Diesel versus gas

[Capt Kangeroo]

Other than the longevity of diesel's, are there actually any meaningful advantages / disadvantages of one over the other. I have owned both and prefer diesels but really don't know much about them. I'm finding that the pickings are very slim in finding diesel powered yachts in the major North American brands so I'm thinking maybe I should take a serious look at gas.


Anyone have any thoughts / comments on this?

 

[psneeld]

Which kind of yachts and how big do you think aren't mostly diesel?

 

[Pete Meisinger]

"Other than Longevity, are there any other advantages" ???

What else is there? Allow me to rephrase your question.

"Other than the fact that gas engines fall apart after around 5,000 hours and need to be replaced while diesel engines will remain serviceable for 5 to 10 times that long".

Are you in it for the short haul or the long haul?

pete

 

[sunchaser]

With dozens of new design "yachts" in the 40-50+ foot range sporting OB power, the OP has raised a good question. Viking has a center console line with the largest at 55' specifically designed for the Merc V12 outboards.

 

[High Wire]

Living on the move or always in marinas? Cruising fast or slow? Planing or displacement? How big of a boat are you looking at?
Gas is good for smaller boats that don’t put too many hours on them. 2000 hours lifetime is pushing the envelope. Even if the engine is babied along at low RPM, the bolt on parts don’t seem to last much more than 10 years or 1000 hours. Just my limited experience.

 

[fgarriso]

Until you state how large, and weight of the vessels you are looking at it would be impossible to make a recommendation on engines.

 

[Comodave]

We were looking to downsize in boats last summer. We wanted to go from diesel to gas in a smaller boat. There aren’t any marine diesel mechanics in our area and I am starting to want someone to do the maintenance for me. But we actually ended up with a bigger boat and diesels that are twice as large as our last boat. Oh well, there just wasn’t a lot to choose from given that we wanted either a Tiara or Formula boat.

There isn’t anything wrong with gas power if you keep up on the maintenance and are careful with the possible fumes. I would not want gas in a boat much larger than about 36’. But that is just my opinion. Diesels are more expensive to maintain but cheaper to run fuel economy wise. But are diesel mechanics available in your area? We looked at a couple of gas powered Formulas, one with worn out gas engines so we were going to repower it. The advantage with gas in a repower was that it could have been done for about $20K. But the boat failed the survey miserably. Another Formula that had recently rebuilt gas engines but had an unscrupulous broker selling it so that deal fell through.

So we ended up with a bigger boat with twin 450HP Cummins in it. But we love the boat and can’t wait until we can launch it. Waiting on the new canvas and seat upholstery work done.

 

[Gdavid]

Other than the longevity of diesel's, are there actually any meaningful advantages / disadvantages of one over the other.

In my opinion the greatest benefit to diesel is safety, the much higher flash point of diesel versus gasoline means that a diesel leak cannot result in an explosion. I would be less concerned about the difference in a new boat where all the tanks are new, fuel lines new and likely fuel injected but my budget had me looking at 30-40 year old boats and I was not willing to consider placing my family on a boat with old gas fuel tanks. If I was looking at either 10 year old vessels or a boat with new tanks and had been repowered with EFI, I would consider gas.

Diesel fuel is more energy dense than gasoline so you will see greater efficiency with diesels which allows both better economy and range, this is particularly important for boats trying to run on plane.

Gas engines are cheaper to replace and in some markets, much easier to get serviced, for running at slow speeds where massive power and torque is not necessary, the lower purchase price of a gas boat could make sense.

 

[kthoennes]

Uh oh, another one of these threads. Shall we summarize to save time? Not my own opinions necessarily, just summarizing the likely responses:

- Diesel fuel is relatively safe. Gas may blow your boat to smithereens and kill you all.

- Diesels have a much, much longer life than gas. (You'll hear 2,000 to 5,000 miles on gas, and reports that there are some diesel engines that have been running continuously for 150 years, 24/7, since Rudolph Diesel fired up his prototype in what, 1870? But then it all depends on maintenance history and use/abuse.)

- Much easier to find parts and service for gas engines, depending on where you are.

- Diesel fuel grows biological crud in the tank that you have to filter and add stuff to clean or prevent. Gas on the other hand has a relatively short shelf life and ethanol blends are damaging, especially to older engines (I know, controversy on that, but I'm in the ethanol-is-bad camp).

