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Old 03-14-2014, 10:40 PM   #1
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Under propped?

Hello,
I have a 1977 Davis (Defever) 40' Trawler. (14"-7" beam, 4' draft)
Single Ford Lehman 120 - 2500 RPM max.
Velvet drive gearbox 2.37-1 ratio, 39,000 lbs+/_
Current prop 25x16 3 blade RH, 1.75" shaft
At slack current I run at 2200-2300 to maintain 8 knots.

I talked to another owner of an identical boat that is over propped with a 25x21 prop and he runs approx. 1600 RPM to get 8 knots, but cannot reach the 2500 RPM's.

So, I am looking to find the optimal pitch/prop type to get off the top end of my RPM's, maintain the 8 knots and help my fuel efficiency.

Any comments would be appreciated!

Jeff
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Old 03-15-2014, 01:09 AM   #2
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Jeff, Thee was recently a running thread on this subject with unending data and information offered. I attempted to locate within this forum category with no luck. It may be an old thread that is brought current yet remains dated, or in another forum.
Hopefully others who participated and read here will be able to direct you to this well reported subject.
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Old 03-15-2014, 01:30 AM   #3
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Jeff

What is your RPM at 7 knots? That is closer to economical hull speed for a DF design with a 36' WL. Trying to push your vessel at 8 knots seems over eager given the 120 maximum HP available. Have you used a photo tach to check RPM?
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Old 03-15-2014, 04:22 AM   #4
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In theory, a properly propped boat will run exactly at the rated RPM of the engine at full throttle. So what RPM are you able to achieve at full throttle? And that will be your answer ref over or under propped. The older slow turning diesels can handle a little over propping as long as you don't push them hard. But I would be inclined to run the engine as it is designed by the manufacturer to run....regardless of what my friends are doing. You are potentially taking longevity out of the engine when you purposely(or accidentally) overprop. How much longevity???? Who knows...and that is my problem with it. You are operating in a realm that was not intended by the manufacturer. So get it properly propped and enjoy the ride.
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Old 03-15-2014, 06:43 AM   #5
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Quote:
Originally Posted by Jeff S View Post
Hello,
I have a 1977 Davis (Defever) 40' Trawler. (14"-7" beam, 4' draft)
Single Ford Lehman 120 - 2500 RPM max.
Velvet drive gearbox 2.37-1 ratio, 39,000 lbs+/_
Current prop 25x16 3 blade RH, 1.75" shaft
At slack current I run at 2200-2300 to maintain 8 knots.

I talked to another owner of an identical boat that is over propped with a 25x21 prop and he runs approx. 1600 RPM to get 8 knots, but cannot reach the 2500 RPM's.

So, I am looking to find the optimal pitch/prop type to get off the top end of my RPM's, maintain the 8 knots and help my fuel efficiency.

Any comments would be appreciated!

Jeff
Can't help with Defevers but...

Another 40 Albin on here runs a 120 with a 2:1 tranny and a 20X16 prop... gets 2500 RPM and around 6-7 knots down at 1600

I have a 40 Albin..not sure of my gearbox and run a 22x17 prop. Not sure I get to 2500 rpm and don't really care too much as I rarely run over 1800...I do cruise at 1600-1700 and get 6.3 knots.

I have a hard time believing your friend is hitting 8 knots at 1600...but I guess it's possible depending on his tranny.
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Old 03-15-2014, 06:59 AM   #6
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>In theory, a properly propped boat will run exactly at the rated RPM of the engine at full throttle.<

This is the method for plaining and fast boats.It is recommebnded by the Mfg as it assures very very low loading at normal cruise RPM.

This gets them by the warrentee period easily.

It is not fuel efficient or a good way to operate an engine , high RPM and minor load, on a displacement boat.

>I talked to another owner of an identical boat that is over propped with a 25x21 prop and he runs approx. 1600 RPM to get 8 knots, but cannot reach the 2500 RPM's.<

Sounds like this owner has installed a proper cruising prop , he should have an EGT gauge and a NEW red line on the tach.

I agree that 8K sounds high for a boat with probably a 36 ft LWL.

7K as suggested may take fewer RPM and 50% less fuel.

With a hand held GPS , simply operate the boat for about 2 min at every RPM going up in 100RPM steps , and plot the results.

Where you see a need fto add big RPM and almost no gain in speed , back off one K and enjoy the quiet and low fuel bill of LRC, Long Range Cruise.

When you locate the LRC , the goal is to prop to have the engine working at 60% to 80% of its rated power AT THAT LOWER RPM!

60% if you go out in blue water and climb 5-10 ft waves at speed. Or it blows 30K+ mos times where you cruise STIDD seats recommended.

