How fast?

[PuffinStuff]

When we purchased our 2012 R-29 the broker recommended max rpms (4000) minus 500 (3500) for optimal high end performance. This tends to end up around 15-16 knots in most conditions and I have respected his advice. I've heard our Yanmar 260 hp is capable of more, but have been hesitant to push beyond the 3500 rpm threshold. That said, I have also heard these engines like to run fast. Is breaching the 3500 rpm pushing too hard?

Thanks,

Paul
Puffin R-29

 

[Levitation]

Your engine has a governor. Full throttle will not damage your engine and most owners manuals will tell you to test the governor at regular intervals to ensure that it is working. And also to ensure that it reaches the governed speed. If it does not then it is time for maintenance.
The manufacturer has tested and determined the RPM the engine can turn without excessive wear or damage. Now, these are not commercial engines that are expected to run at full throttle 24/7. So in your manual there will be a sentence about how long you should run full throttle at a time. There is also a recommendation for continuous throttle RPM. I would believe those over a dealers opinion.
As in all things in life, when in doubt (finally) read the instructions :mrgreen:

 

[Crewdog]

On our R27, 80% is what I usually stay at when conditions are right for high speed cruise. That gives a fuel economy of about 1.9 to 2.0 nmpg.

At wide open throttle, economy goes down to about 1.6 to 1.7 nmpg.

Here's an interesting article about power usage from Power & Motoryacht magazine:
http://www.powerandmotoryacht.com/engines/how-find-best-cruising-speed-your-boat#.VWr4HM9VhBc

And here's an interesting tidbit from the USCG Boat Crew Seamanship Manual, page 10-20 under Margin of Power:

"A margin of power should always be left available for emergencies. The best speed for the vessel
should be determined. A good normal operating limit for semi-displacement vessels is usually 80
percent maximum power, allowing the remaining 20 percent for emergency use."

There's no right or wrong answer I guess, only what is comfortable for the operator.

In most cases, I kind of like the idea of cruising at a speed and knowing there's a little extra power if needed.

 

[Pete]

According to that Power & Motoryacht article the problem with cruising at high rpm isn't catastrophic failure but rather engine wear since, "the harder you push the engine the more fuel it consumes per unit of distance traveled." I find with my 27 that once on plane fuel consumption is around 2.2 mpg (unless there's significant current/adverse weather) and stays at 2.2 virtually up to max rpm.
So if I'm at fast cruise because I want to get somewhere quickly, maybe I need to reconsider my limit of 15% below max rpm.
Pete

 

[Automaticman865]

Here is a great article that puts it altogether for speed, wot, rpms, etc...

http://mvdirona.com/technicalarticles/d ... index.html



Stuart

 

[Hydraulicjump]

Thanks for posting these great articles. For the first two years our boat lived on a trailer. WOT was 4000 rpm, and I traditionally ran our high speed cruise at 3600, 10% off WOT.

Now the tug is in SF Bay full time in a Marina with relentless bottom growth due to an abundance of sea lion poo. Despite bi-monthly cleanings, WOT has slipped to 3900. Do I keep the 10% rule or hold at 3600? Lately, I have been cruising at 3400 rpm just to be sure, but my load on the Yanmar gauge still creeps above 80%. Hard to figure this out!

And the long article that blames the manufacturer for over-pitching the prop is interesting and useful. But even with a load, when the bottom is sparkling clean and the barnacles are off the prop I get 4000 rpm WOT, so I am guessing our props are pitched correctly. It is the load of that scum-covered bottom that is my problem.

 

[pugtug613]

I go by "feel". My R27 likes to run at 12 knots. The fuel economy is good, around 2.2NMPG, and the boat feels like it is just humming along. I push it up to 15 knots on the way home for at least 10 minutes. The fuel economy really suffers! Maybe 1.4-1.6 NMPG. I am from a sail boat, so 12 knots is real fast to me. If conditions are a little snotty, the sweat spot seems to be 10 knots. IMO, don't stress over that stuff. The engine will be fine. Just do your yearly maint. and enjoy your time on the water.