Re-Pitching prop for speed or economy

All,

Have you ever repitched your prop to favor economy or speed? I've heard some pros and cons and would like to know if there are advantages. And if so, is there a sweet spot.

For example, on my single engine Mainship 400 there seems to a sweet high speed cruise at around 13 knots. A lot of folks run it wide open and back off and adjust the trim tabs to hit 13 knots. Not quite planing, but above just pushing water around.

Thoughts?

I thing "sweet" spot is all about smoothness. The engine dosn't shake the boat as much and noise is less too.

But the engine could care less if it's shak'in the devil out of the boat or acting like an electric motor w no reciprocating parts.
I wonder (a tiny bit) if two engines running in exactly the same conditions except one is shaking hard and the other is smooth if there would be any difference how long they both last.one thing for sure is the engine mounts would fail prematurly on the vibrating engine.

Re propping there is a huge amount written about prop loading. I think we started talking prop/engine loading in 2007 and have made at least hundreds of posts since.

For the record I'll take my usual stand and say propping for rated rpm (or 100rpm higher) is the best way to go. The bottom line is that there is so little to be gained and so much to be lost it makes no sense.
Also never heard an engine manufacturer say different. Many, most or all will void your warrenty if you do not prop to rated max hp rpm. I'm a little off now and would like to gain 100rpm.

Seevee said:

All,

Have you ever repitched your prop to favor economy or speed? I've heard some pros and cons and would like to know if there are advantages. And if so, is there a sweet spot.

For example, on my single engine Mainship 400 there seems to a sweet high speed cruise at around 13 knots. A lot of folks run it wide open and back off and adjust the trim tabs to hit 13 knots. Not quite planing, but above just pushing water around.

Thoughts?

Click to expand...

You pitch the engines to make sure they reach rated WOT +3-5% as a brief test when the boat is fully loaded on a hot and humid day. This gives you the correct loading at the lower rpm to ensure the engines are safe. If you tend to run near the edge it is also prudent to add EGT and boost gages which do not cost much and add value.
In some cases when you run a diesel boat that will never utilize near full rpm you can 'cheat' and add a larger prop but you need to 'double secret promise' that the throttles will never be advanced passed a lower hull speed. The advantages of over propping in this way are fairly small as well.

There is a small case to be made for over pitching older normally aspirated engines like the Perkins or Lehmans to gain a bit (and it is a small bit- 10%) of fuel economy and reduce engine noise due to lower rpms while cruising at or below displacement speeds.

But I know your engine- the Yanmar 370 I suspect, as I have had two- one on a MS 34T and the other on my current MS P34. If you are cruising at 13 kts then you are running at about 3,000 rpm and hopefully you are currently pitched a little over rated rpm to about 3,400 rpm.

If you were to over pitch even as little as 200 rpm, ie your wot would now be 3,200 rpm, you would be over loading your engine while cruising at 13 kts and maybe 2,900 rpm. And if you expected to get another half knot or so by running at 3,000 rpm you would really be overloading it.

But if all you ever want to do is to run at 8 kts, then like the NA engines above, you could over pitch by several hundred rpms and gain a bit fuel economy and less noise. But don't ever think about running at 13 kts again.

You may have the Cummins 370 engine, in which case the numbers change by several hundred but the conclusion is the same.

David

Over pitch can cause considerable harm in most of the engines we deal with on this site I know I lost one that way. Smaller engines in sail boats are often over pitched and overloaded and most seem to get away with that. Under pitch does not cause harm and most suggest one or two hundred extra RPM over rated as a safety margin. I purposely run my common rail electronic motors under pitched in order to protect engines and effectively down rate them to less than the original M4 configuration. Another benefit aside from long life of my under loading is very good manners at idle speed in gear. At rated RPM my engines will attain 80-85% load and my fast cruise at 15-16K is at 60-65% load the slow cruise at 9.2K is at 35-40% load. If I were to pitch to full load I might gain 1-2K at top speed where I do not want to travel and their may be a small theoretical boost in economy.

djmarchand said:

There is a small case to be made for over pitching older normally aspirated engines like the Perkins or Lehmans to gain a bit (and it is a small bit- 10%) of fuel economy and reduce engine noise due to lower rpms while cruising at or below displacement speeds.

