Major Fuel Economy Diff: Gas vs. Diesel

[spartonboat1]

I marina mate recently concluded a 480 mile Great Lakes trip. He is running a 38' Carver (I think) with big blocks.

We were discussing the advantages of Diesel vs. Gas, e.g. much high torque in a Diesel, plus better fuel economy.

So I was surprised how much difference. If I run full cruise of 13.0-13.2 kts, I will see 0.7mpg. Higher if I run slower. This is a '72 43'DCFB Hatt with 6-71N's at 310hp. I can't reach the max 2,500rpm, only 2,440 or so. So my cruise is 2,150-2,200., i.e. 200 off the pins, which I understand is recommended max. 1,400rpm to run closer to hull speed of 8.9kts.

He indicated that he runs at 7-8mph, to save fuel. Why not hull speed, I am not sure, but maybe I did not understand. But his Admiral wanted to get back, so they did a push at full cruise. He saw 0.4mpg, which he said is normal. He also agreed that Gas engines "grunt" more when running in a sea of say 2-4'. I can run in 2-4 full cruise, without issue (I know- is slow).

I later overheard him say that he knows a big Hatt with 12v71's that runs slow and still gets better mpg than he does. So there is more and greater differences than I expected: Diesel vs. Gasser

 

[Photolomy]

Hull speed isn't actually the highest mpg. It is just the highest speed before your mpg goes down really fast. Trawlers and LRCs run slower than hull speed, but they are powered that way. Either a single engine or two small engines. We can't effectively idle around because it wouldn't be good for our engines, but at 6 knots my mpg goes up quite a bit.

I read that diesel is about 30% more power than gasoline. Generally it costs more, though in the marine world non-ethanol gas generally costs more.

I originally thought that I would want gas because it is easier to service (they are common engines), but I soon realized how many advantages diesel has over gas on a boat. Especially with regards to safety. Not only is diesel much less prone to fire/explosion than gasoline, they produce very little CO.

 

[spartonboat1]

Hull speed isn't actually the highest mpg. It is just the highest speed before your mpg goes down really fast. Trawlers and LRCs run slower than hull speed, but they are powered that way. Either a single engine or two small engines. We can't effectively idle around because it wouldn't be good for our engines, but at 6 knots my mpg goes up quite a bit.

I read that diesel is about 30% more power than gasoline. Generally it costs more, though in the marine world non-ethanol gas generally costs more.

I originally thought that I would want gas because it is easier to service (they are common engines), but I soon realized how many advantages diesel has over gas on a boat. Especially with regards to safety. Not only is diesel much less prone to fire/explosion than gasoline, they produce very little CO.

True on the Hull Speed, vs idling or nearly idling. There have been posts here about the dangers of "wet stacking", when Diesels are run for long periods at idle.

Also very true on the fire danger! A lit match will go out if thrown into a puddle of diesel fuel, or so it is reported. I do not intend to test that concept!

However, I had a near-miss, when a newly installed fuel line was leaking. After a round trip of only 15 or so miles, I found a gallon or so of fuel, under one of the engines! The Yard was a Hatt yard, but I had a different tech, who I trusted, resolve it.

Finally the Bilge Blower has an opposite use on Diesel vs Gas boats, as I understand it. You definitely run the Bilge Blower for 5 minutes, before starting the engines on a Gas boat. This should not be necessary on a Diesel boat. However, I run my blowers after a run, to pull in cooling air to the engine room to help cool the engines.

 

[madhatter1]

My 34 with Merc 454's got .7 SMPG at 19 SMPH.

With 270 HP Cummins it gets 1.1-1.2 SMPG at 25-27 SMPH

The 46 has a bigger drinking problem than I do!

 

[finatic]

I marina mate recently concluded a 480 mile Great Lakes trip. He is running a 38' Carver (I think) with big blocks.

We were discussing the advantages of Diesel vs. Gas, e.g. much high torque in a Diesel, plus better fuel economy.

So I was surprised how much difference. If I run full cruise of 13.0-13.2 kts, I will see 0.7mpg. Higher if I run slower. This is a '72 43'DCFB Hatt with 6-71N's at 310hp. I can't reach the max 2,500rpm, only 2,440 or so. So my cruise is 2,150-2,200., i.e. 200 off the pins, which I understand is recommended max. 1,400rpm to run closer to hull speed of 8.9kts.

