Visit our Popular Forums

massnspace · 07-03-2024, 10:29

I was just reading on another forum about the reasons NOT to let a marine diesel idle for too long or run under minimal load. Some posters said they tried to run at 2500 RPM to make sure the engine was under load.

My current sailboat is fitted with a diesel/prop that pushes the boat at 5.7 knots at 1500 RPM. Yes, I can jack up to 2500, but at that RPM the boat is shaking and the engine burns a lot more fuel.

So.

Am I OK to run all day long at 1500 RPM? If not, what to do?

David
Seattle

NBs · 07-03-2024, 10:47

Hi,

Can you tell us more details about your engine? If the machine is common rail everything is fine, if it is an old school engine, then, on a general level, it is ok to drive under low load for about 8 hours and then burn the contaminants at half an hour at higher revs, several owner's instructions have mentioned this.

NBs

boatpoker · 07-03-2024, 10:56

Lower RPM with some over prop might be ok.
Engine, gear reduction prop pitch/diameter and hull type will get you more informed responses .. maybe

Paradox111 · 07-03-2024, 13:17

Perhaps to rid shaking of boat
https://www.cruisersforum.com/forums...ns-179781.html

**Dockhead** · 07-03-2024, 13:29

The basic principle is that it's all good provided the cylinders are hot enough. So the specific power of the engine plays a big role here. A big thumper like a Perkins 4.236 needs a significant load to be happy. 3.9 liters displacement, 80-odd hp. A turbocharged Yanmar, say, 2 liters and 100hp, is happy with about 1/2 the load of the Perkins.

Some engines could be OK at 1500 RPM all day long, but we need to know what engine. In any case, if operating at low load, it's good practice to run it up periodically.

SailingHarmonie · 07-03-2024, 16:31

The idea that EVERY engine needs to run at 2500RPM to be happy is just ignorant. There are some diesel engines that have a maximum operating RPM of 1800, and others that are 4500, so we know on the face of it that statement is just stupid

This is one of those issues that gets oft repeated, and everybody “knows” to be true.

I myself, have seen far more problems with engines OVERLOADED with too steep a prop than from too much idle time. Seriously clogged exhaust elbows were a chronic issue on one class of racing one designs that were all FORCED to use a too steep prop by class rules.

The US Army has a HUGE inventory of diesel generators that spend almost all their life running at very low load. Their conclusion after extensive study was that it was not damaging the engines, despite what the mechanic staff believed to the contrary.

Think about diesel trucks in cold weather in a truck stop. They drive for 8 to 10 hours a day, and then idle the rest of the day. They do fine, and last FAR longer than your marine engine. Your average automotive diesel at highway speeds typically is running at VERY low load, yet they have a well deserved reputation for long life.

On a sailboat, there really is no reason to be idling the engine for many hours at a time. If you have an engine well matched to the boat, and a prop well matched to the engine, then just use it to drive the boat. Take good care of it, and follow the manufacturers recommendations in the manual. If you leave the engine idle in for a few hours, it will not damage it.

I have a Volvo TMD22 that has 10,000 hours on it. It just runs. The turbo gets cleaned and rebuilt every 2500 hours or so. We don’t plan on repowering anytime soon, because there is no reason to. It cruises at significantly below its maximum rated RPM, and has for 27 years. What issues should we be expecting to see, exactly, and when?
.

MicHughV · 07-03-2024, 17:15

You can try reading the " the propellor handbook" authored by Dave Gerr.

It goes into great detail about diesel engines, props, etc, ad infinitum. It's a very technical read, but by the time you're done, you'll understand the topic a bit more.

pbiJim · 07-03-2024, 17:58

I usually try to run mine around 1,800rpm, but I will go higher if conditions make it necessary. My prop is both pretty big & a bit over pitched. My motor is marginally sized. In a 30 knot head wind, I can't turn up more than 2,000 rpm, if that. With 10 knots on the nose, I can do more like 2,500 at WOT. Proper RPM range varies with the boat, the prop & the conditions. There is no one size fits all answer for what is the proper RPM for a sailboat diesel.