But I know your engine- the Yanmar 370 I suspect, as I have had two- one on a MS 34T and the other on my current MS P34. If you are cruising at 13 kts then you are running at about 3,000 rpm and hopefully you are currently pitched a little over rated rpm to about 3,400 rpm.

If you were to over pitch even as little as 200 rpm, ie your wot would now be 3,200 rpm, you would be over loading your engine while cruising at 13 kts and maybe 2,900 rpm. And if you expected to get another half knot or so by running at 3,000 rpm you would really be overloading it.

But if all you ever want to do is to run at 8 kts, then like the NA engines above, you could over pitch by several hundred rpms and gain a bit fuel economy and less noise. But don't ever think about running at 13 kts again.

You may have the Cummins 370 engine, in which case the numbers change by several hundred but the conclusion is the same.

David

Click to expand...

David,

Just to clarify the definitions: (see the diagram)

I believe "over pitched" takes more of a bite of water for every revolution
And an "under pitched" takes a smaller bite
(correct me if I'm wrong)


[/IMG]

The over pitched prop will run at a lower RPM, give more speed for a given RPM but have less torque.

The under pitched prop will run at a higher RPH, with less speed, and will have more torque.

I know there are applications where it pays to favor one or the other. Like a tug boat, or a tow boat that really needs the torque to push would favor an under pitched prop.

And a boat that's going a long distance and a fairly constant cruise speed might favor a over pitched prop. (like our Trawlers).

There are also variable pitched props, that can be changed as needed. Not sure if there are cost effective for our trawlers, however.

Now, I understand that we still have to operate within the limits of the engine, and don't want to overload the engine.

Overloading might occur if one has a low pitched prop (over pitched) and tried to accelerate with full power to get away from something quick. Similar to putting your car in 3rd gear and trying to rapidly accelerate away from the stop light. It can be done, but at less power and takes longer to get up to speed.

Underloading might occur if one has an under pitched prop and operating at a low but constant speed at really low power. I don't believe it's that harmful as overloading and can work out fine if we occasionally "race" the engine as prescribed in the engine operations manual.

So, if my goal with to increase speed at a lower fuel consumption, I'd increase the pitch of the prop (overloading) and instead of operating at 3000 RPM for a certain speed, I'd be operating at 2800 RPM.

Does this make sense?

So, I think we can agree that there could be some variables out there, perhaps not enough to matter. But I'm curious if folks have done this, and to what extent and the results.

Seevee:

Your understanding is correct.

But I would say that the risks of overpitching for low speed operation outweigh the advantages, particularly for a high output, turbocharged engine. The advantage is maybe 10% better fuel economy (for a 400 rpm over pitch) and slightly lower engine noise. The risk is destroying your engine if you run it fast for long in an overpropped condition.

You could do it if you only wanted to go 8 kts and never ever ran more than 2,000 rpm.

David

There was a lengthy and highly technical thread on this over at the Hatteras Owners Forum a few years back that discussed this in great detail. It's still available there through search. I believe their bottom line was that one can gain some benefit, but not much and at the risk of damaging the engines if operated incorrectly after repitching the wheels.

The propeller is the load, that's only part of the story.

Good decisions require careful consideration of the available power that the engine can make at that rpm, and what percentage of that available power might be used.

There is not simple economy only by virtue of lowering the rpm.

The diesel engine will burn whatever fuel is necessary to maintain a given rpm, up to its ability to make the max HP for that rpm. Diesel fuel use is not linear according to rpm, it depends on the load the engine is being put under.

Load the engine with more pitch, the engine will burn more fuel at that lower rpm than it did before the pitch was changed. Put the engine in neutral and run at that rpm, it will use a lot less.

Overloading is what happens when the engine is being fully fueled and can't make any more power at the RPM it is being forced to operate at. High heat accompanies this condition.

