Over propped. Is it really a problem?

[DW1979OA]

OP here again. Nice day here in Bremerton. Decided to do a more thorough study of the temperature of the coolant vs the rpms vs SOG. Twin FL120s on a 60000 lb SD boat. Temps for port and stbd engines matched closely. Raw water temp 49.3F Flat water and minimal wind. recently cleaned bottom.
Some current .1 to.3 kts going partially through a narrows that could skew the SOG

1500 177 7.9
1550 179 8.1
1600 178 8.2
1650 178 8.2
1700 180 8.5
1750 180 8.6
1800 181 8.9
1850 183 9.3
1900 183 9.5
1950 183 9.6
2000 186 9.7
2050 187 9.7
2100 189 9.6

Didn't want to push it any harder and what I think I'm seeing is that todays raw water temp is about 4-5 degrees F cooler then we see in the summer. I might not have warmed up the engines entirely before starting my run. Normal temps for the engines are consistently 182-3 at the rpms I travel. Theoretical hull speed (54') is under 10 kts So was able to approach it but just barely before getting hotter then what I felt comfortable.

I've printed out all of the wonderful posts and you all have given me a lot to think about.

Thanks.

jp

 

[LovetoBoat]

Talk to the generator people. A generator runs at a fixed RPM but the load changes from light to heavy. Never been told that my generator’s life will be shortened because of load. In fact all diesel manufacturers reps. Have always said that diesels are good for a given amount of fuel. Use the fuel lightly you get more hours. Use the fuel heavily you get less hours.

Only gonna say this - Everyone I talk to says do not run your generator without a load. You must have a load or you shorten the life of the generator. So there is something to finding a balance. Not heavy and not light... A diesel wants to run but needs a load...

 

[DDW]

OP here again. Nice day here in Bremerton. Decided to do a more thorough study of the temperature of the coolant vs the rpms vs SOG. Twin FL120s on a 60000 lb SD boat. Temps for port and stbd engines matched closely. Raw water temp 49.3F Flat water and minimal wind. recently cleaned bottom.
Some current .1 to.3 kts going partially through a narrows that could skew the SOG

1500 177 7.9
1550 179 8.1
1600 178 8.2
1650 178 8.2
1700 180 8.5
1750 180 8.6
1800 181 8.9
1850 183 9.3
1900 183 9.5
1950 183 9.6
2000 186 9.7
2050 187 9.7
2100 189 9.6

Didn't want to push it any harder and what I think I'm seeing is that todays raw water temp is about 4-5 degrees F cooler then we see in the summer. I might not have warmed up the engines entirely before starting my run. Normal temps for the engines are consistently 182-3 at the rpms I travel. Theoretical hull speed (54') is under 10 kts So was able to approach it but just barely before getting hotter then what I felt comfortable.

I've printed out all of the wonderful posts and you all have given me a lot to think about.

Thanks.

jp

Is there some (rather low) maximum coolant temp required of this engine? I'm not familiar with it. But in the diesels I do own (currently 4) that would not be considered unusual or particularly hot. My Cummins runs at 171 at very low load, but will climb to 185 when pushed to high throttle settings. My Volvo runs at a nearly constant 194, will climb only a few degrees at max throttle. My Ford runs at about 195 but will climb to 210 on a hill. It looks like you aren't going any faster than 9.7 knots. What was the rated max engine RPM again? Is 2100 the max? Unless the manufacturer advised some lower value, I wouldn't be worried about 189 myself.

 

[O C Diver]

OP here again. Nice day here in Bremerton. Decided to do a more thorough study of the temperature of the coolant vs the rpms vs SOG. Twin FL120s on a 60000 lb SD boat. Temps for port and stbd engines matched closely. Raw water temp 49.3F Flat water and minimal wind. recently cleaned bottom.
Some current .1 to.3 kts going partially through a narrows that could skew the SOG

1500 177 7.9
1550 179 8.1
1600 178 8.2
1650 178 8.2
1700 180 8.5
1750 180 8.6
1800 181 8.9
1850 183 9.3
1900 183 9.5
1950 183 9.6
2000 186 9.7
2050 187 9.7
2100 189 9.6

Didn't want to push it any harder and what I think I'm seeing is that todays raw water temp is about 4-5 degrees F cooler then we see in the summer. I might not have warmed up the engines entirely before starting my run. Normal temps for the engines are consistently 182-3 at the rpms I travel. Theoretical hull speed (54') is under 10 kts So was able to approach it but just barely before getting hotter then what I felt comfortable.

I've printed out all of the wonderful posts and you all have given me a lot to think about.

