At full speed I wonder how many gallons to the mile.

[Chiara’s slave]

I thought that the rule of thumb is one litre per hour per ten hp, not one gallon.

that works for my MD2020 which cruising at about 70% of peak revs uses approx two liters an hour.

Yours is clearly 5 times more economical than average, or else you’re looking at peak power and non peak fuel. The gallon per 10 hp works for my Dragonfly and my Ring 6.5. I think, looking again at your post, 70% of peak revs is not at WOT. It’s 9ne hell of a lot less than 70% peak power. I cruise the RIB at 60% revs, noticeably less than half throttle.

 

[LittleSister]

I thought that the rule of thumb is one litre per hour per ten hp, not one gallon.

that works for my MD2020 which cruising at about 70% of peak revs uses approx two liters an hour.

Not if you are using all those ten horsepower!

The revs are near irrelevant for this calculation in a marine diesel. The revs only count when the engine speed is limited by the load, not by the amount of fuel the pump is governed to deliver to maintain the engine speed you have selected.

Most of the time in a cruising boat the engine is delivering way below the maximum power potentially available at the engine revs selected. The engine speed control lever is not operating a throttle! It selects an engine speed, and then the governor and pump provides just enough fuel to maintain that speed.

For example, Bukh says its current DV36ME uses 8 litres per hour or 2.11 (imperial?) gallons per hour at full load (i.e. actually delivering 36hp), which would equate to 2.2 l/h per 10hp (= 0.22 l/h per hp).

Its current DV models' full load consumption is stated to be equal to -
DV24 - 2.4l/h per 10hp;
DV32 -2.3l/h per 10hp;
DV36 - 2.2l/h per 10hp;
DV48 - 2.1l/h per 10hp.

There are, no doubt, more and less efficient boat engines, but they are unlikely to vary wildly from those sort of consumption figures

Bukh gives estimated 'cruising' consumption for each of those models, varying in the range of about 55% to 60% of the full load consumption, but it will of course depend on how much load.

 

[Tranona]

The MD 2020 figures quotes are roughly correct because at 2400rpm cruising speed the power required is around 10hp for the typical boat the engine is fitted to. Kept records of my 2030 for lots of hours and averaged just over 2l an hour. In another boat with a Yanmar 1GM (nominally 9hp) averaged 0.75l/h

All this irrelevant to this lovely Sunseeker (which looks better in the plastic than it does in the photos) because moving it at even cruising displacement speed of 12 knots needs more hp per tonne than our little yachts at 5 knots.

 

[Rappey]

Years ago I had a aluminium "large tinny" and put a 200hp 2.6 litre v6 2 stroke on the back. Hell, why not ? It was fun and terrifying at the same time.
I had two 25litre demountable fuel containers. I set off from Bembridge for portsmouth harbour with a full tank and covered the 5 miles in around 5 minutes.
Checked to see how much fuel was left and it was empty ! ?
I saw a rib with two of these beasts at the fuel jetty. He was filling his rib with petrol, I arrived , refulled my sailing boat and he was still filling after I left.
A 60FT American centre consol running 6 x 450hp outboards at 23 knts would use around 55 gallon/hr .

 

[Telstar26]

One time I was chatting to a Sunseeker employee in Wareham. He told me about a customer that had just bought a new Sunseeker and phoned up after his first trip.
"It's a great boat, no problems except that the fuel gauge isn't working properly"
"What's the problem?"
"Well I filled up then took the boat from Poole to Weymouth to give her a bit of a run. When I got there, the fuel gauge was showing almost empty"
"Oh. Sounds as if the fuel gauge is fine!"

 

[LONG_KEELER]

I thought that the rule of thumb is one litre per hour per ten hp, not one gallon.

that works for my MD2020 which cruising at about 70% of peak revs uses approx two liters an hour.

I've got one as well which used to work out about right.
0.055 Galls Per Hour per HP. As mentioned, only if you are burning full HP.

 

[Chiara’s slave]

I've got one as well which used to work out about right.
0.055 Galls Per Hour per HP.

So .55 gallons per 10hp then, 2.5 litres. Obviously thats a modern diesel, maybe the sunseeker engines ate that good too. All mine are petrol outboards, plus I daresay the rule of thumb is American, yank gallons are smaller. Must be the only thing that is.

 

[LONG_KEELER]

So .55 gallons per 10hp then, 2.5 litres. Obviously thats a modern diesel, maybe the sunseeker engines ate that good too. All mine are petrol outboards, plus I daresay the rule of thumb is American, yank gallons are smaller. Must be the only thing that is.

 

[SaltIre]

I guess that Saturn V is about the same mpg. It carries around 600,000 gallons of fuel (not counting oxidizer, to be comparable). It does around 600,000 miles to the moon and back and a few times round the moon. So, it's about a mile a gallon - much the same!

A pedant points out that the Saturn V doesn't go to the moon and back. The capsule gets detached from it and does - so the Saturn V makes a much shorter journey.

 

[mainsail1]

Sunseeker's own figures for the 100 seem to work out at about 10 litres per mile at 12 knots. At that speed it has a range of about 1300 miles.

 

[burgundyben]

I reckon compared to the cost of capital tied up, depreciation, mooring and insuring, cleaning and maintenance the fuel burn is small fry.

In my 21 years of motorboating fuel has never been the biggest cost, if I get to the end of the season and I've bought a lot of fuel I've had a good year.

Never bought a sail or a winch or a halyard or standing rigging and I re kon on 100ft yacht, same size as the Sunseeker those things can cost a lot!

 

[RobWard]

2600 hp, so the rule of thumb says 260 gallons per hour, 29 knots. Not as bad as I’d have guessed, but so far beyond my pay grade it might as well be powered by a Saturn V.

I've not heard that - my Bav 30 is 18HP and uses <2l PH - quite a lot less?

 

[Stemar]

I've not heard that - my Bav 30 is 18HP and uses <2l PH - quite a lot less?

Your engine will only be giving 18HP (and burning the fuel to produce them at full throttle. At economical cruising speed, it'll be a fair bit less power, so less fuel

 

[ChromeDome]

.. yank gallons are smaller...

Not many things are smaller in yankie doodle land ?

 

[LONG_KEELER]

Your engine will only be giving 18HP (and burning the fuel to produce them at full throttle. At economical cruising speed, it'll be a fair bit less power, so less fuel

It would be helpful wouldn't it to easily calculate how much HP we are using at any given time . I think we could be in for a shock to find that our maximum HP is nowhere near the published rate for the engine. Even taking into account our compromises with prop etc . I think J. Clarkson did a feature on cars using rolling roads.

 

[burgundyben]

It would be helpful wouldn't it to easily calculate how much HP we are using at any given time . I think we could be in for a shock to find that our maximum HP is nowhere near the published rate for the engine. Even taking into account our compromises with prop etc . I think J. Clarkson did a feature on cars using rolling roads.

A modern diesel should know. Derived from the duration of each injection squirt.

A mechanical engine of course you'd need flow sensors on the feed and return.

Engine load of course a function of propeller demand (plus other lesser factors)

 

[jamie N]

Does anyone else reckon that the photos seem to show the boat in a wartime 'dazzle paint'?