Diesel vs. Gas

[Tony B]

1). Aside from the obvious - Carbon Monoxide poisoning or becoming a fireball what else is wrong with gasoline engines.

2). How much more inefficient is gas than diesel? Twice the fuel consumption? More?

*

 

[Marin]

Tony B wrote:
1). Aside from the obvious - Carbon Monoxide poisoning or becoming a fireball what else is wrong with gasoline engines.

They don't last as long (apparently) but the importance of this depends on how long you anticiapte running the boat.

They are more complex, I think, what with carburetors (on some of them) and an ignition system.

But the biggie is as you descibe--- the potential for even the tiniest fuel leak to cause a fire or explosion.* An inboard installation requires all the accessories--- water pumps, etc.--- be spark-proof.* But a lot of cruisers like Tollycrafts and whatnot were and are gasoline-powered and they don't go around exploding with any sort of reguarity.* So I suspect that when used intelligently with a solid safety plan, gasoline engines, like propane galleys, are as safe as anything else.

 

[windmist]

I think I read someplace that a diesel engine is around 30% more efficient than a gas engine.

Ron

 

[Woodsong]

The #1 reason I would tell you not to buy a large gas powered boat is resale, or really, the lack thereof. A gaser will always be a harder and slower sale and price will lag behind diesels when it comes time to sell. Fuel burn is terrible at anything beyond hull speed. My previous boat was a 2005 Silveryon 38 Sport Bridge- nice boat, fancy, lots of room, etc. She was powered with twin 8.1 Crusader High Outputs (425hp each). At hull speed I averaged around 5-6 GPH so really not bad. If I put her up on plane I could easily burn 50 GPH. I do not miss that boat though we sure had fun on her. Now, if you get a houseboat, gasers are acceptable. If you get an express cruiser, gasers at 34-35 are ok. I would not want gasers in an aft cabin or sedan. Typically speaking the biggest reason to go for gasers instead of diesel was the big difference in purchase price between the two. The economy and market have equalized that out so most times you can get a diesel powered boat for the price the same boat with gasers is selling for so go diesel.

The flip side of that debate is if you are getting a high output, turbo, high HP diesel, you can sure do a LOT of very expensive damage to them very quickly and you will pay dearly for those repairs. Gas engines are almost throw away type deal and can put new ones in for like $10-12k each so not a deal killer if you fry a motor.

Having owned both though I can tell you I don't ever see myself owning a gas powered large boat again. Beyond safety, resele, etc. you also have the much better performance of diesels, especially for docking in close quarters.

 

[FF]

Fuel is not the usually major cost of operating a boat.

At slow disaplacement speeds the gassers will be fairly close . perhaps 10-20% more GPH.

BUT the cost of repair on a gasser will be perhaps 1/5 to 1/10 the cost on a diesel.

A factory new short block will run under $4000, If you need one.

Spark plugs are $2.00 each , fancy diesel injectors can run a grand EACH , plus instal.

Purchasing oil in gallons rather than quarts , and oil filters 4x the size does cost more, even antifreeze is different and expensive.

One of the biggest advantages to gas are the lack of problems when the boat is ignored for a couple of months.

A diesel will begin to rust the cylinder walls , unless the Mfg out of service procedure is followed .
This can be a long and costly procedure , you ignore at your peril.

Diesels do not like being run at really low loads , it shortens their service life , but most don't care as a 10,000 hour engine reduced by half is still far more hours than the usual 100 hours a year boat is operated.

The problem with gas is IF you need over 250 HP on a long term basis , hardly a displacement cruiser problem.

 

[Per]

you mentioned already the most important factor:

KA-BOOM!

 

[Doc]

My wife won't sleep on a gas boat....period.

 

[Nomad Willy]

David M wrote:

"A gasoline engine will produce 10-15 horsepower per gallon per hour of gasoline burned. The lower end is represented by carburetter engines and the higher end is represented by high compression, fuel injected engines." I would say that the gas engine produces 5 to 16hp because the temperature of combustion always remains the same regardless of load. So the gas engine is MUCH less efficient at light loads because of the very high heat loss at lower power outputs* ...unlike a diesel. Engine load has a much bigger effect than injection v/s carb. That OK David?

 

[sunchaser]

A diesel powered ski boat or 30 foot Sundancer? Forget it. Diesel and gas both have their place.*It seems the safety issues for gas were*successfully addressed decades ago, now to be replaced by the ethanol mess.

The*costliest (no fatalities)*fires I have personally seen the past few years*happened to*diesel boats* - boat houses in Portland (2006) and Sidney BC (2010). I saw a very old gas houseboat go up at Lake Powell (2005)*during fueling with the gas dock*rendered inoperable for a few weeks.

The*power plant *revolutions today seem to be the ever increasing size of outboards and IPS like drives for both gas and diesel. Some interesting things are taking place in these areas giving builders*new life. Gas will* not go away given its cost advantage as FF notes for smaller craft.

 

[FF]

"Gas will not go away given its cost advantage as FF notes for smaller craft."

