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Old 12-15-2012, 08:24 PM   #1
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Gas v/s diesel for trawlers

Here's Art's comment about gas engines in a trawler.

"Eric Ė I agree with the need to be VERY, Very careful for gas fumes with a gasser (any competent Captain with well ventilated and equipped boat can readily facilitate these cautions)... and ... I agree that gas engines are VERY, Very quiet compared to diesels. What I find troublesome is that most diesel owners erroneously believe gasoline engines use 2 to 3 time the fuel as diesel at hull speed or below hull speed rates... that just ainít so... for planning, semi displacement, and full displacement hulls on boats in the size range up to about 45í. Now, if you get into faster speeds on nearly any boat design or for boats bigger than 45í the torque from large diesel motors is needed for notable fuel consumption reduction compared to gasser. I believe that if you implanted a small, efficiently designed gas engine in your size boat that although you might experience a smidge more fuel usage than equivalent size diesel the simple, inexpensive maintenance and base cost of the gas engine would off-set fuel costs. And the gas motorís quietness is priceless - worth a fortune!

I agree Art and you may be more right than you know about fuel efficiency gas v/s diesel. Actually I think the two engines are just about exactly the same at full load. It has to do w the 15-1 fuel ratio. At full bore it's just about 15-1 for both engines but the gas engine needs that same ratio all the way down to an idle. But as you reduce load on a diesel the mixture gets leaner and leaner until at idle it's 60-1. You can fry an egg on the exhaust manifold of a gas engine idling but you can practically sit on a diesel.

So if a trawlerman was to run his gasoline trawler engine at the light loads that are typical of the way most run their diesels very poor fuel efficiency would result even though the engine would be much happier doing that than the diesel. But if he could and did rum the gasser at 75 to 80% load the gasser may be close enough to a diesel to not need to consider the difference in fuel consumption.

Older gas engines wouldn't tolerate the continuos heavy load but perhaps new engines could and would.

And of course the safety issue hasn't changed ..... or has it???
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Old 12-15-2012, 09:12 PM   #2
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Safety, based on awful things I have seen, remains a deal breaker issue for me,though fuel injection systems and associated advances must have improved gas engine safety.
Even with fuel injection, which used fine (perishing) rubber hose for the final connection to the injector, a good reason for Jaguar cross flow heads was fuel could not drip on the exhaust manifold. Remember the proud boast of the Jag owner,"drove Sydney to Melbourne and back, and it only caught fire once".
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Old 12-15-2012, 09:51 PM   #3
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Here are two real examples of a gas vs a diesel:

My 8,000 lb Cape Dory 28 with a 275 HP gasser burns 11 gph to go 12 kts.

My 15,000 lb Mainship Pilot 34 with a 370 HP Yanmar diesel burns 12 gph to go 14 kts.

So, about the same Nm/gal, but the Mainship is almost twice as heavy.

Slow both down to 7 kts and the CD will burn 4 gph (low power gassers are atrociously inefficient) and the Mainship will burn at most 3 gph.

The only way to come close with the gasser would be to use a 50 HP outboard to go 7 kts. That would load the gasser decently and you might get below 3 gph.

Modern gassers make 12-13 HP per gallon at wot, 4 stroke FI outboards get 10 and modern diesels do 18-20.

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Old 12-15-2012, 10:16 PM   #4
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Well, doesn't diesel contain more energy than gasoline for a given volume?
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Old 12-15-2012, 10:36 PM   #5
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march,
Well I'm see'in the OB as having such a poor showing because of the seawater cold cooling system. I thought many times in the past of converting an OB to FWC and getting the temp up to a reasonable level.
But again if it's to be done successfully I think the gas engine will need to be run quite to very hard.
But Art's right it would be wonderful to have very quiet and nice and smooth engines for our boats.
Something else to consider is that the old Lehman's and Perkins engines we (mostly) have probably only make 15hp per gal hr so the gas engine would have a much easier time of matching that or coming reasonably close.
However I don't see anybody doing that as most here are reluctant to run an engine hard not to mention the mistaken idea that they need mucho extra power to outrun storms.
With the combination of all things to be considered taken into account I think this is just an interesting discussion of how feasible the concept could be.
And speaking for myself I have no intention of selling this idea but just to pool information and try to look at it objectively.

Mark says "doesn't diesel contain more energy than gasoline for a given volume?" ... As far as I know it's close to the same but w gas engines much much more of the heat (energy) goes out the cooling system unless they are run very hard as in 75 to 85% of max load.
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Old 12-15-2012, 11:47 PM   #6
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Mark says:
"Well, doesn't diesel contain more energy than gasoline for a given volume"

Yes, per gallon about 147,000 BTU for diesel and 125,000 BTU for gas.
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Old 12-16-2012, 03:01 AM   #7
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Gas vs Diesel:

Diesel every time for me. I once pulled a bloke and his wife out of the water after a petrol(gas) refill accident, all it took was one spark in the engine bay.

