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Old 07-14-2017, 08:53 AM   #1
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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?
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Old 07-14-2017, 09:23 AM   #2
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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.
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Old 07-14-2017, 09:23 AM   #3
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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?

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.
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Old 07-14-2017, 09:32 AM   #4
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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
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Old 07-14-2017, 06:32 PM   #5
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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.
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Old 07-15-2017, 10:06 AM   #6
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Originally Posted by djmarchand View Post
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
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][/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.
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Old 07-15-2017, 11:12 AM   #7
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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.

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Old 07-15-2017, 11:23 AM   #8
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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.
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Old 07-15-2017, 11:24 AM   #9
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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!!!

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Old 07-15-2017, 11:28 AM   #10
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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.
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Old 07-15-2017, 11:59 AM   #11
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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
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!
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Old 07-15-2017, 12:17 PM   #12
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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.
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Old 07-15-2017, 12:25 PM   #13
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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.
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 :-)
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Old 07-15-2017, 12:27 PM   #14
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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.

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Old 07-15-2017, 12:32 PM   #15
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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.
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Old 07-15-2017, 12:35 PM   #16
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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.
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Old 07-15-2017, 12:47 PM   #17
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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.
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.
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Old 07-15-2017, 12:54 PM   #18
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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.
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.
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Old 07-15-2017, 01:10 PM   #19
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Dumb questions... what's a CRD?
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Old 07-15-2017, 01:33 PM   #20
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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.
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