Thanks.

jp

Some of this will depend on where the sensor is and how much is exhaust manifold versus cylinders. I'm not familiar with the exact water flow path of a FL. As an example, water temperatures are less critical if engine block temperatures are 175 and exhaust manifold is 195. Obviously the thermostat is in the engine block, but the exhaust manifold water is coming back into the engine to go through the thermostat. If the temperature sensor was measuring cylinder head temperature I would be more concerned than a combination that included the exhaust manifold.

Ted

 

[klee wyck]

Two questions:
Am I over propped?
Should I care?

Engine: Mercedes OM 403 V10 that displaces 16L/1000cid, HP300 (18hp/liter)
Rated RPM is 2300 - I can get around 1900 at full throttle, mid load, clean bottom. I cruise at 8 knots and 1500 rpm. Boat, classic heavy full displacement, weighs 135K and 54 feet long. Pushing the throttle past 1600 rpm gains you absolutely nothing unless you like large waves. Coolant temps runs 174-177 over any range that I have ever operated in. EGT runs around 750 once warmed up and under decent load. Does not seem to vary much over the range I operate over but this is literally a thermometer on the dry stack just above the manifold.

Seems like I run either 25% or 45% of max power output depending upon whether you are using rated rpm or observed rpm max. I may only care about engine life as long as I own this boat and that may be forever, but as I own it there is no risk of running at full throttle since there is no reason to run at full throttle.
I think this may be all of the data needed to answer the two questions as I understand them.
I have my view on this.
What is yours?

 

[rslifkin]

If sounds like Libra is overpowered enough that you're probably in a fairly safe range, as you're using significantly less than max power. Although I'd be curious how it would behave (including fuel consumption) with a bit less prop pitch.

 

[klee wyck]

If sounds like Libra is overpowered enough that you're probably in a fairly safe range, as you're using significantly less than max power. Although I'd be curious how it would behave (including fuel consumption) with a bit less prop pitch.

Well, I can tell with some certainty that I would not consider for a minute trading the wheelhouse 58-decibel low rumble of this engine turning 1500 rpm while turning the high pitch, 38"-wheel 500 rpm in exchange for 10 less gallons of diesel consumed in an entire year of cruising year at higher, noisier rpm and zero measurable positive effect on the life of this engine while in my care.

Careful what you wish for.....

 

[tiltrider1]

Only gonna say this - Everyone I talk to says do not run your generator without a load. You must have a load or you shorten the life of the generator. So there is something to finding a balance. Not heavy and not light... A diesel wants to run but needs a load...

Yes, do not run a generator unloaded. The generator won’t come up to proper temperature causing unburnt carbon deposits to form.

Now how dose a generator cycling between 40% and 80% at a fixed RPM differ from being slightly over or under propped?

 

[Jeff F]

Although I'd be curious how it would behave (including fuel consumption) with a bit less prop pitch.

I've used three different props differing in pitch in my current boat over 15k miles, and have ECM supplied data like fuel consumption, % load, etc. As far as I can tell there is no difference between them in terms of fuel consumption. Fuel consumption/speed has not changed even though fuel/rpm and % load changes. This is true across my 6-9 knot cruising speed range.

If there is a fuel consumption difference it's negigible.

 

[Jeff F]

Most Trawlers can and do get away with some degree of overprop, without short term damage, but the problem I see is that if/when you are pressed into running the motor hard, old man Murphy will be right there!
Bad weather, strong current, or towing can put the overloaded motor in the red zone quickly, results can be catastrophic.
I like knowing that my rig is capable of whatever comes my way, and it actually consumes a little less fuel at cruise, despite higher rpms than it did when it was overpropped.

I had exactly the same line of thinking when I put a Cummins BTA in my last boat. I wanted access to the full range of throttle, and deliberately underpropped it a bit.

I think the decision should be dictated by the use case, including worst-case scenarios. There is no universal right or wrong here.

 

[O C Diver]

Two questions:
Am I over propped?
Should I care?

Engine: Mercedes OM 403 V10 that displaces 16L/1000cid, HP300 (18hp/liter)
Rated RPM is 2300 - I can get around 1900 at full throttle, mid load, clean bottom. I cruise at 8 knots and 1500 rpm. Boat, classic heavy full displacement, weighs 135K and 54 feet long. Pushing the throttle past 1600 rpm gains you absolutely nothing unless you like large waves. Coolant temps runs 174-177 over any range that I have ever operated in. EGT runs around 750 once warmed up and under decent load. Does not seem to vary much over the range I operate over but this is literally a thermometer on the dry stack just above the manifold.

