Diesel RPM?

[Brunton's Propellers]

https://bruntonspropellers.com/autoprop/


Automatic Variable Pitch


No need for manual pitch adjustment the Autoprop does for you!

[Dockhead]

Originally Posted by sinnerman

Boy, this question brought out a lot of responses...

Basic reasoning I understood was that diesels like to be run under load.

If you want to cruise the boat at 1500 RPM, then since the engine is under load (actually moving the prop and pushing the boat) it is functioning as designed.

What they do not want you to do is have the engine running at idle for hours on end just to charge the batteries..

Supposedly this will "glaze" the cylinder walls.

Of course this could all be hearsay

MY two cents

Cheers...

There are tons of discussion about this in the archives.


Diesel engines need enough load to get to a reasonable cylinder temperature. Low cylinder temperature can lead to different problems. How much load that is varies. This is particularly dangerous for constant speed generators because at higher RPM, more air is going through the cylinders and you need more load to burn enough fuel to warm them up.



Turbo engines are least affected by this because they are smaller displacement for a given HP rating, so less air and less fuel burn needed to keep them warm. So you can run them safely at a smaller % of their rated power, than a naturally aspirated engine.

[Dockhead]

Originally Posted by Brunton's Propellers

https://bruntonspropellers.com/autoprop/

Automatic Variable Pitch

No need for manual pitch adjustment the Autoprop does for you!

I have many thousands of hours of experience with Autoprops on two boats over nearly 30 years. Probably 10,000 hours altogether.

Certainly these props are good for your main engine as they will pitch up to keep a load on. Brilliant for motor sailing.

However, they require a lot of maintenance and need regular overhauls. The bearing races get brinelled rather quickly as they use ball bearings as thrust bearings. Then you get sea water in the mechanism because it is hard to seal.

I spend at least a few hours every year working on mine. I think this year I'm due for another overhaul. It's not that hard to do but takes time with the boat out of the water, and the parts are costly. I get less than 1000 hours between complete overhauls, which requires changing out all of the bearings and seals. At first I was sending my prop back to Brunton's for this, but eventually realized that the labor to remove the prop and box it up for shipment, and drag it to the post office, was more than the labor to overhaul it myself on the shaft, not to mention time spent waiting for it to come back, and the cost. Plus I trust my own torque wrench more than theirs; see below.

Parts are readily available (the most important thing) but are expensive.

People complain about customer service, which unfortunately you will likely need a good bit of with one of these. I've had mixed results. They are responsive, but not always helpful. I had an unhappy incident with them where they overtensioned the blade nuts when they overhauled my prop the last time, which spoiled the bearings within a couple of hundred hours. They refused to take any responsibility, and refused even give me a new set of bearings for me to install myself, even though it was well within UK warranty period (actually worse than that -- they promised to refund the cost of the bearings if I sent them the spoiled ones back to show them what had happened -- I did so but then they refused to do the refund). That made me really angry, and I stopped sending my prop to them and starting doing it myself.

Autoprops have a reputation for throwing blades, which some people curse them for. I have never had this problem, and the locking mechanism on the later, sealed model, like what I have on the current boat, seems robust enough. Touch wood I don't worry about this too much. I think this was a problem only with certain early models.

Another peculiarity of these is that it takes some time for the blades to adjust when going from forward to reverse or back, so you need to use a lot of throttle and wait a bit, which can complicate harbour maneuvers.

We have a Gori two-speed prop on the Discovery 67 we sailed across the Atlantic a couple of years ago which does not have this problem. It bites fiercely in reverse, which is very useful in harbour maneuvers. The two speeds are not automatic, and it's just two speeds, but the "overdrive" gear gives much of the benefit of the Autoprop's variable pitch.

Worth considering as an alternative. But I wouldn't want a prop without SOME kind of variable pitch, on a cruising sailboat where you do much motor sailing, or slow motoring in calm water. It's really very, very beneficial. And not just to your main engine.

[malbert73]

Originally Posted by Lee Jerry

Ditto.



