50m Superyacht consumes only 3x the fuel of my 30' ?

[Anthony]

According to this YBW news story

'At 12 knots, the aluminum superyacht consumes 98 litres per hour'

Thats only 8L per Nm, my 30' sports boat consumes around 2.5-3L/Nm

OK, the Superyacht is only doing 12kn compared to the 20kn I plane at, but that still seems amazingly efficient.

Hmmm, maybe time to rip out my petrol lumps, put a big electric motor in and upgrade my generator!

Ants

 

[BruceK]

If you play with the numbers

At 12 knots, the aluminium superyacht consumes 98 litres per hour, excluding the generators, and at 10 knots, this again reduces to 57.5 litres. This is achieved with a pair of MTU 12V 2000 M61 engines, 600 kW each. The vessel has a cruising range, at 12 knots, of 3,750 miles; this is achieved with a fuel capacity of just 45,000 litres.

45000 / 3750 you get 12 litres per mile. If the numbers are correct then the generators account for 33% of the fuel burn. To my mind at least the figures don't add up.

 

[longjohnsilver]

If you play with the numbers



45000 / 3750 you get 12 litres per mile. If the numbers are correct then the generators account for 33% of the fuel burn. To my mind at least the figures don't add up.

Did the same calculation. What we haven't been told is whether they've included a fuel safety margin, usually approx 25% which would then make their figures much as reported. But that does seem extremely efficient!

 

[Capital_R]

That does seem okay.

A boat with 163' LOA might have a LWL of about 145'-155' and therefore a hull speed of just over 16 knots.

Roughly estimating the hull forms, displacement and prismatic coefficients, and accounting for shaft and gearbox losses, and belt drives odd the engine (but more likely generator burn on a super yacht) - then I guess, with two shafts and a 4 or 5 blade prop, somewhere between 950hp and 1000hp at the prop will push it at 12 knots.

That usually means a little more (5% or so) more at the engine - so between 950-1050hp.

Given that, there's a rough rule of thumb that says a diesel, almost no matter its size, burns 10l of fuel for every 100hp it outputs.

That slaps the yacht at 95-105 l/hr, or about 8l/NM.

Remember, your motor boat, with the assistance of lift and water force, is actually driving your boat right out and up on top of the water. Holding it 18-36 inches higher out of the water than is natural by gravity. The super yacht is just letting the water slide on by (hopefully) with little vertical mass movement in any direction.

And if the boat has an extra 10' of waterline length - the fuel requires to meet 12 knots of boatspeed would drop by almost 15% to 800hp at the shafts, give or take.

Reduce that speed to 10 knots...and a boat with a 50m waterline is pulling down less than 500hp at the prop(s) (combined) and getting almost half the fuel efficiency as you do.

At 9 knots, it's less than 4l/mile.



Happy motoring!

R.

 

[Capital_R]

If you play with the numbers



45000 / 3750 you get 12 litres per mile. If the numbers are correct then the generators account for 33% of the fuel burn. To my mind at least the figures don't add up.

I just saw the 10knot speed quote...

This let's us narrow in...

It seems to be describing a vessel with a 151' LWL, beam of 28', depth of probably less than 8' - and a displacement/list ship weight of about 500 Tonnes?

Jist by the math?

 

[Chris_d]

Given that, there's a rough rule of thumb that says a diesel, almost no matter its size, burns 10l of fuel for every 100hp it outputs.

That slaps the yacht at 95-105 l/hr, or about 8l/NM.

R.

I always thought it was about double that, approx 5gph or 22lph

 

[Divemaster1]

I've always used between 195 - 200 gram per HP hour... so I use 200 gram ... or 0.2 Kg ... therefore 100 HP = 20 KG

Marine diesel have a weight of approx 0.86 KG per litre (actual figures are between 0.848 and 0.874 pending on grade), and therefore you have 20 KG / 0.86 = 23.25 L per hour when you draw out 100 HP of a diesel engine...

If you use the lower estimate of 195 gram per hp hour, your figure is approx 22 L per hour ...

Remove my rough estimates (safety factor) and user factory spec., you will get lower ... but not much lower figures .... http://www.barringtondieselclub.co.za/technical/fuel-consumption.html