Enjoy,
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Old 03-15-2014, 07:01 AM   #7
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You are potentially taking longevity out of the engine when you purposely(or accidentally) overprop. How much longevity???? Who knows...and that is my problem with it. You are operating in a realm that was not intended by the manufacturer. So get it properly propped and enjoy the ride.
Heed that advice as I can speak from experience what over proping can do, and it is expensive. And as FF said my boat is planning and fast, sorta.
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Old 03-15-2014, 07:28 AM   #8
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really depends on the engine and where you are willing to operate it...and of course...reasonableness in the overpropping.

the OP does have a single 40 foot trawler with a 120 in it...he's never gonna see even reasonable semi-d speeds unless he want's to spend all day on the pin...so if he's willing to accept less than 8 knots and wants 6-7 knots overpropped so he can run down around 1600 versus 1800 or so...I doubt the old Lehman would even know or care....but staying down there all day long (which is pretty easy in his setup) and cruise for 10's of thousands of miles.....he should be fine ad many have done it and pretty much is where I am too.
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Old 03-15-2014, 09:29 AM   #9
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Another 40 Albin on here runs a 120 with a 2:1 tranny and a 20X16 prop... gets 2500 RPM and around 6-7 knots down at 1600
That would be me
And I don't know much about 40 Defevers except that I cruised with one for a few days in 2012 up in Canada (Ralph Yost who recently sold his boat).
He ran at about 1800 rpm normally, and was doing that when we cruised together and I could NOT keep up with him. I was doing probably 7 + knots and I estimate he was doing 8 or close to 8 cause he "dusted" me.
I DO know he had a 2.57 tranny and a (maybe) 24 inch dia prop. Larger dia prop is more efficient, plus the different ratio, so maybe that was the reason he gave me ample time to view any deadheads he may have licked up ahead of me.
As far as fuel use (in miles per gallon) I believe we were very close.
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Old 03-15-2014, 10:02 AM   #10
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The inexpensive simple solution would be to have your prop pulled (by a diver) and take it to a reputable prop shop. If it's in good condition and the original pitch was close to 16, have it reconditioned and add a couple of inches of pitch. A good prop shop will tell you whether that is reasonable based on the original pitch and the condition of the prop. With the price of fuel, adjusting the pitch for better economy seems a very modest cost. While you may think you want more than a couple of inches, the first 2 will make significantly more difference than the next 2 as far as efficiency.

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Old 03-15-2014, 10:16 AM   #11
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Quote:
Originally Posted by FF View Post
>

This is the method for plaining and fast boats.It is recommebnded by the Mfg as it assures very very low loading at normal cruise RPM.


Enjoy,
FF, I will disagree based on semantics. I know what you are getting at. It assures "design loading" not some reduced power type of loading. Can a Lehman handle some over propping? Probably. But how much?

And is the term "cruise prop" an accepted industry term and actual type of prop or is it something invented on the dock with a little backwater engineering?
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Old 03-15-2014, 10:29 AM   #12
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The norm is to prop so the engine gets a little OVER the rated RPM at full throttle with the boat FULLY LOADED. This approach ensures that you can run the boat at any RPM without overloading the engine. That's what all the engine manufacturers specify and require for warranty conformance.

If you want to diverge from that, there are lots of tricks that a small number of people play. One is to load the engine more heavily at lower RPMs (like your friend is doing), but that will overload the engine at higher RPMs so it needs to be accompanied by a way to reduce your max RPMs so you stay out of the overload RPM zone.

This is playing with the difference between the load that a prop presents to an engine, and the load that the engine can sustain. At lower RPMs, the engine can sustain a higher load than the prop presents. However at higher RPMs, that same prop will overload the engine. So you have two choices.

1) You can prop to reach max allowed load at max RPM, which allows for engine operation that is within specs at all RPMs. This is what all the manufacturers recommend/require.

2) You can prop for higher load at lower RPMs, but need to simultaneously limit MAX RPM so you never go into overload on the engine as you increase RPMs.
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Old 03-15-2014, 10:40 AM   #13
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Jeff:

Assuming that you are now propped right, ie you can just barely make 2,500 rpm at wot, then propping down a couple of inches is at most going to pick up 10% fuel efficiency. It probably isn't going to harm your engine to run at 1,600-1,800 rpm to get 8 kts, but you can never run at wot.

Is that worth it, in terms of prop rework cost, loss of flexibility vs fuel saved. It all depends on how many gallons of fuel that you now burn each year. If you operate at 100 hours a year and you currently burn 3 gph, then the savings are probably worth it to repitch your prop. A repitch job will cost about $500 and you might save $120/yr.

Also, if fuel efficiency is your quest, you might think about a new prop altogether. Talk to a pro at a good prop shop. Larry at Accutech Marine in New Hampshire is one such guy.

By specing a prop just right for you boat you could easily pick up 10% fuel consumption in just prop efficiency in addition to possibly operating at a more efficient engine rpm.

But you have to burn a lot of gallons to make it pay.

David
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Old 03-15-2014, 11:05 AM   #14
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2) You can prop for higher load at lower RPMs, but need to simultaneously limit MAX RPM so you never go into overload on the engine as you increase RPMs.
Okay, I will play along. What is our max RPM limit??? That is somewhat of a rhetorical question. My only point here is it is gonna take some guessing and assuming to get what you want. And then in the end, you are going to have to believe in your guesses and assumptions. If I were buying a boat with a 2600RPM engine and went to full throttle on sea trial and all it produced was 2200RPM and a bunch of black smoke, I would immediately walk away from the deal.

