Bigger engines consume more diesel under cruising?

[Macwoody]

Originally Posted by MicHughV

diesels are not like gasoline engines.....their fuel burn rate can differ even at the same rpm...depending on the load...

diesels also have different ratings....continuous...intermittent, etc....too involved to explain here...which affect fuel consumption....different diesels engines are designed to operate at specific rpm's...bigger diesel engines typically have much lower rpm's than smaller diesels.....it's a very involved science...


a 70 hp sailboat diesel at cruising rpm may only be producing ....say 40 hp...
a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced....therefore a 70 hp engine at cruise speed will burn about 2 gallons/hr..this is not set in stone, but a rough guideline....it's not uncommon for a 70 hp sailboat engine to consume less.....having a turbo charged engine is another variable.

diesels " burn" fuel, and this is a critical difference compared to a gasoline engine...for a diesel to " burn" fuel efficiently it must be loaded correctly....for instance, running a diesel engine to to turn the alternator to charge the batteries is not a very efficient way to run a diesel...

hope this provides some guidance...I have found the 1 gallon/hr per 20 hp produced is fairly accurate for sailboat diesels.......there is often a misconception about diesel engine horsepower figures....think of it this way....it's work produced....kinda like riding a bike....on a flat road,you can pedal at reasonable speed all day long, but going uphill at the same speed requires your legs to work much harder....and you will soon be exhausted....

On top of the above and since nobody else commented on this part yet, I would add that you would need to make sure you use a variable prop to use the power you would have at similar RPM. I have a 166HP Steyr diesel, which at 1600 RPM pushes my 60ft, 28ton alu monohull to about 8.5knts cruising speed and uses between 8-10 liters an hour at that speed (mainly depending on wind direction and strength where that ends up). I agree with everyone that sticking 2 of those huge engines in that cat makes no sense though!

[MicHughV]

true....cruise ships...being ever mindful of operating costs, implement this....the older cruise ships will have their twin diesels run at a constant rpm, but adjust the propellor pitch to suite their needs....
modern cruise ships...power those ships with electric motors, which in turn are powered by a diesel/electric generator, but in both cases, best fuel economy is attained by a constant rpm...

back in my youth I flew model r/c airplanes. an expensive hobby regarding fuel costs..then the market came out with "diesel" model airplane engines, touting their fuel efficiency, so I got one. I think it was a 1cc engine as I recall, but model airplane diesel fuel was also expensive, so I learned to make my own..consisting of castor oil, ether and kerosene. The ether I would source from a local pharmacy, always getting the sideways eye, when purchasing this. Tried using regular diesel fuel without any success.

I've read you can pour just about any flammable oil in a diesel and it will run, butter, lard, cooking oil, etc. Never tried it, but intrigued. Have my exhaust fumes smell like french fries could not be viewed as a bad thing.

[mikedefieslife]

Depending on the cat involved the OP might do well to take a look at the equivalent power cat if available. It might very well be a massively different design, but the older ones weren't too much different from their sailing brethren.

Looking at the specs of that, might give him a better idea of the type/size of engine he could fit. Just spec it down a little to compensate for the weight of the sailing items.

My old Snowgoose 35 has a Yanmar 3GM30. Apparently the same hull (minus the sailing gear) was used for a one-off power boat that would do 25knts on the plane. No idea of the engine size or placement though.

[cottonsail]

We just went from a Westerbeke W-40(37HP) to a Volvo D2-50F (49 HP). Same 33' 15000 lbs displacement sailboat . W-40 was run about 2150 rpm and about 6.2 KTS burned 1 GPH. The Volvo runs about 2750RPM at 6.9 KTS and burns 1.06GPH. Just my experience.

[a64pilot]

Diesels and especially turbines can run off of most any flammable liquid to some extent.
But that’s true of most engines, Diesel engines originally ran off of petrol. Later many were manufactured so they had spark ignition and initially ran on petrol and were later switched to Diesel after they warmed up.
Many Diesel engines after WWII I think burned “tractor gas” which was kerosene and the cheapest available fuel, and I think many gasoline engines ran off of it too after being initially started on petrol and warmed up.
The Ford Model T would run off of petrol, benzine, ethanol and even kerosene. I think something had to be modified for it to run on kerosene though, maybe the carburetor?