Re-pitching is not to be taken lightly for the novice, there is exposure to shortened engine life for the uninitiated.

Keep reading!!!

RB

Yep, if you ever intend to run at a power setting above hull speed, you want to prop for rated rpm at full power plus 50 to 100rpm. 13kts is certainly above hull speed.

Overpropping for nmpg is not a good idea on such a boat. Maybe on a displacement hull, provided operator understands the situation and knows that the higher power settings are a no-go zone. The advantage there is less noise from lower rpm.

The effect on nmpg is trivial. Given a certain cruising speed, it takes the same hp regardless of rpm, and the burn rate on a diesel is highly tied to load, not rpm. On a gasoline engine the burn rate has a strong correlation to rpm, but not on a diesel.

djmarchand said:

Seevee:

Your understanding is correct.

But I would say that the risks of overpitching for low speed operation outweigh the advantages, particularly for a high output, turbocharged engine. The advantage is maybe 10% better fuel economy (for a 400 rpm over pitch) and slightly lower engine noise. The risk is destroying your engine if you run it fast for long in an overpropped condition.

You could do it if you only wanted to go 8 kts and never ever ran more than 2,000 rpm.

David

Click to expand...

David,

Good points.

I'm assuming that whatever changes were done would be within limits of the engine with regard to loading and rpm limits. And, for me, operating at higher speeds occasionally will be part of the plan.

I agree, see very little advantage in over propping (using a high pitch) with low speeds. The advantages would come at higher speeds, where one gets either better fuel economy or better speed.

I see this in aviation, but only with constant speed props (variable pitch) and the effect id dramatic. Even the planes that have a fixed pitch prop can be adjusted for a high torque prop (under pitched) or a cruise prop (over speed). And if you can get the airplane off the ground the fuel savings and speed is big.

Guess we boaters don't enjoy this advantage. Also, what's the skinny on variable pitched props for boats? They are expensive upwards of $8 to $10K, and doubt one would fit my profile, but curious how effective they are.

Thx for the good comments!

Boat diesel propping is very different from aviation gasoline or gas turbine propping. Diesel have a relatively flat consumption curve vs rpm at a specified hp output. Gasoline and gas turbine engines rpm has a big effect given a specified hp output.

If you have a 370hp diesel and want 200hp down the shaft, it does not matter more than a couple percent whether you get it at 2500 or 2700rpm.

Ski in NC said:

If you have a 370hp diesel and want 200hp down the shaft, it does not matter more than a couple percent whether you get it at 2500 or 2700rpm.

Click to expand...

I think that's the key concept that folks new to this have trouble with. Fuel consumption correlates to load, not rpm. And speed correlates to load. RPM is just reflects the gear ratio.

When I came into this world and had the same questions I puzzled over prop torque curves until I understood them completely, and then it became clear. YMMV but it worked for me

It is difficult to draw parallels between a controllable pitch prop in air, powered by a gasoline engine with a TBO of 2,000 hours and a diesel powered boat that we expect to get 10,000 hours or more out of.

Controllable pitch props exist, but maybe not in the sizes that recreational trawlers would need. In any case they would be expensive.

You said "I'm assuming that whatever changes were done would be within limits of the engine with regard to loading and rpm limits. And, for me, operating at higher speeds occasionally will be part of the plan."

Over pitching an engine is never "within the limits of the engine with regard to loading and rpm limits". All engine manufacturers will void your warranty if not pitched to reach rated rpm at wot. Any overpitching is at your risk.

And since you will be "operating at higher speeds occasionally", then don't do it.

David

On a FD hull yes of course you can get slightly lower fuel burn. Most any other huul too.

David has been using 10% for overpropping fuel savings. I think that may, just may be right in an ideal situation but usually I belive it's closer to 5% and even less in a lot of cases. Could be even higher in extreme configurations. There's so many variables. Engine, gear ratio, size of prop, closness to hull, keel and most importantly how deeply overpropped. But for a "blanket number" I'll bet 5% would be closer. I don't really know though.