Seems like I run either 25% or 45% of max power output depending upon whether you are using rated rpm or observed rpm max. I may only care about engine life as long as I own this boat and that may be forever, but as I own it there is no risk of running at full throttle since there is no reason to run at full throttle.
I think this may be all of the data needed to answer the two questions as I understand them.
I have my view on this.
What is yours?

I think your boat and mine are in about the same engine parameters. I looked for a 4:1 gear briefly when flattening the pitch didn't work. Glad I didn't find one. My boat could be pushed with about 80 to 90 HP (WOT) with a 4:1 gear. With the old engine (6CTA 450 HP Cummins) the boat could reach 10 knots with about 350 to 400 HP (ridiculous). Instead, I cruise at 6 to 7 knots between 1,200 and 1,500 RPM. The percentage of load relative to RPM is between 40 and 50%. I cruise 178 to 182 engine temperature, going to maybe 185 at 8 knots. And like Libra my cruise noise level is very nice regardless of where you are in the boat. Running the boat up to WOT out of gear has become intolerable to me. Cruising at 1,700 to 1,800 to make 8 knots is unpleasant compared to what I'm use to.

I have almost 6,000 hours on the engine now and do oil analysis with every oil change. Can't imagine (barring a catastrophic failure) it won't run 20k to 30K hours before rebuild. Pretty sure you're in the same situation.

Ted

 

[PierreR]

I have a difference in burn rate and a difference is visible turbulence behind my boat between my two engines. When I pulled the boat out of the water after 4,000 miles for winter storage I noticed that I have two different prop designs on my boat. This was not caught on survey. The matching propellers are hanging in the lazarette.

 

[rslifkin]

Yes, do not run a generator unloaded. The generator won’t come up to proper temperature causing unburnt carbon deposits to form.

Now how dose a generator cycling between 40% and 80% at a fixed RPM differ from being slightly over or under propped?

A generator is generally spec-ed for continuous duty. So it's meant to handle sustained periods near full electrical load. However, there's usually some surplus engine power to handle starting tough loads like air conditioning, so even a steady full load on the generator is unlikely to be full load on the engine (and it's certainly within the design specs).

 

[rslifkin]

I've used three different props differing in pitch in my current boat over 15k miles, and have ECM supplied data like fuel consumption, % load, etc. As far as I can tell there is no difference between them in terms of fuel consumption. Fuel consumption/speed has not changed even though fuel/rpm and % load changes. This is true across my 6-9 knot cruising speed range.

If there is a fuel consumption difference it's negigible.

That's an interesting data point to have. It looks like the impact on fuel consumption varies between engines. No difference on your boat, but Delfin saved fuel by reducing pitch.

 

[tiltrider1]

A generator is generally spec-ed for continuous duty. So it's meant to handle sustained periods near full electrical load. However, there's usually some surplus engine power to handle starting tough loads like air conditioning, so even a steady full load on the generator is unlikely to be full load on the engine (and it's certainly within the design specs).

Cat 3208’s are Truck engines, Boat Engines and Generator engines. What is the difference Between running a Generator at 1500 rpm’s at 80% load and running a boat over propped at 1500 rpm’s that causes the engine to be at 60% load?

 

[SteveK]

cat 3208’s are truck engines, boat engines and generator engines. What is the difference between running a generator at 1500 rpm’s at 80% load and running a boat over propped at 1500 rpm’s that causes the engine to be at 60% load?

0+0=00

 

[PierreR]

Cat 3208’s are Truck engines, Boat Engines and Generator engines. What is the difference Between running a Generator at 1500 rpm’s at 80% load and running a boat over propped at 1500 rpm’s that causes the engine to be at 60% load?

210hp vs 375hp. Same engine natural or turbocharged. Not the same duty rating. Also two different rpm's on the generator.

 

[psneeld]

And yet my 3208s were 320hp. The higher hp/later year 3208s had a few engineering differences, not sure if the reengineered ones were ever used in 210hp service and if so, capable of and using less hp.... who would know unless you had the short block data?

 

[rslifkin]

Cat 3208’s are Truck engines, Boat Engines and Generator engines. What is the difference Between running a Generator at 1500 rpm’s at 80% load and running a boat over propped at 1500 rpm’s that causes the engine to be at 60% load?

Difference in power rating. In the marine world, a 3208TA is rated for 375 to 435hp at 2800 RPM. The highest rating I can find for a generator application is 299hp at 1800 RPM. For the 435hp marine rating, it appears the engine can produce somewhere around 325hp at 1800, so there's a bit of de-rating for the generator version compared to what it's capable of. And the data I'm seeing doesn't specify if the generator engine received any changes such as upgraded exhaust valves to handle higher continuous EGTs.

