[C320-list] Correct Engine RPM's

[Scott Thompson]

Max continuous rating on the Yanmar 3GM30F is 3400 RPM. "Appropriate" 
cruising RPM depends on a lot of things. Many sources say that a diesel 
engine will run best and last longest if pushed to its maximum rating, 
in which case the 3400 would seem to be the recommendation. However, you 
may not be able to run that hard if any of the following apply: prop is 
overpitched; prop is dirty; bottom is dirty; boat is overloaded; or boat 
is towing something.  So some experimentation is in order. If you push 
the engine too hard you will get black smoke out the exhaust, indicating 
incomplete combustion, which is bad for the engine, and you will 
probably also have a lot of vibration and noise, which is uncomfortable. 
So many back off a bit. If I'm going 6-7 knots with a "clean" exhaust I 
don't try to push it any further.  My tachometer usually reads about 
2600 at that point, but it reads low (per my original survey) and so I 
think my actual RPM is about 3000. (No RPM recommendations are 
meaningful unless you know whether your tachometer is accurate.)

My sense is that recommended prop pitches often assume that you will not 
run at full RPM, so that the prop is overpitched a bit. I'm pretty sure 
that my Gori two-blade folding prop is overpitched even though I 
followed the manufacturer's recommendation on pitch.

You might start by asking the guy who installed your prop and set the 
pitch what assumption he was making about RPM, and did he account for 
the proper reduction gear ratio of your transmission.


On 6/12/2019 4:43 PM, jackbrennan wrote:
> On the Yanmar, peak sustained running is 3,200 rpm with a top of 3,600 rpm.
>
> I've seen owners recommend all kinds of numbers, even 3,200, but I think that's way too high.
>
> On longer motoring cruises, I like to range between 2,200 and 2,400 rpm. That gives a speed of roughly 5.5 to 6 knots at half to 3/4s of a gallon.
>
> 3,000 might get me to 6.5 knots, but fuel consumption will be way up, probably more than a gallon an hour.
> Jack Brennan
> Sonas, 1998 Catalina 320
> Tierra Verde, Fl.
> Dolphin Cruising Club of Tampa Bay
>
>
>
>
>
> Sent from Yahoo Mail on Android
>   
>    On Wed, Jun 12, 2019 at 3:15 PM, Mark Seyler<mseyler at cox.net> wrote:   I've always been told that most diesel engines are happy running at about 70-80% of the rated maximum, or "red line" on the tachometer.  On my Perkins the red line is 3,600 RPM, so I usually run around 2,500 - 2,700 RPM. I don't know the red line for the Yanmar, but the math should be the same.
>
> The other question that may apply is what gives the best fuel economy.  I only run my engine to get in and out of my marina, and don't burn enough diesel to care that much.  But if you are cruising longer distances, that might make a difference.
>
> Mark Seyler
> S/V Reality,
> Catalina 320, #232
> New Orleans, LA
>
>
>
> -----Original Message-----
> From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf Of Roger Nitkin
> Sent: Wednesday, June 12, 2019 1:14 PM
> To: c320-list at lists.catalina320.com
> Subject: [C320-list] Correct Engine RPM's
>
> We recently installed a new MaxPropEasy feathering prop to replace the original bronze two-blade prop on Summer Time.The MaxProp supplier pre-set the blade pitch for our Yanmar 3GM30F and boat.
>
> Does anyone have any guidelines on appropriate cruising RPM's, or how we can figure this out ?Thanks !
> RogerSummer Time, 1997, #429Westport, CT
>    

-- 
Scott Thompson
Surprise, #653