Diesel Engine

OK, here is another layman's question.

Here is a quote from an online post:
"By the time a boat reaches 16, 000 lbs. or around 36 feet, it is approaching the limit where a gas engine can power it efficiently"

I don't really care about the numbers, whether it should be 36' or 40', but I don't fully understand the "efficiently" part.

Let's say I have a gas engine rated at 300hp. Whether I use it to push a 5000lbs boat or 16000lbs, why does it make any difference? The heavier boat is just going to go slower at the same rpm, right? If I don't care about the speed, where does it "inefficiency" come from?

I understand if I want to push the two boats at the same speed, of course the heavier boat will require more RPM on the same engine, hence more wear and tear. But suppose I always drive the two boats at the exact same RPM, will there be difference as the engine life? Wear and tear etc?

Without getting complicated...yes to diesels for bigger and slower boats...but not always...


All but a few models you buy will have the right engine for the boat in it.


Some can come with either...then it's a matter of how you use the boat.


Diesels are usually a bit more efficient and they produce more power lower in their rpm curve.


That's all I will say as there are so many variable that people will beat to death.


Again...most boats you look at ...you will get an OK engine with it unless the owner has changed something. Sometimes for the better, less often for the worse.

For one, the added torque required to move the mass of the boat, beginning with a standing start. Which is easier for you to move across the floor at all, a box that weighs 100 lbs, or one that weighs 200?

Might want to give Gerr's "The Nature of Boats' a look see.

But would I be correct to say that if I always drive at the same RPM, also assume I rum them the same amount of time, the weight of the two boats don't matter?

I know this is not realistic, just trying to understand the basic concept.

36' is often given as a "break point" where diesel makes sense. But it is really a range, and depends highly on the type of boat. And depends on the type of use.

Above 36', the weight of the diesel is less a factor, and it is easier to isolate people areas from diesel noise.

Above 36', the boats often are used for long range, like actual long trips or 100+ mile fishing trips. Diesel range and efficiency a big plus.

Folks often sleep in the bigger boats, and the low CO emissions of a diesel is a safety plus.

Gas engines commonly available from around 150-350hp. Diesels range from 5hp sailboat pushers up into the thousands of hp. Need a reliable engine over 350hp, going diesel.

Trawlers it makes no sense to install gas, although you can certainly have a good experience with a gas powered boat operating like a trawler.

No hard and fast rules. But in general I do agree that around the mid 30's is the transition from where gas is better to where diesel is better.

There are several reasons why people go with diesel over gasoline engines.

There is about 11% more energy in diesel fuel per gallon.

Diesel engines last much longer on average.

Diesel has a lower chance of combustable fumes.

Back in the mid 1990's, they started marketing diesel fuel by the BTU, and that's when diesel prices spiked, truck drives protested, etc... even with a higher price per gallon, the economy is there over a gasoline engine.

I personally would put the switch point at outboards = gasoline, inboards = diesel if nothing other than safety factor. That's just me.

As you surmised, it all depends on one's definition of efficiency. Tollycraft built some pretty good-sized cruisers equipped with gasoline engines. Were they efficient? They were considered so at the time.

Elco and Higgins built 80 and 78 foot respectively PT boats during WWII that were powered with three Packard V-12 gasoline engines that initially developed 1,200 hp each. Each new boat had to prove it could go some 42 mph or the Navy would not accept it. The Elco boat carried 3,000 gallons of fuel and it was good for one eight-hour mission.

Where they efficient? Not by our cruising standards but they were for the mission they were designed to perform.

And efficiency can be more than just fuel burn for a given distance. Engine cost, engine longevity, frequency of required service and maintenance, reliability, cost of parts, and so on can all contribute to an engine's overall efficiency "rating."

RPM is not the only factor, either. A diesel can usually produce more torque than a similarly-sized gas engine, which means it may be able to power a prop with more pitch at a given rpm than a gas engine. More pitch can equal more boat speed particularly with the heavier, slower kinds of displacement or semi-planing boats we're talking about on this forum.

