Gas engines / verses Diesel on larger boats ??

[sunchaser]

Just a personal thought based on no evidence.

My rule of thumb is under 30 feet-gas, over 30 feet-diesel.
But of course there will be exceptions

Exceptions galore. Reading the boat mags, touring marinas, accessing offshore fishing grounds or visiting launch ramps is revealing. Outboards over 30 feet are skyrocketing in use. It is amazing how things have changed. Whether Grady White, Tiara, many Al builders, Backcove, MJM or Ocean Alexander the outboard vessel options are endless.

Understandably, the majority of boaters who populate TF prefer old slow boats with diesel inboards outnumbering gas probably 20:1. It is likely fair to say our pocket book, age and or cruising style determines our vessel of choice.

A common conversation for those over 30' and outboard inclined is discussing the pros and cons of a Seven.

 

[Art]

I had diesel boat and it was OK to deal with, engine rather expensive though in the long run. My wife hates the smell of diesel fuel and exhaust. Gotta say; those two odd/unpleasant smelling items are not my favorite either!

We've had plenty of good running well-constructed gassers; three vintage craft right now that we keep in really good condition:
- 1977, 34' Tollycraft tri cabin with twin screw 255 hp 350 cid Mercruiser straight drive. Planing hull that can lope along just below hull speed [of 7.58 knots] at 6.5 to 7 knots and get 2 +/- nmpg or plane-out at 16 to 17 knots and get 1 +/- nmpg. Going really slow, 4.5 to 5 knots using only one engine... nearly 3 nmpg! This well outfitted and comfortably laid out "cruiser" pleasure boat is a fun play-toy to own. As are the next two boats mentioned.
- 1989, 19' Blue Water cuddy cabin ski boat with 225 hp Mercruiser I/O
- 1975, 15' Crestliner "Stinger" 4 seater with 50 hp Johnson O/B - Our tow behind runabout.

For people who are only going to use their boats for fun [one to two hundred hours run time per year] difference in general cost of engine is night and day [gas being the less expensive drive unit]. IMO for those folks good gas engines are the way to go!

For people who are going to cruise many hundred and into the thousands of hours per year, IMO good diesel engines are the way to go!

For boats up to 36' +/- I believe that correctly sized, geared and propped gas engines can provide plenty of power. That includes D, SD, and P hulls.

For boats in the above 36' range... diesel engines seem to best fit the bill. Although, I'm not saying that in boats up to 45' or slightly above... that some gas engine layouts would not also be suited for cruising conditions.

IMO - Boat above 50' = Diesel for sure!

We use our boats for local fun... not coastal cruising. If we ever decide to go long range then our boat would probably be in the 50' to 60' size category; diesel powered for sure!

 

[moparharn]

I am a little surprised

that discussion surrounding torque is not a bigger part of this conversation. Torque is far more important than HP. Diesels are selected principally for their torque curves. Keeping a boat at a selected speed is a function of torque. You can buy 350 hp small blocks just as easily as big blocks, so why are big blocks the engine of choice? Torque and engine weight/durability at high output. My last diesel truck made 250 hp and 750 pound feet of torque. I could pull a mountain with that engine. A 250 hp gas motor would not even budge it. Pulling a boat through water is no different than pulling a trailer. Torque is king and horsepower is almost meaningless when it comes to constant load.
Fuel efficiency, torque, and longevity is why I would want a diesel. If I cannot afford it then a Crusader EFI big block is my second choice.
I have been around boats nearly sixty years and have never seen or had first hand account of one blowing up. I was a dock boy for two years of this as well. I hear of houses blowing up all the time, yet we don't heat with diesel for safety reasons. Gas is cheaper and easier. Kind of the same with a boat. Bill

(Opinions expressed are those of a moron and should be taken with some level of doubt)

 

[Nomad Willy]

Jackhulse wrote;
“If you want to go with 2 couples then you would be looking at a 40+ or larger boat to have two state rooms and a 50' boat to have two heads with showers. Even a 60' boat will not have the room of a 2 bdr condo.”

Folks do it fairly often on an Albin25. Has a little aft cabin. I see more big boat mentality.

 

[rslifkin]

My last diesel truck made 250 hp and 750 pound feet of torque. I could pull a mountain with that engine. A 250 hp gas motor would not even budge it.

Actually, that's not the case. The gas engine would require lower gearing to get the same torque at the wheels to move the load, but if both make 250hp, both are capable of doing the same work (once the gearing is adjusted to the engine's power curve) and both would move the load at the same speed.

