Over propped. Is it really a problem?

[DW1979OA]

I'm getting ready to do a scheduled haul out in the next six months. The props have a few dings, but nothing serious according to the divers that recently cleaned the bottom. The boat is powered by FL120's and with a clean bottom both engines reach 2250 rpms at full throttle giving 9.3 knots. 2400 rpm would be expected if things were correct. The temp also climbs past where I like to run the engines, so don't stay at rpm for long.

With 3-4 months growth on the prop and bottom, I lose about 100 rpm and drop speed by more then a knot.

The boat came to me over propped, and I've learned to run at 1750 rpms at 8 knots. Talking to a couple of brokers, they say that being over propped is not that big a deal. Getting to the time where age is catching up with me and considering selling in the next 2-3 years.

Some of my thoughts are that without a clean bottom, all boats are over propped due to additional drag. I'm in an area that a clean bottom doesn't last very long. Is getting the props tuned up worth it in that case?

jp

 

[Comodave]

I have my props in right now being repitched to get me some more RPMs since the Cummins guru says it is extremely important to get full RPMs. Not sure how critical it is on the 120s. Maybe ask Brian at American Diesel since he is the guru on Lehmans.

 

[Delfin]

I'm getting ready to do a scheduled haul out in the next six months. The props have a few dings, but nothing serious according to the divers that recently cleaned the bottom. The boat is powered by FL120's and with a clean bottom both engines reach 2250 rpms at full throttle giving 9.3 knots. 2400 rpm would be expected if things were correct. The temp also climbs past where I like to run the engines, so don't stay at rpm for long.

With 3-4 months growth on the prop and bottom, I lose about 100 rpm and drop speed by more then a knot.

The boat came to me over propped, and I've learned to run at 1750 rpms at 8 knots. Talking to a couple of brokers, they say that being over propped is not that big a deal. Getting to the time where age is catching up with me and considering selling in the next 2-3 years.

Some of my thoughts are that without a clean bottom, all boats are over propped due to additional drag. I'm in an area that a clean bottom doesn't last very long. Is getting the props tuned up worth it in that case?

jp

If you run around at fractional power loadings, there probably won't be much damage from lugging the engine, maybe none. Then again, if you take a bit of pitch out, there won't be any overloading, your idle speed won't be four knots, the engine will run cooler and your fuel economy will improve.

 

[O C Diver]

At 150 rpm the over pitching is a rounding error. Your tachometer could be off that much. An engine design of that vintage just isn't going to be that critical as far as turning rated RPM.

I would be more concerned of a rise in coolant temperature being a function of a fouled heat exchanger or other cooling problem.


Ted

 

[Jeff F]

The boat came to me over propped, and I've learned to run at 1750 rpms at 8 knots. Talking to a couple of brokers, they say that being over propped is not that big a deal.

If that's the speed you always run I'd agree. No need to pull the props just to repitch.

 

[Pascall]

Depending on the type of ship and how you sail it.
We have a water displacement ship and never sail at full throttle.
When choosing the drivetrain, we deliberately chose a heavier propeller, max prm 2600 which is 200 prm less than required.
Advantage, less revs at cruising speed, less noise and the engine is put under a small amount more load which is good to prevent the contamination of the engine/turbo.

I hope the translation makes it understandable.

Greeting,

Pascal.

 

[Slo-Mocean]

At 150 rpm the over pitching is a rounding error. Your tachometer could be off that much. An engine design of that vintage just isn't going to be that critical as far as turning rated RPM.

I would be more concerned of a rise in coolant temperature being a function of a fouled heat exchanger or other cooling problem.


Ted

I'm a huge advocate for being under pitched but that mostly applies to high horse power lightweight diesels or planning vessels .

Like OC diver said that old school motor is not as affected unless you are running it full throttle than you may get overheating or cylinder wash from excessive fueling .

I suspect as OC that your heat exchanger or mixing elbow most likely needs cleaning .

 

[DW1979OA]

Thanks for the inputs. The tacs are hall effect digital so probably pretty close to accurate.
The over heating at upper rpm loading is a concern that I will continue to investigate, although it has been this way for as long as I can remember. Temps run 183 up to 1900 rpm before starting to climb. I chicken out holding at full throttle for more then a couple of minutes.

