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Old 01-28-2016, 04:43 PM   #1
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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?
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Old 01-28-2016, 04:54 PM   #2
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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.
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Old 01-28-2016, 04:58 PM   #3
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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.
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Old 01-28-2016, 05:36 PM   #4
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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.
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Old 01-28-2016, 05:55 PM   #5
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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.
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Old 01-28-2016, 06:05 PM   #6
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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.
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Old 01-28-2016, 06:06 PM   #7
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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.
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Old 01-28-2016, 06:21 PM   #8
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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.


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Old 01-28-2016, 08:28 PM   #9
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Quote:
Originally Posted by chicagoq View Post
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.
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.
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Old 01-28-2016, 09:17 PM   #10
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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.
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Old 01-28-2016, 11:27 PM   #11
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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?
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Old 01-29-2016, 01:05 AM   #12
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Quote:
Originally Posted by chicagoq View Post
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?
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.
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Old 01-29-2016, 07:10 AM   #13
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Quote:
Originally Posted by chicagoq View Post
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?
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.
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Old 01-29-2016, 08:31 AM   #14
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"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.
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Old 01-29-2016, 10:22 AM   #15
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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
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Old 01-29-2016, 01:58 PM   #16
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Quote:
Originally Posted by chicagoq View Post
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?
As long as they were setup properly, yes. Like SKI said, the engines don't care.
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Old 01-29-2016, 02:34 PM   #17
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Thanks for your replies. I understand this much better now.
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Old 01-29-2016, 08:22 PM   #18
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How does the higher torque ,the diesel has play in to this?
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Old 01-29-2016, 08:41 PM   #19
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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.
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Old 01-29-2016, 08:47 PM   #20
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Torque = spin slower, spin bigger and move the same cheaper. No explosions.
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