Wide open rpm 6lp

[Jkraft]

I know this topic has been beat to death but now is my time to make a decision. My 315 yanmar makes exactly 3800 to the pins when full of fuel and loaded down heavy. I usually run it 3200-3300 rpm giving me around 17.5 knots. Should I be tweeking the prop to get 3950 out of it over the winter or leave it alone? The prop is in good shape with no chips or Dings in it now.

Thanks

[Norseman]

Looks like you are “over-propped”.
Somewhere in the Yanmar literature it says you should obtain minimum
3950 RPM at full throttle.
(My Yanmar is the 6LP-STE, which is built in Japan and rated @300 HP, the 6LPAs are built in the US and rated @315, but same engine, the HP rating was just obtained at different temperatures)
Max should be 4050, or they give that range. (3950-4050)

If you really want to dig in a bit deeper, sign up for Boatdiesel.com, pay your $25 annual fee and do a forum search on Yanmar 6LPs.
Tons of information and horror stories of what happens when you abuse the 6LP with over-propping.
My prop is fairly new and has been fine tuned to yield 4050 RPM at WOT. To get 17 knots cruise I need to turn 3600.
I also have a spare prop with a slightly coarser pitch, have not tried it yet, but expect to see 4000 at WOT.

[Jkraft]

Norseman, I agree, and I am a member of boatdiesel as well. I guess my main question was that if I am making 3800, 3950 is only 150 rpm off, I am pretty darn close. But am I close enough until I am due for my next prop tune? Or should I get it addressed now? (3950 is where I would like to be ideally)

Also, I am working my way through the boat since I just got it a couple of weeks ago. The turbo has likely never been cleaned and I am being told I it is due for injectors. Maybe I should address those two items before messing with the prop? Just looking for advice, really.

[Norseman]

. Maybe I should address those two items before messing with the prop? Just looking for advice, really.

Not sure about those 2 items, not a diesel mechanic.
Only advice I can give, is to stay within manufactures limitations on RPM.
I take that stuff pretty seriously after having flown airplanes commercially for 37 years, every engine manufacturer have limitations on the motors, whether it is RPM, temperature, torque, EPR, (Exhaust Pressure Ratio) etc.
A sane pilot would never willingly exceed those limitations, the engine could blow up, quit, whatever, and you lose your job, or your life.
To me it is black or white, either you operate within the factory parameters, or you don’t. (Only 150 RPM off?)
Like being pregnant really, either you are, or you are not.

[Jkraft]

Thanks Norseman! I will get it addressed.

[Norseman]

Thanks Norseman! I will get it addressed.

Good idea.
Nothing wrong with your injectors or your turbo as you get the boat speed: 22-23 knots, but at the wrong RPM. I also get 22 knots at wide open throttle, but at 4050 rather than 3800.
Some folks get away with over propping for years, some don’t..
(Perhaps like smoking cigarettes, some heavy smokers die at 95 with no effects from smoking, some at 40 from lung cancer.)
Do you feel lucky?

[Jkraft]

Norseman, when I go to the prop shop do you think I should tell them to make it reach 3950 or 4050?

[Norseman]

Norseman, when I go to the prop shop do you think I should tell them to make it reach 3950 or 4050?

I would shoot for in between: 4K then you will still be within the green band if they are off a few RPM.

This is my primary prop, 4050 RPM @ WOT.

91620FDF-400A-4A9B-AF4D-16BEF5858A52.jpeg

Not sure if you can use the same numbers or not, but it is a start.

[RobS]

Here's the propping sheet from Yanmar, follow it.

I see no mention of tachometer accuracy in this thread. Do not evaluate your WOT rpms and prop load until you have verified your tachometer using a photo-tach (laser).

[Norseman]

. I see no mention of tachometer accuracy in this thread.

Naturally, but it goes without saying:
Always calibrate the tach before doing anything else ( And clean the prop, clean the bottom, etc. )

Here is an inexpensive photo tach I got on Amazon:

4164F2B7-D73E-40B8-B2FB-57CD79F87B3B.jpeg

[RobS]

Great, you've got your bases covered. Nothing "goes without saying" when you are trying to help others, too risky as we all have different levels of experience and that is why we are all here.

[Jkraft]

I’m glad you mentioned it. I’m going to photo tach it tomorrow morning

[Jkraft]

Here is an update. It is making a true 3805 rpm wide open but once she came out I see that I I’ve a pretty good bend in one of the blades on the prop. Going to have this corrected and try again

[furball]

My two cents on this, if you can turn 3800 fully loaded at the end of the season, likely a bit of growth on the bottom, etc. I would consider that about perfect. Spring launch, perfect bottom, lightly loaded, I would get about between 3850-3875 WOT. End of the year, heavy, a little dirty, 3800. That is where Yanmar says it should be worst case. I wouldn’t consider that over propped on a 6LP-STP 315HP motor. My RPM numbers come from a digital Flo-scan tach. Definitely have the prop addressed but I would run it another season before changing anything with those numbers. I would start every season documenting my RPM/Speed numbers. Full fuel and empty everything else. I would typically run from 3200-3600 rpms in 100 rpm jumps and WOT. Write the numbers down and during the year you can compare to the base line. I now measure the amount of stuff on the boat in Knots . It does give me a feel for how much weight and growth are effecting performance.

[Jkraft]

Thanks Chief! I’m hoping that twist in my prop was part of the issue.