Impossible boat fuel mileage, 50 footer goes 29 knots at 5.5 liters per nautical mile??

[sdowney717]

Fantastic Boat Trip to Skellig Michael with John Brennan - YouTube

at 7:14 into video, he shows the screen with speed and fuel consumption. Has twin diesels.
at 5 minutes in, he says it is a semi displacement hull?
How can this large boat go almost 30 knots per hour in the ocean using such small amount of fuel?
5.5 liters is 1.4 gallons per nautical mile and the boat is going almost 30 nautical miles per hour.

That is better mileage than many large boats get.
What am I missing here.
There are couple videos on this boat, one shows the layout and engine room.

 

[Mr Efficiency]

5+ litres per nautical mile I would say isn't that wonderful.

 

[philSweet]

It suggests around 450 hp at the shaft. Maybe a bit more if dead on the engine sweet spot. At about 45 pounds per hp at 30 knots, that's a 20,000 pound boat. A Sea Ray 500 weighs double that. It burns 47 gal/hour per engine at 28.6 knots. Maybe that's one engine? Or the 50'er isn't really a 50'er footer, and the tide is favorable.

 

[sdowney717]

it is a 15 meter boat made by Redbay
More detailed info and walk through on the boat at the dock.
Meet The Owner : John Brennan - YouTube

At 20 minutes in is the engine room. I think he says 3 Yanmar G70 engines?

 

[BlueBell]

That's ~160 litres per hour...
Certainly not miraculous

nautical miles per hour

Also known as "knots".
.

 

[philSweet]

The engines are Yanmar 8LV 370s. He's got three of them. From Yanmar's marketing dept curves, it looks like he's got them a bit over-propped and is making about 165 hp each at 3000 rpm, which coincides with the fuel burn and would push a 22,500 boat.

https://www.yanmar.com/marine/wp-content/uploads/2020/06/8LV370_Power_curve_1280x1024-768x614.jpg

https://www.yanmar.com/marine/wp-content/uploads/2020/06/8LV370_Fuel_curve_1280x1024-768x614.jpg

 

[cracked_ribs]

Seems reasonable, in the sense that I would have no problem with those numbers as long as it wasn't my visa we filled up on.

Did you maybe get the units reversed in your head and think they were claiming 5.5litres per hour?

Now that would be impressive!

Although it could be done. I don't think with Yanmars but there's a Volvo product that could get you to 30 knots on a 20m boat, burning even less.