Just as an exercise, lets suppose that when I do haul the boat and get an experts opinion on the condition of the props, and lets say that there needs to be work done. Would any of you consider "under proping" by say 100 rpm. To maybe compensate for less then pristine bottom or other reasons?

FL120's are happiest at around 1750-1800 rpm. Kinda the sweet spot that gets me 7.5 to 8kts. With the current set up more rpms just don't generate much more speed. This is on a heavy 50+5 ft displacement hull boat (1979 Ocean Alexander Mk 1).

 

[Comodave]

Could be other reasons for the temp rise, heat exchanger, etc. As to the correct RPMs get a photo tach for $25 from Amazon and check them to make sure.

 

[Jeff F]

FL120's are happiest at around 1750-1800 rpm. Kinda the sweet spot that gets me 7.5 to 8kts. With the current set up more rpms just don't generate much more speed. This is on a heavy 50+5 ft displacement hull boat (1979 Ocean Alexander Mk 1).

So what's your motivation for changing anything? You can certainly depitch by a couple of hundred rpm, but that means your new boat speed at 1800 is whatever you get now at 1600. If you want the same speed 2000 rpm is your new cruise.

Whether that's good or bad for the engine health or fuel consumption has been endlessly debated here. I think that in your case it really doesn't matter ?

 

[Slo-Mocean]

Thanks for the inputs. The tacs are hall effect digital so probably pretty close to accurate.
The over heating at upper rpm loading is a concern that I will continue to investigate, although it has been this way for as long as I can remember. Temps run 183 up to 1900 rpm before starting to climb. I chicken out holding at full throttle for more then a couple of minutes.

Just as an exercise, lets suppose that when I do haul the boat and get an experts opinion on the condition of the props, and lets say that there needs to be work done. Would any of you consider "under proping" by say 100 rpm. To maybe compensate for less then pristine bottom or other reasons?

FL120's are happiest at around 1750-1800 rpm. Kinda the sweet spot that gets me 7.5 to 8kts. With the current set up more rpms just don't generate much more speed. This is on a heavy 50+5 ft displacement hull boat (1979 Ocean Alexander Mk 1).

Kind of a heads up about full throttle overheating .

For 15 years we had a over 3/4 throttle over heating problem with the 38 hp Westerbeke in my dads bought new Hughes 48 sailboat. It wasn't until he sold it that the buyers surveyor picked up on that the thrull hull was of size but the reducer to the smaller seacock was just enough to cause reduced cooling flow over 3/4 throttle causing over heating.

Just something to think about. You need to sort out your over heating problem and if the over heat is happening so suddenly, it has nothing to do with over propping and lugging especially not having a turbo .

Nothing kills motors more than water intrusion or lack of oil pressure than over heating.

Once or twice is survivable, after that without oil changes your risking thin oil damaging rod bearings , wrist pins or cam lobes.

 

[O C Diver]

Thanks for the inputs. The tacs are hall effect digital so probably pretty close to accurate.
The over heating at upper rpm loading is a concern that I will continue to investigate, although it has been this way for as long as I can remember. Temps run 183 up to 1900 rpm before starting to climb. I chicken out holding at full throttle for more then a couple of minutes.

Just as an exercise, lets suppose that when I do haul the boat and get an experts opinion on the condition of the props, and lets say that there needs to be work done. Would any of you consider "under proping" by say 100 rpm. To maybe compensate for less then pristine bottom or other reasons?

FL120's are happiest at around 1750-1800 rpm. Kinda the sweet spot that gets me 7.5 to 8kts. With the current set up more rpms just don't generate much more speed. This is on a heavy 50+5 ft displacement hull boat (1979 Ocean Alexander Mk 1).

Lots of possibilities can cause elevated temperature levels in those engines. I would look first, and do if they haven't been done in the last 7 years, cleaning of the heat exchanger on both the coolant and raw water side. Sediment from the engine block collects in the bottom of the heat exchanger and reduces the amount of heat transfer.

Cleaning or better yet replacing the transmission coolers. Unless cleaned with acid, about a third of the transmission cooler tubes are inaccessible to be cleaned by pushing a rod through the tubes. Reduced flow through the transmission cooler reduces flow through the engine heat exchanger. As most transmission coolers lack an anode, they have a finite life. A new transmission cooler is about 10% of the cost of rebuild a transmission that has had salt water intrusion.

