Gas verse Diesel Engines

[Don L]

I my search for a maybe future 40-50o trawler I have been disregarding all boats with a gas engine.

Is this something to not be concerned about?

 

[Pete Meisinger]

Huge concern, especially in a boat of that size and weight. First of all, I doubt you will find many gassers in that size and weight range, that should tell you a lot.

Just check the archives for endless discussions on gas vs diesel. No need to reopen an old wound.

pete

 

[Gdavid]

There is a time and place for gasoline engines but I really don't think they make much sense in a 40-50' boat. If I recall correctly you are also looking at boats from the 80's and up, that would mean some pretty old tanks, which I would not be comfortable with in a gasoline powered boat.

I really think the market is softening and deals will be around. Are you selling your sailboat first? Buying a trawler before selling it is a guarantee to tank the boat market.

 

[Don L]

Huge concern, especially in a boat of that size and weight. First of all, I doubt you will find many gassers in that size and weight range, that should tell you a lot.

Just check the archives for endless discussions on gas vs diesel. No need to reopen an old wound.

pete

I wouldn't have asked if there was not gas powered boats in the size range. And I DID do a search based on the word "gas" and got no results because I knew what would happen..

 

[rslifkin]

The short version is that diesel fuel is safer, but with appropriate precautions and procedures, gas isn't unsafe. You're more limited on what equipment can be installed in the engine and tank spaces with gas.

Operationally, gas engines are cheaper to buy and may be a little cheaper to maintain than an equivalent hp diesel. But they'll burn more fuel and getting diesel like reliability is dependent on having a reliable ignition system and keeping it in good condition.

We've got gas engines in our boat (twin 454s). Fuel wise, our slow cruise is 6.5 kts or a little more burning about 5 gal/hr. A comparable boat with diesels will cost at least twice as much to buy, but would burn more like 2 - 2.5 gal/hr at the same speed. Realistically, the fuel safety concerns and fuel consumption are the only downsides for us. The gas engines push the boat along fine, they're reliable, and they're geared and propped well to have good low speed thrust.

Generally, for trawler use, I'd prefer diesels. But if the price is right and the boat is well powered with gas engines (and has adequate fuel range for your needs), I wouldn't necessarily run from the gassers.

 

[Comodave]

I wouldn't have asked if there was not gas powered boats in the size range. And I DID do a search based on the word "gas" and got no results because I knew what would happen..

I find using google search with trawler forum in the search wotks better, at least for me.

 

[sunchaser]

There are many gas boats in the 40-45’ range. Nice ones too. Vessels like Mainship, Baylinet, Tollycraft, Trojan, Carver etc. Then comes the current outboard revolution in all manners of speedy craft.

 

[tiltrider1]

After 45’ you are not likely to finD a gas boat this is due to the torque curve of a gas engine. From 38’ to 45” gas boats work fine if you keep things light. If you make them heavy you will have difficulty getting on plane.

If you never plan to travel faster than hull speed gas engines are fine.

Fact, a gallon of gas does not make as much power as a gallon of diesel. This means fuel economy will always be worse than diesel.

Fact, for some reason gas is sold at marina’s with road tax included. Diesel is sold with out road tax. This makes diesel look cheaper at the pump but if you apply for the road tax credit then both fuels are similarly priced. Granted, applying for tax credit is a pain so advantage diesel.

Diesel is safer than gas and 4 engine aircraft are safer than two engines. When was the last time an airline bought a 4 engine aircraft. So safer sometimes is just a buzz word when both options are plenty safe enough.

So pro’s for gas that are often over looked. Gas is Quieter and stinks less. Gas is significantly cheaper should there be a catastrophic failure(very low probability but a fact).

Finally, while based in false logic, the public perceives that diesel is superior to gas and as such will pay more for a diesel boat. This false perception is still a fact and means you will need to pay less for a gas boat as you will need to price it for less to sell it.

 

[mcarthur]

I my search for a maybe future 40-50o trawler I have been disregarding all boats with a gas engine.

Others have given useful information, but to help your search since this is an international forum, you could add "petrol" to your search terms since in much of the world "gas" refers to things like propane, LPG, etc.

 

[Portage_Bay]

I'm joining the party mildly in favor of gas powered boats.

Depending upon your intended use a gas boat can work out because they can be much cheaper to purchase. The lower price can buy a lot of gas and or engine work.

Regarding safety. If gas were as dangerous as some think the beer soaked go-fast day boats and weekenders would be going up in balls of flame all the time. They are not. You do have to be anal about maintaining the fuel system to prevent leaks. And any thing you install in the engine and tank spaces MUST be ignition protected.

Reliability. It seems to me that electronic ignition and fuel injection on a gas engine should be as trouble free or troublesome, depending upon your point of view, as electronic engine controls on a modern diesel. Long term reliability of the non electronic parts of the engines I think diesel wins.