He indicated that he runs at 7-8mph, to save fuel. Why not hull speed, I am not sure, but maybe I did not understand. But his Admiral wanted to get back, so they did a push at full cruise. He saw 0.4mpg, which he said is normal. He also agreed that Gas engines "grunt" more when running in a sea of say 2-4'. I can run in 2-4 full cruise, without issue (I know- is slow).



I later overheard him say that he knows a big Hatt with 12v71's that runs slow and still gets better mpg than he does. So there is more and greater differences than I expected: Diesel vs. Gasser

How did you come up with a hull speed of 8.9kts?

 

[jim rosenthal]

I have had the same boat for nearly thirty years. (36 Hatteras Series 1 convertible) When I bought her, she had gas engines- 454 Chevrolet Mercruisers. First repower was with Cat 3116s, which were not at all satisfactory. But the performance and mileage went up dramatically. Second repower was with Cummins B 370s, what I should have bought to begin with. Performance and economy are amazing. They run clean, they are much quieter than the Cats were, and the transom is free of any soot. I would estimate the boat uses 55-60% of the fuel at cruise that it did with gas motors.

 

[Hat43c1981]

As a bit of comparison. I had a 34’ Silverton Flybridge with twin 454’s that got right around .6-.7 MPG. at a cruising speed of 20 knots and weighed about 16,000 pounds loaded. Our 43’ Hatteras convertible with twin Detroit’s 6V92’s twin turboed gets the same fuel economy at practically the same cruising speed as the Silverton. The weight difference between the two boats is about 32,000 pounds. I was amazed at these numbers when I calculated them. Our Hatt weighs 3x’s what the Silverton did and gets the same fuel economy.

 

[Freeebird]

Sorry I'm late as the token Hatt gasser as I just now noticed this thread.

I've said it before, and I'll say it again. If everything is as it should be, gas power is not much (if any) less efficient on certain boats up to around the 40' mark. I've posted my numbers before, but I'll post them again for the naysayers.

My 41DC cruises 20MPH @ 3,000 and gets .8 MPG with full fuel and full water. Back it down to hull speed, and it gets 1.15. WOT comes in at around 27MPH @ 4,400. Find me a diesel version of a 41 or 43 that yields those numbers in either speed or economy on plane. Hull speed fuel numbers won't be that much different when equipped with the DD's that came in them originally. Wanna talk about rebuild costs when things go south?

My previously largest gasser was a 34' Sea Ray Sedan Bridge with the same 7.4 MerCruiser inboards that are in my Hatt. That boat liked 3,500 and cruised about 30MPH while getting about that same .8MPG the Hatt does. I never had the inclination to see what hull speed would yield, but running at 3,000 required a bit of tab where 3,500 didn't.

All that being said, my next boat will be diesel, and it will have a cockpit.

 

[krush]

My 41c does much better than your numbers. 1nmpg at 18knots. I don't know what that is in funny units LOL.

At hull speed, I'm probably around 3-4 nmpg. Diesels are much more efficient at lower loads than gas engines. It's all about combustion and thermodynamics.

 

[whoover]

Oh my, pull up a chair and break out the popcorn. Krush, did you post that on purpose?

Walt Hoover

 

[RyanB]

Freebird

How many hours are on your 7.4's, that are getting these fuel numbers? We're looking at pulling the trigger on a 36 Sedan '86, with 350hp Crusaders. The crusaders have around 1700 hours on them (probably my biggest concern with boat). I've read/been told everything from they're on borrowed time, to they're half used up, to you can expect 4000+ hours out of them. I don't truly buy the later 4000 hour point, but curious as to what others get with gassers in their heavy boats.

I'd be thrilled to get .8mpg at a cruise of 20knts. I've read the 36 C's & Sedans are more like .5-.6 mpg. I'm not planning on cruising long distances yet, more trolling and short trips, but would be nice to know the range.

 

[Photolomy]

This is why we choose diesel.

https://www.dailymail.co.uk/video/news/video-2238720/Video-Woman-thrown-sea-explosion-motorboat-Italy.html?ito=email_share_article-video-channel

 

[krush]

Oh my, pull up a chair and break out the popcorn. Krush, did you post that on purpose?

Facts are facts. Numbers and thermo laws are what the are. However, gas engines do make sense for the majority of pleasure boaters. The cheaper parts and maintenance costs greatly outweigh the increase in fuel consumption for people that don't put many hours on their engines.