Navigation with one engine

[Notorius]

My boat is a Uniesse 40, equipped with twin CAT 3208 DITA 435 HP. Even though it's a fast boat, I really enjoy cruising at 8 knots, around 1000 RPM.


However, on long trips, this could be problem for the engines, as the turbochargers probably aren’t operating properly at 1000 RPM. After 1–2 hours of cruising at that speed, if I suddenly accelerate to full cruising speed (2400 RPM), I get a large cloud of black smoke for about 30 seconds or more.

A friend of mine, who has the same engines, prefers to cruise using only one engine at 1400–1500 RPM, turning the other one off. At 1400 RPM, the turbocharger is engaged. He has shaft brakes installed between the gearbox and the shaft to prevent potential damage to the gearbox of the engine that's turned off.

To me, that seems like a strange solution.

Is anyone else here cruising with only one engine, even though they have two?

 

[Clark]

I normally always use two, but some friends doing America's Great Loop have cruised on one engine to get better fuel economy. The water supply for both my driplesses are fed from both engines and my transmissions are lubricated with the engine off and can free spin in neutral without the engine running. When I have a failure and have to run on one engine, I've found that free spinning results in faster speeds and better fuel economy than locking the shaft.

 

[tiltrider1]

Cat recommends running 3208’s at 1500 RPMs. Your friend is achieving this.

 

[Notorius]

Cat recommends running 3208’s at 1500 RPMs. Your friend is achieving this.

Thank you. Do you know a forum or a web page with this kind of info?

 

[Frosty]

A friend of mine, who has the same engines, prefers to cruise using only one engine at 1400–1500 RPM, turning the other one off.

To me, that seems like a strange solution.

Caveat: I'm not on a boat with twin diesels. That said, to me* this seems like a good solution. You are putting half the hours on your engine pair, and running the active engine in the more optimal RPM range. What's not to like?

*this assumes that like other posters above, you have taken possible transmission/lubrication issues into account

 

[SteveK]

Cat recommends running 3208’s at 1500 RPMs. Your friend is achieving this.

Curious, where did you find that? My ingrained thought is to run an engine continuously 50-75% of maximum RPM as a rule of thumb. So 1500 fits that, but I did not know Cat recommended it as a minimum.

 

[Notorius]

the question is exactly about the gear. Better to brake it? Or it is not necessary? I have ZF gear of 1992.

 

[rslifkin]

Curious, where did you find that? My ingrained thought is to run an engine continuously 50-75% of maximum RPM as a rule of thumb. So 1500 fits that, but I did not know Cat recommended it as a minimum.

Every engine has a range of outputs where it's happy to run continuously. Above that you'll start to run into durability or lifespan issues. Below that it's carbon buildup or other issues. Some of the load loading issues can be managed by just running it up periodically, but not necessarily all of them.

The minimum continuous output of an engine will vary widely between designs. Some have no problem getting hot enough and burning cleanly enough under pretty light loads, but others need significantly more load. I remember one poster on here indicating that Cummins had confirmed it was no problem to run his QSBs around 1000 RPM for extended periods.

 

[tiltrider1]

Curious, where did you find that? My ingrained thought is to run an engine continuously 50-75% of maximum RPM as a rule of thumb. So 1500 fits that, but I did not know Cat recommended it as a minimum.

I have the original owners manual for my 3208’s. It’s clearly stated in the operations manual along with oil changes every 250 hours.

 

[Mac2]

What transmission do you have? If you can free wheel, without damaging the transmission, the only other concern is the shaft seal overheating on the freewheeling engine that is shut off.

 

[Ithaka7]

I prefer to cruise at planing speeds but sometimes run at below hull speed at night. I can get 8 knots with two engines at 900 rpm or one engine at 1,200 rpm. The engines are Cummins 6CTA, the manual says avoid running for a long time below 1,000 rpm. Hence, I do one engine mostly. Never had any issues with the gear boxes at these speeds, they stay cool, incl. the free spinning one.