- If you google my gas Marine Power 454's, you can get a modern replacement for about $10-$15,000 (new, not used or rebuilt). An equivalent diesel -- and for our model boat they came with optional Yanmar 4LH-STE 240's (among others) but that's 20 years ago -- but if you look for a 240hp Yanmar now, I see they start around $17,000. On the shelf of course, not in your boat, and that doesn't include shipping.

- Bigger power boats with diesels *generally* command higher prices than gas, sometimes much more, because diesels last longer and can be more reliable, hence the higher value.

- We have a frequent discussion about fuel on this forum but now that gas and diesel are both an eye-watering $5 per gallon on the street around here, it seems equally painful to me, lately anyway. I drive a diesel Ram 2500 and a gas Escalade -- they both get around the same mileage for the same price.

I like my boat a lot and don't have a minute of regret, but if I could wave a magic wand and make our boat diesel instead, I would, if that tells you anything. Although then I couldn't get it serviced at my marina and they don't sell diesel fuel at the gas dock so I'd have to get it trucked in. There's a Bayliner 3988 right next to us at the marina and boy the powerful low rumble of those engines is really impressive. Almost like Harley Davison sexual.

Did I miss anything?

 

[Capt Kangeroo]

Good to see some of the old crowd still here! I appreciate the comments, a few points I hadn't considered like access to diesel mechanics & safety. The boats I'm looking at now are your run of the mill Silvertons, Carvers, Chris, etc in the 40 to 45 range but nearly all are gas. Don't really care about speed.



Kthoennes, brilliant summary but now we have nothing to discuss. Perhaps to restore the momentum I'll change the title to "Do old diesel engines make good anchors"

 

[rslifkin]

I'd love a pair of diesels in my boat. Mostly for fuel economy. The less flammable fuel would be a nice bonus.

Realistically, the gassers I have are thirsty, but reliable and they push the boat just fine. It'll cruise at 17 kts without beating on the engines. The recommended max continuous power gets us about 18.5 kts. I figure fuel burn around 30 gal/hr at 3300 rpm / 17 kts. At a nice slow cruise of 1300 rpm, we burn around 5 gal/hr traveling about 6.5 kts.

Durability wise, one engine got a long block after an oil cooler line failure on plane took out the main bearings in 1 engine. So the engines have about 1800 and about 300 hours. The 2 run pretty much identically, both turn up to the same RPM at WOT, so power output is still about equal. Compression is still good on the older engine and while it consumes sightly more oil than the newer one it's still pretty minimal.

So I'd say that at 1800 hours that engine is far from worn out. Maintenance pretty much consists of spark plugs every year, oil changes, impellers, etc. Only extra compared to diesels are exhaust risers every few years (my manifolds are in the coolant loop).

 

[SteveK]

The boats I'm looking at now are your run of the mill Silvertons, Carvers, Chris, etc in the 40 to 45 range but nearly all are gas.

I wonder if the price of fuel is the reason gas boats are mostly for sale in your area. Gas Boats in that size must carry a banker on board.

 

[Slowmo]

The lifespan of an engine in a boat is affected by many things. There have been plenty of diesels that required overhaul after just a few thousand hours and there are plenty of gas engines that have lived thousands of hours. As others have pointed out replacing a gas engine is a fraction of what it costs for a diesel. While I like my Cats I've learned there are advantages for gas engine including: they are quieter, they smell less, and they're cheaper to maintain. Safety with gas requires you to take some extra precautions but these aren't overwhelming, but they can't be ignored. Fuel consumption of gas engines is higher, but then the purchase price of the boat is lower so if you don't cruise long distances the fuel consumption difference may not be meaningful.

 

[BruceK]

Welcome Aboard. To add to the above, I was involved in the aftermath of a fatal gas boat explosion I would not want anyone to experience. I think they`ve become safer, but...
Have chartered gas boats years ago from the reputable builder/charterer Halvorsen, but still had my nose down the bilge before every start.

PS We spell it Kangaroo.

 

[rslifkin]

Welcome Aboard. To add to the above, I was involved in the aftermath of a fatal gas boat explosion I would not want anyone to experience. I think they`ve become safer, but...
Have chartered gas boats years ago from the reputable builder/charterer Halvorsen, but still had my nose down the bilge before every start.

PS We spell it Kangaroo.

Appropriate safety precautions, equipment, etc. are critical on a gas boat. Topping it all off with a healthy dose of paranoia is also a good idea. Take a gas boat, add some shoddy maintenance and an idiot operator and things can go very wrong (we've all at least seen video of it).

I'm definitely on the paranoid side. In person bilge inspection before the first start of the day. Blowers on before every start, fume detector installed and used. And an in person check of the engine room and around the fuel tanks after fueling. Bilge checks are always done before turning on the blowers to minimize the chance of missing anything that's not right.