Engine operating costs. Gas engines are far cheaper to repair or replace but they don't last as long.

Gas boats can be quieter. Much quieter. In my previous boat a 40' Tolly with 454s we could have a nice conversation in the main cabin at cruise speed standing right over the engines. In the current boat a 42' Californian with CAT 3208s anything above 1400 RPM is uncomfortably loud. Anywhere near the rated RPM of the CATs is louder than I want to listen to except for a short run every day to keep the engines clean and happy.

 

[rslifkin]

My gas engines are quite reliable with carbs, but electronic ignition. Same with the generator. It's really the ignition system that's the big historical weak point of gas engines, but even by the 80s, things had come a long way. The Mercruiser Thunderbolt ignitions are particularly good, but most of the other electronic setups out there are reliable as well.

As far as torque curve and getting a heavy boat moving, it really depends on the engine choice and how the boat is geared and propped. Something like the basic low output (330 - 350hp) 454s make plenty of torque. A gas boat will need deeper reduction gears than diesels because the gassers are higher revving. And once you need more than somewhere in the 350 - 400 hp per engine range, there just aren't any good, durable choices for gas engines, so it's time for diesels.

Our boat is a little over 27k lbs fully loaded (and 38 feet length on deck), tops out around 25 kts loaded, fast cruise is 17 - 18 kts at 3200 - 3300 RPM. Slow cruise is ~6.5 kts at ~1250 RPM (which is nice and quiet even with poor engine room sound insulation, mostly thanks to huge 2 stage water lift mufflers).

We've got 2.57:1 gears and 22x26 props, so bigger props and lower gears than most of the express style gas boats this size and more on par with the props you'd see on a similar size boat with diesels. As a result, low speed thrust is not an issue. We do about 4.3 kts idling in gear and unless it's very windy, it's fairly rare to need any extra throttle while docking. The boat also has no trouble getting up on plane. Drop the trim tabs a bit, push the throttles forward to approximately the fast cruise position and she just climbs right up and settles in. No plowing and struggling or any drama, just a few seconds of waiting while it finishes accelerating so you can tweak the throttles to where you really wanted them. None of the small gas boat "go WOT and back off when it pops up" stuff, although it will pop up on plane a bit faster if you do.

All of that said, I'd love diesels in this boat, but it would be 90% for the better fuel economy (and corresponding longer range). From an operational perspective (other than fuel economy) and powering the boat appropriately, the gassers are fine.

 

[sunchaser]

By the late 90s marine gas engines were TBFI which pretty much eliminated carbs. Resto mod car rebuilders have gone the same route. Todays EFI marine gas engines are wonderful units providing great reliability and power.

All this said, on older gas powered cruisers a rebuild and modernization on the critical components is quite doable. That is if one is leaning that way.

 

[Hippocampus]

Three times in decades of boating I’ve seen gasoline powered vessels explode. I have much less concern about fire than explosions. Fire you have time to either control or get off the the boat. Explosion you don’t. With outboards there’s no ignition, spark possibility or other concerns beyond what you would have with a propane cooker. The fuel is stored no where near the combustion. As long as piping is intact risk is low. With any type of gasoline cruiser the fuel and its piping is near by in an enclosed space allowing for aerosol. One of the most potent military bombs is an air/fuel bomb. And gasoline but not diesel can be used. You can put lit matches out in diesel. You can safely run a diesel engine with raw diesel in the bilge if necessary. Boat fires may kill people. Boat explosions do kill people. So beyond efficacy ( really not much different if judge btu against btu) energy density (way in favor of diesel) cost ( not much different if DYI and considered by service life) the major drawback is safety to my thinking. I have no interest in waiting for blowers to do their thing. Nor stressing about living with a potential bomb. I love gasoline outboards. Great hole shot, much more user friendly than electric for dinghies, trivial explosion risk. But for any type of cruiser think diesel is worth the additional outlay. BTW none of the explosions I saw were at the fuel dock. One was during approach to the Jamestown bridge. Another at trolling speed off Browns Bank. The third at the rip between mainland and MV.

 

[Pete Meisinger]

A simple answer is that a gas engine will be ready for a rebuild at 5,000 hours. A diesel will go many, many times that.

Try "Gas vs Diesel" search in the archives.

pete

 

[rslifkin]

On the safety aspects, fume detectors in the tank spaces and engine room go a long way towards ensuring you'll catch a problem if one shows up while you don't have your nose in the bilge. A gas engine room should also have good natural ventilation so there's always some air moving through there if there's a breeze outside. Yes, you have to wait a few minutes for blowers to run before startup, but that's not a big deal. There's always something to do (like unplugging shore power) between flipping the switch and being ready to start the engines. At anchor, I keep the fume detector on 24/7 to give some confidence in the event that an emergency start is needed.