US Military runs all its Diesels off JP-8 which is basically kerosene.

[MicHughV]

I'm liking the fuel that smells like french fries... I think I might be on to something here.

I've come to the conclusion, that few people understand diesel engines, their fuel burn rate, horsepower and torque ratings, etc, etc.....at the end of the day, for the individual boater and their respective craft...it just is what it is...nobody is going to change out their current engine to another model for any reason just because someone here sez so........we are sailors...cheap by nature....our diesel selection invariable revolves around price, not performance.

but...where was I .....french fried exhaust fumes...yes, I see a market here..

[Shaneesprit]

I only have a 25ft CAT but with 25hp I can tell you that the engine has more than enough power, - Too much in my opinion, but changing it for a smaller engine isn’t enough of a weight saving to make it worthwhile.
Ideally you want the engine that fits the size of boat and no more.

I wonder what idea started this thread?

[Shaneesprit]

If you want to run on cooking oil, the recipe is very simple, just add 25ml of white spirit to every 25Litres.
However there are some drawbacks, namely in that, most diesels not like it, the fuel pumps cannot cope with the consistency, some engines fair better than others, as a rough guide, the High Pressure injection pump needs to be inline type rather than rotary.
That’s why the favourite car of cooking oil enthusiasts is an old Volvo.

[Boatfreak]

I cannot say if this is true or not but I was told this story by the owner of a boatyard just before he presented his bill. He told me that he had inherited a large motor cruiser with two 450hp engines from his father. He spent about 6000 euros on the engines to get them in good shape and used the boat for a season. When it came to the annual service he found that the water pump impellers were 450 euros each. He sold the boat and installed a swimming pool at his house.

The moral of the story which was for my benefit, was if you can't afford to own a boat don't have one, don't complain about the cost of running it.

The moral of the story for the OPs benefit is a larger engine will not make much difference to the speed but be much more expensive to service. An impeller for my Yanmar 2GMs is about 20 euros.

[Shaneesprit]

Maybe the moral of the story is not to not have a boat but to have a boat you can afford!
In reality most boat owners probably struggle with the cost of ownership, that doesn’t mean you shouldn’t have a boat, but they always cost more than you budgeted.
It’s all a balancing act between the boat the porsche and which house I can afford. - in that order lol
Hopefully I will get to use a couple of litres of diesel soon, I have an 8 week season this year starting soon.

[cottonsail]

Originally Posted by Shaneesprit

I only have a 25ft CAT but with 25hp I can tell you that the engine has more than enough power, - Too much in my opinion, but changing it for a smaller engine isn’t enough of a weight saving to make it worthwhile.
Ideally you want the engine that fits the size of boat and no more.

I wonder what idea started this thread?

I have a 33' sailboat with a 15K displacement. The original engine was a Volvo MD2B, 21 HP. It was seized up when I bought the boat. Junked it. I put in an overhauled Westerbeke W-40(37HP). This engine was instant starting and had reasonable power. Cruise speed was 6.2KTS. Too many auxiliary problems over the next 5 years. This year I replaced the Westerbeke with a Volvo Penta D2-50(49HP). Cruise speed is now 6.9 at the same 1 gal per hour as the Westerbeke. The engine weights was the VP was 20lbs heavier. The new engine is absolutely sweet, but I would not recommend Volvo Penta due to their horrible customer support. They treat giving out spec's like they are giving military intelligence to the enemy.

[Grimgrak]

Wow scary your like a mind reader!
Perhaps this would be better? https://www.yachtworld.com/boats/200...maker-3649230/

Originally Posted by Thumbs Up

Pretty sure that the OP is asking about this one:https://www.multihullsolutions.com.a...63-phoenix-xi/
I an not so sure about this statement: "a typical rule of thumb is that a diesel will consume about 1 gallon/hour for every 20 hp produced." My Volvo md2030s burn about half a gallon of fuel per hour (each engine). They are rated at 29HP. I run them at around 2800 rpm. I make 5.5 knts with one and over 7 knts with both so am getting closer to double the efficiency than that rule of thumb on these non turbo engines.