Logic behind fuel efficiency?

[jsc7]

I don't really understand the interest in fuel efficiency for larger ships (above 50 feet).

For example, I was learning about the FPB long-range cruisers and fuel efficiency is a major design objective, and to achieve this serious speed compromises are made. The cruising speed of the FPB 64 is about 9 knots. Compare to a much smaller 40' semi-displacement vessel that can easily cruise at 15 knots or more.

I don't really get this. For someone spending up to $3 million on a ship, why would they care about a few thousand in fuel costs? Heck, I would expect they spend more on food than on fuel. It's a trivial cost in the grand scheme of things. What is the rationale?

[Cotemar]

For cruisers its not really about the speed or even the fuel cost.

Its all about the distance you can go without stopping for fuel.

[MV Wanderlust]

Originally Posted by Cotemar

Its all about the distance you can go without stopping for fuel.

That.

[UNCIVILIZED]

The above mentioned bits are definitely a factor. But you also have to consider that on many "small" boats, once you start getting into the double digits speed wise, the noise level & motion can get to be a biti much for the crew. Especially since you don't have the steadying effect of sails on the boat's ride. So going too fast in some conditions can be fairly wearing on the crew. And the other bit is that in boats that size, the amount of power necessary to break the 10-12kt barrier is exponentially greater than going from say 6kts to 9. As you're having to overcome the boat's natural hull speed (peak drag level).

[30West]

My 300-gallon tanks can carry my 40' boat 187 miles at 25 knots, or 600 miles at 8 knots. Most planing hull cruisers actually get pretty good mileage when cruising below hull speed, not burning all that much more than displacement hulls. I've been looking for fuel burn numbers on a variety of hull shapes at 40', they all seem very similar at 8 knots. Diesels burn slightly less than modern gas engines, but not dramatically, the big savings seems to be at higher throttle settings. Single engines save significantly over twin. I'm not finding many true displacement hulls with twin engines in this size to compare numbers, the few I've found seem to burn about as much fuel as planing hulls at low speeds.

I'm still doing my reading on this, but it seems there isn't much difference in efficiency among big boats at low speeds, more marketing than anything. At high speeds there are huge differences. So big boats that can go fast can slow down and enjoy excellent range. Big boats with planing hulls might not be as comfortable in big waves, but they can better avoid those big waves by going fast.

[Stu Jackson]

Consider the speed of major containerships and cruise liners. They would or should WANT to go faster to speed up commerce, one would think. But the reality of physics, the interaction of the water and the hull, have determined over the years the most appropriate speed to balance fuel economy and vessel motion.

[a64pilot]

Range,
Dock neighbor, the kind that has to act superior, you know the type.
Has a 50ish old Hatteras is talking to me and finds out we are leaving this Summer to cruise, with hopefully the South Pacific as the destination.
His comment, of course I couldn't go as economically as you. I smiled back and told him he wouldn't make it, he didn't have the range, it takes something like a large Nordhaven to have the range, and they are out of my tax bracket.

Ran into a couple in a very nice 65' Viking convertible, very nice boat, well into the millions I'm sure, carried 2,000 gl. fuel
Based on his stated fuel consumption and cruising speed it had a 500 mile range, then had to belly up for another $5,000 in Diesel or so.
I did the math on range, I didn't mention it, they were nice, down to earth people.

[barnakiel]

You can go furthest while going slow - because at slow speed you only have friction while at more speed you have a mix of friction and turbulence. Turbulence is % more drag.

However, most people will accept a lower limit as they do not care to go 'too slow'.

Look here:
http://www.hallberg-rassy.com/filead...0propeller.pdf

to see how going slowest makes miles. Just no-one (except maybe in a dire emergency) cares to go at 2 knots!

b.

[jsc7]

That makes sense. A semi-displacement cruiser will max out in the hundreds of miles, not enough to get across an ocean, so it is fine if you want to go up down a coast, but when you want to go to Iceland, then the sailor has to plan on throttling down to hull speed. Of course, if the boat is long enough, that speed can creep up. For example, the FPB 115 in construction is estimated to have a cruising speed of 14 knots.

I have heard that there is a radically new design with knife blade prows that cut through waves and allow the possibility of breaking the hull speed barrier. Of course, if you are knifing through 30 foot swells, I hope your bridge is watertight, because you are going to be a submarine at that point.

[30West]

Originally Posted by a64pilot

Ran into a couple in a very nice 65' Viking convertible, very nice boat, well into the millions I'm sure, carried 2,000 gl. fuel
Based on his stated fuel consumption and cruising speed it had a 500 mile range, then had to belly up for another $5,000 in Diesel or so.
I did the math on range, I didn't mention it, they were nice, down to earth people.

I just looked up an older Viking 70 test just to see, they don't have numbers for trawler speeds. At 13.5 knots it gets .5mpg, I would bet it gets over 1mpg at 8 knots, with 2,700 gallons of fuel. Another older Viking 61 test has it getting close to 1mpg at no-wake speed, that is with turbo V-12s. If you are willing to slow down to trawler speeds, most boats are capable of having good range and efficiency.

