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


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  #11  
Old 04-06-2010
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MobjackB32,

It sounds like the load that you are placing on the engine would be considered acceptable. The issue of whether you are lugging the engine (prop pitch/ gearing) is still unanswered.

As suggested by k1vsk, the first thing to do is to verify that your tach reads properly. Next, you need to make sure that you can actually hit your maximum rpm while in gear, not just in neutral. If you cannot, this means that you are putting too great of a load on the engine. This load could be due to marine growth on your hull and or propeller (check this first) or due to improper gearing or propeller pitch. If you can't reach redline rpm in gear and your bottom is clean, check to make sure that the transmission that was replaced had the same gearing as the current one. If all of that checks out, then look to your prop pitch.

Good luck.
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Old 04-06-2010
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Originally Posted by MobjackB32 View Post
I just bought a 1981 Bristol 32 with a Universal 5424 that has 1150 hours. Bringing her home (30 hours) I noticed she hit cruising speed 5-6 mph at 1400-1500 rpm. I had heard that crusing rpm should be over 2000 on this engine. Is there any harm in having the crusing rpm between 1400 and 1500? Will it lug the engine or cause undo wear, overheating, etc? She has run this way for almost 30 years it would appear.

Thanks,

Jeff
Jeff,

2800 is max rated RPM for the 5424. You'll want to get a photo tach to confirm your actual RPM though.

You should ideally be able to hit max rated RPM at max throttle with a clean bottom and prop in smooth current free water. 1500 is quite low for cruise RPM. You should be turning 2200-2250 for your cruise RPM..

Universal & Westebeke are very picky on proper RPM. In the spec sheet for all their new engines they will have a statement like the one below.

From the M-25 XPB Spec Sheet:


"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle"
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Last edited by Maine Sail; 04-06-2010 at 05:09 PM.
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  #13  
Old 04-06-2010
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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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  #14  
Old 04-06-2010
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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.
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Old 04-06-2010
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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.

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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.
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  #16  
Old 04-08-2010
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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.
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Old 04-09-2010
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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.
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Old 04-09-2010
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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.
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Old 04-09-2010
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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?

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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.
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