Diesel RPM?

[boatpoker]

Originally Posted by Lee Jerry

Thanks for the direct answer. However, none of those are given as true, and are quite possibly not true. I'm curious if you have any proof or citations in support. The first two are most likely to be the same between the two props (to the level that one could notice or reasonable measure), with smaller but equal chances that either could be slightly better than the other (proper vs over-).

For example, the over-propped boat will probably have a slightly higher minimum speed in gear (at idle speed, depending on the engine design) - that doesn't seem like a good thing to me in tight quarters. I think it will also take an extra second or two to get good flow over the prop when switching gear as the higher pitch churns the water more. Again, not as good.

As for fuel consumption, there are several ways to address and I'm not sure which is best. How about: at a given boat speed, the over- will operate at a lower RPM, but (essentially) the same power and therefore (essentially) the same fuel consumption. Again, the details could make either prop slightly better. Basically, the resistance is the same and the propulsive coefficient is essentially the same, therefore the FC is the same.

And then there are the negatives: mainly the reduced range of performance as conditions become more demanding, but also reduced power available, reduced RPM range available, higher probability of cavitation (at all speeds),...

I am quite content that my 20,000hrs. of logs in the triangle between Duluth MN, Antigua and Honduras proves me right. You are entitled to your opinion.

[donradcliffe]

Originally Posted by Lee Jerry

And all that has absolutely nothing to do with the question I asked.

What do you think the benefit(s) is (are) of over-propping? If you could give a direct answer to that question, I can (most likely) explain why that is not the case and also give a handful of reasons for why it is bad (including the one you already ignored).

The benefit of over propping is fuel economy.

I have a 3 liter 6 cylinder turbocharged Mercedes in my RV. One of the neat features is an instantaneous fuel economy computer and gauge. It calculates MPG based on the fuel burn rate and the vehicle speed.

I have a 5 speed transmission, so I can manually select one of 5 engine speeds to see the impact on miles per gallon. Normally I leave the selector in drive and let the computer do the choosing, and it always selects the gear with the lowest engine speed but enough torque to maintain speed. If I manually go to a higher engine speed, there is a dramatic drop in mpg with constant speedometer and load. I have more available torque and more reserve power, but I am paying a penalty in fuel usage.

In a boat, you can change engine rpm at constant load by changing the pitch and diameter of the prop, and the transmission drive ratio. There will still be the same tradeoff between reserve power and fuel burn.

When you talk about propping to barely achieve some rpm, are you testing with the boat tied to the dock, or in open water? There is quite a difference.

[Wotname]

Originally Posted by Lee Jerry

..........

Well, I don't know what anyone else thinks is "close enough." But I'll ask you the same question: what is the benefit(s) of being over-propped? (Other than not buying a new prop.)

????? Dunno why you are asking me this question. I never claimed there was any benefit other than (with respect to the OP of this thread) not having to fine tune his prop; even then, I qualified my claim with being slightly over propped.

[Lee Jerry]

Originally Posted by boatpoker

I am quite content that my 20,000hrs. of logs in the triangle between Duluth MN, Antigua and Honduras proves me right. You are entitled to your opinion.

How many of those 20,000 hr were with the "proper" propeller and how many over-propped?

[boatpoker]

Originally Posted by Lee Jerry

How many of those 20,000 hr were with the "proper" propeller and how many over-propped?

Any response to you is likely to be rewarded by your euphenism of "untrue".
Simply not worth anymore responses.
Have at it.

[Lee Jerry]

Originally Posted by donradcliffe

The benefit of over propping is fuel economy.

Yes, that appears to be the claim. I remain unconvinced.

Best case is you can (might) do that for one condition, but you make it worse in the others. Proper matching gives best all around performance (including all around fuel econ).

Originally Posted by donradcliffe

I have a 3 liter 6 cylinder turbocharged Mercedes in my RV. One of the neat features is an instantaneous fuel economy computer and gauge. It calculates MPG based on the fuel burn rate and the vehicle speed.

I have a 5 speed transmission, so I can manually select one of 5 engine speeds to see the impact on miles per gallon. Normally I leave the selector in drive and let the computer do the choosing, and it always selects the gear with the lowest engine speed but enough torque to maintain speed. If I manually go to a higher engine speed, there is a dramatic drop in mpg with constant speedometer and load. I have more available torque and more reserve power, but I am paying a penalty in fuel usage.

The changes from a gear shift in a car is an order of magnitude (or more) greater than the effect of changing the pitch of a prop. In other words, not a particularly useful comparison IMHO. It's also not surprising that shifting from the gear the computer chooses (i.e. from the proper gear) at that time results in worse performance. Finally, you don't say what happens if you switch to a lower engine speed.

Originally Posted by donradcliffe

In a boat, you can change engine rpm at constant load by changing the pitch and diameter of the prop, and the transmission drive ratio. There will still be the same tradeoff between reserve power and fuel burn.

This may be semantics, but the engine rpm changes because the (engine) load has changed.

In order to decrease fuel consumption, you need to either increase the propulsive coefficient (prop efficiency and other similar items) or the engine has to produce power more efficiently. As I said above, these are possible (to a small degree), but it's also possible to make them worse and most likely to stay essentially the same. So which one are you doing by over-propping and how do you know?