The shape of the power curve does matter with a boat, although unless you build a seriously high revving performance engine, it's unlikely to be much of an issue unless the boat is bordering on underpowered, where it may struggle to get up into the higher RPM at all if the props have good bite at low speeds and the engine doesn't have enough low rpm torque.

The small block vs big block thing comes mostly from one big thing: it's easier and cheaper to build a durable 350 - 400 hp big block with a wide powerband than doing it with a small block.

Personally, if I had to build an ideal gas boat engine, it would be something very durable but small displacement like a beefed up 4.8 liter LS motor or similar. And then add a turbo to get the desired power output with a wide powerband and without needing to rev it to the moon. If done right, it should be able to make easily the same power as a lot of the big blocks do with as good or better powerband and by virtue of being smaller displacement with boost, it should be noticeably more efficient at low power outputs than the big block would be (due to lower pumping losses).

 

[Art]

that discussion surrounding torque is not a bigger part of this conversation. Torque is far more important than HP. Diesels are selected principally for their torque curves. Keeping a boat at a selected speed is a function of torque. You can buy 350 hp small blocks just as easily as big blocks, so why are big blocks the engine of choice? Torque and engine weight/durability at high output. My last diesel truck made 250 hp and 750 pound feet of torque. I could pull a mountain with that engine. A 250 hp gas motor would not even budge it. Pulling a boat through water is no different than pulling a trailer. Torque is king and horsepower is almost meaningless when it comes to constant load.
Fuel efficiency, torque, and longevity is why I would want a diesel. If I cannot afford it then a Crusader EFI big block is my second choice.
I have been around boats nearly sixty years and have never seen or had first hand account of one blowing up. I was a dock boy for two years of this as well. I hear of houses blowing up all the time, yet we don't heat with diesel for safety reasons. Gas is cheaper and easier. Kind of the same with a boat. Bill

(Opinions expressed are those of a moron and should be taken with some level of doubt)

I would say you are more "More-On" than many -

 

[moparharn]

I suppose with a sixty speed transmission you might have a point, but in a practical sense your case is hard to make an every day appeal for. Is is not practical in any sense to leave torque out of the equation. The higher torque motor will greatly outperform the lower torque motor of the same horsepower. It will have the economy and longevity that is "real world" for marine applications. Horsepower is for salesman. Not really sure why more is not made of Torque. In the sixties the race was on for horsepower, which is why it was rated differently back then. Horsepower sold cars and it continues to be a buzz word today. Try selling a work truck on horsepower. Trawlers are work trucks. Donzi's are performance cars. I say all of this in a friendly and practical manner and not a combative and theoretical manner. I am also not claiming to be 100% correct, just curious as to why torque is being left out as much. Why will a Tesla smoke a Porsche twin turbo? Torque Same thing with Elon Musk pulling a train with his pickup truck and also hitching it to an F-150. Torque.
In short, diesel is torque and torque is good. Thats all.

Actually, that's not the case. The gas engine would require lower gearing to get the same torque at the wheels to move the load, but if both make 250hp, both are capable of doing the same work (once the gearing is adjusted to the engine's power curve) and both would move the load at the same speed.

The shape of the power curve does matter with a boat, although unless you build a seriously high revving performance engine, it's unlikely to be much of an issue unless the boat is bordering on underpowered, where it may struggle to get up into the higher RPM at all if the props have good bite at low speeds and the engine doesn't have enough low rpm torque.

The small block vs big block thing comes mostly from one big thing: it's easier and cheaper to build a durable 350 - 400 hp big block with a wide powerband than doing it with a small block.

Personally, if I had to build an ideal gas boat engine, it would be something very durable but small displacement like a beefed up 4.8 liter LS motor or similar. And then add a turbo to get the desired power output with a wide powerband and without needing to rev it to the moon. If done right, it should be able to make easily the same power as a lot of the big blocks do with as good or better powerband and by virtue of being smaller displacement with boost, it should be noticeably more efficient at low power outputs than the big block would be (due to lower pumping losses).

 

[psneeld]

If torque were the only factor...no one would have ever used turbine engines for anything requiring "power".



But the icebreaking ship I was on prefered using one turbine versus multiple diesels of even greater horsepower to break ice.....so take the "torque" theory and do whatever you want with it...but like all discussions....specifics work, generalizations are only that. There can be gasser applications where torque is only a fraction of the need.

 

[moparharn]

"Personally, if I had to build an ideal gas boat engine, it would be something very durable but small displacement like a beefed up 4.8 liter LS motor or similar. And then add a turbo to get the desired power output with a wide powerband and without needing to rev it to the moon. If done right, it should be able to make easily the same power as a lot of the big blocks do with as good or better powerband and by virtue of being smaller displacement with boost, it should be noticeably more efficient at low power outputs than the big block would be (due to lower pumping losses)."

