PDA

View Full Version : Yanmar 6LPA optimum RPM



MrFishit
11-28-2007, 02:54 PM
I was wondering what would be the optimum RPM to run this motor at for best life expectancy and fuel economy?

Right now this thing gets 2 MPG or 8 GPH at 2800 RPM which is not even close to the advertised numbers. I know this all depends on prop size and all that but I never messed with any of that from the factory.

Mistress
11-28-2007, 03:28 PM
What's rated max rpm? What rpm are you currently achieving at WOT?

Advertised numbers from where (Shamrock or Yanmar)?

Kurt
11-28-2007, 03:43 PM
According to the Yanmar numbers, you should be burning around 7 GPH @ 2800 RPM. Max for the engine (WOT) is 3800 RPM and if you are propped to be able to reach that or a bit above, you should be able to run that engine at 3400 RPM (~12 GPH) till the cows come home. Max continuous RPM is 3600.

Max torque band occurs between 2400 and 3000 RPM (pretty flat) with the actual peak at around 2700-2800, where you're running it.

Mistress
11-28-2007, 04:25 PM
According to the Yanmar numbers, you should be burning around 7 GPH @ 2800 RPM. Max for the engine (WOT) is 3800 RPM and if you are propped to be able to reach that or a bit above, you should be able to run that engine at 3400 RPM (~12 GPH) till the cows come home. Max continuous RPM is 3600.

Max torque band occurs between 2400 and 3000 RPM (pretty flat) with the actual peak at around 2700-2800, where you're running it.

What Kurt said...except I might be a bit more conservative and say 3200 max sustained cruise.

The key to obtaining the fuel (gph) numbers from Yanmar is to be properly propped (able to achive your 3800 and then some). If she'll run up to near 4K the Yanmar numbers should be quite accurate.

FWIW...I don't even consider Shamrock numbers of burn rate and speed. I've found them to be way optomistic across the board. But then that seems to be the industry standard (inflate the mpg's).

MrFishit
11-28-2007, 06:34 PM
Thanks that is re-assuring. To be honest I have been afraid to go above 3000 rpms just because my previous experience with diesel engines in trucks, etc. is that they are low reving and it's not good to red line them. I guess this does not apply to marine engines.

But I have pushed it before just to see what it had and it went to 4000 on the tach at about 38mph. It's interesting how the faster it goes, I can actually see the temp decrease meaning the load on the engine is less at higher rpms since the hull gets on plane. The waves in the Pacific are almost always never helping the fuel situation either.

Please bare with me and my obvious questions sometimes, but this is my first diesel boat and I am very curious about a lot of things and having tons of fun learning.

Kurt
11-28-2007, 07:38 PM
It may be that you are just below plane speed at 2800 RPM (always on the wrong side of the curve) - it calcs out that you are only doing 16 MPH (<14 Kts) @ 2800 RPM (based on the numbers you reported) - the hull may be plowing in displacement mode and that requires more power. If you can reach 38 MPH @ 4000 RPM, I would think that you should be getting more than 16 at 2800.

If you like cruising @ 2800 (engine sound, for instance), perhaps you should try running the RPMs up to 3200 - 3400 to get the boat on plane and then back off to 2800. Fiddle with the trim tab settings a bit to see if there is a better position that keeps you on plane at the lower RPM.

I'm just guessing that this is what's happening.

So far as not red-lining the engine, these lighter weight Yanmar marine Diesels are higher revving than what you'd expect in a truck. I'd bet that the Cummins 210 HP in Mistress's boat is probably rated at around 2800 - 2900 RPM @ WOT. The newer Yanmar/BMW engines have a 4000 RPM WOT rating.

Mistress
11-28-2007, 09:32 PM
Max rated rpm on my 210 is 2600. I think the hotrodded Cummins versions (HO, etc.) are up to 3400, maybe more.

If you can hit 4K, loaded, there might be a little headroom on the prop. I'd invest in a pyrometer before I started playing around with props though. That will tell you what headroom might really be there. The flipside is you're engine isn't even breathing hard at spec'd cruise rpm.

In a commercial environment, I'd opt for a lower revving bigger engine for "X" horsepower. I think in a sportboat that doesn't see in excess of 500 hours per year, they'll both last about as many years if treated right. I suspect mine will rot apart long before I wear it out. On any of the marine dieses in sportboats, something else usually takes the engine out (marinized part failure, waterladen fuel, etc.) vs. wearing the bottom end out.

I agree that 14 knots on most any of these hulls is not where you want to be unless other factors dictate. Not really on plane, but way faster than hull speed...pushing gobs of water. I do like the way my keelboat handles at these non-optimum speeds though in comparison to more high performance hulls.

My Mack really likes 18-20 knots for econo cruise @ 1900 rpm. No flowmeter to give numbers though. I'm routinely at 3+ mpg overall. If I could carry enough fuel it would make it to Hawaii and back without a hiccup. This is Pacific Ocean speed, not FAC lake speed. Lake speeds would be increased by ~2 knots for a given rpm. Still a far cry from the published Shamrock data of the time.

messi
12-03-2007, 10:21 PM
I wouldn't be afraid of letting her spin. I've had several yanmar tractors, and they will run for thousands of hours a few hundred RPMs off max engine speed.

I've a Cat 3208 in my dump truck, and it is governed @2800. It spins 2600 to run the truck at 55mph in 10th gear, and it will purr like a kitten there. It took a little getting used to for me to run something that close to maximum speed, but that is how they are designed to be run. Because of that, your actually hurting them more by trying to baby them.

anchorinn
12-03-2007, 11:10 PM
I have two of these in a 2003 29. We cruse @ 3200-3400. 2400 is just on plane. WOT is 4000. Our fuel consumption is 1.1 to 1.3 kmpg depending on the seas. Dry weight is 12000 lb. These engines love to run. And you are right about the temperature when we are in ruff seas and run just off plane they run warm 190 to 195 but on plane @ 3200 temp 180. We have 1481 hours to date on the starboard engin and 1479 on the port. Most for our trips are over 40 miles.

There a great motor, run em up, keep them happy

ege :cool: :D :cool:

seatux
12-17-2007, 12:15 AM
I have two of these in a 2003 29. We cruse @ 3200-3400. 2400 is just on plane. WOT is 4000. Our fuel consumption is 1.1 to 1.3 kmpg depending on the seas. Dry weight is 12000 lb. These engines love to run. And you are right about the temperature when we are in ruff seas and run just off plane they run warm 190 to 195 but on plane @ 3200 temp 180. We have 1481 hours to date on the starboard engin and 1479 on the port. Most for our trips are over 40 miles.

There a great motor, run em up, keep them happy

ege :cool: :D :cool:
Mine is a 2003 290 WA also. I ran from Boca Pass (Port Charlotte Fl ) to Marathon @ 2800 RPMs and got 2 MPG even. On return trip I pushed it up to 3000 RPMs and it droped to 1.7 MPG. Since then i've added a second thru hull transducer, first one was a 600w wide beam, this is a 1000w narrow beam with a huge fairing block. It has droped my top end by 5 MPH! And thus my MPG has also been hurt. Our normal dive trips average 120 to 150 miles a day, so even tho she loves to hum at 3000 to 3200 R's I run the 2800 for economy.

ku2c
12-17-2007, 09:31 AM
Fishit,

Check your tach against a photo tach - I found mine read 10% low, in other words your 2800 could be ~3000rpm. Mack-Boring told me 3900 - 3950 is WOT.
Most times I cruise around 3000-3200rpm - 20-22kts.

Pete