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Old 09-08-2018, 08:06 AM   #21
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As most discussions here on TF......absolutes and 100% and always and every other expression alluding to the same has to be applied to a specific set of parameters.

Which, for the most part is never done.

So whether overpropping can be or can't be done safely and for efficiency is more of when and where....more so than never or always......
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Old 09-08-2018, 08:28 AM   #22
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Over-propping will show up during a proper sea trial. It can kill a deal. The current owner of the vessel in question should be requested to prop the vessel correctly to ascertain if full rated RPMs can be safely obtained under full tank scenarios.

If the owner refuses, the potential buyer then must shoulder the risk or walk. A good broker knows this buying dance.
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Old 09-08-2018, 10:12 AM   #23
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FF and the rest are right ....... to a point.

The problem is no one knows where (what rpm) the overloading starts.
If you run your boat through it’s rpm range while tied to the dock the upper end of what you can achieve (rpm wise) is overloaded. Close to the top is very overloaded. The problem is you don’t know where the overloading starts. Lets say you can achieve 1900rpm WOT. Where does the overload start? At 1800? Probably quit before that. May be at 1600, 1700 or 1500.

It is true at a certian rpm (lets guess 1400 in the above example) the engine runs as if it were at a 2500 rated rpm. That is at the same load. And then you run at 1200 thinking you’re saving fuel. At some point you are. But at a very narrow range .. but what rpm is that? Probably arange of 100rpm approx. Very likely less than that

The following is mostly subjective but my boat has been overpropped 125rpm for quite awhile and now (a month or two ago) I put the old prop back on and I really like the way it runs. Runs right up to rated rpm. She just easily sings along seemingly w less effort ... and that part is true as I’ve been cruising at the same rpm. I like it. Actually I’d rather be 100rpm underpropped.
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Old 09-08-2018, 02:32 PM   #24
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Originally Posted by Nomad Willy View Post
FF and the rest are right ....... to a point.

The problem is no one knows where (what rpm) the overloading starts.
If you run your boat through it’s rpm range while tied to the dock the upper end of what you can achieve (rpm wise) is overloaded. Close to the top is very overloaded. The problem is you don’t know where the overloading starts. Lets say you can achieve 1900rpm WOT. Where does the overload start? At 1800? Probably quit before that. May be at 1600, 1700 or 1500.

It is true at a certian rpm (lets guess 1400 in the above example) the engine runs as if it were at a 2500 rated rpm. That is at the same load. And then you run at 1200 thinking you’re saving fuel. At some point you are. But at a very narrow range .. but what rpm is that? Probably arange of 100rpm approx. Very likely less than that

The following is mostly subjective but my boat has been overpropped 125rpm for quite awhile and now (a month or two ago) I put the old prop back on and I really like the way it runs. Runs right up to rated rpm. She just easily sings along seemingly w less effort ... and that part is true as I’ve been cruising at the same rpm. I like it. Actually I’d rather be 100rpm underpropped.
It depends on your engine.

I have a tier 2 engine which essentially means mechanically injected electronically controlled pump. Because a computer is controlling the injection rate (fuel consumption) and knows the rpm, it can determine load or more importantly % of load for any RPM. There is a gauge on my engine display that shows % of load. So, all I need to do is keep below 80%. From 80% to 100% the computer tracks time value and after so many minutes, it electronically slows the engine down to below 80%. Usually I'm around 50% at cruise RPM, so the load isn't an issue. If I increase speed from 7 to 8 knots, I'm running at 70% of allowable for that RPM. I'm still ok to cruise continuously at that setting. If I go over 80%, I think I can do that for 30 minutes. 100% load I believe is limited to 15 minutes. I never reach these percentages as the speed increase is less than a knot, so I'm not absolutely sure on the time values.

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Old 09-08-2018, 04:52 PM   #25
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Over propping will allow the engine to have more load at lower RPM if one wants to run slow that will help get to the optimum operating temperature at at lower RPM. But I agree it probably will not want to run at full RPM
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Old 09-08-2018, 07:21 PM   #26
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I am in the process of moving down (in speed that is) to a 40 foot Marine Trader Labelle. At the sea trials the engines did not meet the recommended full max. rpm of 2500. The motors are 212 hp Sabre Marine 6 cylinder, turbo(ed). The PO had replaced the original (at least when he bought the boat) 3-blade props with 4-blade ones (D 24 P20 ), which I have. His thoughts were he was buying the boat for the Great Loop and he would get better efficiency at 8 knots with this arrangement. I have the 3 blade props. I plan on doing much more extensive cruising where efficiency at a higher speed would be welcomed. My experience with other machinery is that they are more efficient if run at their "sweet spot". However I have no clue as where it is at this time.
So after all this my question is do I change the props. Here is the performance numbers at the trials.