 

[mvweebles]

No idea why people get worked up about gas on a boat.

Peter
PS - this is a friend's boat that was purchased new and well maintained. Amazingly they replaced it with another gasser.

 

[Nomad Willy]

Pete Meisinger wrote;
“ Are you in it for the short haul or the long haul?”

Most everybody on TF is “in it” for the short haul whether they know it or not.

Sure there are a few that keep their boat for decades. But I think (guessing) that most rec trawler owners after a few years sell or trade their boat for the one they want ……. and can now afford. Most people’s piggy banks get bigger as they get older and thus their buying power gets bigger too.

Notice the “Interesting Boats” thread. Almost all the boats posted about there are big and expensive. It’s what skippers here want. The “two foot itus” disease is alive and well and looks to be very enduring as also.

If one owns a boat that needs an engine replacement .. being a big ticket item said owner will not want to face this problem again so the “forever engine” is very popular in skippers minds.

How many boat owners are replacing their engine that they bought and installed? I’ve never heard of one. PO is talked about extensively.

So in this question about longevity almost everybody is making the assumption that they will pilot their boat for many many years and need an engine known to last 10,000hrs +hrs.

And of course there is the extreme bias that diesels are the engine to have and you’ve got to be stupid to want to even think about gas engines. Most yachts that were cruisers (not sailboats) predominately were gasoline powered. I remember the 62’ 1952 or so “motor yacht” was powered by gasoline engines. Three of them. Flat heads at 160hp each. These were very big engines and very durable .. to what extent I’m not sure but in those days marine engines had very special valves, rings and other high wear parts not found in automotive use.

My point though is that very very few of us have the need for an engine of extreme durability. And engine that is inexpensive and easy to work on is probably of more value than having an engine that will last 15,000 hrs.

 

[Comodave]

We have had many gas powered boats. But I also took extreme safety precautions due to a boat at the marina my parents boat was docked at when I was a freshman in high school. A 40’ boat with about a dozen people on board docked, fueled up and blew up. It killed 3 or 4 people, mangled another 3 or 4 and the rest were unharmed. It made a powerful impression.

 

[SteveK]

A comment was made that gas boats sell for less than their diesel sisters. And that the difference can be justified by higher fuel cost. True.
The GB is my second diesel. I prefer the purr of the diesel over the scream of the gas.

 

[tiltrider1]

Gas engines do some things better than diesels engines and vis versa. Really depends on the application. Since Capt Kangeroo has 441 posts here I am going to put this thread down as trolling. I think a bunch of you just got sucked in.

 

[ranger58sb]

OP, there are also a boatload of previous threads on the topic, so reading those could keep you busy for a week or so, too...

While I like my Cats I've learned there are advantages for gas engine including: they are quieter, they smell less, and they're cheaper to maintain.

I wonder if that's really a comparison to some diesels in some boats?

A pair of big-ish two-stroke DDs with typical "mufflers" are usually pretty loud, or at least distinctive.

OTOH, the various engines in the three different diesel boats we've owned don't seem to me to be any louder than many gas boats, including the one we had once. And ours have always been much quieter than the gas go-fasts that come around from time to time...

-Chris

 

[Hippocampus]

Thinking on the future. Carbon neutral fueling seems more compatible with diesel like technologies. In fact even now biodiesel is a reality and can be run in standard engines. Ethanol has even less energy density than gasoline. Functional and economic breakpoint is around 15%. Above that adding more ethanol just doesn’t make sense. There’s a sweet spot for adding carbon molecules to the hydrocarbon chain where ignition point, vapor point, and energy density are good for internal combustion engines using liquid fuels.
So looking to the future although e engines have multiple benefits for vessels requiring high energy density availability due to concerns about range (displacement hulls) or power/weight such as planing hulls there remain problems with e engines. Unlike cars/trucks refueling is an issue both in terms of time needed via alt energy as well as availability. So bunker fuel of some sort is required.
Wouldn’t be totally surprised by the return of some form of steam engine in moderate to larger boats. Of course any boiler driven engine is much less fussy about purity of the fuel and even solid fuels are potential sources of power.
However gas is problematic now and for the future.

 

[rslifkin]

With poor engine room sound insulation, diesels are often a bit louder, as they tend to radiate more mechanical noise. But that's an easily fixable problem, so in my mind, not a difference worth caring about.