[MarkJ]

PBH 110 has 14 or speed in a displacement hill because that's what it's hull speed is.

That's why a 40 footer at displacement speeds will go 8 knots.


Other point you don't realise is a full displacement boat can be HUGE compared to a same size semi-displacement boat.

So a 40 foot displacement boat maybe 20 tons of huge living area, 3 decks, but a 40 ft semi or planing boat may be 5 or 10 tons and small inside.

[a64pilot]

Originally Posted by 30West

I just looked up an older Viking 70 test just to see, they don't have numbers for trawler speeds. At 13.5 knots it gets .5mpg, I would bet it gets over 1mpg at 8 knots, with 2,700 gallons of fuel. Another older Viking 61 test has it getting close to 1mpg at no-wake speed, that is with turbo V-12s. If you are willing to slow down to trawler speeds, most boats are capable of having good range and efficiency.

They told me they cruise at I think it was 30 kts (could have been faster though), I remember distinctly though that their fuel burn was 120 GPH.
I've run smaller Sport Fishermen, I hated being off plane, first it was so Sloooow and secondly you would roll your guts out, have to to troll of course, but your fishing then.
Boat was just much more comfortable just as it cracked over on plane, and you were making ground.

I have run the numbers twice on two different Soprtfishermen, max range is idle, the faster you go, the less the range, I had hoped there would be a mileage increase just as she got on plane, but it wasn't so.
Any significant sea state and a Soprtfisherman is a miserable boat to be on going slow, and they don't steer worth a crap either.
I assume that is true for most any boat designed to be on plane? Sort of like being stuck in a traffic jam in a sports car?

[Valmika]

The Nordlund 72 I was Captain on would burn about 8 GPH at 6.5 Kts but would do 20 kts in a pinch. Twin Cat 2 strokes. It wasn't too bad in a a bit of a swell as it had Naid stabilizers. Fuel capacity was 1600 Gallons.
It's all about the range.

[TrentePieds]

Here is a nice little 50-footer for you to learn about fast powerboats. Sisterships come up for sale now and then in Scowegia, as these boats are becoming obsolete.

https://www.youtube.com/watch?v=YsFvxLTDdaQ

About 20 tons displacement, 1,200 HP, 40 knots, fuel tankage about 350 gallons, range about 200 NM.

The physics of high speed power boats are such that all high-speed designs and their performance at high speed tends to be much of a muchness. You may deduce from that, that fuel burn for any 20 ton fifty-footer at 40 knots tends to be 2 gallons per nautical mile.

Your comment about not worrying about diesel being $6/gall (which it is here) if you've spent $3M on the boat is well taken. Why would you worry about it costing $2.5K to run 200NM? Which in flat water you could do every five hours. But the replies you've had,i.e. that it's all about range, are really much more apposite. If you would have 2K NM range out of this boat you'd obviously have to carry ten times the tankage, i.e 3,500 gallons. That would increase her loaded displacement by about 12 tons. Do you think you could still drive her up on the plane and do 40 knots?

As I said above, high-speed power boats are flat-water boats. High speed is not really attainable with safety in open water, but you'll come to appreciate that intuitively once you've been aboard a boat doing 40 knots in a one foot slop :-).

As for "knifing through 30 foot swells" - dream on! And don't believe everything you hear!

Back to Skeene's Elements of Yacht Design :-)!

TP

[Cadence]

Originally Posted by TrentePieds

Here is a nice little 50-footer for you to learn about fast powerboats. Sisterships come up for sale now and then in Scowegia, as these boats are becoming obsolete.

About 20 tons displacement, 1,200 HP, 40 knots, fuel tankage about 350 gallons, range about 200 NM.

The physics of high speed power boats are such that all high-speed designs and their performance at high speed tends to be much of a muchness. You may deduce from that, that fuel burn for any 20 ton fifty-footer at 40 knots tends to be 2 gallons per nautical mile.

Your comment about not worrying about diesel being $6/gall (which it is here) if you've spent $3M on the boat is well taken. Why would you worry about it costing $2.5K to run 200NM? Which in flat water you could do every five hours. But the replies you've had,i.e. that it's all about range, are really much more apposite. If you would have 2K NM range out of this boat you'd obviously have to carry ten times the tankage, i.e 3,500 gallons. That would increase her loaded displacement by about 12 tons. Do you think you could still drive her up on the plane and do 40 knots?

As I said above, high-speed power boats are flat-water boats. High speed is not really attainable with safety in open water, but you'll come to appreciate that intuitively once you've been aboard a boat doing 40 knots in a one foot slop :-).

As for "knifing through 30 foot swells" - dream on! And don't believe everything you hear!

Back to Skeene's Elements of Yacht Design :-)!

TP

I hadn't heard scowegian in years. My father would use it.