1100-6.8 Kts
1400-8 kts
1800-9.2 kts
2100-13.4 kts (however I saw 14.5 on my smartphone app)

Any advice is welcomed as I am new to big boat performance.



IMHO, there is the question about accuracy of the tachometers. It could be out of adjustment. Also, I would rev the engine in neutral to see that the governor in the injection pump is not limiting it below the 2500 RPMs.


Lastly, based on my experience with my 120 HP Lehman, going from a 3 to a 4 blade propeller resulted in a reduction of 240 RPMS at FOT. The fuel efficiency performance was not changed but the sweet spot I found it to shift down from 1800 to 1650-1700 RPMs. Of course, YMMV (Your methods may vary).


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Old 09-08-2018, 07:50 PM   #27
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OK Ted,
How many of us have engines like that?
My comments were directed at the older mechanical engines like my 2003 Mitsu.
What elements of engine operation are considered to arrive at “% of load”.
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Old 09-08-2018, 08:20 PM   #28
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What elements of engine operation are considered to arrive at “% of load”.
Most engines built in the last 20 years have manufacturer's performance graphs. One is usually a horse power curve. Often there is a fuel consumption curve. It would be my assumption that a manufacturer develops that information through testing. If you assume the line represents 100% of the load for a given RPM, then it should be pretty easy to develop a computer program that determines the percentage of load for that RPM based on the ammount of fuel consumed. As an example my engine might be able to develop 80 HP (100% of load) at 1,500 RPM with a 4 GPH fuel consumption. In my boat, when cruising at 1,500 RPM, I'm only burning 2 GPH. Theoretically, if I'm burning half the fuel, I should be running at 50% of load.

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Old 09-08-2018, 08:29 PM   #29
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Installing a propulsion system that can result in overheating the engine at normal/capable engine speeds is a harmful waste of time. If one wants optimal fuel-consumption/speed efficiency, move at a knot below hull speed.
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Old 09-08-2018, 09:57 PM   #30
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It's really not that hard. The engine has a torque/rpm curve. The prop has an rpm/torque curve. You overlay them, the gap between them is the excess torque available. The prop torque over the engine torque at any point is the percentage load. Where the curves cross is the max rpm achievable. You just need to decide what constitutes overload. The 80% assumption is just a guess, and there is no one right answer.

Determining the fuel economy is much more complicated, requires the fuel map, and can probably really only be determined with tests.
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Old 09-08-2018, 11:25 PM   #31
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This is an interesting discussion. I just went back and read it from the start.
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Old 09-08-2018, 11:38 PM   #32
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This is an interesting discussion. I just went back and read it from the start.
And then posted by mistake :-)

I completely get the over propping argument. But implicit in doing that is an acknowledgement that the motor is too powerful for the intended purpose. You really do forfeit the ability to get maximum power from the motor. I'd have trouble making that trade-off.
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Old 09-09-2018, 12:05 AM   #33
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Motors are always too powerful for the prop, at some rpm. That is because the engine torque curve is always convex, and the prop torque curve always concave. They are not a good match for each other - equal only at (at most) 2 points. This is very unlike the same engine mounted in a truck, with a choice of 6 gears to match available torque to load.

A variable pitch prop would allow much better matching of load to available torque. Most airplanes have variable pitch props for exactly this reason.
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Old 09-09-2018, 05:47 AM   #34
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"A variable pitch prop would allow much better matching of load to available torque. Most airplanes have variable pitch props for exactly this reason."

This is true but the savings from perhaps $10,000 to $15,000 investment in a VPP would not pay financially.

It would be great for engine life , fuel burn and noise while cruising , but its an expensive luxury.

"You really do forfeit the ability to get maximum power from the motor. I'd have trouble making that trade-off."

Most displacement boats have a "hull speed" where added power results in bigger and bigger waves , and fuel flow but only a minor speed increase.

Sinking the stern as the bow attempts to climb its own wave is the usual result of the oversized engines stuck in many trawlers.