In the 40-ish foot range, particularly when looking at 80s builds, there are more gas powered boats out there than there are people that want them. So they're cheap. They built tons in the 80s, as the choices for diesels that would fit some of those boats weren't as good as what became available later. And gas was cheap. As an example, my own boat could be bought new with 340hp gas engines or 230hp Volvo diesels that added 2000 lbs to the boat and reduced its maximum cruising speed to "barely planing". More powerful choices existed back then, but they were either even heavier or wouldn't physically fit. I know of one that was built with the diesels. Considering fuel was cheap at the time, it's no surprise.


Others have made a very good point on the durability thing. Many boats just don't get used enough to worry about wearing out engines. And it's pretty common for a part failure not caught in time or some neglected maintenance to kill an engine before it would have otherwise worn out.

 

[ABfish]

I could make a case for installing a single 6.0 liter GM Vortec engine in a displacement hull.

These 360 hp engines produce around 120 hp at 2,000 rpm. A 40 ft. displacement hull needs 50 hp for hull speed, so at 2,000 rpm you'd be operating at 50% on the torque curve. You could set a rev limiter to keep engine speeds below 3,000 rpm to prevent transmission damage, but still have plenty of reserve power.

So you'd have an iron block, aluminum head, fuel injected engine cruising at 20 percent of maximum horsepower. Quiet, smooth, no smoke, cranks the first time every time. One half the cost of a new Tier 4, 100 hp diesel. Less than 600 lbs. fully dressed.

Parts are cheap and available, as are techs with the expertise to repair them. Engine management software can be read with a $100 OBD reader. These engines regularly go 300,000 miles or 7500 hours in pickup truck duty.

 

[rslifkin]

I could make a case for installing a single 6.0 liter GM Vortec engine in a displacement hull.

These 360 hp engines produce around 120 hp at 2,000 rpm. A 40 ft. displacement hull needs 50 hp for hull speed, so at 2,000 rpm you'd be operating at 50% on the torque curve. You could set a rev limiter to keep engine speeds below 3,000 rpm to prevent transmission damage, but still have plenty of reserve power.

So you'd have an iron block, aluminum head, fuel injected engine cruising at 20 percent of maximum horsepower. Quiet, smooth, no smoke, cranks the first time every time. One half the cost of a new Tier 4, 100 hp diesel. Less than 600 lbs. fully dressed.

Parts are cheap and available, as are techs with the expertise to repair them. Engine management software can be read with a $100 OBD reader. These engines regularly go 300,000 miles or 7500 hours in pickup truck duty.

At that point, I'd be inclined to go for something like the package Indmar sells for ski boats, provided they could de-tune it a little to not need premium gas. They marinized the Ford 2.3 liter turbo 4 cylinder from the ecoboost Mustang. Makes about 300hp, but because it's small displacement, it should be more efficient at low power outputs like the above displacement cruising example (compared to a bigger gas engine that will suffer more pumping losses in that scenario). https://www.indmar.com/engines/23l-ecoboost/


The pumping loss problem from gas engines being throttled is a big part of the efficiency issue with them. I've run the numbers for my boat and with modern diesels it would burn something like 30 - 40% less fuel on plane (because of diesels being more efficient and the fuel having a higher energy density). But because diesels tend to scale better efficiency wise from light to heavy load, it would burn 50 - 60% less fuel at displacement cruise compared to my current engines.


Don't forget that marinizing an engine adds a good bit of weight in terms of cooling and exhaust parts. The weights I can find for off the shelf marinized 6.0 LS motors (from Crusader and Ilmor) are a bit over 900 lbs. Those may be the iron head version of the 6.0, however. Aluminum heads should save 50-ish lbs. Add another 150 lbs for a transmission. My old 454 big blocks with closed cooling come in right around 1000 lbs (plus transmission) for comparison.

 

[BandB]

The biggest argument made by the anti-gas group is always safety. These are the same people who often have propane stoves and propane grills and tanks aboard and have gas outboards for their dinghies. I'm not sure how they fully reconcile all that in their minds.

Safety with gas is an issue but safe handling of gas is easily done with some care. The fact is there are many times more gas boats than diesel and we do have fires and explosions but we also have fires aboard diesel boats. Is diesel safer than gas? Yes. However, there are many factors to safety aboard.

Most of this conversation centers on inboards. Significant cost savings on gas. Significant cost savings on replacement. Fuel savings on diesel will take lifetimes to make it up. Speaking of lifetimes, the life of an engine is brought up. Yet, few will ever exceed the life of a gas or diesel engine.