I can accept that we are now generally on the same page (or close enough that I don't want to repeat) with one big exception: you aren't changing the engine load by over-propping. The load is determined by the cruise speed. It is the same with both props at the same boat speed (with the usual "all things being equal" caveat).



I have to add that I don't understand the highlighted bit. But it's been a long day and I've had a couple adult beverages, so...

Yes agree an increase in cruising speed increases the load on an engine.

The faster you try to push a displacement hull there’s a swift rise in power required.

But you cannot be suggesting the load on the engine is the same with overpropped vs conventionally propped scenarios? That’s like saying the load on a truck diesel going up a hill is the same whether they are in 3rd or 5th gear. It’s not. That’s why there are gears- to better match power required with load needed. A conventionally pitched prop is like a car on the highway that never can use the last gear- the engine must run faster and less efficiently

So, to restate, there are 3 solutions to achieve proper engine loading.
1. Smaller sized engine with a prop pitched for full RPM (like the old auxiliaries) where the power supplied matches the power required at slower efficient speeds. Your top end power ends without a reserve.
2. Have a larger power plant with prop pitched for full rpm, and be forced to motor at faster, less efficient, wake producing cruising speeds to properly load the engine. (Even overpropped, my EGT shows that my engine is on the light side of the load at a relatively efficient 7 knots). This can result in double the fuel burn for only 5-10% more speed. What happens is most cruisers don’t actually run their engines up at this speed due to noise, fuel burn, etc- and instead are “killing their engines with kindness”. But, this does allow full HP to be extracted from the engine
3. De rate an oversized engine by mildly overpropping to better match efficient cruising speed with corresponding engine load at lower RPMs. Accept the loss of top, rarely needed HP. Install an EGT to safely monitor for overload (and underload- which still is easy to do even over propped)

I do like the idea of the Gori overdrive prop. I like my Flexofold but the two speed would be great and achieve both objectives. Really wish there was a transmission with 2 gear ratios as that would also allow best of all worlds

[skipperpete]

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”?

In response to you first post……YES, it is ok!

[skipperpete]

Originally Posted by donradcliffe

This makes absolutely no sense at all. By 95% of high idle do you mean to say 95% of WOT in neutral?

Kiwijohn got it right, that’s more or less what Yanmar states in the “ installation manual” and gives the following seatrial test for a fairly typical 4LH-STE
Observed high idle…3675-3725 rpm
Engine speed at maximum output…. 3300 rpm
Observed seatrial engine rpm….. 3,350-3,500 is ok
Below 3,300 rpm is overpropped.
Above 3675 rpm is underpropped
This is what Yanmar expects and they even include the propeller dimensions to help the installer get it right. Not saying I totally agree with Yanmar but it’s what they want to see as the upper limit….. the recommended cruise rpm is somewhat lower.
In the same manual yanmar generally expects an EGT of between 600°C and 650°C as the absolute maximum limit, this extends across the entire range of small Yanmars and includes the 3GM series.
I’ve enjoyed following the informed and amicable debate between Malbert73 and Lee Jerry, lots of useful information regarding a fairly contentious issue ….but I guess the OP’s answer was provide by Boatpoker with a staggering 20,000 hrs on a grossly under loaded 120 hp 6-354 Perkins in a small motor cruiser.
In my experience a lightly loaded engine with a jacket water temp of 80°C has the same operating jacket water temp as a fully loaded engine despite the obviously higher EGT so I’m not sure how the cylinder walls could vary too much below 80°,C assuming the thermostat is installed and working.

[Lee Jerry]

Originally Posted by malbert73

Yes agree an increase in cruising speed increases the load on an engine.

The faster you try to push a displacement hull there’s a swift rise in power required.

But you cannot be suggesting the load on the engine is the same with overpropped vs conventionally propped scenarios?

No, I'm not suggesting that...I'm stating that. And I have about a dozen times.

To be clear, I am using "load" to be the same as power. %MCR. If you mean something else, you'll have to define it.