In addition, the 299hp generator application is on a generator rated for 200kw standby (intermittent use at variable load) or 180kw prime power (continuous use with periods of time potentially at full output). Even accounting for a bit of inefficiency in the generator end, a continuous 180kw load would be demanding somewhere around 250 - 260hp. That's about 80% of the max output at 1800 RPM, which is quite a bit. However, we don't know what the expected lifespan of the 3208 is in generator service. It's entirely possible that CAT was targeting a 5000 hour lifespan as a genset, while someone using it for propulsion in a boat would want to see 10,000 hours (which would potentially require lighter loading to reach the design life). As a reference point, the highest number of hours on any 3208 powered generator I can find for sale is just over 1300.

As another data point, for the NA version of the 3208 industrial engine, I can find 2 ratings. 175hp at 2800 (good for 125hp at 2400 continuous duty) or 210hp at 2800 (good for 150hp at 2400 continuous duty).

Depending on the engine, an increase in load beyond the recommended operating conditions doesn't necessarily mean the engine is going to explode imminently. There may be a danger zone where long periods of operation in that range can cause some ugly failures, but otherwise, it may just be a case of "add more load and receive a shorter lifespan before it's worn out". Depending on the engine design and what components are first to wear out, the same power at lower RPM (higher load) beyond a point may be saving a little wear on one part, but causing more wear on another.

 

[Jeff F]

In addition, the 299hp generator application is on a generator rated for 200kw standby (intermittent use at variable load) or 180kw prime power (continuous use with periods of time potentially at full output). Even accounting for a bit of inefficiency in the generator end, a continuous 180kw load would be demanding somewhere around 250 - 260hp. That's about 80% of the max output at 1800 RPM, which is quite a bit. However, we don't know what the expected lifespan of the 3208 is in generator service.

Given that in propulsion these engines are typically rated for continuous use at 80% of the maximum output at ~200 under max rpm (putting out a lot more power) I don't understand why you'd expect a lower lifespan. I'd put my money on the genset.

 

[rslifkin]

Given that in propulsion these engines are typically rated for continuous use at 80% of the maximum output at ~200 under max rpm (putting out a lot more power) I don't understand why you'd expect a lower lifespan. I'd put my money on the genset.

I'm not finding any mention of CAT saying you can cruise a 3208 at 2600 (200 RPM below max). Every mention I've found of rated RPM for them shows 2800 as rated RPM, but I haven't seen anything mention a continuous rating above 2400.

 

[Jeff F]

Given that in propulsion these engines are typically rated for continuous use at 80% of the maximum output at ~200 under max rpm (putting out a lot more power) I don't understand why you'd expect a lower lifespan. I'd put my money on the genset.

And closely related: If you accept that when right-propped it's within guidelines to operate at 80% capacity at the top end, how could you possibly object to running at 80% capacity at some other lower rpm/hp range? That's what a VP prop offers, and what may be achieved with overpropping.

 

[rslifkin]

And closely related: If you accept that when right-propped it's within guidelines to operate at 80% capacity at the top end, how could you possibly object to running at 80% capacity at some other lower rpm/hp range? That's what a VP prop offers, and what may be achieved with overpropping.

It's not automatically wrong, but it's a case of "some engines will be fine with it, some won't". So without some kind of guidance from the manufacturer that it's an acceptable operating condition, you're wandering into uncharted territory.

Heavy load at low RPM means the power delivery is less smooth (less power pulses) which changes the stresses on the rotating components. Think about lugging a car with a manual transmission. Too much load at low RPM leads to an unhappy engine (and on some engines can cause main or rod bearing issues). The question is all about "how much is too much?" and in the absence of enough information to answer that, it's better to play it safe.

For those worrying about noise, I've always found a diesel under more load at low RPM to put out more vibration and low frequency noise, which is harder to isolate. So while the engine may be louder while lightly loaded at higher RPM, that noise is likely to be easier to isolate with some sound isolation for the engine room.

 

[Nomad Willy]

The elephant in the water is boat load or/and generator load.

A generator load can be varying just as an boat load can be varying. But the rating may be a constant load. You may run a boat engine at 80% from after it’s warmed up to marina maneuvering speed or/and do something similar (load-wise) w a generator. Neither one is 100% at working load.

For pleasure boat running I’d just count the 80% load time.
But 80% is a heavy load. It would be easy to apply more like w a fouled bottom and running at the 80% rpm would overload the engine if it was dialed in as 80% for cruising.