Check the exhaust mixing elbow for corrosion /obstruction. Replace the thermostat, flush the engine block, and replace the coolant.

I wouldn't under prop the engines. Spend your money on routine maintenance to the cooling system. Over time, all boat engines operated in saltwater need to have periodic coolant system maintenance done. Finally, consider what cruising speed is. This is the point in your boat where you get probably 90% of your speed for <50% of your engines maximum fuel burn. Let the next owner more than double the fuel burn for the last knot.

Ted

 

[PPandE]

FWIW if I were shopping for a boat like yours and the one I was interested in ran hot and low rpm’s with nicked up props, I’d keep looking. On the other hand, recently tuned and repaired props, temps and revs to spec and a clean bottom would be a plus. Would also give the impression of a more loved boat overall.

 

[TBill36]

Depending on the type of ship and how you sail it.
We have a water displacement ship and never sail at full throttle.
When choosing the drivetrain, we deliberately chose a heavier propeller, max prm 2600 which is 200 prm less than required.
Advantage, less revs at cruising speed, less noise and the engine is put under a small amount more load which is good to prevent the contamination of the engine/turbo.

I hope the translation makes it understandable.

Greeting,

Pascal.

Can't agree with running a diesel at an overloaded condition being "good". You are brurning more fuel than the combustion process is designed for and adding soot and unburned contaminants to the engine. In this case it may be minor, but never good.

 

[Marco Flamingo]

Ask the yard and/or the local prop shop how much to remove and examine. My RPMs were okay, but the price was really nothing to have it pulled and examined. Turns out the shop wasn't that local, but they came by once a week on a scheduled route so I didn't have to deliver it to Tacoma. Because I was 100 low on RPMs, I asked that any necessary tweaking be towards reducing pitch. Turns out pitch was okay (4 blade) but it was slightly out of balance. Hardly any charge. Now I know what I'm dealing with in the future (report figures are kept in the log book). Might even be that I'll recupe that money if next buyer appreciates good record keeping.

 

[sunchaser]

FWIW if I were shopping for a boat like yours and the one I was interested in ran hot and low rpm’s with nicked up props, I’d keep looking. On the other hand, recently tuned and repaired props, temps and revs to spec and a clean bottom would be a plus. Would also give the impression of a more loved boat overall.

100% agree. When our boat (truly a unicorn) went through sea trial last year prior to sale, the new owner's mechanic was ecstatic that the engines made 50+ RPM over rating with no over heating, strange happenings or vibration issues. I'd not consider buying a vessel that falls below rated RPM when sea trialing as there are too many questions raised.

Since the OP is pulling the boat it may well be a simple matter (or not) to pull the props and re-pitch keeping in mind overheating can be a sign of several non prop related issues.

 

[LovetoBoat]

I have found this article to be useful for discussions like this.


https://www.sbmar.com/featured-article/propellers-move-boats-engines-just-turn-them/

 

[jungpeter]

FWIW if I were shopping for a boat like yours and the one I was interested in ran hot and low rpm’s with nicked up props, I’d keep looking. On the other hand, recently tuned and repaired props, temps and revs to spec and a clean bottom would be a plus. Would also give the impression of a more loved boat overall.

Hi PPandE,

Positively right on. Attempting to sell a boat with over-pitched props and attendant overheating issues is a very poor idea. Attempting to tap dance around over propping issues revealed during survey is seldom successful, The adage “pay a bit now, or a LOT later” comes to mind.

Ditto for continued operation for the original poster. Living with a foul bottom and dinged props (and certainly overheating) seems unwise.

Regards,

Pete

 

[Pascall]

Can't agree with running a diesel at an overloaded condition being "good". You are brurning more fuel than the combustion process is designed for and adding soot and unburned contaminants to the engine. In this case it may be minor, but never good.

