MPG gas vs. diesel

[BobH]

For the exact same boat, one gas and one diesel, which would get the better MPG?
Thanks,
Bob

 

[ofer]

are you really asking that?

diesel by far

 

[rslifkin]

The diesel boat will generally get significantly better mpg. Exactly how much better will depend on the engines in question for each fuel and how heavily loaded they are at a typical cruising speed.

My own 38 foot gas boat burns ~5 gal/hr at 6.5 - 7 kts, ~30 gal/hr at ~17 kts. That's with mid 1980s engine tech, so newer gas engines would do a little better. Powered to run similar speeds with modern diesels, I'd expect the fuel burn at 17 kts to be closer to 20 gal/hr and the fuel burn at 6.5 - 7 kts to be in the range of 2 - 2.5 gal/hr.

Diesels don't lose efficiency nearly as badly as gas engines do when running lightly loaded, so for a boat that's powered for planing speeds, the gas vs diesel difference gets bigger as you go slower. For my boat, at planing speeds, the diesels would use 60 - 70% of the fuel the gas engines do. At low speeds where the engines are very lightly loaded, the diesels would be using 40 - 50% of the fuel the gas engines do.

Of course, depending on your intended usage and the boats in question, the choice isn't always as simple as fuel consumption. Sometimes a boat you like is only offered one way or the other (or one option is better powered than the other). And gas boats are typically cheaper to buy as well.

 

[tiltrider1]

A gallon of diesel has more energy in it than a gallon of gasoline. How you use these fuels determines the performance.

As weight becomes more critical you will find gas becomes more efficient. Boats favor diesel, trucks favor diesel, cars split diesel for economy, gas for performance, aircraft favor gas in piston engines but turbine engines use diesel.

In a boat diesel typically has a 20% advantage.

 

[BobH]

That's what I thought but I wanted to verify with the experts. Coming from a sail boat that carried 210 gal for a 80 HP engine, fuel range was never an issue.
Thanks for some actual numbers rslifkin.
Bob

 

[kthoennes]

I have twin 454 gassers and I'm very happy with them, but this one isn't a close call -- diesel.

 

[BobH]

We're thinking of maybe doing the Atlantic ICW again and there range is really important.

 

[rslifkin]

We're thinking of maybe doing the Atlantic ICW again and there range is really important.

Having enough tankage for anywhere on the east coast isn't too hard regardless of the fuel the boat burns. With my boat, I figure planning range at slow cruise is 350-400 nm. About 160 nm at fast cruise. We carry 420 gallons of gas, range is based on burning 300 gallons with the remaining 120 gallons as reserve.

Having more range may be convenient in terms of choosing where to stop for fuel, but I can't think of anywhere on the east coast you can't go with 300nm of range. Even on the west coast there are only a few places my boat wouldn't have the range for (the Baja run being the big one).

 

[High Wire]

Diesel 40% better than fuel injected gas inboards. Carb gas 50 percent. Carb 2 stoke outboards 60%+. Nothing official just observing over 40 years.

 

[BobH]

We've done the East Coast several times but in a sailboat. Mostly in the ditch, no hurry to get anywhere, some offshore legs. Carried 110gal diesel for a 56HP Perkins. Had a watermaker so we very rarely visited a fuel dock, anchored out almost exclusively. Hence, not really up on refueling options along there.

 

[fgarriso]

There is more Btu or energy per gal of diesel than gas.

1 gallon of gasoline = 120,476 Btu. 1 gallon of diesel fuel = 137,381 Btu

 

[ranger58sb]

We've done the East Coast several times but in a sailboat. Mostly in the ditch, no hurry to get anywhere, some offshore legs. Carried 110gal diesel for a 56HP Perkins. Had a watermaker so we very rarely visited a fuel dock, anchored out almost exclusively. Hence, not really up on refueling options along there.

Fuel available approx. every 12.732 statute miles...



-Chris

 

[Takoradi]

For the exact same boat, one gas and one diesel, which would get the better MPG?
Thanks,
Bob

If you're deciding on purchasing based on gas v diesel, there's more to the decision than your energy source. David Pascoe's book "Mid Size Power Boats" has a very informative section on your issue. Diesel engines are more efficient and diesel fuel has about 12% higher energy density so you'll pay less for diesel fuel overall for the same number of miles driven. Other important considerations are longevity and maintenance. Since most boats on the secondary market have many hours on the engines, longevity should be a concern. You will pay dearly if you have to repower diesels should they completely fail. Gas engines are a fraction of the cost to replace. Diesels are harder to maintain with fewer trained mechanics available. Gas is easier to maintain, smaller in footprint and there are mechanics everywhere.