Fuel explosions do happen, but they're rare, and usually a product of a neglected boat with significant fuel system problems, an operator that takes no safety precautions (such as a bilge inspection after fueling), and often a mistake (like gas into a rod holder that drains to the bilge).

The actual amount of fumes required to get anywhere near the lower explosive limit is surprisingly high. I always have the blowers and fume detector on when changing fuel filters, but I've never set off the fume detector in the process (even though I'm dealing with a filter in a bowl of raw gas while screwing on the new one and then carrying the old one off the boat). And the detector goes off long before the LEL. And the blowers scavenge fumes pretty effectively from the lower areas of the engine room even though it takes a few minutes for a full air change. By the time I carry the filter off the boat and come back aboard to change the next one, there's no gas smell remaining in the engine room.

A simple answer is that a gas engine will be ready for a rebuild at 5,000 hours. A diesel will go many, many times that.

Try "Gas vs Diesel" search in the archives.

pete

True, although many people will take a very long time to put on enough hours to actually wear out either. And many diesels don't make it 5000+ hours either. Not because of wearing out, but because something gets neglected and kills them. If you're going to put enough hours on the boat quickly for engine lifespan to be a big concern, fuel consumption of gas vs diesel is likely a bigger concern.

 

[Don L]

Well as the OP I went and did a general google search. Other than initial lower cost I don't see any advantage to the gas engines for my target boat range. That initial new boat lower cost doesn't appear to be translating to the 1980s used boat prices near as I can tell. Also gas pretty much disappears above a 42' boat and I am probably looking bigger than that anyway.

As someone who has experienced driving a car when it blows up from a gas leak problem I feel it would take a pretty big $$$$ difference for me to risk gas. Plus I spend enough time taking the carburetor on my outboard apart to want to add a main engine to it.

Thanks for the thoughts

 

[rslifkin]

Well as the OP I went and did a general google search. Other than initial lower cost I don't see any advantage to the gas engines for my target boat range. That initial new boat lower cost doesn't appear to be translating to the 1980s used boat prices near as I can tell. Also gas pretty much disappears above a 42' boat and I am probably looking bigger than that anyway.

As someone who has experienced driving a car when it blows up from a gas leak problem I feel it would take a pretty big $$$$ difference for me to risk gas. Plus I spend enough time taking the carburetor on my outboard apart to want to add a main engine to it.

Thanks for the thoughts

Yeah, once you get bigger than 40 - 45 feet, the gas boats pretty much disappear anyway. Nobody wanted gas in a slow boat, and there weren't viable gas engines to use for a fast boat at that size range.



Cost-wise, for an 80s 40 footer, it's typically 2x or more the price for one with diesels. But if you're looking bigger, you're unlikely to find many gas boats anyway.

 

[Nomad Willy]

Huge concern, especially in a boat of that size and weight. First of all, I doubt you will find many gassers in that size and weight range, that should tell you a lot.

Just check the archives for endless discussions on gas vs diesel. No need to reopen an old wound.

pete

“gassers” not fitting the personality expectations kills most of gasoline engine thoughts by rec trawler skippers.
A well maintained and operated gasoline engine is probably better than many diesel engines. Remember the fifties w gas powered trucks, many of them onderpowered grinding down the highway? Making screaming noises working hard? I do. And many of them lasted years and years. But in those days most knew to warm up an engine before high load and high rpm.

Trawler skippers don’t like gas engines because they don’t sound like a tug boat belching black smoke. It’s mostly in the perception.

 

[Nomad Willy]

Don L wrote;
“ Well as the OP I went and did a general google search. Other than initial lower cost I don't see any advantage to the gas engines for my target boat range.”

Try noise, vibration and engine sensitivity to water in fuel. All big issues.

 

[tiltrider1]

Three times in decades of boating I’ve seen gasoline powered vessels explode. I have much less concern about fire than explosions. Fire you have time to either control or get off the the boat. Explosion you don’t. With outboards there’s no ignition, spark possibility or other concerns beyond what you would have with a propane cooker. The fuel is stored no where near the combustion. As long as piping is intact risk is low. With any type of gasoline cruiser the fuel and its piping is near by in an enclosed space allowing for aerosol. One of the most potent military bombs is an air/fuel bomb. And gasoline but not diesel can be used. You can put lit matches out in diesel. You can safely run a diesel engine with raw diesel in the bilge if necessary. Boat fires may kill people. Boat explosions do kill people. .

Let’s be fair. With modern fuel injection, electronic ignition and fume detectors, when was the last time a gas boat blew up. On the same vane of safety, diesel engines can experience a runaway. There is a case of an individual being killed by shrapnel from an exploding run away diesel engine.

