Diesel engine/prop/hull efficiency questions

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.

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

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

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.

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.

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.

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

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.