SeeVee wrote;
"I'm assuming that whatever changes were done would be within limits of the engine with regard to loading and rpm limits. And, for me, operating at higher speeds occasionally will be part of the plan."

Absolutely not unless I'm not reading this right. Any overprop at all and at WOT the engine will be overloaded.

Nomad Willy said:

SeeVee wrote;
"I'm assuming that whatever changes were done would be within limits of the engine with regard to loading and rpm limits. And, for me, operating at higher speeds occasionally will be part of the plan."

Absolutely not unless I'm not reading this right. Any overprop at all and at WOT the engine will be overloaded.

Click to expand...

Eric,

I hope I posted that correct... I DO plan on going fast at times, not WOT, as Yanmar prohibits this. However, 290hp at 3100rpm will work, or about 80% of total power. That's the power area of the most benefit of a coarser pitch change. However, seems like there's little benefit with a lot of risk from the comments here. So be it, but just looking for folks that have done this pitch changing... or perhaps have a variable pitch prop.

eyschulman said:

Over pitch can cause considerable harm in most of the engines we deal with on this site I know I lost one that way. Smaller engines in sail boats are often over pitched and overloaded and most seem to get away with that. Under pitch does not cause harm and most suggest one or two hundred extra RPM over rated as a safety margin. I purposely run my common rail electronic motors under pitched in order to protect engines and effectively down rate them to less than the original M4 configuration. Another benefit aside from long life of my under loading is very good manners at idle speed in gear. At rated RPM my engines will attain 80-85% load and my fast cruise at 15-16K is at 60-65% load the slow cruise at 9.2K is at 35-40% load. If I were to pitch to full load I might gain 1-2K at top speed where I do not want to travel and their may be a small theoretical boost in economy.

Click to expand...

Not to get too far off topic here, but...

Eyschulman, I completely understand your setup and rationale. Am following a similar path by replacing my BT 210 hp with a BT 250 hp and keeping the same drivetrain.

But if I were replacing it with a CRD I think I'd be less concerned with the risks of overloading just because there is a wealth of operational data that would tell me at a glance if the motor was at or near an overload situation. It wouldn't happen without my awareness. Appreciate the idle speed benefits, but apart from that it seems like you're needlessly giving up top end and putting up with rpm related NVH - noise/vibration/harshness - that maybe you don't have to. Not critical here, just curious.

I guess my question is whether a set of good practices for a mechanical motor are applicable to a modern CRD. Seems from my armchair position that the CRD needs less structural protection from the harms of overloading.

Dumb questions... what's a CRD?

Common rail (electronic) diesel.

Just to expand, the biggest threat to motor health is often the nut behind the wheel. Let's assume here that we're dealing with contentious experienced operators that care about longevity and reliability. I'd feel much more comfortable with a 'close to the edge' CRD installation than I would with a mechanical.

Seevee said:

Dumb questions... what's a CRD?

Click to expand...

CRD=Common Rail Diesel....IOW electronically controlled. His point being that one has the data to derive the load to determine if the engine is overloaded or not.

With all that said, I just recently added an inch of pitch to my props to gain a little performance. A little history....

I bought my boat that had low time engines(300) hours and without a sea trial. I bought the boat extremely discounted because of this. I was having to replace the props before even running the boat. Research showed the boat left the factory(one of the benefits of buying American mass produced boat) with 23x27 props. I used prop calculators and then went more conservative and equipped with 23x24. I have Cummins 330B with a rated max RPM of 2800. My boat ran 3050RPM with those props. So fairly underpropped. 4 years later, after boating in water that averages about 7 feet, I have beat the props up a bit and decided to have the retuned. I have always wondered what adding an inch of pitch would do. So I figured I had quite a bit of room to add pitch and I did. I just added an inch. Could I see a benefit in performance??? Actually, yes I can. I usually cruise around 2300RPMs. That used to yield about 16 knots. Now it yields about 17 knots.

Everyone said I would lose about 75RPMs at WOT. And everyone was pretty much correct on that. Hard to tell with accuracy without digital tachs. But now WOT is about 2950...Maybe a bit more. I would be tempted to add another inch but I am not. I have gained a little performance and am still fairly underpropped. I run on plane whenever I am not in a no wake zone...so pretty much all the time. So the added insurance of still being underpropped while gaining a bit of performance is reassuring.