This sounds like the Ford Ecoboost. 365 horsepower and..........420 pound feet of torque.
A purpose built "torque" motor designed with highly specialized parts to withstand the load. It is easier and cheaper to go big block, but you are correct in the turbo direct injection application. The eco boost is a great motor, but it is expensive to build and is successful based on its superior torque curve. Bill

 

[moparharn]

Ok, just one more generalization. Are most ships built diesel electric or turbine? Diesel.

If torque were the only factor...no one would have ever used turbine engines for anything requiring "power".



But the Icebreaker I was on prefered using one turbine versus multiple diesels of even greatr horsepower to break ice.....so take the "torque" theory and do whatever you want with it...but like all discussions....specifics work, generalizations are only that.

 

[rslifkin]

Thinking about it, in trucks, high torque engines are favored over higher revving, low torque with short gearing for 2 reasons: not revving the crap out of the engines tends to lead to somewhat longer lifespan (but that's also design dependent). And the average person absolutely hates revving the crap out of an engine all the time, as they perceive that as being underpowered. So when they only need 50% of the engine's output for the current task, they'd prefer the one that'll do it at 2000 RPM instead of the one that requires a downshift and revving to 4000 even if either one would do the job just as well.

 

[rslifkin]

Ok, just one more generalization. Are most ships built diesel electric or turbine? Diesel.

But that's mostly for efficiency, not due to power / torque output. Turbine efficiency isn't great with variable loads, while a very large, slow turning diesel can be very efficient.

 

[psneeld]

Ok, just one more generalization. Are most ships built diesel electric or turbine? Diesel.

This was not diesel electric...it was straight gearbox.


And sure if you don't need huge amounts of HP for many ship applications, turbines are not suitable.... but that wasn't my point...


It's that torque isn't the only superstar in engine selection...it's just one factor....more so in some applications, less so in others.

 

[moparharn]

I get your point. The problem with rpm is longevity. The effective loads on nearly every part of the engine go up exponentially, never mind linear. Internal combustion reciprocating engines do not last in high rpm conditions. If a Nascar engine is lucky enough to finish the race it is rebuilt immediately with nearly all new parts.

Thinking about it, in trucks, high torque engines are favored over higher revving, low torque with short gearing for 2 reasons: not revving the crap out of the engines tends to lead to somewhat longer lifespan (but that's also design dependent). And the average person absolutely hates revving the crap out of an engine all the time, as they perceive that as being underpowered. So when they only need 50% of the engine's output for the current task, they'd prefer the one that'll do it at 2000 RPM instead of the one that requires a downshift and revving to 4000 even if either one would do the job just as well.

 

[moparharn]

"It's that torque isn't the only superstar in engine selection...it's just one factor....more so in some applications, less so in others."

Ok, but this is a trawler conversation and that is what is guiding my thinking in regards to the OP. For my trawler I would want torque.

For the record, I don't even own a trawler. Yet. Please see comment about moron.

 

[smitty477]

I get your point. The problem with rpm is longevity. The effective loads on nearly every part of the engine go up exponentially, never mind linear. Internal combustion reciprocating engines do not last in high rpm conditions. If a Nascar engine is lucky enough to finish the race it is rebuilt immediately with nearly all new parts.

"I get your point. The problem with rpm is longevity."
Assuming that the higher rpm also has higher loads that would be correct.

 

[moparharn]

I guess I was thinking that this applies regardless of engine load. RPM causes greater stress on the engine parts. The con rods and fasteners elongate, as does the crankshaft and its fasteners, valve spring fatigue, etc etc.

"I get your point. The problem with rpm is longevity."
Assuming that the higher rpm also has higher loads that would be correct.

 

[rslifkin]

I guess I was thinking that this applies regardless of engine load. RPM causes greater stress on the engine parts. The con rods and fasteners elongate, as does the crankshaft and its fasteners, valve spring fatigue, etc etc.

That's absolutely true, but unless we're talking very high sustained RPM (as in Nascar), it can be mostly mitigated with engine design. And putting out lots of power at low RPM has its own set of stresses that have to be designed for.

 

[smitty477]

I guess I was thinking that this applies regardless of engine load. RPM causes greater stress on the engine parts. The con rods and fasteners elongate, as does the crankshaft and its fasteners, valve spring fatigue, etc etc.

