Fuel conspumtion 55' as opposed to 40'

Hi all. A question which has no doubt arisen a few times and of which there are numerous variables and answers but I shall seek opinions anyway. I currently own and liveaboard my C Kip 40 with twin ford Lehman 120hp engines but after a cramped few days with guests I've decided that when I retire in 6 years time I definitely need a bigger boat! Looking to upgrade to something like a Hershine 529, Tiger Marine 55, Hatteras 53 or 58 yachtfish or indeed any similar vessel that may catch my eye. I generally cruise at around 7 knots in the c kip burning somewhere around 20 litres ( 6 us gallons ) per hour. All the abovel boats have quite a few more horses and are capable of travelling quite a bit faster but my question is if I keep the speed down to similar 7-8 knots, obviously I'll use more fuel, but how much more do you think? When I retire I'm off to cruise the Mediterranean and, whilst I'm in no hurry and can sit and wait for the next pension cheque to buy some more fuel, I don't really want to burn huge amounts more for a similar speed. I know that if I am throttle happy then it'll cost but speed for speed will a 55 burn much more than a 40? Let the debate begin! Thanks Iain.

We are aboard a 52' yachtfisher with twin Cat 3208TA (375 per side). At 7-8 knots, we burn about 6 gal per hour. At 22 knots- I simply throw the money over the side!

Strange things can happen. We cruised our 32' Ennos Sapphire (semi displacement) at 7.5 knots and burned 10 litres of diesel per hour.
We now cruise our 42' Nordic Tug (semi displacement) at 8.0 knots and burn 8-9 litres per hour. Strange things can happen.

A whole lot is different. Generally the beam/length ratio is lower for the longer boat, and if it's newer the displacement/length is probably lower as well. The operating speed/length ratio is going down significantly as you increase waterline length by close to 20'. The increase in fuel use will mostly depend on how huge the weight (displacement) increase will be.

Note what Conrad posted, fuel use can go down for equal or greater speed in the longer boat......

Simply try to discover the actual weight (displacement ) of the boats.

The tons carved in the main beam is volume not weight , dont be fooled.

Divide the displacement by 2240 and multiply by 2hp (slow cruise ) or 3hp (inshore , fuel available)

The number you get will be the approximate HP to cruise the boat .

Divide the HP number by 16 (std engine) or 20 (turbo after cooled electronic injection) and you will have the approximate GPH the boat should use.

2x the displacement will use 2x the fuel but if the LWL is longer at least the speed will be higher

That's handy.

For a 44,000-lb. boat, I get 2.45 GPH at the slower (2 hp) speed. Is that per engine or total of both on a twin?

Thnx

FF said:

Simply try to discover the actual weight (displacement ) of the boats.

The tons carved in the main beam is volume not weight , dont be fooled.

Divide the displacement by 2240 and multiply by 2hp (slow cruise ) or 3hp (inshore , fuel available)

The number you get will be the approximate HP to cruise the boat .

Divide the HP number by 16 (std engine) or 20 (turbo after cooled electronic injection) and you will have the approximate GPH the boat should use.

2x the displacement will use 2x the fuel but if the LWL is longer at least the speed will be higher

Click to expand...

That's pretty slick Fred. Using your calculations my NT should use 8.7 litres/hr, which is exactly what my last measured fuel use was.

For a 44,000-lb. boat, I get 2.45 GPH at the slower (2 hp) speed. Is that per engine or total of both on a twin?

That is the total HP required to push the hull.

Two engines doing the work if one usually carry about a 15% fuel penalty.

The engines internal friction /resistance has to be paid for.

The twins are usually more lightly loaded , so less efficient .

A center line prop can have a larger diameter , spining slower , is more efficient.

If it will help we have a Jefferson 52 Marquessa which is very similar to a Hatteras 53 but in our opinion a more roomy interior. We have Detroit 6V92s rated at 550 HP each. At the 7 to 8 knot range we burn around 7 to 8 gallons an hour total. At 10 knots we burn about 12 gallons an hour. Above that it really goes up very fast. We were very pleasantly surprised when we went from our 45 fast trawler to the Jefferson. I had the same concern you have but we actually do as good or better at the speeds we like to go. We also have the benefit of the speed if we have a need but seldom do.