Well, the word "efficiently" can have a lot of meanings: 1) absolute efficiency in terms of the number of hp a gallon per hour of fuel can make, 2) miles per gallon, or 3) "economically".


All modern recreational gasoline engines: Volvo Penta, Mercruiser Crusader, etc. make about 11-13 hp per gph at wide open throttle (wot). We can't know what they make at reduced throttle because the manufacturer's won't publish that information. But I will bet it is somewhere near that value, maybe less at lower rpms.


A modern marine diesel engine makes 19-20 hp per gph at 50% load on up to 100% load. It drops a bit at low load, maybe down to 16 hp per gph at 15% load. A gasser would be half of that, at best at that load. On an old gasser I had with a Flowscan it made about 6-8 hp per gph at low load, but EFI has improved that some.


So you can put an 8.2 liter gasser (Mercruiser) in a 45', 40,000 lb. trawler and push it along just fine at displacement speed. It will burn almost twice the fuel of an 8.3 liter diesel (Cummins). And you can put a 4.5 liter V6 gasser (Mercruiser) in a 35', 20,000 lb trawler and it will also burn about twice the fuel of a 4.5 liter diesel (John Deer).


Whether that is "efficient" and at what break point in length or displacement it is no longer "efficient" is up to you to decide.


David

chicagoq said:

But would I be correct to say that if I always drive at the same RPM, also assume I rum them the same amount of time, the weight of the two boats don't matter?

I know this is not realistic, just trying to understand the basic concept.

Click to expand...

There are a couple ways to interpret "But suppose I always drive the two boats at the exact same RPM".

Warning-Big generalization... A typical gas marine engine will normally run at a higher RPM for a given HP than a diesel. Transmission gears ratios and propellers are also set up differently for gas vs diesel. Not really sure where you want to go with this. So trying to compare by same RPM is kind of useless. Maybe you wanted to compare an identical gas boat vs diesel boat at the same speed through the water?

Warning-Another Big generalization...
A carb gas engine develops about 10 hp per GPH or 30 GPH for 300 HP
A modern turbo diesel engine develops about 20 hp per GPH or 15 GPH for 300.

I think your question has nothing to do w rpm.
HP is hp and to maintain a given speed on a given boat one needs X amount of hp to maintain a given speed. A boat needs torque to bring the boat up to speed but to my knowledge hp is what keeps the boat moving.

But the big difference in power diesel and gas is that the diesel can usually maintain producing more power continously. So you will need a more powerful gas engine to match the diesel cruising hour after hour. But to maintain a given speed you need X amount of hp. What type of engine, other than the ability to maintain that power it makes no difference what kind of fuel is being burned or what rpm the engine is being run at. It's just the power that can be delivered.

This I think is true but if it's not I'd like to know what part of it is not and of course why.

I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?

chicagoq said:

I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?

Click to expand...

If the engines are the same, running same rpm, and making the same hp, the only difference would be the heavy boat going much slower. Big key part is them both making the same hp, which would depend on the propellers being correctly selected for each boat.

The engines do not know what boat they are pushing, they only know at what point on the hp/rpm curve they are running, could be a skiff or a barge, does not matter.

chicagoq said:

I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?

Click to expand...

Diesel has more energy per gallon.

A diesel can usually extract a greater percentage of energy from a gallon..

Combine those 2 properties and you get in theory better efficiency..but in all...it's just not that simple when deciding which engine type is better for a boat.

Making a diesel engine that is simple, reliable and meets EPA standards....very hard....look at Volkswagon's mess.

First and foremost, good diesels tend to be heavier and more costly than gas. So until you get to a boat size where the weight and cost are less of a large percentage...gas often wins.

Another similar comparison is Inboard versus Outboard power. Outboards were the domain of smaller boats and inboards for larger ones. As outboards became more efficient and bigger in horsepower rating...not up to 30 something feet, you might see either power type in them.

So again in a most general way...up to 35-40 feet, it really depends on the boat and personal preference what it may be powered by....almost never will you see something larger as there just aren't gas engines designed for those HP loads unless it is a slow boat with small gas engines in it....usually something an owner did, not a manufacturer.