Like everyone has said, there is no free lunch. If you are already correctly propped, DO NOT DO IT if you are trying to squeak out performance on the top end. You will roast that motor!!!!

Baker said:

CRD=Common Rail Diesel....IOW electronically controlled. His point being that one has the data to derive the load to determine if the engine is overloaded or not.

With all that said, I just recently added an inch of pitch to my props to gain a little performance. A little history....

I bought my boat that had low time engines(300) hours and without a sea trial. I bought the boat extremely discounted because of this. I was having to replace the props before even running the boat. Research showed the boat left the factory(one of the benefits of buying American mass produced boat) with 23x27 props. I used prop calculators and then went more conservative and equipped with 23x24. I have Cummins 330B with a rated max RPM of 2800. My boat ran 3050RPM with those props. So fairly underpropped. 4 years later, after boating in water that averages about 7 feet, I have beat the props up a bit and decided to have the retuned. I have always wondered what adding an inch of pitch would do. So I figured I had quite a bit of room to add pitch and I did. I just added an inch. Could I see a benefit in performance??? Actually, yes I can. I usually cruise around 2300RPMs. That used to yield about 16 knots. Now it yields about 17 knots.

Everyone said I would lose about 75RPMs at WOT. And everyone was pretty much correct on that. Hard to tell with accuracy without digital tachs. But now WOT is about 2950...Maybe a bit more. I would be tempted to add another inch but I am not. I have gained a little performance and am still fairly underpropped. I run on plane whenever I am not in a no wake zone...so pretty much all the time. So the added insurance of still being underpropped while gaining a bit of performance is reassuring.

Like everyone has said, there is no free lunch. If you are already correctly propped, DO NOT DO IT if you are trying to squeak out performance on the top end. You will roast that motor!!!!

Click to expand...

Great illustration of this. Now do you cruise at 16 or 17 knots? What drives this somewhat arbitrary cruise speed decision? Let's assume that 16 knots had similar fuel consumption and motor wear and tear costs across the prop change.

Nomad Willy said:

SeeVee wrote;
Absolutely not unless I'm not reading this right. Any overprop at all and at WOT the engine will be overloaded.

Click to expand...

And in extreme cases the motor could be overloaded farther down the rpm/throttle range. This whole max rpm test is just to gather a single data point that more or less guarantees against overloading at any throttle.

Seevee said:

Eric,

I hope I posted that correct... I DO plan on going fast at times, not WOT, as Yanmar prohibits this. However, 290hp at 3100rpm will work, or about 80% of total power. That's the power area of the most benefit of a coarser pitch change. However, seems like there's little benefit with a lot of risk from the comments here. So be it, but just looking for folks that have done this pitch changing... or perhaps have a variable pitch prop.

Click to expand...

Seevee,
Just my opinion but running at 80% load is far too much load if overpropped.

How do you mean "290hp at 3100rpm"? By whose measure? Under what circumstances? How will you know you're making 290hp if your overpropped?

Jeff F said:

Great illustration of this. Now do you cruise at 16 or 17 knots? What drives this somewhat arbitrary cruise speed decision? Let's assume that 16 knots had similar fuel consumption and motor wear and tear costs across the prop change.

Click to expand...

Mentally I have been thru that. I still cruise at 2300RPM...which now yields 17 knots. So the question you are getting at and I would like to know is...am I now working the engine harder therefore more fuel therefore more wear? I know likely your answer(and mine) is YES...I am. But where is that "line"? And why not just underprop and run it at a higher RPM? I would imagine higher RPM would still cause more wear regardless of load based on RPM alone. Maybe the engine runs more efficiently at my "new load"(ie heat) than at the old one..... I guess there is really no way to know without data. I am installing pyro gauges and boost gauges...so that will help a little. But since I have no baseline from the previous prop pitch I will have no comparison.