That’s like saying the load on a truck diesel going up a hill is the same whether they are in 3rd or 5th gear. It’s not.

No, if you recall my earlier post, hills were analogous to wind/waves. The car analogy below is better.

That’s why there are gears- to better match power required with load needed.

OK. But to be pedantic, I would say it might be more accurate to say it is to match torque delivered and demanded. (Notwithstanding that load is a function of power.)

A conventionally pitched prop is like a car on the highway that never can use the last gear- the engine must run faster and less efficiently

Yes, I generally accept this analogy. But the load/power is the same in both gears (determined by the speed / drag), only the efficiency might change. And the "less efficient" isn't as much as you think. (Especially for the boat side of the analogy, anyway.)

So, to restate, there are 3 solutions to achieve proper engine loading.
1. Smaller sized engine with a prop pitched for full RPM (like the old auxiliaries) where the power supplied matches the power required at slower efficient speeds. Your top end power ends without a reserve.
2. Have a larger power plant with prop pitched for full rpm, and be forced to motor at faster, less efficient, wake producing cruising speeds to properly load the engine. (Even overpropped, my EGT shows that my engine is on the light side of the load at a relatively efficient 7 knots). This can result in double the fuel burn for only 5-10% more speed. What happens is most cruisers don’t actually run their engines up at this speed due to noise, fuel burn, etc- and instead are “killing their engines with kindness”. But, this does allow full HP to be extracted from the engine
3. De rate an oversized engine by mildly overpropping to better match efficient cruising speed with corresponding engine load at lower RPMs. Accept the loss of top, rarely needed HP. Install an EGT to safely monitor for overload (and underload- which still is easy to do even over propped)

I would state the options differently:
1. Reduce installed power appropriate to the desired speed.
2. Increase speed appropriate to the installed power.
3. Run at cruise speed for a while, then run at higher speed/load for a "short time" to increase operating temp. Repeat as necessary.
(All are propped properly.)

I do like the idea of the Gori overdrive prop. I like my Flexofold but the two speed would be great and achieve both objectives. Really wish there was a transmission with 2 gear ratios as that would also allow best of all worlds

No comment at this time.



PS - I will be offline for several days. Going sailing - go figure - so will reply (as necessary) when I return.

[Bowdrie]

This is anecdotal. there is a 1952 ex-Norwegian fishing boat near me that I've worked on a few times, and knew the owner who brought it over from Ireland where he had purchased it.
It originally had some 2-clyinder monster with a controllable pitch prop, that lasted ~30 years or so.
The replacement is a 6-354T Perkins with a fixed prop. with that package it's been across the Atlantic 3 times, up/down the Eastern US, thru the Canal and up here to the PNW, never missing a beat, engine still just fine.
I've "spun-it-around" a couple times, the engine will not quite reach full RPM, but it's close and it holds a stable water temp.
I did a "one-off" engine/prop install in the middle '70s. It was a 3-clyinder Volvo with a Hundested controllable pitch prop.
The technique was to set the desired RPM, and then increase/decrease pitch until the EGT/cylinder head temp, (I forget which measurement they used,) was in the "green".
I have no idea if Hundested still makes what I installed, (~a 22" prop,) but it operated well and was pretty simple.

[skipperpete]

Yes, Hundested still makes propellers, actually the shafts and pitch control units as well but mainly for markets other than the cruising yacht market. There’s no better way to understand how small the amount of adjustment there is between the correct pitch and the wrong pitch than with a Hundested CPP. …. But these propellers are expensive. VERY expensive.

[gulfislandfred]

Aside to all the above:

Somewhere, way back on this thread, someone said that if you run the engine hard for a half hour to and hour, once in a while, it will clear out the built up deposits in the combustion chambers.
I think most engines like a hard run, to get everything up to temperature.
Two friends who had diesels that ran well for years, one in a commercial fishing boat, and one in a Dodge pickup truck, had pyrometers in their exhaust stacks, and made sure to get the engine up to temperature at least once every week or so.

