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Old 06-30-2019, 09:34 PM   #1
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Why do folks over prop

I run into this frequently with boaters, they brag about how their boat is over propped not realizing the damage that can be done to a motor that functions at lower RPM pushing more weight.

So why do people over prop?
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Old 06-30-2019, 10:04 PM   #2
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I think they do it to get better fuel economy at lower RPMs. Yes you should not go to higher RPMs with an overloaded prop, but at lower RPMs it should not hurt the engine.
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Old 06-30-2019, 11:13 PM   #3
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Overprop can often get the same boat speed at a lower rpm which can some times reduce noise and/or reduce fuel use.

But you are correct that if they then run to close to the new max rpm they may do some damage.

One of the BEST ways to avoid that damage with an overprop is an EGT guage. The temp. will tell you when you are pushing to hard.

I ran and actually still do somewhat run like that but the catch is I do have that EGT guage and I pay attention. I did take an inch off but could still use 1 or 1.5 inches off.
But she runs well and as long as I use that EGT I won't do damage.
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Old 07-01-2019, 06:36 AM   #4
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Over propping will not only lower the wear and tear and engine noise but will give better fuel burn for the same speeds , in effect making the fuel supply bigger ,increasing the vessels range.

Sure some dumbo could harm the engine by not understanding what using the engine power at lower RPM does if they simply drove a full throttle all the time.

But as most trawlers have repurposed farm equipment or small truck motors which are not industrial rated the same damage from running full throttle would occur.

"But she runs well and as long as I use that EGT I won't do damage."

A knowledgeable skipper understands engine operation,and throttles accordingly.

For sport fish and other boats that run on top of the water the props are selected with that purpose in mind .

At low speeds while accelerating they are usually overloaded , witness the black overload smoke as they accelerate.
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Old 07-01-2019, 06:55 AM   #5
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One of the few advantages of a Tier 3 motor with a computer is that you can have a percentage of load gauge. In essence, it tells you the percentage of load you running at for that particular RPM. So, if you changes props, you can see the increased load and fuel consumption for the same RPM. My boat is over propped. Being able to cruise at 1,500 RPM with a 45% load (for that RPM) generates less noise and possibly slightly better fuel consumption. The computer also prevents you from running at loads above 80% for extended periods by reducing RPM to lower levels after a time period has been exceeded.

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Old 07-01-2019, 07:24 AM   #6
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I run a Perkins 4-236 with a Hurth gear. The PO told me the boat is over propped. I cruise at 1600 to 1700 rpm, don’t know if that’s good or bad, basically not at all clear on the meaning of over propped and how one arrives at deciding you’re there. Been running the same way for 11 years with no engine or transmission issues.
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Old 07-01-2019, 08:13 AM   #7
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If someone were doing extensive open water passages, which in general we try to plan for downwind, then a slight overprop would be an advantage. However, in reality when you have seas and winds, especially on the nose, then it's healthier for the engine to be slightly underpropped, which alot of manufacturers recommend.



But in reality how many different pitch props are you going to carry in your inventory and how do you change them easily underwater?
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Old 07-01-2019, 08:32 AM   #8
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An over propped condition is when the engine won't reach rated maximum rpm at wot due to the prop having too much pitch. Let me give an example:

The Cummins 6BT engine often used in trawlers from the late 80s to 90s is rated for 210 hp at 2,600 rpm. If the prop has too much pitch, it will take too much of a bite in the water and the engine won't have enough power to reach its rated 2,600 rpm. It is like going up a hill in a heavily loaded car in high gear. The solution is to downshift which is analogous to reducing pitch on your prop.

So say it only reaches 2,200 rpm at wot. If you only run it at 1,600 rpm or less, that is way down on the prop power absorption curve and probably won't harm the engine. You will get a little better fuel economy and lower noise by running in that condition.

But don't try to run that engine at 2,200 rpm which would be fine if it weren't overpropped. It will put too much load on the engine and will result in reduced life.

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Old 07-01-2019, 09:37 AM   #9
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An over propped condition is when the engine won't reach rated maximum rpm at wot due to the prop having too much pitch. Let me give an example:

The Cummins 6BT engine often used in trawlers from the late 80s to 90s is rated for 210 hp at 2,600 rpm. If the prop has too much pitch, it will take too much of a bite in the water and the engine won't have enough power to reach its rated 2,600 rpm. It is like going up a hill in a heavily loaded car in high gear. The solution is to downshift which is analogous to reducing pitch on your prop.