Baker said:

I would imagine higher RPM would still cause more wear regardless of load based on RPM alone.

Click to expand...

Thanks for your response. Good thoughtful answer.

I think that many of us approach this with intuitions that are not quite right. My first hands-on experience with turbodiesels was operating a travel lift close to 30 years ago. The way we ran it was to idle at startup and during breaks, but the rest of the time left the throttle set to a screaming rpm. I was too junior to argue, but it seemed all wrong at the time.

I now understand why we did that. It's all about load with the motors we're talking about. RPM is almost a distraction. Still an armchair POV but I think it's the right one.

Seavee:

I agree with Eric, 290 hp is too much. I try to limit mine to 225 hp at 2,800 rpm.

Davjid

Jeff F said:

Thanks for your response. Good thoughtful answer

Click to expand...

And in the end, I am running a 2800RPM engine at 2300RPM and it is still underpropped so mentally I still feel fine about running it that way. It should last a very long time at that load.

Seevee said:

David,

Good points.

I'm assuming that whatever changes were done would be within limits of the engine with regard to loading and rpm limits. And, for me, operating at higher speeds occasionally will be part of the plan.

I agree, see very little advantage in over propping (using a high pitch) with low speeds. The advantages would come at higher speeds, where one gets either better fuel economy or better speed.

I see this in aviation, but only with constant speed props (variable pitch) and the effect id dramatic. Even the planes that have a fixed pitch prop can be adjusted for a high torque prop (under pitched) or a cruise prop (over speed). And if you can get the airplane off the ground the fuel savings and speed is big.

Guess we boaters don't enjoy this advantage. Also, what's the skinny on variable pitched props for boats? They are expensive upwards of $8 to $10K, and doubt one would fit my profile, but curious how effective they are.

Thx for the good comments!

Click to expand...

Perhaps reveres the way you think about this to see that the diesel behaves much different than a fuels engine that requires a very specific air/fuel ratio.
"I see this in aviation, but only with constant speed props (variable pitch) and the effect id dramatic."
You see this in the aviation because the air fuel ratio of the engine is very tight and running that engine at a higher rpm burns more fuel whether that fuel is needed to produce the work or not - therefore the variable speed prop raises or lowers the rpm to match power needed by attempting to match fuel needed.
Conversely with your marine diesel you engine will not burn fuel base upon rpm - it is very happy and efficient running a very high air to fuel ratios as high or higher than 90:1. So you do not need a variable prop to match loads as the governor on the diesel engine does that for you. A practical example from one of my past engines is that it would burn up to 17 gph at full load of 3,000 rpm - but if you take that same engine up to 3,000 in neutral (high idle) you would burn about 0.5 gph.
"Guess we boaters don't enjoy this advantage."
Yes - you get this advantage by utilizing a diesel and the governor adjust the fuel used for you better than a variable prop.
Enjoy you boat and prop it light for a long life.

Baker said:

And in the end, I am running a 2800RPM engine at 2300RPM and it is still underpropped so mentally I still feel fine about running it that way. It should last a very long time at that load.

Click to expand...

Gotcha, and agreed.

I don't want to take over here, but want to present one more thought experiment using your specific case. Think it's at the heart of Seevee's question.

If you wanted to fully optimize for operation at 17 knots and pledged never to try to exceed that, what would the optimum rpm be? Basically you can adjust the gear ratio through prop changes to lock into that optimum rpm.

Here's the range I think you have. You can gear down (underprop) to the point at which you're running at 2600 rpm, which is the maximum spec for continuous operation. Or you can gear up such that the load is still within the motor's power curve, but closer to the limit. Let's call that 2100 rpm.

Where is the 'sweet spot'? Are there tradeoffs in motor longevity or fuel consumption? Going for 2100 introduces risks. An extra load or adverse conditions might do harm. Your 2300 seems very safe from that perspective. So let's limit the practical target range to 2300 - 2600. Why would 2300 be any better than 2600? Is is possible that 2600 might in fact be better in terms of motor health, or even fuel consumption? Is there any downside apart from noise? Maybe a true expert can answer.

I'll shut up now