Diesel engine/prop/hull efficiency questions

Hi,

CaptJ-Breaking my own decision to say no more on this subject I cannot watch more drivel and garbage be posted without responding.

I understand how propellors are loaded as well as vessel speed is not directly related to RPM either

If this were a true statement then surely you would understand that if a vessel does say 10 knots at 1000 rpm ( I am using engine rpm here) that this is a direct relationship between engine rpm and vessel speed?

I haven't seen one factual published article to back up your claims.

Have you seen anything FACTUAL to back up Browards claims?

You are the one behind the smoke screen in this thread.

What I meant is, Engine RPM and vessel speed are not linear and they are not always the same. If there is current, wind, or waves the same vessel may do 8.5 knots or 12 knots at 1,000rpm's when it does 10 knots in calm water with no current. Not always 10 knots. Just like a boat may do 16 knots at 1950rpm's or the same boat could do 21 knots if the trim tabs are properly adjusted. It's variable within reason as is load and fuel consumption. A yacht can be 10% heavier or ligher simply depending on the amount of fuel, water, and blackwater that's on board. There are A LOT of variables that determine how much a diesel engine will consume per hour in a yacht application. In rough seas when the yacht is climbing up a wave, fuel consumption and load will go up even at the same rpm's.

I believe Browards claims and it should be considered factual unless there is published evidence that anyone can provide to prove otherwise. Not formula's. They're a reputable company and I don't think they would publish lies with no research to back it up.

"I believe Browards claims and it should be considered factual unless there is published evidence that anyone can provide to prove otherwise."

Well Cap, let's look at the published figures again. That Broward holds 9800 gallons with long range tanks. The generators are 65kW Kohlers. Kohler publishes a fuel consumption figure of about 3 gph at 75 percent output. So, for the time it takes to travel 7800 miles at 10 knots one generator will burn 2340 gallons of fuel. That leaves 9800 minus 2340 or 7460 gallons for the engines. To keep running for the 780 hours it takes to go 7800 miles at 10 knots those engines will have sip fuel at a rate of 4.78 gallons per hour each. Those are good engines but they aren't THAT good.

Are those published enough for you?

Diesel Food for Thought

Not even pretending to hang with the 3 "heads" represented here, I have some smaller recreational data for your consideration.

Having logged over 9,000 miles on Tiara's 5800 Sovran, powered by 3 Volvo D6's at a rated 435 chp each, I have witnessed at idle of 600 rpm, in gear moving at 4.9 knots, the combined consumption ranging from .9 to 1.2 gph. This is for a 50,000 lb. wet planing hull.

Further, the single 17kw Onan genset drinks about 1.2 gph with minimal load.

Don't expect an engineer's reply if you challenge me. All I can say is that I am within 2-4 gallons of expected fuel when taking on 500+ gallons, so the burn meters would seem quite accurate.

Evan

Ps: This thread is better than taking a class and serving 180 days plus in the ER. Thank you.

Evan,
Your signature makes me ashamed of myself. I only own one set of clothes which require me to move when the weather changes.

Nice engines. They are a good example of what common rail and electronic control can do with a smaller engine. Compare the fuel consumption on those with the generator and they look really good.

Run the Generator at 600 rpm's and see what it burns. hehehehe

Hi,

You might have trouble seeing that as you won't be having any electricity at that speed so why would you run it at 600 rpm?

It was a joke, that is why it has the "hehehehe" after it.

Hi,

I think you are the joke here, maybe that's what the J after Capt stands for

You know something K1W1, just because I may or may not agree with your thoughts and ideas, I do not resort to personal insults. Maybe you should take a real long look at your maturity level. This is a forum and anyone is welcome to present their thoughts and ideas, that is how new ideas come about and new things are introduced.

I have run a lot of yachts at displacement speed and have a ton of actual real life experience doing this and I am certain. I can honestly say or even put money on the fact that you have never done a long trip over 1,000 NM in a yacht with a planing hull at displacement speeds.

Load is a variable on a yacht, just like it is on a Generator. A generator (most) are designed to run at 1800rpm's or could be 2100rpms constantly. However, their fuel burn of GPH will change drastically depending on whether they have a 25% load or 90% load. It is exactly the same with a larger diesel. One cannot look at a load chart from a dyno and compare it directly to the real world on a yacht and expect it to be accurate. On a yacht, load constantly changes and to a degree you can change that. The fuel displays do not lie, just like the guy with the 57' Carver with 3 -435hp engines, at idle all 3 engines burn 0.9-1.2 GPH, so if 1305hp burns lets say 1.2gph, why couldn't 4000hp in 2 diesels burn 3 times that (let's take the larger figure 3x1.2gph= 3.6gph) if both engines have the same type of fuel injection (common rail). That IS simple physics. Heck, even a 20 degree difference in air temperature can change fuel consumption 5-10%.