2-3 GPH is 30-50HP , propping to dump 120 or 135 or more HP into the water may only result in a 1 or 2K speed increase , bit BIG stern waves.
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Old 09-09-2018, 08:40 AM   #35
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Installing a propulsion system that can result in overheating the engine at normal/capable engine speeds is a harmful waste of time. If one wants optimal fuel-consumption/speed efficiency, move at a knot below hull speed.
EXACTLY!

Thanks Mark... for crossing the t's and dotting the i's in a simple true, factual answer to this subject. Your statement is KISS to the utmost!!

I've been reading this thread's "what if this" and "why not that" for some time.

Takes a mellow guy like Mark to make a short statement that in this case pins the tail on the donkey...!
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Old 09-09-2018, 09:26 AM   #36
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As with most overpropping threads, what's missed by the members with planning hulls is that the hull (hull speed) is the limiting factor. Consider my boat. With 135 HP, my boat can reach a speed of 8.6 knots. To reach 10 knots requires 450 HP (original engine in my boat) and a huge set of trim tabs to go above 10 knots.

My John Deere develops 135 HP at 2,600 RPM. If I prop my boat to reach 2,600 RPM, it means my 8 knot cruise goes from 1,700 RPM to about 2,200 to 2,300 RPM. My 7 knot cruise goes from 1,450 RPM to 1,800 to 1,900 RPM. First, there's no reason to endure the additional noise and secondly, does anybody really believe running an engine 500 RPM faster all the time improves life expectancy (all other factors being equal).

Really fail to understand why people can't understand that it's ok to to have a self imposed maximum RPM other than the manufacturer's. Finally, why is it ok to use a variable pitch propeller to maximize load / speed at reduced RPM but not a fixed pitch propeller at the same pitch?

Ted
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Old 09-09-2018, 11:30 AM   #37
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1100-6.8 Kts
1400-8 kts
1800-9.2 kts
2100-13.4 kts (however I saw 14.5 on my smartphone app)

Any advice is welcomed as I am new to big boat performance.
There is something wrong with these numbers. You increased rpms by 300 (1100-1400) revolutions and gain 1.2 kts.

Then you increased your rpms by 400 revolutions (1400-1800) and gained 1.2kts.

Ok, that sounds normal.

Then you increased your rpms by 300 revolutions (1800-2100) and gained 4.2 kts. This isn’t possible! There is something wrong with the data.

I don’t know if the tach stuck at 2100 or if something is wrong with the speed data.
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Old 09-09-2018, 11:41 AM   #38
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Finally, why is it ok to use a variable pitch propeller to maximize load / speed at reduced RPM but not a fixed pitch propeller at the same pitch?

Ted
Of course it is OK. An engine is only overloaded if it is overloaded. The pitch of the prop is one factor of many. I'm sure there are owners who should not have any setup that ever allows engine mismanagement, as they will mismanage it. Other owners will never have a problem with the same setup.

My boat seems to be overpropped by about 200 rpm (2800 max vs. 3000 spec). I chose to leave it that way, as full throttle operation accounts for less than 1% of operating time. The engine is loaded more at 1400 - 1600 with this prop, where 90% of the running is done, and this load is welcome as it is still only around 35% of available torque. Operating slightly overloaded 1% of the time will have negligible effect on engine life, and less than the effect of reduced piston travel at normal cruising speed.

I did not suggest that variable pitch props were economical when their cost is considered. Just that without them, the physics of props virtually guarantees lower load operation at part throttle.
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Old 09-09-2018, 11:41 AM   #39
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I find nothing wrong with overpropping on a trawler that has excess HP. Ted's posts explain it well. Running hull speed with a larger than needed engine I would rather have it at lower rpm at higher load to keep firing temps and pressures up. To a point...

But it does leave a caveat: If operator was heavy on the throttle with it overpropped, it can harm the engine. Some engines more susceptible to that than others.
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Old 09-09-2018, 02:43 PM   #40
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I think you would not be wise to fit 4 blade propellers, as you will overload the engines at higher RPM. My 38 year old 38ft 16.5 ton DW semi displacement trawler yacht has identical twin Ford Sabre 212BHP engines, swinging 26 inch 3 blade propellers. I make similar but slightly higher speeds up to 2100 RPM, and can make 18Kn at 2400RPM without smoking, and holding 90c temperature. (The manufacturers claimed 19KN).
By running at 7kn the fuel consumption is increased threefold, from running at 12kn on the plane.
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