My boat is 30 + years old, all sorts of added wiring & electrical equipment in the engine room. There is a bit of rust on top of the tanks looks OK though, the fuel lines are definitely not new, but again look OK. The eutectic compressor kicks in and out 24/7(on shore power), the water heater looks a bit dodgy etc. Gas(petrol) tanks in that environment, with the family on board, not for me.
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Old 12-16-2012, 10:09 AM   #8
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Diesel engines meter fuel directly into the combustion chamber at a fixed rate that is optimized for combustion at the peak of efficiency. It maintains this ratio at all times.

Gasoline engines (most) rely on atomization and metering of the fuel far from the combustion chamber and the ratio is constantly changing trying to correct to the varying intake air pressure.

Gasoline engines are simpler to manufacture, easier to maintain, but across the board less efficient to an equivalently sophisticated diesel engine.

Diesels are inherently safer as well.
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Old 12-16-2012, 10:49 AM   #9
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Some Sailor said:

"Gasoline engines (most) rely on atomization and metering of the fuel far from the combustion chamber and the ratio is constantly changing trying to correct to the varying intake air pressure."

Times are changing. Aside from the energy differential favoring a given volume of diesel fuel, gas engines can indeed and are beginning to approach diesels in their fuel efficiency. Gas engines are now adopting fuel metering designs to allow them to meter fuel right at the combustion chamber. Buzz words, identical to diesel, are:
  • Piezo Injectors
  • High Pressure Injection
  • Common Rail
BMW seems to be leading this new technology but others are quickly following to get ready for the new CAFE standards. Expect this diesel mimicing stuff to add significant cost to new cars.
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Old 12-16-2012, 11:06 AM   #10
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Hence I said "... to an equivalently sophisticated diesel engine"

Diesel has a higher energy density than gas by volume. Gasoline is 34.6 MJ/L and diesel is 38.7 MJ/L (Megajoules per Liter).

All things being equal from a technology standpoint, diesel wins in efficiency as a fuel.
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Old 12-16-2012, 11:30 AM   #11
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Somesailor said:

"Diesel has a higher energy density than gas by volume. Gasoline is 34.6 MJ/L and diesel is 38.7 MJ/L (Megajoules per Liter)."

Yes, I said the same in post # 6. And over the road diesel costs about 10 to 20% more at the pump in the US largely wiping out the energy differential.

I arr a singtis two, I no sumpin bout megajewels vs BTU!
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Old 12-16-2012, 11:44 AM   #12
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Efficiency by volume also helps with range.

I buy my trawler fuel on the water. Our on-the-water fuel prices are only about 5% apart here in the NW. ($3.91 for gasoline and $4.11 for diesel)

The 11% difference in fuel energy still applies. (all things being equal)
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Old 12-16-2012, 11:47 AM   #13
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Andy,
We know all the old stuff about gasoline and safety. The only question about gas and safety is ... is there anything new.

SomeSailor wrote
"Diesel engines meter fuel directly into the combustion chamber at a fixed rate that is optimized for combustion at the peak of efficiency. It maintains this ratio at all times."

Not at a fixed reta at all. Gasoline is at a fixed rate ... (15-1) and diesel varies from 60-1 to 15-1 unless I've got this wrong as I'm going from memory and long past studies. The ratio of fuel to air does remain constant in a gas engine though ... I'm sure of that. Thanks for not beating the safety issue.

Sunchaser,
They've been working on direct injection for gasoline engines for a long time I though. Didn't the 300 SL Gullwing Mercedes have direct injection?

There also is (I believe) the issue of how long the pressure from combustion lasts and how close to the ideal position of the crankshaft it can be programed? In other words can the crankshaft be positioned in the ideal place when the pressure is maximized and is the duration too long or short to be of best advantage? I think diesels have a shorter pressure period (combustion length) and wonder if that leaves much of the time the lineup of crankshaft and other parts are in a seriously advantagous position to make power somewhat wasted.

But the real efficiency revolves around the ability for each engine to make power without wasting heat. All heat that does not make power is waste. Like the pistons moving back for another stroke and the friction of all the moving parts. All the heat in the exhaust and cooling system discharges is totally a waste. This is the reason diesels are so much better (more efficient) at lower loads. They generate less heat to loose.

I did not intend to post a thread about what engine (gas or diesel) individuals would put in their own boat or recommend to a friend. But what engine would be better in all asoects other than safety that is a given.
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Old 12-16-2012, 11:53 AM   #14
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Quote:
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The ratio of fuel to air does remain constant in a gas engine though ... I'm sure of that. Thanks for not beating the safety issue.
You're correct. I was backwards. The exception would be direct injected gasoline motors, but my old Lehmans aren't quite up for that.
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Old 12-16-2012, 02:09 PM   #15
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I have been following tests reported on boattest.com for years. They test everything from bowrider outboard models to 80' performance cruisers with thousands of horsepower. They measure fuel consumption vs rpm with on board instrumentation and report that data vs rpm and kts in a table associated with the boat test report.