"By the time a boat reaches 16, 000 lbs. or around 36 feet, it is approaching the limit where a gas engine can power it efficiently"

This is for PLAINING boats where massive power in a diesel can be had from truck engines instead of running a huge gasser .

For 16,000 lbs in a displacement boat 16 to 25 hp would be required to cruise , no problem for a modern gasser.

To have "efficiency" with todays fuel injected gassers is easy.

In past times the carb would be replaced with a proper sized unit .

With fuel injection and all the complexity of a computer system 12-14 HP per gallon can be achieved.

This compares well with the 16-18 from a properly set up diesel and compares really well with an oversized diesel operating at frequently at 10% to 25% load.

Factor in the higher cost of diesel operating and maint , or GASP,, engine replacement and gas becomes the "efficient " choice.

$5,000 for a gasser perhaps $20,000 to $30,000 for the diesel, you chose.

A modern gasserr will see 3000-4000 hours before simple replacement .

At 75 to 200 hours of recreational annual use , that's a bunch of decades.

For the commercials that run 2000+ hours a year the diesel choice is easier.

RPM isn't what's important. The power capability of the engine and prop power delivery to the water is. An out of gear engine can run at say 2000 RPM and use little fuel, gas or diesel. A loaded engine OTOH will use the amount of fuel it takes to produce the power to spin to the same 2000 RPM.


Efficiency is a function of engine design and fuel used.
If you look up BSFC (Brake specific fuel consumption) you will see that there is a variation in power produced per unit of fuel depending on the type of engine. IN general diesels produce more power per unit of fuel than gasoline engines. So a diesel pushing a boat would be expected to use less fuel than a gas engine pushing the same boat, regardless of RPM

chicagoq said:

I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?

Click to expand...

As long as they were setup properly, yes. Like SKI said, the engines don't care.

Thanks for your replies. I understand this much better now.

How does the higher torque ,the diesel has play in to this?

bluemain,
I think basically the only difference is in the continous ratings.
If you needed 100hp continous you'd need a 140hp diesel engine but probably a 180hp gas engine. The gas engine still being far cheaper. And after you've chosen the 180hp gas engine you may have the same torque as the 140hp diesel.

I'd say torque means little and perhaps nothing. Torque helps start the boat moving but adds nothing to maintaining a steady cruise. That's my opinion but I'm not really sure about the torque.

Torque = spin slower, spin bigger and move the same cheaper. No explosions.

One more thing, sorry about being obnoxious, but go out on somebodies else's boat and see what you think (or charter one). Experiencing something can assist in decision making. If you do not like it and it is not what you want opinions are not worth squat.

My present car is a slightly larger turbo diesel version of its gas NA predecessor, engine is 200cc bigger. The diesel version is much more tractable, more torque, unfazed by hills,uses much less fuel. Boats is boats, cars is cars, but it tells me something.

Gasoline engine is relatively inexpensive to initially purchase (4 to 5X less than a diesel engine), and generally simpler to maintain during its useful life, and equally less expensive to replace after some 3000 to 5000 (generally no more) hours of carefully maintained and generally easy use. At equal HP output... Gas engine will consume a bit more fuel per hour than applicably sized diesel engine.

Diesel engine is relatively expensive to initially purchase (4 to 5X more than a gasoline engine), and generally more difficult to maintain during its useful life, and equally more expensive to replace after some 5000 to 15000 (or more - depending on engine type) hours of carefully maintained and generally easy use. At equal HP output...Diesel engine will consume a bit less fuel per hour than applicably sized gas engine.

Therefore... for your choice of engine type: You can take the statements above (YRMV - compared to my experience, with gasoline and diesel) as well as the hundreds of other variables in a particular boat's (and boat owners's) "needs" for one or the other type engine to make your decision.