I have seen many examples where lower rpm and overloading (overpropping) have caused serious problems with marine engines.
In most cases higher rpm causes no harm but overloading to make lower rpm does.
RPM alone cause little stress.

 

[Art]

Metal stretches. Different metals and metal unit configurations/sizes have different stretch [i.e. hold it snug into place] coefficients. Thus... there are "torque" wrench foot pound values that engineers apply to bolts on nearly any mechanical apparatus' fastener or stressed part thereof.

If the constant rpm [or temporary rate of acceleration] torque stays inside the engineered torque range of any part of the apparatus then the apparatus' designed life span will continue to be satisfied.

If the torque is constantly [or even momentarially, again and again] elevated higher than what it has been engineered to experience then metal failure may be experienced sooner rather than later.

Example: Recent post mentioned race cars engines being totally rebuilt between races. That is because, no matter how well engineered and no matter how good the material used... during the race... torque applied to all parts of the engine is higher than can be used if the engine was to last for more than one race. In other words... the engine is driven to it's absolute limits of endurance - in attempt to WIN the race!

Now - Back to normally used engines. Well designed engines, of whatever size or type, will simply last longer with less breakdowns if they are used "gently" and serviced "correctly". But - if they are pushed too hard or their service is neglected - Poof! A new expensive engine is needed!

Each user of an engine needs to calculate how "gentle" of use and how often of maintenance they will provide their engine. Following general manufacturer guidelines usually works well. Just remember... the more gently of use and often of maintaince we give our engines usually = how long our engine will run well for us!

YRMV

 

[psneeld]

"It's that torque isn't the only superstar in engine selection...it's just one factor....more so in some applications, less so in others."

Ok, but this is a trawler conversation and that is what is guiding my thinking in regards to the OP. For my trawler I would want torque.

For the record, I don't even own a trawler. Yet. Please see comment about moron.

Well keep reading because pretty much no one agrees with what a trawler is from a 20 something outboard to a 100+ mega yacht.


So your definition of trawler and torque necessity might jive just right...but generalizations here don't fly in my book because they don't.


Be specific to boat type and engine type and your thrusts may just might fall into high probabilities.


Even if I totally agree with your and my definition of trawler and best engining of said trawler...it doesn't make it an absolute.

 

[moparharn]

“And putting out lots of power at low RPM has its own set of stresses that have to be designed for.”

So this leads me to a question. I assume what I have quoted is the reason Diesel engines are so “robust”.
My question- I have heard that horsepower per pound of engine weight is one way of judging how long the engine will last all things being equal.
Is this true? Is this something you guys look at? Or gals.
It seems diesels have big heavy castings. Is this because of the tremendous torque they create? An honest question. I am not leading anyone down some rabbit hole. I really do not like to argue or get into any internet conflict. Unfortunately my communication style is not as soft as it probably should be. My apologies for any anger I may have inspired. Bill

 

[Art]

“And putting out lots of power at low RPM has its own set of stresses that have to be designed for.”

So this leads me to a question. I assume what I have quoted is the reason Diesel engines are so “robust”.
My question- I have heard that horsepower per pound of engine weight is one way of judging how long the engine will last all things being equal.
Is this true? Is this something you guys look at? Or gals.
It seems diesels have big heavy castings. Is this because of the tremendous torque they create? An honest question. I am not leading anyone down some rabbit hole. I really do not like to argue or get into any internet conflict. Unfortunately my communication style is not as soft as it probably should be. My apologies for any anger I may have inspired. Bill

Rule of thumb I go by, for best chance of a lasting engine life: Total HP capability of engine should not be more than 75% of engine's cubic inch.

 

[moparharn]

Thank you. So for your purposes engine weight is not a consideration, at least in as much as cubic inch may have a relation to engine weight. Do the two usually move together in your experience?

 

[rslifkin]

Conventionally diesels are heavy for a couple of reasons: big, beefy castings to support the forces involved in making lots of power at low RPM. Low power density leads to large displacement relative to power output, so that drives up size and weight.

On the other hand, a lot of the higher revving, smaller displacement diesels that don't make as much low end torque (and are probably also not quite as durable) fall much closer to a gas engine in terms of weight vs power.

As an example, the 370hp Yanmar 8LV weighs almost exactly the same as a fully marinized 454 big block. Another would be the Cummins QSB 6.7 vs the John Deere 6068. The Cummins revs higher (3000 and 3300 for the higher ratings vs 2800 for the highest rating of the JD). The Cummins is also a little over 300 lbs lighter, despite having almost the same displacement. And yet the highest rating for the Cummins is 550hp compared to 400 for the JD. And both are known for being quite durable when operated reasonably.

