Fuel Consumption

What speed, what hull form, what displacement ???? they all have an effect on fuel consumption. They can all be twin engine with stabilizers.

The faster you wish to go the more horsepower you need, The heavier a vessel is the more horsepower required to achive the same speed and the hull form also has a huge amount to do with it.

Broad spectrum there then.
An averaged specific fuel consumption of 210 grammes of fuel burnt every hour, for every kiloWatt of power required would cover it. It wont help you much, but in its way its an accurate answer

Soft chines doing 20 knots? Is that 1400 HP per engine?
If you have soft chines, I think that's technically considered a full displacement.
Semi displacement is when you have hard chines but still can not operate in the planing speed spectrum (due to lack of HP / bottom shape / keel configuration, etc...).

I got to say one thing, at 75 feet fuel consumption is going to be the least of your expense...

Here is one example

I ran a 76' Lazzara motor yacht, displacement about 100,000 pounds. She had 1150 h.p. MTU's. Cruising at 20 knots it consumed 75 gph. Hope this helps.

And just to compare my numbers with that Lazarra,

I ran my 132 ft, about 1.5 million pound steel trawler accross the Atlantic at about 9.5 kn. average, consuming almost 25 gallons an hour.

OK. It's apples & oranges but jeeh, what a difference full displacement makes huh?

Thorwald

Although I am running a boat slightly larger with a soft chine "semi displacement" hull, my contribution can give some clue in regards to the propulsion and hull design, which may effect fuel consumption considerably.

The boat is 95 feet long and running on Cat 32 acerts with 1825 hp each. She has a top speed of 26+ knots. She has Servo gear pitched propellors. Servo gear had design input into the hull for better efficiency. When we travel at 1100 rpm at max pitch we cruise at 11 knots and we burn 30 liters per engine per hour. She is equipped with stabilisers as well.

On an 85´displacement yacht we had a consumption of 60 liters per hour at 9 knots including one genset.

Engines were 2 x 420 hp Mercedes MTU 12 cyl and we had 2 x 33 Kw Northern Lights.

If I recall right, the consumption was cut in half at 7 knots.

If you go to the websites of different engine manufacturers you should be able to find a fuel consumption graph for the engines in the boats that you are looking at. The engines do not know what lind of boat they are in, they only react to RPM and load.

64' Vantare (planing hull), 41.5 tons displacment, twin DD 8V92s @ 735 hp ea.

WOT = approx. 20 kts. @ 76 gph (both engines combined)
1200 RPM = 11.3 kts. @ 17 gph (both engines combined)

Huge difference

1993 Azimut 76 - Deep vee hull - Naiads - 1000HP MTU 12V183 on fresh rebuilds - Slow cruise 13 kts. 44 gph 450 nm range 1500 gal. cap.
Fast cruise 17-18 kts, WOT 21+ kts. Clean bottom & props - full water, light fuel, 8 people
Mfg'ers specs: 45 tons displ.
Sea trialed two weeks ago

Your mileage may vary

that is correct, but different hulls can achieve different speeds at a given rpm and load and this is indeed related to the efficiency of the design.

Hi Nilo.

The size of your boat is about the same as what I am building right now (completed in Nov this year), but your boat is 3 feet longer. I was wondering what is your exact top speed and speeds and fuel consumptions at 1500, 1600, 1800 and 2000 RPM? I would very much appreciate it if you can give me some indications at these various RPM ranges. I know that the actual performance in my yacht WILL DEFINITELY be different from yours but I can get a rough idea of what kind of operating costs it will have.

THANKS A LOT!

During sea trials we have recorded 26.8 at 10% load and 26.2 at 50% load factors. I do not have readily available the figures for different RPM levels and relative consumptions, but will let you in a short while.