The engine is loaded more favorably at cruising speed, the speed we always sail, which is about 60% of the power of the engine.
Also shown by the figures, 4 liters of consumption per hour at 12.2 km per hour for a boat of 11 meters long and 14 tons heavy, those are excellent figures!
He doesn't smoke at all, and the exhaust temperature is perfect, and it's the exhaust temperature that decisives, the rest is secondary.
Exhaust temperatures that are too high are usually due to too high back pressure of the exhaust and, to a lesser extent, a screw that is too heavy.
A dry exhaust is therefore the best solution.
Our situation, we have a water-cooled exhaust, but to reduce the counter-drug, 85% of the cooling water for the exhaust goes overboard, the remaining 15% cooling water is more than sufficient to cool the exhaust and reduce the noise.
We finished at 85% because we reached the perfect exhaust temperature at the time.

Mvg,

Pascal

 

[Nick F]

I, personally, do not see a problem in "overpropping" the boat if normal cruising power is well below max engine power (almost always the case for trawler type boats).

Below is a graphical illustration of where changing to a heavier prop takes you. The values are all imaginary, but the shapes of the curves are typical (ex: speed to the power of 2.7 for prop power). I have assumed 40hp as the power required for cruising speed.

I have created a "sweet zone" for continuous operation. This zone is bounded on its 4 sides as follows:
Top: max continuous power of engine
Bottom: 20hp minimum operation to maintain minimum operating temperatures.
RH side: max revs of 200 rpm below engine max
LH side: min revs to ensure good lubrification, etc.

Looking at the graph, point A is the operating point with the "normal" propellor and point B is the operating point with the "overpropped" propellor.

As you can see, both operating points are comfortably within the sweet operating zone.

Beyond normal cruising - say, max continuous power - you can get 62hp @ 2200rpm with the normal prop. With the overpropped boat you will be limited to 57hp @ 1830rpm.

 

[Nomad Willy]

If you run around at fractional power loadings, there probably won't be much damage from lugging the engine, maybe none. Then again, if you take a bit of pitch out, there won't be any overloading, your idle speed won't be four knots, the engine will run cooler and your fuel economy will improve.

Yes yes and yes. Even better if you’re 50-100rpm underpropped. And when your bottom get’s a bit fouled you won’t be overpropped.

Edit;

If you’re significantly off from good proportion re pitch/blade area you should, with a good prop man, cut a bit of area off the prop tips. There’s an ideal relationship between prop pitch and prop blade area and speed. To get the rpm up to where it should be reduce; # Of blades, blade area or reduce blade pitch. Don’t loose track of the fact that you need both blade area and pitch to push water. If you have the right # of blades (almost always 3 for a rec-trawler ) you need less pitch or blade area. Get w the prop man to decide if you need less pitch or blade area.
Too many rec-trawler skippers have too much blade area and too little pitch loosing efficiency/speed through too much friction between the prop blades and the water.

 

[DDW]

Can't agree with running a diesel at an overloaded condition being "good". You are brurning more fuel than the combustion process is designed for and adding soot and unburned contaminants to the engine. In this case it may be minor, but never good.

A normal diesel never "burns more fuel than the combustion process is designed for" unless it is loaded beyond the torque spec for that rpm. This never happens in a boat at part throttle as the prop torque absorption curve is below the engine torque output curve everywhere except at the maximum rpm achievable, which is where the two meet.

Diesels are not throttled, so until they reach max torque there is an excess of oxygen and a deficit of fuel.

 

[rslifkin]

A normal diesel never "burns more fuel than the combustion process is designed for" unless it is loaded beyond the torque spec for that rpm. This never happens in a boat at part throttle as the prop torque absorption curve is below the engine torque output curve everywhere except at the maximum rpm achievable, which is where the two meet.

Diesels are not throttled, so until they reach max torque there is an excess of oxygen and a deficit of fuel.

There is still a difference between max output at a given RPM and that output that's intended to be used continuously at that RPM. The additional output beyond the continuous point is only meant to be used for short periods such as while accelerating through that range.

 

[sunchaser]

I have found this article to be useful for discussions like this.


https://www.sbmar.com/featured-article/propellers-move-boats-engines-just-turn-them/

This is a good read for the real scoop

 

[Jeff F]

There is still a difference between max output at a given RPM and that output that's intended to be used continuously at that RPM. The additional output beyond the continuous point is only meant to be used for short periods such as while accelerating through that range.

I think that any talk of accelerating through a range is specific to planing boats, and so not applicable to discussion in displacement boats like the OPs.

 

[rslifkin]

I think that any talk of accelerating through a range is specific to planing boats, and so not applicable to discussion in displacement boats like the OPs.