Originally Posted by donradcliffe

When you talk about propping to barely achieve some rpm, are you testing with the boat tied to the dock, or in open water? There is quite a difference.

Not sure where this came from. Propeller matching is done at sea. Except maybe for a tug...

[Lee Jerry]

Originally Posted by Wotname

????? Dunno why you are asking me this question. I never claimed there was any benefit other than (with respect to the OP of this thread) not having to fine tune his prop; even then, I qualified my claim with being slightly over propped.

I don't recall the OP ever asking about prop match; that was only brought up by others.

[Lee Jerry]

Originally Posted by boatpoker

Any response to you is likely to be rewarded by your euphenism of "untrue".
Simply not worth anymore responses.
Have at it.

I'm not pretending to know or be stating the truth. I'm asking you to support your claim that I do not accept. I have to indicate that lack of belief with some word(s); what ones would you prefer?

So you ran all 20k hr with the same "wrong" prop and actually have no point of comparison, is that about right?

[Loredo]

Can we please put this "ideal diesel RPM" thing put to sleep once and for all?
It is nonsense!
Also prop-match without supporting data makes no sense. Where do we get the "data" from? An Exhaust-Gas-Temperature sensor is the only answer and the only way to KNOW if an diesel engine is overloaded or not. Extremely simplified, "ideal" cruise speed EGT is in the 600 to 800 degrees Fahrenheit range. Below that and the cylinders are cold, above that for long periods and the engine will suffer in the long run. Ideally we'd need to monitor the coolant temp, the oil temp and the EGT. Only that will provide the picture about the working load on the engine. Anything else is just gossip.

[donradcliffe]

Originally Posted by Lee Jerry

Not sure where this came from. Propeller matching is done at sea. Except maybe for a tug...

The only time most people will use ALL the power in your engine is when you are motoring into strong headwinds and seas. When your boat speed through the water drops to 2 knots or lower, you may wish that you had "matched" the prop at the dock. Again, there is a fuel economy penalty for 99.99% of the time you use your engine

[donradcliffe]

I obtained much of my prop matching knowledge from discussions with a freshly minted Naval Architect who wrote his thesis on prop matching. He was motoring down the ICW and lost his prop outside the boatyard I was in. He ordered a maxprop to replace it, and then agonized about the proper pitch setting. He pitched the prop, relaunched the boat, and was disappointed in his sea trials. He was contemplating rehauling the boat to adjusting the prop pitch by 2 degrees. With the wisdom that comes from grey hair, I told him to wait until the next time he needed to haul the boat.

[dannc]

If the OP is still reading this discussion, one way to get real data about what is happening with your engine and it's operation, is to do a used oil analysis(UOA). I have been doing UAO for my truck and tractor(Yanmar) engines for decades with Blackstone Labs.

Once upon a time I was getting fuel in the oil in both engines. The fuel in the truck oil showed up when I had to drive into an area with lots of stop and go traffic which meant quite a bit of idling. The total drive time was 60-90 minutes with 15-30 minutes being idle time depending on traffic. At least 30 minutes of the drive was pretty much at a steady state of 55 mph. Once I stopped driving into that area, the UOA was back to normal. I was hoping that the long drive at a steady rate would burn off any contaminants, which may have happened, but just not enough.

On the tractor, I just increased the throttle from 1500-1600 to 1800-2000 RPMS. Engines WOT is 3200 if I remember right.

The fuel in the oil was still within specifications but I was glad to see the UOA go back to 0%.

I was recently reading a Yanmar common rail engine manual and they recommended revving up the engine every few hours of operations. That recommendation surprised me.

[skipperpete]

The OP was more interested in lack of load and under propping, not overloading.

[massnspace]

Originally Posted by skipperpete

The OP was more interested in lack of load and under propping, not overloading.

That’s correct.

Thread drift is one of the things I don’t enjoy about this forum, but I’ll take the good with the bad.

Thanks to those who have contributed towards my original question.

As far as oil analysis…..I just changed it. How long should one wait before attempting that?

David
Seattle

[Lee Jerry]

Originally Posted by donradcliffe

The only time most people will use ALL the power in your engine is when you are motoring into strong headwinds and seas. When your boat speed through the water drops to 2 knots or lower, you may wish that you had "matched" the prop at the dock. Again, there is a fuel economy penalty for 99.99% of the time you use your engine

You're making my point. That's when you most DON'T want to be over-propped, because it reduces your RPM and HP available at those higher loads.

You still haven't explained how over-propping gets better fuel economy: is it the prop producing thrust more efficiently, or the engine producing power more efficiently, or something else?

Originally Posted by donradcliffe

I obtained much of my prop matching knowledge from discussions with a freshly minted Naval Architect who wrote his thesis on prop matching. He was motoring down the ICW and lost his prop outside the boatyard I was in. He ordered a maxprop to replace it, and then agonized about the proper pitch setting. He pitched the prop, relaunched the boat, and was disappointed in his sea trials. He was contemplating rehauling the boat to adjusting the prop pitch by 2 degrees. With the wisdom that comes from grey hair, I told him to wait until the next time he needed to haul the boat.

That's reasonable. Glad you didn't tell him to leave it because it was close enough.