But the fuel tank seems very small. Is it just one 250L tank or are there actually two 250L tanks? In which case you would be looking at around 200 miles ish.
 
One; a previous owner downsized the fuel tank from the original 430L one, possibly to add a holding tank which never actually happened. Restoring the original fuel tank size could be on the cards if I pull the trigger.
 
Real world range will leave a prudent margin for unforeseen adversities. So, for example, you might prefer to think what 70% of the theoretical range will allow.
 
PCUK said:
My tank on an 8.5 metre sports cruiser is 450 litre with a single engine, so to me 250 litres is far too little for your proposed vessel. My previous 12mt vessel carried 1250 litres.
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I had a 210 litre tank on my 7m sports cruiser with a 200 Verado on it.

Yes diesel should be more efficient, but a 250 litre tank is going to hamper your range

Why not buy a boat that hasn't been modified?

Could you give us a clue as to what sort of budget you have?

The market is very depressed currently and your hard earned cash will go a long way....
 
Certainly at max revs the ChatGPT fuel burn seems very optimistic for a twin engine 10m cruiser on the plane doing 26kn (the 26kn thing might also be a tad optimistic)! At lower revs the numbers seem to be a bit more realistic, but still slightly optimistic.

Depending on your intended usage a 250l tank is definitely small for longer range cruising on a boat that size and with those engines and to my mind would severely limit your ability to use the potential speed given the much higher consumption. For inland cruising pottering around at displacement speeds, fine, it will go for days, but not out at sea doing 15kn+! My 8m semi displacement single engined Saga 26HT has a 200l tank, and I consider that to be on the small side!
 
porphyro said:
My plan was to moor at Chatham, 20 minute drive from my wife's parents and reasonably easily accessible from where we are in London.
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Two proper hold your hand "marinas" in the locale, both with a lock to transit each time you go in or out. Both at about £4500 PA ??? for 10m.

Down the food chain.
A couple of somewhat less posh boatyards offering a marina type service, lift outs etc , along with several smaller operations offering little more than pontoon moorings.

Cheapest option of all , boat clubs , all will require commitment from prospective members to help with maintaince of club facilities and not merely joining to escape marina pricing, usually with a vetting interview involved to see if you fit in with the ethos of the club.
Applicants with smaller craft might get berth very quickly, anything over 10m and its dead mans shoes, years rather than months.
The Mudway offers lots of boating 365, ideal area to brush up your initial boating skills , upstream , an hour or two gets you above the tidal lock to fresh water and quite possibly the entire navigable upper Medway to Tonbridge, 8 locks, dependant on airdraft.
Down stream takes you to a selection of tidal creeks and backwaters to sharpen your knowledge regards the importance of tides. :)
None of this will trouble those limited fuel capabilites.
Only when you wish to venture further afield out of the river would this be of any concern, requirement to ensure any destination has fuel to ensure your return back to base, going to guess the boat is more suited to days out with the odd weekend.
 
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