All modern diesels ranging from the Volvo D3, Yanmr BYs, Cummins QSB/C, Volvo D9, Cat Acerta, etc all make about 20 hp per gallon per hour of fuel burned at maximum hp at WOT. This isn't too surprising as it is exactly what the engine manufucturers publish in their specifications.

Gasoline inboard and outboard manufacturers don't publish this data (wonder why) but it is available at wot from the boattest data.

A modern Volvo, Mercruiser or Crusader V6 or V8 engine with electronic fuel injection makes 12-14 hp per gph at wot. These are the same base engines that are in GM cars and trucks.

Within a few years I suspect that the GM equivalent of Ford's Eco Boost engine (dual turbochargers on a small V6) will hit the marine market and improve that figure by 10% just like Ford has done with its over the road Eco Boost engines. This improvement comes from making more horsepower from a smaller displacement engine, thus spreading the parasitic losses over a bigger number. Whether those engines will last is another issue.

But don't expect that common rail, direct combustion chamber injection will come to gasoline engines. Gasoline burns much too quickly. If you want to do that, burn diesel. That is what it is made for.

And lightly loaded gasoline engines are horrible fuel wasters. The numbers get down to about 5 hp per gph at 25% load whereas a modern diesel will drop to only 15 hp per gph at that loading rate.

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Old 12-16-2012, 06:53 PM   #16
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Quote:
Originally Posted by Andy G View Post
Gas vs Diesel:

Diesel every time for me. I once pulled a bloke and his wife out of the water after a petrol(gas) refill accident...The eutectic compressor kicks in and out 24/7(on shore power).
With you on diesel, I was involved in litigation over Avis (Aust)`s corporate boat "SIVA" exploding in Quakers Hat Bay. Not easily forgotten.
Sorry your eutectic is still troublesome. It should not be on 24/7. After a 3 hour initial pulldown mine needs an hour or so am & pm to maintain. Refrigeration theory is complex (to me), but excessive run time can cause problems (sorry for the generality,it`s somewhere in Calder`s book but hiding at present, something about causing vacuums).
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Old 12-16-2012, 08:40 PM   #17
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D Marchand said:
"But don't expect that common rail, direct combustion chamber injection will come to gasoline engines. Gasoline burns much too quickly. If you want to do that, burn diesel. That is what it is made for."

Bosch is now on their third generation of Gasoline Direct Injection via common rail. For the past 7 years GDI has been in automotive use. The big fight now is piezoelectric vs solenoids for injecting gasoline into the cylinders. Tens of billions are at stake with Delphi betting on solenoids for their NA uses. Small displacement turbocharged GDI engines are now common in BMWs product line. 3 cylinder gasoline engines are even appearing - a major gas engine evolution is underway in automotive use.
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Old 12-16-2012, 08:43 PM   #18
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Over the years I have had several friends with inboard gas eengines that crapped out. I dont have the numbers or any studies to quote but there sure seems to be a lot of gas engines that get pulled and rebuilt or replaced. I just dont see the reliability factor. If your search on yatchworld for boats you will find the gass engine boats are cheaper than the same model with diesel.
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Old 12-16-2012, 09:24 PM   #19
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fryedaze,
Of course the're cheaper. They cost less to begin with.

But ther'e nicer if ya don't blow yourself up and I don't know any that have.

Gasoline engines are FAR more tolerant of water in the fuel and what else can go wrong w fuel injection and fully electronic ignition? Seems those two items have leveled the playing field to me ... as far as dependability goes.

If you bought a gas engine and had it rebuilt in 10 years and your'e 20 years down the road and your friend bought a diesel who would be out the most money? Probably the gas eng owner but it could be close and it could even be cheaper. Just a thought. I own a 73 Buick and appreciate how much more dependable the newer gas engines are.
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Old 12-16-2012, 09:38 PM   #20
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We've had at least one gas boat blow itself up in the time we've been in our marina.

As to the high number of gas engine breakdowns Dave alludes to, that's certainly what we hear on the radio up here. But..... most of these boats are runabouts for want of a better term, most of them inboard-outboards, and most of them are Bayliners.

I suspect the reason is very simple--- there are more of this type of boat up here than any other kind. Everyone and their grandmother has one parked in the driveway or in the backyard. The owners use them and park them and use them and park them and they don't for the most part get any maintenance or servicing until something breaks. And since they're all single engine, many of them with no kicker engine, they get on the radio and call for help.

The other very numerous boat type up here are the twenty-something foot Bayliner cruiser-type or fishing-type boat (and other makes, too). Same deal--- single gas inboard-outboard. A co-worker has one, and while he's never had a breakdown to my knowledge, he's had a number of "ran real bad" stories at work with the subsequent "got a mechanic down to check it" follow-ups.
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