Quote:
| Originally Posted by Marmot "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?
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.