I’m say’in it’s basically fly stuff. 70% load is a good target that is very practical and hard to get into trouble load-wise.

Some adjust to X number of rpm (hndrederds) down from max WOT. I ran my W30 at 500rpm down from max WOT as my max cruise rpm. I almost always ran at 2300 (700rpm down) .. under normal conditions but if I was bucking a big tide in Knight Inlet I may run for several hours at 2500rpm .. or I may anchor up.

Lots of people (and especially boat or airplane pilots) tend to over do technical and numerical maximum’s. But the more you know the better … knowing too much thinking and reading into stuff can enter the red zone of overload haha.

However I suspect wandering into red zones powering your boat is less likely that wandering into under loading problems like a FL at 1400rpm and 150 degree thermostat.

 

[Nomad Willy]

rslifkin wrote;
“ For those worrying about noise, I've always found a diesel under more load at low RPM to put out more vibration and low frequency noise, which is harder to isolate. So while the engine may be louder while lightly loaded at higher RPM, that noise is likely to be easier to isolate with some sound isolation for the engine room.”

It’s true that under most all running conditions the more diesel engine combustion noise you hear the better. I had a 6 cylinder car that was very noisy re engine noise. I usta love coming to a hill so I could press down on the accelerator pedal as it was so much quieter.

Engine noise and plenty of rpm in a rec-trawler is more likely to be a good sign than bad.
Lack of noise in a rec-trawler is likely bad unless it’s valve noise or piston slap.

It’s too easy to think a noisy engine is working itself to death.

 

[Choices]

I stand on the other side.

Expecting a 40-50 year old diesel motor to run at it new design/stated specs is a wonderful thing. But why test at that rating as a consideration for a purchase?

Would you make a condition of buying an all original 63 Vette that it hits and maintains max rpm without overheating a criteria for buying? Would you even care if it runs and drives the way you plan to operate it?

If it can't make full rpm that is one problem. A separate problem is it overheats. Doubtful the two are related. But a full throttle test will show both.

Remember, a full throttle test is considered a destructive test. It's just a matter of how much.

 

[PierreR]

What is really being argued here. I get confused about what is really being said. I get the idea that some here know little about the difference between engine power curves and propeller power curves. Some appear to actually be talking about overloading an engine above it's power curve and well below its maximum power output while others are talking about overloading near maximum outputs on the engines. While others still are talking about noise and vibration.
Now I don't know a lot of people who run their boats at anywhere near 80% of max rated power. 65% year but 80%? not for very long.
At my normal cruise power my engines are putting out 31% of their max rated power. The propeller is consuming 47% of the rated power of the engines at the rpm I am running at. At max loading the engine at my normal cruise rpm my engines would be putting out 57 % or total rated power.
My engine do not turn up to full rated rpm and their isn't a chance in hell of me overloading them at even 80% of power. On top of that, I don't believe the tacs.
I am going to have all four of my props into the shop for checking and reconditioning and come out with propspeed on them and I am not going to worry to much about it. I will find out which set of props is best for my boat and install that set.

 

[PierreR]

I stand on the other side.

Expecting a 40-50 year old diesel motor to run at it new design/stated specs is a wonderful thing. But why test at that rating as a consideration for a purchase?

That depends. Mine did not run up quite to full rpm and I purchased the boat but, My boat was much heavier than the test boat and equipped with stabilizers.

 

[Jeff F]

It's not automatically wrong, but it's a case of "some engines will be fine with it, some won't". So without some kind of guidance from the manufacturer that it's an acceptable operating condition, you're wandering into uncharted territory.

You're right. I keep forget about modern high performance diesels with recreational ratings. In that case ther are likely many uncharted territories for the uninformed. But nobody has these engines in non-planing boats.

 

[cafesport]

So our boat is and was powered by a pair of generator engines with 2.88 gears. On survey she was found incapable of running WOT due to being over propped.

I spoke with the manufacture then ran through the curves for the engines and found that by propping for hull speed at 1800 rpm the engines would be happiest (go figure…a generator engine) and fuel mileage would probably improve.







Now with full fuel and a clean bottom we run hull speed with both engines loaded at 30 percent and singularly at 60 percent, leaving the additional power for towing, foul bottoms, currents and weather.

Previously being over propped and running on one engine pushed the loads well into the 100 percent zone. Also with the increased slippage due to prop blades turning 100 rpm slower, the nmpg at hull speed was a bit less.

Bottom line, as long as you know that you are over propped I think its a manageable situation. Monitor loads and egts and you should be fine. The problem is most owners don't, so sticking to the WOT rpm metric prevents most problems for the manufacturers and owners alike.