Technically, diesel boats are safer. Technically, not boating is extremely safer than operating a diesel boat. You can’t ignore the huge risks that come with boating and then try to make a case for the small amount of risk incurred when adding gas.

 

[sunchaser]

Don L wrote;
“ Well as the OP I went and did a general google search. Other than initial lower cost I don't see any advantage to the gas engines for my target boat range.”

Try noise, vibration and engine sensitivity to water in fuel. All big issues.

Eric
Add to it more weight, often hard to find parts and maintenance costs. For most on this forum a 100 hours per year is a lot which hardly requires a presumed long lasting diesel. But finding a good gas powered “trawler” is difficult.

 

[Don L]

Let’s be fair. With modern fuel injection, electronic ignition and fume detectors, when was the last time a gas boat blew up.

I hear of a boat catching fire that the fuel dock a few times a year. Just saying.

 

[rslifkin]

I hear of a boat catching fire that the fuel dock a few times a year. Just saying.

Watch how most owners treat fueling and it's honestly surprising it doesn't happen more often. They usually just pump fuel in, hope it's the right hole, then throw off the dock lines, hit the blowers and immediately crank the engine(s).

On the other hand, the right way to do it is to fuel with the fume detector on, blowers off. When finished fueling, the tank area and engine room get inspected and sniffed for any gas fumes or issues. Then turn the blowers on, wait a couple minutes, then start the engines and depart.

 

[psneeld]

More importantly, along with propane explosions and the greatest number of sinkings....how many were operator error.

Stats are so misleading...especially in the boating world.

 

[psneeld]

Watch how most owners treat fueling and it's honestly surprising it doesn't happen more often. They usually just pump fuel in, hope it's the right hole, then throw off the dock lines, hit the blowers and immediately crank the engine(s).

On the other hand, the right way to do it is to fuel with the fume detector on, blowers off. When finished fueling, the tank area and engine room get inspected and sniffed for any gas fumes or issues. Then turn the blowers on, wait a couple minutes, then start the engines and depart.

While I agree that it' funny how it doesn't happen more, but my point is.....

I have lived at several marinas with busy fuel forks and have been at plenty more marinas with busy fuel docks. I did both....rescue with the USCG and Assistance towing and seeing more than one boat blow up at a marina in a lifetime to me would signal to fire the fuel dock employees, give better training or shut down the fuel dock because something is wrong.

I am not saying multiple fires/explosions don't happen at the same fuel dock, but it's pretty rare to happen regularly.

 

[JESSEDIVER49]

Gas versus Diesel

#1 reason Flammability-
Diesel fuel has a flashpoint between 52 and 96 °C (126 and 205 °F). In comparison, gasoline is a flammable liquid with a flashpoint of -40°C (-40°F). It gives enough vapor to create a burnable mixture in the air or the bottom of a boat. For lower flammability risk, a marine diesel engine is the safer option.

Also, Marine diesels have lower fuel costs, better efficiency and will last as much as 3-4 times longer than gasoline engines. Gas engines' exhaust have greater amounts of CO than diesels, about 20X.

The one advantage of a gas engine is low initial cost.

 

[Carl Martin]

Another problem with gas is the short shelf life. Even with Stabil it still degrades in a relatively short time.

 

[Hyperion]

If gas were as dangerous as some think the beer soaked go-fast day boats and weekenders would be going up in balls of flame all the time. They are

This is because 99% of them use outboards and the big danger with gasoline is that the fumes are not flammable, they are explosive. In a large boat with inboards I could dump 50 gallons of diesel in my bilge and leave it for a month then light a match and nothing would happen. You do the same with 1 gallon of gas and leave it overnight and a small spark comes off your starter and the boat will explode into smithereens.

 

[tiltrider1]

This is because 99% of them use outboards and the big danger with gasoline is that the fumes are not flammable, they are explosive. In a large boat with inboards I could dump 50 gallons of diesel in my bilge and leave it for a month then light a match and nothing would happen. You do the same with 1 gallon of gas and leave it overnight and a small spark comes off your starter and the boat will explode into smithereens.

You are fear mongering. If gas was dangerous insurance companies wouldn’t insure them. Next you are going to tell me anything less than a Nordhavn is to dangerous to go boating in.

 

[BruceK]

Gas vs diesel is a well plowed field on TF. As part of the plowing, I`ve contributed my story of the cruiser exploding on Sydney Harbor. If it happens, it can be really bad. A baby died, my clients were projected upwards, fractured all 4 limbs,had months in hospital, and among other things one had enduring weeping sinuses caused by dirty bits of boat blasted into her body tissues. She was a model. Their marriage broke down. I was at the photo shoot of disfigurements for her case. It was awful.

I concur EFI has helped, but it can leak too. Gas on boats is an avoidable risk.