It's more of a concern with a planing hull, but it applies to any boat. If you start out moving along at idle doing, say, 2.5 kts and then throttle up to your normal cruise speed somewhat quickly you'll experience it. Engine load will be higher for the first couple of seconds as the boat gains speed and reduce as you reach a steady state. The engine (and therefore prop) RPM will increase when you increase throttle, but turning the prop at a given RPM imparts more load if the boat is moving slower (hence why a correctly propped boat won't reach full RPM if you went to WOT with the boat tied to a dock).

You can also notice this when you first put the boat in gear at a stop. You'll hear the engine load up as it goes into gear, but as the boat comes up to its normal speed at idle you'll hear the load reduce slightly.

But the point isn't about how much extra power you need for acceleration, but a question of what parts of the engine's operating range are intended for continuous vs short term use (this will vary significantly from one engine design to another).

 

[Nick F]

Looks like my graph failed to post - trying again:

I, personally, do not see a problem in "overpropping" the boat if normal cruising power is well below max engine power (almost always the case for trawler type boats).

Below is a graphical illustration of where changing to a heavier prop takes you. The values are all imaginary, but the shapes of the curves are typical (ex: speed to the power of 2.7 for prop power). I have assumed 40hp as the power required for cruising speed.

I have created a "sweet zone" for continuous operation. This zone is bounded on its 4 sides as follows:
Top: max continuous power of engine
Bottom: 20hp minimum operation to maintain minimum operating temperatures.
RH side: max revs of 200 rpm below engine max
LH side: min revs to ensure good lubrification, etc.

Looking at the graph, point A is the operating point with the "normal" propellor and point B is the operating point with the "overpropped" propellor.

As you can see, both operating points are comfortably within the sweet operating zone.

Beyond normal cruising - say, max continuous power - you can get 62hp @ 2200rpm with the normal prop. With the overpropped boat you will be limited to 57hp @ 1830rpm.

 

[Jeff F]

I
But the point isn't about how much extra power you need for acceleration, but a question of what parts of the engine's operating range are intended for continuous vs short term use (this will vary significantly from one engine design to another).

Again, this only applies to planing boats. In a displacement hull with fixed prop the mid-rpm torque capabilities of a diesel don't matter, because the full available power in that range is never needed on a continuous basis.

 

[Greg QS]

Over propped is over propped through the full hp range. When over propped you are operating the engine outside of the manufactures specifications. The manufacture spends millions of dollars on the designs of these engines. They have specific rules for the install and loading of the engine. Specific rules for the amount of fuel , air, and boost mix to make them run efficiently and reliably for years. When in an overpropped condition you are operating without maximizing the fuel burn. At the price of fuel alone, why not maximize the efficiency of the engine.

At any given hp demand you burn the same fuel. So if you pull 150 hp at 1000 rpm or 150 at 1200 rpm you will burn the same fuel. The differences is you have more boost, oil circulation, water cooling raw and freshwater circulation. If you have egt readings you will see you exhaust is cooler at the 1200rpm as to the 1000rpm at the same fuel burn and hp draw . You get better combustion with the propper air mix. What does this mean? Better fuel milage! You are also operating the engine in a constant slightly overheated state. Now throw in a damaged prop, bottom growth , fowled coolers, and the heat only goes up and Fuel efficiency further down

If you do the prop repair it would be great to see how this works in real data.
Take you fuel burns and speeds accurately and egt readings on the old setup. Then take the same when finished. You should see at any given speed better fuel milage. At say 8k , 10k you will see better fuel milage

 

[Greg QS]

Another way to look at overproping. Lets forget all about all the engineering and mumbo jumbo and personal . opinions.

Has anyone ever had a fuel mileage problem in there car and go to the shop and they miraculously fix it a a minimal cost? I have. The problem was a dirty air filter.

When you run a boat overproped at any rpm you are intentionally suffocating the air for the engine resulting in poor fuel combustion. = Bad fuel milage! You are running the engine without the proper fuel, air combination to maximize the power from the fuel.

Since boats realy don't get fuel milage. I think a lot of boater forget that a small change in the mileage , like .2 , is a 20% increase in fuel milage. Looks so small on the screen but its huge in the wallet.

So maximize your fuel milage and prop your boat properly.