**Wotname** · 07-03-2024, 18:31

Quote:

Originally Posted by massnspace

My current sailboat is fitted with a diesel/prop that pushes the boat at 5.7 knots at 1500 RPM. Yes, I can jack up to 2500, but at that RPM the boat is shaking and the engine burns a lot more fuel.

So.

Am I OK to run all day long at 1500 RPM? If not, what to do?

David
Seattle

As indicated above, it depends on engine and the load.

If you have say a 80 hp Ford Lehmann in 40+', 15 ton sailboat, 1500 rpm is fine (even 1200 is fine).

If you have say a 27 hp Yanmar 3GM30 in 28', 3 ton sailboat, 1500 is significantly under-loaded and will result in exhaust mixing point restriction and general carbon build up. The combustion chamber pressures and temperatures will be way too low for a long life.

What to do? As indicated upthread, open up the throttle every couple of hours for say 10 or 15 minutes and run at max (intermittent) RPM. The boat won't go any faster but it will create a significant bow and stern wave which indicates the engine is getting loaded up with the corresponding increase in combustion chamber pressure and temperature. For the 3GM30, max (intermittent) RPM is 3,600.

As a side benefit, any increase noted in the coolant temperature when running at WOT indicates a developing problem in the cooling system (engine or raw water side).

massnspace · 07-03-2024, 19:52

The diesel is a Universal M-40 (32 HP I believe), and the boat is a 32’ long by 10’ beam long keel cruiser that weighs 15,000 lbs.

Again. I cruise all day long at 5.7 knots when at 1500 RPM. Is that OK? Is the motor “under sufficient load”?

flightlead404 · 07-03-2024, 20:06

Quote:

Originally Posted by SailingHarmonie

The idea that EVERY engine needs to run at 2500RPM to be happy is just ignorant. There are some diesel engines that have a maximum operating RPM of 1800, and others that are 4500, so we know on the face of it that statement is just stupid

.

wow! Do you think you can tone it down a bit. Posts like yours make this forum very unpleasant at times.

How about "The idea that every engine needs to run at 2500rpm to be happy is a bit of a myth".........."on the face of it that statement is false"

I'm sure you enjoy attacking people and making yourself feel like a bigger, smarter, better looking, suave, debonair and orders in french......but its not working and frankly detracts from the intent of a collegial group of sailors on a forum..

You're not alone in this of course, but damn dude, try to chill out.

**Wotname** · 07-03-2024, 20:38

Quote:

Originally Posted by massnspace

The diesel is a Universal M-40 (32 HP I believe), and the boat is a 32’ long by 10’ beam long keel cruiser that weighs 15,000 lbs.

Again. I cruise all day long at 5.7 knots when at 1500 RPM. Is that OK? Is the motor “under sufficient load”?

Yes.

Assuming you prop is in the ball park, I would suggest running at 1500 is pretty close to ideal. See attached power / torque curves for the M-40. I have added the red lines.

So it is being loaded at ~19.2 hp from a max of 32 which is ~61% and running near the peak of the torque curve. If it was my boat, I'd be very happy.

To check how close your prop is to ideal, run it at WOT in calm water with a CLEAN prop and bottom. If you can make 2600 rpm or better, the prop is suitable.

Because I'm a belts and braces guy, I'd still run it at WOT for say 10 mins every 2 hours.

Bowdrie · 07-03-2024, 20:52

Many of the relatively little engines we use for propulsion are used as gensets in power boats, and they run forever at 1,800, much of the time with little load, and they do just fine.
Load comes more into the crosshairs as in "over-loaded", whether it's a propulsion engine that's fighting against too much pitch/diameter prop load or a genset that's pushing all the fuel it can trying to maintain that 1,800 rpm.
The other issue is heat, the closer the engine stays to proper temp, (at whatever load,) the better, the same with oil temp.