And just to clarify, when I said what I said up above, notice I said "in theory". My boat has two higher performance diesels and this is an area we have to be a stickler about. My engines turn a hair over 3000RPMs and they are 2800RPM engines. And as far as warranties go ref engine manufacturers and boat manufacturers, the engine manufacturer approves the installation. It is as simple as that. And if the installation is approved by the engine manufacturer, then it is warranted.
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Old 03-15-2014, 11:08 AM   #15
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Underpropped?

50rpm underpropped is ideal IMO.

Extremely close (+50 or -100rpm) to that is next to ideal.

Anything else is unacceptable.

twistedtree wrote;
"little OVER the rated RPM at full throttle"
"That's what all the engine manufacturers specify and require for warranty conformance."
"You can prop to reach max allowed load at max RPM, which allows for engine operation that is within specs at all RPMs. This is what all the manufacturers recommend/require."

I'm buy'in that 100% and think it's the only safe way to prop/load one's trawler engine.

Good to finally hear of a Lehman running 22 and 2300rpm. May be underloaded though as Jeff didn't indicate if he was propped right.
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Old 03-15-2014, 11:19 AM   #16
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Over propping may also lead to high at idle speeds. Do you want to go 4 knots in the marina and have to take it in and out of gear often? I am going to de-pitch my prop a bit next time I go on the hard due to this.
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Old 03-15-2014, 11:31 AM   #17
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Okay, I will play along. What is our max RPM limit??? That is somewhat of a rhetorical question. My only point here is it is gonna take some guessing and assuming to get what you want. And then in the end, you are going to have to believe in your guesses and assumptions.
Actually, I think it's all science if you have the engine's power curve. I've attached an example that shows the full throttle power curve and the approved prop curve for a Cummins QSC 500, which happens to be what's in my boat.

I've marked in a more aggressive prop curve in red (pardon my sloppy art work, but it should convey the idea). With the approved prop load show in the black line, you can see that it intersects the engine's full power curve at max RPM. The entire prop curve is below the engine's max power curve.

In the fictitious red prop load that is more aggressive, it now intersects the engine's max power curve just below 2000 RPM instead of 2600 RPM. Above 2000 RPM, the prop is overloading the engine. This more aggressive curve is OK provided you never run the engine above 2000 RPM, so that's the new red line for the RPMs.

I should have said this in my previous post, but I am NOT an advocate of doing this. I bring it up just because it's what the original poster's friend was doing, and what he was asking about. And people like Dashew talk about this too, so I'm trying to shed some light on exactly what they are doing, and where the hazards lie.

You said that if you came across a boat that only turned 2200 RPM that was speced for 2500 RPM, you would walk away. So would I, and pretty much every surveyor would flag it as a problem that is likely to have caused engine harm over the years. I think it's a bad idea.
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File Type: pdf Pages from QSC500Performance.pdf (32.2 KB, 79 views)
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Old 03-15-2014, 11:50 AM   #18
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Thanks for the work TT. Do you think Cummins would sign off on that?? I know you and I are in the same camp. I guess the gist of my question is Cummins is a very interactive company and very hands on with their customers. I am just wondering if anyone out there would approve of doing something like that.

And back to your graph, I understand the theory of it...now how would you execute it?

And slightly off topic....what will your QSC be rated at???
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Old 03-15-2014, 12:06 PM   #19
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Thanks for the work TT. Do you think Cummins would sign off on that?? I know you and I are in the same camp. I guess the gist of my question is Cummins is a very interactive company and very hands on with their customers. I am just wondering if anyone out there would approve of doing something like that.

And back to your graph, I understand the theory of it...now how would you execute it?

And slightly off topic....what will your QSC be rated at???

No, I'm quite sure Cummins WOULD NOT sign off on it. The fundamental problem, as you point out, is that there is no good way to enforce the new RPM red line. I assume that's what you mean by "implement it". People talk about putting a cork under the throttle lever, or other form of mechanical stop. Others say they will just never run over the new limit, Scout's Honor. I doubt that would satisfy Cummins, and I'd take their position if I were in their shoes. They are very clear how the engine needs to be propped, and very clear that it voids the warranty if you do otherwise. So to me it's pretty cut and dry.

My QSCs are rated at 500HP, and it's a pleasure boat rating, or whatever term they use. The limit is full RPM for 1 out of 8 hours, with all other operation at 200 RPM or more below rated RPM. So in my case it's 2600 RPM for 1 of 8 hours, and 2400 RPM or below the rest of the time. In practice I run below 1400 RPM 90% of the time, and occasionally get wild and run 2200-2300 when I want to just cover ground.
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Old 03-15-2014, 12:12 PM   #20
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And slightly off topic....what will your QSC be rated at???
For clarity, the QSCs are on my Grand Banks. The Nordhavn has a Deere 6090 M2 rated at 325hp. The M2 rating is full throttle for 16 out of 24 hours, and 100 RPM below that for the balance. So that's 2200 RPM for 16 out of 24hrs, and 2100 RPM for the rest. At 2100 RPM it's continuous rated.
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