Three years ago, I was looking at two 1984 Uniflite 42's in Seattle. One was diesel, had had several owners and was in horrible shape. The other was a gasser and was being sold by the original owner. While I preferred diesel, I went with the gasser and am happy with my choice. Original owner had gone to the Uniflite factory and supervised the build and had taken care of the boat. My friends in the marina all have diesels and while I envy them their higher mileage and lower fuel costs, I do not envy them the costs they've all had getting their engines up to par and keeping them there. Also I like it that I can do a lot of work on my 454's myself, which I prefer both for cost and for understanding my boat. NONE of my friends ever do any of the work on their diesels and I daresay don't understand them terribly well.

So a lot of the decision will be based on what you're comfortable with and can afford. If you like to tinker, perhaps a gasser is a good call. If you don't mind the higher cost of engine replacement and maintenance, perhaps diesels are. There just isn't a "right" or "wrong" answer. Both engines will get you from A to B, they'll just take different routes to do so. Hope that helps you in your progress. Pascoe is a great resource and the rest is just my personal history with the decision. Good luck.

Tak

 

[Nomad Willy]

MPG gas vs. diesel
For the exact same boat, one gas and one diesel, which would get the better MPG?
Thanks,
Bob




Something I read a long time ago is that they both burn about exactly the same amount at full throttle. Anything less and the diesel gets the nod but the diesel is much better at low rpm.
It’s a heat issue. Any heat loss from an engine is lost power and lost efficiency.
You can hardly keep your coffee warm on a diesel at idle.

 

[rslifkin]

MPG gas vs. diesel
For the exact same boat, one gas and one diesel, which would get the better MPG?
Thanks,
Bob




Something I read a long time ago is that they both burn about exactly the same amount at full throttle. Anything less and the diesel gets the nod,

Gassers still burn more at full throttle as the fuel is less energy dense and the combustion cycle isn't quite as efficient. Most gas engines are at their peak efficiency at an RPM near their torque peak and under as much load as they can handle before the fuel mixture needs to go super rich to keep things from melting down (this will be at less than WOT). Above or below that, efficiency drops off. And at light loads, it drops off badly compared to the efficiency curve of a diesel.

 

[Pete Meisinger]

If someone is happy with twin 454 gassers he must really like leaving money at the gas dock.

pete

 

[plittle2005]

If someone is happy with twin 454 gassers he must really like leaving money at the gas dock.

pete

Yeah, but only part of the money he didn't leave with the seller!

 

[Nomad Willy]

rslifkin,
Yup things haven’t changed that much. And if they have it’s probably dure to turbocharging.
As to all these wonderful things happening at peak torque rpm I’ve always thought it was at least close to an old wife's tale.

 

[rslifkin]

rslifkin,
Yup things haven’t changed that much. And if they have it’s probably dure to turbocharging.
As to all these wonderful things happening at peak torque rpm I’ve always thought it was at least close to an old wife's tale.

The best efficiency point for the engine will be at some moderately high load level near peak torque rpm. But once you install the engine in a vehicle, raw engine efficiency is far from the only factor in overall efficiency. So the point of peak engine efficiency is rarely going to be the most economical speed to run at. On my boat, I'm likely close to that peak efficiency point at our 17 kt fast cruise, but we burn less than half the fuel per mile if we slow down to 6.5 kts (even though the engines aren't nearly as efficient in terms of fuel in vs HP out). But the reduced power demand at the lower speed is so dramatic that it outweighs the reduced engine efficiency.

 

[Nomad Willy]

Yes rs, hull efficiency is a biggie. But the elephant in the room will probably be weight.

But if it was’nt for the fire hazard of gasoline I’d definitely wanna has a gasoline engine.

 

[rslifkin]

Yes rs, hull efficiency is a biggie. But the elephant in the room will probably be weight.

But if it was’nt for the fire hazard of gasoline I’d definitely wanna has a gasoline engine.

The weight difference has gotten a lot smaller with modern turbodiesels. The gassers are still lighter, but the lighter duty diesels (still typically beefier than the gassers) aren't all that heavy. In some cases it's surprisingly equal. A 350hp 454 and a 370hp Yanmar 8LV are almost identical in weight, for example.

 

[Barking Sands]

One of the main reasons for diesel efficiency vs gas is compression ratio. This is also one of the reasons gas engines have become so much more efficient over the last 20 years. Engineer's were able to increase typical compression ratios of gas engines from a typical 8.5 to 1 all they way up to a now fairly common 12 to 1 or more. Fast computers controlling air fuel ratio and timing along with knock sensors and multiple fuel maps for varying conditions have brought gas engines a long way. But diesels just have too much going for them. Especially for boats.