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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel > Cruising RPM
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


Thread: Cruising RPM Reply to Thread
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Topic Review (Newest First)
04-11-2010 07:44 PM
klem
Quote:
Originally Posted by lancelot9898 View Post
You've got distance and time to factor into the equation.
Are you saying that you are trying to balance fuel consumption versus your own time? I find the easiest graphs to read the ones that compare fuel consumption/power versus rpm. On most diesel engines, this will occur near the torque peak. This can be manipulated a little through things like timing but not much. At high rpms (which I would consider 80% to be), you are putting a lot of energy into simply spinning the engine which is also not running as efficiently. 80% is perfectly acceptable if you want to get somewhere relatively quickly but it is not going to yield optimal fuel economy.
04-09-2010 10:40 PM
lancelot9898
Quote:
Originally Posted by Omatako View Post
Really?

You've got distance and time to factor into the equation.
04-09-2010 10:37 PM
lancelot9898
Quote:
Originally Posted by sailingdog View Post
WOT will vary if you're tied to the dock as the prop isn't moving through the water as it should be and will generally encounter more resistance trying to move the water with the hull not moving.... so will reach a lower max RPM.
That's exactly my point and the reason to suspect that additional rpm would be gained by motoring into a contrary current.
04-09-2010 05:59 PM
Omatako
Quote:
Originally Posted by lancelot9898 View Post
I do agree that approx 80% of WOT setting is a good number for cruising and saving on fuel.
Really?

04-09-2010 03:43 PM
sailingdog WOT will vary if you're tied to the dock as the prop isn't moving through the water as it should be and will generally encounter more resistance trying to move the water with the hull not moving.... so will reach a lower max RPM.

Quote:
Originally Posted by lancelot9898 View Post
Using a photo tach is a good idea since my tach was out of calibration with actual readings lower than what I was reading. I have a Perkins 4-108 which was new in 1985 and I've been basically running with a too low WOT for the past 25 years so I'm not really sure how dire the consequences for over proping is on engine life. I do agree that approx 80% of WOT setting is a good number for cruising and saving on fuel. I also noticed that WOT varies with current. Tied at the slip my WOT setting is around 200 rpm lower than when I'm in open water. One thing I want to see is how this setting changes with current. My theory is that going against a current, my WOT rpm will be higher. How much is the real question. I do have a max prop and have varied the pitch to see how performance changes. FWIW WOT rpm is 2300 rpm and I cruise at 2000 rpm making 6 plus knots with a 37 ft Tayana.
04-09-2010 01:53 PM
lancelot9898 Using a photo tach is a good idea since my tach was out of calibration with actual readings lower than what I was reading. I have a Perkins 4-108 which was new in 1985 and I've been basically running with a too low WOT for the past 25 years so I'm not really sure how dire the consequences for over proping is on engine life. I do agree that approx 80% of WOT setting is a good number for cruising and saving on fuel. I also noticed that WOT varies with current. Tied at the slip my WOT setting is around 200 rpm lower than when I'm in open water. One thing I want to see is how this setting changes with current. My theory is that going against a current, my WOT rpm will be higher. How much is the real question. I do have a max prop and have varied the pitch to see how performance changes. FWIW WOT rpm is 2300 rpm and I cruise at 2000 rpm making 6 plus knots with a 37 ft Tayana.
04-08-2010 08:57 PM
ericroline Do a speed RPM graph to find out the most efficient region for the engine. Diesels like to run at 80% of rated HP for the most efficient and long life area. I will run 2800, 2900 rpm for long periods backing off to around 2200, 2400 just to prevent running at one speed all the time.
04-06-2010 10:44 PM
sailingdog The hull is clean and so is the prop... so the prop is mismatched and you're not able to get anywhere near what the WOT RPMs should be... I'd say you're over-pitched by about THREE inches, since each inch will drop or raise the RPMs about 200 RPM. That would bring you up to 2600 RPM WOT... which is not bad for an engine that has a redline of 2800 RPM.

This would bring your cruising RPM up to about 2200 RPM, as suggested by Maine Sail. This should also lower your speed when idling.

Quote:
Originally Posted by MobjackB32 View Post
The hull is clean and the prop was checked and cleaned 1 hour before the trip started. WOT in gear produces 2000 rpm, out of gear 2500+ rpm. She also runs about 3 mph at idle. the prop is large and 3 bladed. Large meaning just fits in the space within the full keel.
04-06-2010 10:26 PM
MobjackB32 The hull is clean and the prop was checked and cleaned 1 hour before the trip started. WOT in gear produces 2000 rpm, out of gear 2500+ rpm. She also runs about 3 mph at idle. the prop is large and 3 bladed. Large meaning just fits in the space within the full keel.
04-06-2010 04:59 PM
T37SOLARE If the alternator has been upgraded then the RPM's on the tach are likely off. I'd get a hand held tach to confirm it correct before you make any drastic changes to the prop.
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