Stack temperature is fairly easy to monitor. Real data in real time.

[Bowdrie]

Originally Posted by skipperpete

Yes, Hundested still makes propellers, actually the shafts and pitch control units as well but mainly for markets other than the cruising yacht market. There’s no better way to understand how small the amount of adjustment there is between the correct pitch and the wrong pitch than with a Hundested CPP. …. But these propellers are expensive. VERY expensive.

Yeah, for me it was "just a job", that the foreman gave me.
All the stuff. prop. shaft. control wheel. stern bearing, inner bearing, etc. all showed up in a couple of wooden crates.
It was impressive, and it all went together with the Volvo engine without any real difficulty.

[malbert73]

Originally Posted by Lee Jerry

No, I'm not suggesting that...I'm stating that. And I have about a dozen times.

To be clear, I am using "load" to be the same as power. %MCR. If you mean something else, you'll have to define it.



No, if you recall my earlier post, hills were analogous to wind/waves. The car analogy below is better.



OK. But to be pedantic, I would say it might be more accurate to say it is to match torque delivered and demanded. (Notwithstanding that load is a function of power.)



Yes, I generally accept this analogy. But the load/power is the same in both gears (determined by the speed / drag), only the efficiency might change. And the "less efficient" isn't as much as you think. (Especially for the boat side of the analogy, anyway.)



I would state the options differently:
1. Reduce installed power appropriate to the desired speed.
2. Increase speed appropriate to the installed power.
3. Run at cruise speed for a while, then run at higher speed/load for a "short time" to increase operating temp. Repeat as necessary.
(All are propped properly.)



No comment at this time.



PS - I will be offline for several days. Going sailing - go figure - so will reply (as necessary) when I return.

“ Yes, I generally accept this analogy. But the load/power is the same in both gears (determined by the speed / drag), only the efficiency might change. And the "less efficient" isn't as much as you think. (Especially for the boat side of the analogy, anyway.)”

Really? Load on the engine is the same regardless of gear ratio (cars) or prop pitch (boats)?

Ok, like Boatpoker up thread, I’m out.
Need to kick spring commissioning into high gear anyway.

[Adelie]

Originally Posted by Lee Jerry

...

That is a gasoline engine. So while the trends are similar, I believe the possible efficiency gains are much smaller (~half) with a diesel. Your lines show ~6.7% improvement, generally. Even so, while "generally" true, there are points on that map where the over prop is the same or slightly worse consumption. You need to have the map and know all the operating points to ensure you are improving consumption at the intended speed by overpropping.

....

As you say diesel and gas engine fuel maps are similar which is my opinion from looking at a bunch of them. The overprop shows consistent fuel efficiency gains except at 20-22hp where the lines are about even. The little bubble is specific to this engine and there might or might not be something similar on a different map, diesel or gas. Regardless it is gas not diesel, they are similar.

Below are some more fuel maps, both diesel & gas.

Originally Posted by Lee Jerry

Yeah, OK, but not really germane to the conversation.

The discussion of Controllable Pitch Props (CPPs) are germane to the discussion since this discussion has touched on why Overpropping might be desirable. At any desired horsepower/speed the pitch can be adjusted (overpropped) to minimize fuel use. I added a LAVENDER line to the previous fuel map to show the difference. At 40hp & 30% there is a 25-30% fuel efficiency advantage, At 20hp it's about 25%, at 10hp its 20%.

An overpropped Fixed Pitch Prop (FPP) will have more modest gains, but still consistent across the most of the range of powers.

[jen1722terry]

Per NBs' comment: what is a 'common rail" deisel?

Thanks in advance

[Adelie]

Originally Posted by jen1722terry

Per NBs' comment: what is a 'common rail" deisel?

Thanks in advance

Common tail is a fuel delivery system. It uses much higher pressure pumps and solenoid valves than regular fuel injection which results in better power and fuel efficiency for the same sized engine. The tradeoff is greater required repair costs and a the injectors are smaller and more prone to blockages. Fuel filtering becomes way more important.