Happy Boat-Power Daze! - Art

PS: As mentioned several times before in this thread - I too believe that at the point of some 20K +/- lbs dry and 40' +/- feet long gasoline engines should probably be supplanted with diesel engines. Much past that weight and size boat will almost surely function better with diesels. Our 34' Tollycraft tri cabin at 20K lbs loaded and with twin 255 hp Mercruisers functions perfectly well to fulfill our desires.

I'd suggest the advantages of diesel are not limited to larger boats.

Our boat has been built with 315hp 454 gas, and with 260hp Volvo diesel. It gets about 40% more miles per gallon with diesel. A buddy with a 2859 Bayliner repowered from 454 gas to Yanmar 6LP diesel, and saw about the same advantage in fuel economy and thus range.

Even though we are only 26 feet, with limited fuel capacity (ours is 110 gal) the efficiency of diesel vs gas can be a huge advantage. Cruising the distances of the Inside Passage, diesel gives us far greater range on a tank, and thus much more flexibility in the routes we choose to wander. For us, the $7,500 higher initial cost for the diesel has been well worth it.

Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.


For the OP, best to stick with generic concepts until he has a specific boat or powerplant in mind.

torque turns the prop. At any speed the torque is continuously needed to spin the prop. Torque does the work HP is simply the measure of how much work is done over time.
Greater torque means moving more water is possible.

psneeld said:

Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.

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My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.

bayview said:

torque turns the prop. At any speed the torque is continuously needed to spin the prop. Torque does the work HP is simply the measure of how much work is done over time.
Greater torque means moving more water is possible.

Click to expand...

bayview that's a statement. I've heard it many times. I don't believe it. Torque is force like voltage. There can be lots of torque w/o any movement of the shaft but no movement will happen w/o work being done. If your boat is moving work is being done and that's hp .. not torque. Torque is a static value. Torque increases the shaft speed or accellerates a car or starts a boat moving but hp is what poweres the boat.

An old wives tale said that long stroke engines had more torque. Don't belive that either. Buick went from very long stroke to short in 1953. All the short stroke Buick engines made more torque than the old long strokers that preceeded them. The same is probably true for Chevy's. I've always said it SEEMED the short strokers didn't have much torque because the torque was overshadowed by all the increased power .. hp.

But like you I do believe torque has something more to do w power than a static force. I think I used to know but have long since forgotten. I'm hoping someone can relight my extinguished flame.

psneeld said:

Comparing an efficient diesel to an inefficient gas and running the boat differently isn't necessarily a fair comparison.


If you compare state of the art and power the boats so the performance is exactly the same and run exactly the same....then it is more fair.


For the OP, best to stick with generic concepts until he has a specific boat or powerplant in mind.

Click to expand...

Agreed - especially for larger boats; e.g. those over 32' and/or over 20K lbs loaded. Of course hull shape plays heavily into efficiency levels too. Each boat/engine combo always has its own tricky nuances.

Boattest.com recently did a review (well actually more of a features recital) of Mercury's new 3.0 liter 260 hp marine diesel engine which was derived from Volkswagen's TDI automotive diesel.


They "compared" it to a Mercury 6.2 liter 320 hp gasser which delivers roughly the same performance as the diesel. What was interesting was the pricing quoted:


The MSRP for the Mercury Diesel 3.0L V-6 260 is $33,533.
The MSRP for a Mercury 6.2L 320 ECT TowSports gas inboard is $20,550.


At least for these pairs of diesel/gassers the price is not 4-5 times higher for the diesel. OTOH, I don't think that the Mercury diesel will last the 5-15,000 hours quoted above. I think that if used the same way, both the Mercury diesel and the equivalent gasser will probably last roughly the same length of time. I did note that the EPIC 23V that boattest used to compare the two engines was propped very lightly for the diesel, which probably helps with longevity.


I also take issue with the "a bit more fuel for the gasser" statement above. It is a lot more fuel. The Mercury diesel produces 20 hp per gph of fuel at wot. The equivalent gasser produces about 12 hp per gph. At about 90 hp where boattest reported the "best cruise" for that engine, the diesel burns 4.3 gph, the gasser would burn 7.5 gph, 74% more.


David