The digital displays on the 16v2000's I have 6 months of experience with burned 6-7gph (combined) at idle, 6 if we were light and going with the current, 7 if we had 2200 gallons of fuel on board and going against the current. This is factual and something I saw with my own eyes, numerous times in real life. I'm sure Broward Marine took their figures directly from seatrials on THAT particular boat on a SEATRIAL. Not some wacky formula they used and converted that from a figure at 1200rpm's.

"One cannot look at a load chart from a dyno and compare it directly to the real world on a yacht and expect it to be accurate."

You are correct. That is why the spec sheet for a marine engine includes what is known as a propeller curve. It graphs the horsepower absorbed by a "well matched" propeller. Well matched meaning one that will absorb the full power output at the rated rpm, or a bit less to provide for hull fouling and other variables so as to reduce the chance of overloading the engine.

You can reference the propeller curve to obtain fuel flow across the engine's power range down to what is normally accepted as the lowest practical operating level. You will also notice that at the lowest power level the curve levels out. That is because the engine uses a finite amount of fuel just to turn itself over, that is related to mechanical efficiency and internal friction. The figures you get off the ECM display may not be very accurate at low power levels with the power lever in the idle position. At that point fuel flow meters tend to be very inaccurate as are all analog devices at the extremes of their range. If the fuel flow is derived from a map it is referencing the idle consumption, not the 50 or 70 hp it takes to turn the gear, shat, and prop at 600 rpm. Simple physics tell us that no high speed diesel is 49 percent efficient and has lower BSFC at idle than at cruise power.

Hi,

Welland accurately said Member Marmot.

CAPTJ- Mine was a joke as well.

Tell me how a 2100 rpm generator can output 50 or 60 HZ?

Is it by poles in the generator or an external regualtion?

I honestly have no idea. I don't work on the electrical end of generators. what I do know on the smaller ones say under 30kw that I am familiar with is that the same model will be rated a different killowatt output for 50hz and 60hz. It can be done with a transformer just like shorepower is, that I know. Whether or not the manufacturers do it that way is beyond me. I know that it can be done by spinning the generator less RPM's and the hertz will be less. But in reality I could not say for certain.

Hi,

Protest as much as you want , truth is you are just copying what others have written, without knowing the full research or experiences that gave their "online" solution to your current in adequacy.


You could easily provide the answer to my last question, it's a pity the answer is as easily search able by simple terms which is the same place that I estimate 99% of your previous posted statements came from.

Diesels are inherently more efficient under 1000 rpm's then at cruise. At idle just sitting there with nothing attached they burn about 1 gph, if that, just to turn them. Even in gear on a yacht, the range is always much more at under 1000 rpms then at most efficient cruise speed every single time....... Look at these fuel speed graphs from motorboating magazine (granted smaller engines around 500hp, but physics are physics.) A larger boat like the Broward could be even more efficient because the longer the boat the higher the displacement speed can be. Anyways, here are published articles from a reputable independant source.

THE columns far right with the MPG keep getting pushed to the left, I calculated the MPG at various points with a calculator.

Here is a 48' Searay- http://www.motorboating.com/articleHtml.jsp?ID=1000066679
550hp cummins

RPM KNOTS GPH DB-A
600 5.4 1.3 67 4.15 MPG
900 7.5 3.2 70 2.34 MPG
1200 9.4 7.6 77 1.24 MPG
1500 10.6 15.0 79 .71 MPG
1800 10.9 26.0 85
2100 21.0 34.9 85 .60 MPG
2400 27.9 36.9 87 .76 MPG
2700 34.1 43.7 87 .78 MPG
3000 40.2 60.1 91 .67 MPG


Here's a 48' Fairlane
575hp volvo D9 EVC
http://www.motorboating.com/articleHtml.jsp?ID=1000065239
RPM KNOTS MPH GPH
800 7.4 8.5 3.8 2.24 MPG
1000 8.1 9.3 6.8 1.37MPG
1200 9.3 10.6 11.6 .91mpg
1400 10.2 11.7 17.4
1600 11.9 13.6 23.6
1800 16.4 18.8 30.0
2000 22.0 25.3 37.0 .68 MPG
2200 25.8 29.7 43.8 .68 MPG
2400 29.0 33.3 51.6 .65 MPG
2506 30.3 34.8 56.0

I gave a Cummins distributor a call, and got a chance to talk with a pretty cool fuel guy. Anyways, I did find out that Cummins and all the major engine manufacturers pulled all of this BSFC information because the fuel maps are becoming much more complicated now that engines are more intregrated with electronics. He was also saying that it's becoming much more difficult to convey this information, and it would be impossible to supply an accurate map....He said BSFC at a few RPM points are useless information unless you have the complete fuel map to get the most out of that information.