All that said, everything else equal, I'd prefer diesel. It's just that everything isn't often equal and sometimes the most attractive boat in other ways is gas. I would not be scared of gas. The first 5 boats I owned were gas. All my boating to the age of 42 was gas. No one on the lake we were on even sold diesel.

Now, for new boats and future, another huge factor has entered, the replacement of gas inboards with outboards. In a boat under 50', huge advantages in original cost, in cost to repair, in cost and ease of replacement and in space utilized. I just pulled up a Tiara 43LE with twin Mercury 600's. What a perfect boat for a young working couple on Lake Michigan. Or for a young family, how about a Searay Sundancer 390 with triple Mercury 300's.

Trawler boaters are a very small niche of boaters and even on this forum, there are many who don't own trawlers even in the broader sense, but most do trawler type cruising.

Another personal love I have for the proliferation of outboards. People don't start their boating lives with large diesel driven trawlers or yachts. The rising prices of inboard boats, gas and diesel, was restricting entry into boating. We need new boaters. You want marinas and services, new boaters are key. You want someone to sell your trawler to in years to come, new boaters today are your customer.

 

[ABfish]

No doubt a small diesel, properly sized for the application, is much more efficient. What I'm talking about is life cycle cost, considering initial cost, cost of service, and cost of fuel. The small, forced induction gas engines are super efficient, but the turbo and associated piping are complex and another potential point of failure. On the other hand, the idea of a marinized Ecoboost in a displacement hull is intriguing.

My business neighbors are landscape companies. One is large with multiple locations, the other is a mom-and-pop. The large company stopped buying medium duty diesel trucks, due to to reliability issues. The owner said he'd rather pay the gas bills than have the down time. The small company stopped buying diesel engine mowers. He said he could buy 2 gas mowers for the price of one diesel. He buys a gas mower, runs it daily for 2 years, and sells it for scrap. In both cases, conventional wisdom would tell you that diesel is the way to go.

 

[rslifkin]

No doubt a small diesel, properly sized for the application, is much more efficient. What I'm talking about is life cycle cost, considering initial cost, cost of service, and cost of fuel. The small, forced induction gas engines are super efficient, but the turbo and associated piping are complex and another potential point of failure. On the other hand, the idea of a marinized Ecoboost in a displacement hull is intriguing.

The smaller turbocharged engine is more complex, but for the use case where you're running lightly loaded most of the time and only want to use all of the power occasionally, it's probably a better fit. Potentially better enough to justify the complexity. It's not like modern turbos are unreliable anyway.

 

[kthoennes]

Yep, good post (B&B's above). I have a little more experience with diesels now, helping my dock neighbor with the 3988 and my diesel truck. But for a very long time diesel was a strange, scary and foreign territory to me. I rebuilt the gas engine in my Chevy Nova SS when I was 18 but had never touched diesel. Those of us with a higher or broader experience level on engines may not be intimidated by diesels -- and the parts, service, and fuel availability issues that come with them -- but just as B&B says, for lots of boaters there's an intimidation factor to diesel. I know sailboaters overcome that fear all the time and just dive into their little engines under the companionway stairs. I'm not saying it can't be overcome, but it's there for lots of entry stage power boaters.

 

[BandB]

My business neighbors are landscape companies. One is large with multiple locations, the other is a mom-and-pop. The large company stopped buying medium duty diesel trucks, due to to reliability issues. The owner said he'd rather pay the gas bills than have the down time. The small company stopped buying diesel engine mowers. He said he could buy 2 gas mowers for the price of one diesel. He buys a gas mower, runs it daily for 2 years, and sells it for scrap. In both cases, conventional wisdom would tell you that diesel is the way to go.

Once again, "conventional wisdom" isn't always.

I can't speak to the diesel trucks, but can to mowers. We have a lot of landscapers and mowing services as customers and virtually no requests for diesel. Now we have seen a huge swing to electric which for a commercial use requires a lot of extra batteries. However, the equipment has been very trouble free. Then the noise benefit. A few years ago, a young couple in college, started a lawn service in our area. They used all electric equipment. Neighbors would notice. People would drive by. Soon they had 15 or 20 homes in our area, then 50, then hundreds and many crews. People love every aspect of it, environmental and the lack of smell and noise. Now, selling to commercial users we've noticed a huge shift. Even have one huge customer that does every type service imaginable and they do large acreages and small yards and now their rule they follow is large gas equipment when over 100' from any home, but electric when within 100' so in most city residential areas.

Another reason for very little diesel demand on even the larger mowers. They don't trust their crews with two different fuels to put the right fuel in the right equipment.