The Boating Forum - 6 cyl diesel what cruise rpm.

greetings from cork southern ireland, thanks guys for sending all these hurricane leftovers our way. after months of never ending frustration our new osprey 26 xpd pilothouse has arrived .and after 100hrs of wreck fishing up to 60 miles offshore we are blown away by the boats rugged no frills build and sea keeping ability fuel capacity is 565 ltrs which gives us a theoreical range of nearly 500 nmls! electronics consists of ray L1250 plus,RL70RCRC 2kw radome,DSM250,AIRMAR B260.NOW HERES THE QUESTION!!
"we ase powered by a yamaha me 421 sti i/o " which basicly is a toyota landcruiser 100
4.2L diesel which has been marinised by yamaha (europe only i believe thanks to a trade agreement with mercury mariner) max specified rpm is 3600/3800 rpm
however were slightly overpropped and pulling max 3500 rpm .we normaly drive her at a
somewhat relaxed 2800rpm which provides 25kts cruise(3500rpm gives 33kts max)..what is an acceptably safe cruise rpm?

That motor is sold in the US as the Yanmar 6LP. I had a boat rigged with one. You really should correct the overpropping. I assume you are saying you can only reach 3500 fully loaded with fuel and gear. About 1-2" of pitch needs to come off the props, which is a bit of a challenge with an i/o without replacing the props, but very important to the engine's longevity.

Once you prop so you can reach 3800 at WOT, you can safely cruise at 3300-3400 all day every day. I usually ran mine at 3250-3300 unless I was in a rush. If you have a pyrometer and a fuel flow meter, I think you would see that the motor is overloaded at your 2800 cruise now with a high exhaust gas temperature and a higher fuel burn at that rpm than the design curve.

Yanmar tachs are notorious for being 75 to 125 rpms off. Get a Phototach and make sure you know what the true rpms are. Then, change the props so you reach the engine's rated rpms + 50. This will allow your engine to live a long life. Next add boost and EGT gauges. These will tell the real story on how the engine is loaded. Write down all of these numbers in a log at 100 rpm intervals (when the engine is new). These baseline numbers will be incaluable later down the road if you experience problems that require the warranty to be invoked. Once everything is dialed in correctly, you can cruise at the rpm Yanmar suggests and won't have to worry about the engine grenading. 3300 would probably be fine.

You will also probably pick up some fuel economy when you get her propped right.