Hi,

pulled all of this BSFC information

Do you have any idea what BSFC actually means?


You must really have some influence to spout and believe that.

BFSC is a mathematical equation not a salesmans version of affairs so swim carefully.

Brake specific fuel consumption (BSFC) is a measure of fuel efficiency within a shaft reciprocating engine. It is the rate of fuel consumption divided by the power produced. BSFC allows the fuel efficiency of different reciprocating engines to be directly compared.

Any engine will have different BSFC values at different speeds and loads. For example, a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak. However, the numbers often reported for a particular engine are a fuel economy cycle average statistic.

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

Under 1,000 RPM's a diesel is nearly unthrottled and a Detroit Diesels torque peak is 1200 rpm's

"Diesels are inherently more efficient under 1000 rpm's then at cruise."

If that were not in black and white before my eyes I would not believe anyone could or would say something like that. Do you know what "efficient? means? The most efficient diesel in the world only turns about 100 rpm at full speed so I guess that is under 1000 rpm but if you are talking about an engine that delivers its lowest BSFC at 1600 rpm or 3000 rpm then you are a bit off wouldn't you say?

"Any engine will have different BSFC values at different speeds and loads."

Gee, you just got that? It was among the first things I told you earlier ... it is one of the reasons why your engine is NOT efficient at idle speed and burns far more fuel per horsepower than at cruise power. BSFC goes way up at each end of the power curve. A diesel working at low power burns more fuel per horsepower, not less. It is less efficient, that is a fact, it is not conjecture or opinion. Your 16V2000 engine is not 49 percent efficient at 600 rpm. That is a fact, it is not conjecture or opinion. Its mechanical efficiency at that speed is nearly off the scale low because most of the fuel it burns is used just to keep it rolling over, any power beyond that comes at the cost of much more fuel per hp than those developed at 1600 rpm. That is fact, it is not conjecture or opinion.

Do you know what BSFC is and what it means? How fast a boat goes and how many gallons per mile it consumes is not a direct measure of an engine's efficiency or performance, it is a measure of the boat's efficiency and performance. How much fuel the engine burns to produce a given amount of power is the measure we are talking about.

I suggest that if approaching this from the diesel side is too much at this time, then find a good text on propellers so you can get some idea of how much power it takes to turn a prop in water. When you have a solid feel for that, then get a diesel text and work on how many BTU's it takes to produce the amount of power the prop needs.

I figured one would "assume" the statement was regarding the diesel we are talking about in this entire thread, the 16v2000. But one could also assume we are talking about a modern diesel that would be installed in a yacht, since this is a yacht forum. And since the majority of diesels installed in yachts top out at around 2350, one could assume that also. It's quite obvious I wasn't talking about Rudolph Diesels 1 cylinder diesel that was designed to run on peanut or vegetable oil, or any other non common diesel that is not installed in a yacht application.

The power it takes to turn a given prop is a variable as well. Considering that propellors are not 100% efficient. Even the salinity and temperature of the water has an effect on how much HP it takes to turn a given propellor a given shaft speed. Also, it will take a different amount of HP to turn the same propellor if there is a different reduction in the gearbox. Shaft length, shaft diameter, number of cutlass bearings, number of struts all have an effect on how much horsepower it takes to turn a propellor in a yacht. How can you use the same formula across the board? It's impossible.

The 75' Sportfish I was on used 2.5" shafts for 16v2000, 2000hp diesels. Out of the water I could turn the propellor with one hand. Going to 2.5" shafts increased speed 2 knots on that boat, without compromising any strength. The shafts were splined as were the propellors and there was no key way greatly increasing shaft strength.

Sea Torque (stuart, Fl) has come out with a product to replace normal stuffing boxes, it is proported to decrease fuel consumption and increase horsepower 8%. So how can you use the same formula for every single yacht? What if Broward has installed these as well as other technological breakthroughs.