So say it only reaches 2,200 rpm at wot. If you only run it at 1,600 rpm or less, that is way down on the prop power absorption curve and probably won't harm the engine. You will get a little better fuel economy and lower noise by running in that condition.

But don't try to run that engine at 2,200 rpm which would be fine if it weren't overpropped. It will put too much load on the engine and will result in reduced life.

David
Thanks David, great explanation.
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Old 07-01-2019, 11:00 AM   #10
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The terms "overpropped" and "underpropped" are a bit sloppy as technical terms in trawler type boats. But useful. Very useful and important for planing boats.

On a trawler type boat, most operate with engines significantly more powerful than needed for cruising. Lots of trawlers run their engines at like 30-50% of what they are capable of. Like a Lehman 120 burning 3gph when at full power it is like 7gph. On a sportfish, engines typically run around 80% of max. Also those are turbo and charge air cooled. On a sporty you have to have the propping right.

On a sporty, if you can make a little over rated rpm at full power, then at cruise you can be pretty certain the engine is at a happy load spot.

On a trawler, it is different. Most are not capable of planing, so as you increase power up to full power, the shaft hp curve takes a steep upward curve. Running at a power setting above hull speed is really an economic no-go zone. Digging a big hole and wasting fuel.

On a trawler using the sporty criteria for correct propping is not always the best approach. And on most trawlers the engines are not super high output TA versions (some are!!) and thus the engines are not super sensitive to overload.

Each engine has a performance map, often called a BMEP vs RPM map, with BSFC curves present. The point of propping is to put the engine into a happy spot on the curve for all expected vessel speeds. Unfortunately, engine MFR's don't like to publish the charts and us engine guys end up having to snoop around for bits and crumbs to get any idea of them. Some MFR's are better than others (Cat, Cummins).

On a trawler you can never go wrong propping it for rated rpm + a bit at full power, even though full power usually is a silly place.

The problem is when power is backed down to a sane cruise, the engine is no longer in a happy spot on the performance chart. Rpm can be too high and load too low.

So it makes sense in some boats to add pitch and/or diameter to increase specific load and drop rpm. Can be a good bit quieter. Fuel burn likely a little better. A little.

But you don't want to increase prop too much. Any black smoke and engine is not happy.

And you do then need to avoid running at high power as engine will then be overloaded. And in an emergency you may need to do exactly that.

Double edged sword.
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Old 07-01-2019, 11:44 AM   #11
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Commercial fishermen troll for salmon or jig for tuna, often overprop. They overprop by using a much bigger wheel, not more pitch. As others stated, the economy at lower rpm is greater than with the correct prop and trolling/jigging speed is more consistent in ocean swells because of the bigger wheel diameter. Many tuna/salmon boats have 2 props and change for each season. I had a 55' boat and got more than 2 nm/gallon at running speed of about 9kts. Twin engines went about 20 years between rebuilds. I never attempted wot. I could do 14kts with a big fuel bill.
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Old 07-01-2019, 12:17 PM   #12
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I have found over the years that this calculator to be VERY accurate.

https://www.vicprop.com/displacement_size.php

Just insert the REAL numbers
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Old 07-01-2019, 12:20 PM   #13
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On a trawler type boat, most operate with engines significantly more powerful than needed for cruising. Lots of trawlers run their engines at like 30-50% of what they are capable of.

On a trawler using the sporty criteria for correct propping is not always the best approach.

On a trawler you can never go wrong propping it for rated rpm + a bit at full power, even though full power usually is a silly place.

And you do then need to avoid running at high power as engine will then be overloaded. And in an emergency you may need to do exactly that.
Double-edged sword is right! Like all compromises in boat design, it's about what suits your use. Commercial trollers and trawlers have a specific usage. Coastal recreational cruisers have theirs. For bluewater globe trotters however, the range of conditions to be subject to is wide. For that specific use IMO it's best to just be correctly propped. I recall Robert Beebe in his book also concluded that his slightly over-propped boat was not the best choice (for global passage making).
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Old 07-01-2019, 12:31 PM   #14
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I have found over the years that this calculator to be VERY accurate.

https://www.vicprop.com/displacement_size.php

Just insert the REAL numbers

I believe the calculations are based on Gerr A formula. A good estimation for “normal” boats but not necessarily for extremely thin, fat, bulbous bows or extreme designs.
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Old 07-01-2019, 01:16 PM   #15
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This has been beat to death for 100 years. It is not rocket science guys.
It is just simple math that most people can do with a calculator.
the chart below is actual numbers from my boat.
It has the correct 4 blade prop and will run at 10.84 knots using 310 HP WOT.
Back it off to 9 knots and it requires 165 HP.
To cruise a hull speed 8.89 knots with10% reserve power for variable loading and sea states requires 174.5 HP.
I have twin 155 HP engines @2200 RPM, but cruise at 8.9 knots the engines only turn 1750 RPM.