At lower RPM, it's a tradeoff of more force being applied to the pistons, rods, crank, bearings, etc. per cycle vs the forces involved with spinning all of those parts faster, cycling valve springs more often, etc. at higher RPM. Within a reasonable range, either one can be designed for. For the most part, trying to keep RPM down too much will lead to adding a bunch of weight. And letting RPM get too high makes it hard to keep valve springs alive, etc. or at least drives the design cost up too much. And in some cases, having a wider usable RPM range can benefit the application even if it doesn't improve the engine design.

 

[Art]

Thank you. So for your purposes engine weight is not a consideration, at least in as much as cubic inch may have a relation to engine weight. Do the two usually move together in your experience?

That depends on how the engine is set up.

 

[Art]

Conventionally diesels are heavy for a couple of reasons: big, beefy castings to support the forces involved in making lots of power at low RPM. Low power density leads to large displacement relative to power output, so that drives up size and weight.

On the other hand, a lot of the higher revving, smaller displacement diesels that don't make as much low end torque (and are probably also not quite as durable) fall much closer to a gas engine in terms of weight vs power.

As an example, the 370hp Yanmar 8LV weighs almost exactly the same as a fully marinized 454 big block. Another would be the Cummins QSB 6.7 vs the John Deere 6068. The Cummins revs higher (3000 and 3300 for the higher ratings vs 2800 for the highest rating of the JD). The Cummins is also a little over 300 lbs lighter, despite having almost the same displacement. And yet the highest rating for the Cummins is 550hp compared to 400 for the JD. And both are known for being quite durable when operated reasonably.

At lower RPM, it's a tradeoff of more force being applied to the pistons, rods, crank, bearings, etc. per cycle vs the forces involved with spinning all of those parts faster, cycling valve springs more often, etc. at higher RPM. Within a reasonable range, either one can be designed for. For the most part, trying to keep RPM down too much will lead to adding a bunch of weight. And letting RPM get too high makes it hard to keep valve springs alive, etc. or at least drives the design cost up too much. And in some cases, having a wider usable RPM range can benefit the application even if it doesn't improve the engine design.

Yeah - What he said!

In a nutshell [jokingly]: An engine's attributes for size, weight, hp and torque... "Angle of the dangle is directly proportional to the hypotenuse of the square!"

 

[Toki]

So, I think this thread has drifted enough for me to ask a semi-unrelated question.

My Ranger 27 contains the Volvo-Penta D3-220. This same basic engine is available from 110 HP up to my 220 HP. I've heard differing explanations of how the same engine can span such a range of rated HP. Here's some questions:

I'm going to assume the guts of these engines is all the same, so I'd further assume the D3-110 running at full load would be MUCH more durable and long lasting than the D3-220 running at full load. Sound about right?

Lets say I typically run at a rate that requires 100 HP. Which will be more durable and long lasting, the D3-110 running at 100 HP or the D3-220 running at 100 HP?

 

[rslifkin]

So, I think this thread has drifted enough for me to ask a semi-unrelated question.

My Ranger 27 contains the Volvo-Penta D3-220. This same basic engine is available from 110 HP up to my 220 HP. I've heard differing explanations of how the same engine can span such a range of rated HP. Here's some questions:

I'm going to assume the guts of these engines is all the same, so I'd further assume the D3-110 running at full load would be MUCH more durable and long lasting than the D3-220 running at full load. Sound about right?

Lets say I typically run at a rate that requires 100 HP. Which will be more durable and long lasting, the D3-110 running at 100 HP or the D3-220 running at 100 HP?

If they're actually identical guts, then in theory, you're correct. And for 100hp steady output, it would come down to whether there's a durability difference due to different operating RPM at the 100hp load point (being that they'd be propped differently unless there's a huge rated RPM difference).

However, they're not necessarily identical engines. It's possible one is aftercooled and one isn't. And if they do anything like Cummins historically has, the guts may be a little different on the higher output versions.

 

[mahal]

Only problem is, even with that savings, assuming a full re-power (engines and generator) could be done with modern diesels in the 350 - 400 hp ballpark for $120k, it would still take something like 3000 hours of running to make it up in fuel cost (using the somewhat high prices at my home fuel dock of $3.99 diesel, $4.39 gas). At 50/50 running slow vs on plane, the gas engines will burn something like 56,250 gallons in 3000 hours, while the diesels would burn 31,800 gallons. That's a cost of $246,937.50 for gas, $126,882 for diesel, or a $120k fuel savings in 3000 hours.

The resale value of your boat should increase with the repower, so maybe you don't need to recoup the entire bill.