Its not the crafts size , its the HP output required.

Gas , to me , is superior on most recreation vessels , until you hit that 250 hp area, or long 24/7 power requirements.

For 100 or 200 hours a year there is zero justification to a diesel, if costs are a concern.

BOOM ! is hardly a problem since the 1930's , and for anyone "worried" ,the real question is,

have you purchased a boat built with Fire Retardant Resin , or metal , to be "safe"?

 

[Tony B]

Sooooooooooooooo...........
Can I conclude that if I were to buy a used 44' houseboat with twin 120HP gas engines to run inland say from Galveston, Tx. to Kentucky Lake in Kentucky every year at around 6 Kts that gas engines wont be too bad of an investment? I would still like diesels instead but in the 'used' market you take what you can get at what might appear to be a good deal on the boat.
As this thread goes on, I am becoming a little more comfortable with the idea of gas on a house boat. If I get a trawler, it will definitely be diesel. Now if I can just sell my sailboat. This is not exactly a good time of year nor a good year for selling boats.

 

[Nomad Willy]

DavidM,

How much less efficient are 4 stroke OBs than gas inboards? As I see it the OB is less efficient for two reasons. Sea water cooling that necessates running the coolant 130 degrees or less and relatively small props. The prop has other + and - features also.

What do you think David?

 

[FF]

"If I get a trawler, it will definitely be diesel. "

Why?

The only reason I can think of is fuel management , the Fed mandated garbage gas does not store over a month.

But that is easily handled with tank management.

Remember its no sweat to run a gas tank bone DRY , and not have any restart hassle , no injectors to bleed.

In Fl and other civilized areas NO ETHANOL gas is available at dockside pumps.

 

[Tony B]

DavidM wrote:
Tony:

Are your twin 120 hp engines outboards? Worse yet, are they two stroke?.........

David

*Yes they are inboards. One guy I talked to said he has a 44' houseboat for sale with twin 120HP engines. He said he only gets about 1/2 gal / mile at 8 knots. I couldnt live with that.

Then there were a few 42 or 44 footers on Yachtworld.com with single 120 or 150 Volvo's

I wonder what the fuel consumption would be on those at about 6 or 7 knots.

The reason for my questions about single vs. twin and gas vs. diesel and engine sizes and all engine related questions is because we plan on running around 3,000 miles per year for the next five years....assuming all goes well in life. Fuel consumption, engine maintenance and engine life may become a big factor in my decision making. There are some diesels on houseboats and am considering them also.

After hiring a surveyor and almost buying a trawler with teak decks I am shying away from teak decks. That can limit the market quite a bit.

Tony B

 

[Tony B]

David

Many thanks to you and others for taking the time to simplify this whole thing. It now is starting to make sense as I see the whole picture.
I can live with 2 MPG pushing a 40 to 44 foot vessel.

The reason I picked 6 kts was simply because I thought that slightly below hull speed would be more fuel efficient than at hull speed. Was I wrong?

Thanks in advance

Tony B

 

[FF]

"The reason I picked 6 kts was simply because I thought that slightly below hull speed would be more fuel efficient than at hull speed. Was I wrong?"

Hull speed is mostly as sailboat concept , as the wind is free , so paying to make huge waves is easy and fun.

Tho many sailboats in a 20K breeze may not be as much fun , esp with a spinnaker to get the HP.

The SQ rt of the water line length (SL) is usually a cheap place top operate.

The lower the speed the less important the shape of the hull, but the more important the surface area.

Most any hull that doesn't look Wierd will usually do fine at SL x 1

 

[Nomad Willy]

FF wrote "Hull speed is mostly as sailboat concept"

Absolutely the most false thing I've ever heard on this forum.

 

[timjet]

nomadwilly wrote:
FF wrote "Hull speed is mostly as sailboat concept"

Absolutely the most false thing I've ever heard on this forum.

*Or how bout "Running a twin single engine is NOT more fuel efficient".

 

[FF]

FF wrote "Hull speed is mostly as sailboat concept"

Absolutely the most false thing I've ever heard on this forum.


THe concept as shown on the previous graph is most trawler crawlers will chose either

6 kts SL= .96 hp= 15 or 7 kts SL= 1.12 hp= 27 And burn 1 or 2 gph


The "9 kts SL= 1.44 hp= 85" is seldom in the trawler folks cruising plans. Twice the burn for a K or 2 , not common.

The only times I have been to "hull speed" on displacement trawlers is on the WOT engine check, 5 -10 min max,2x a year.

The sailors have no problem going as Fast as possible , even hull speed ++ , since they are NOT paying the fuel bill.

 

[sunchaser]

FF's point of "1.0" flies away when tank testing occurs and propulsion systems are taken into account. How is that some sailboats sail much faster when better sails, more efficient rigging, the proper use of tell tales and smarter crew are employed? Or*why does a narrow MV hull**generally require less power than a wider hull to achieve the same speed?

Unless bluewater cruising, I forego the better mileage 6 knots allows and steam along at 8 or so knots to allow cocktail hour to begin and hour or two earlier.