See the chart below


Data Input

Waterline length in feet:44 feetBeam at the waterline in feet:16 feetHull draft in feet (excluding keel):6 feetVessel weight in pounds:80000 lbsEngine Horsepower:155 HPNumber of engines:2Total Engine Horsepower:310 HP Engine R.P.M. (max):2200 RPMGear Ratio:2:1Shaft R.P.M. (max):1100 RPM Number of shaft bearings (per shaft):2Desired speed in Knots:9 knotsHorsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s). Total available horsepower at the engine(s):310 HPTotal available torque ft/lbs at the engine(s): 740 ft/lbsHorsepower loss of 3% per gearbox:- 9.3 HPHorsepower loss of 1.5% per shaft bearing:- 9.3 HP Total horsepower available at the propeller(s):291.4 HPTotal torque ft/lbs available at the propeller(s):1391 ft/lbsSpeed & Power Calculations

Basic displacement speed and horsepower requiredDisplacement hull speed (1.34 X sqrt of waterline length):8.89 KnotsMinimum horsepower required at propeller(s) for Hull speed:174.5 HP Calculations based on desired speed and available HPHP required at propeller(s) for desired 9 knots speed:165 HPEstimated maximum speed with existing 310 horsepower:
This is the speed we will use for the propeller size.10.84 Knots At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel. Propeller Size

Number of bladesDiameter (inches) Pitch (inches)2 Blade27.1X17.53 Blade25.8X17.34 Blade24.3X17.0 The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.
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Old 07-01-2019, 01:38 PM   #16
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I believe the calculations are based on Gerr A formula. A good estimation for “normal” boats but not necessarily for extremely thin, fat, bulbous bows or extreme designs.

Nor I expect for duoprops.

My boat was over propped by the previous owner for gas savings (gas engine) but I'm betting at most he saved maybe 30 gallons per year. Most under use their boats so I don't get the hopes of getting reduced fuel, I'd get it if their boat was used constantly.

The big problem I encountered with his over propping was prop walk. And my prop didn't walk, it ran. Not only was the pitch more aggressive but the diameter was increased as well.

When I left and returned on the great majority of trips from and to my home marina, I was faced with a beam sea and usually a beam wind. So the boat definitely gravitated to starboard, then when corrected it would over correct to port, all the time. So when I re-powered and re-legged, I opted for Merc's duoprop system. So I also experienced increased prop walk due to the modifications made to the old leg's propeller.
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Old 07-02-2019, 06:02 AM   #17
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Overloading long term is bad for an engine , but a quick check does no harm.

Run the warm engine at full throttle , note the RPM and color of the exhaust.

Pull back till the exhaust is clear , or if its clear at full throttle , a further pull back of 10% will usually be max cruise RPM with out overload.

I prefer 300 RPM pullback to be safer.

Should the max RPM after pullback be way more RPM than you usually cruise at , you might contemplate a bigger prop to better load the engine , where you do cruise.

Should you install a cruising prop , just be sure that full throttle will give another 10% or 300RPM for a test.

Not as accurate as an EGT gauge (auto units work fine and are about $125 .")

Running an engine at a slightly higher load and lower RPM will usually be quieter on board ,and wear the engine less.
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Old 07-02-2019, 08:02 AM   #18
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Not as accurate as an EGT gauge (auto units work fine and are about $125
I've always learned that an EGT is an important tool for monitoring your engine's health. However, I remember in Nigel Calder's book he recommended a gauge on each cylinder, eg. 6 in your engine room, to truly see the health of each individual cylinder.

Honestly, would you think that's overkill or very important?
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Old 07-02-2019, 09:23 AM   #19
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I have known many ancient boats with ancient engines without EGT gauges.


Any engine run near max rpm that isn't a continuously rated diesel, certainly is a candidate.


Info is a good thing...but....a gauge on every cylinder is more than most have....if it were such an important thing I would have thought I would have seen it at least once in my career...and this is the first time I ever heard it recommended, let alone see it.
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Old 07-02-2019, 10:48 AM   #20
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I think I lost my copy of Marine Diesel Engines when I went overseas, but if anyone has a copy it would be good to look up that section.
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