Engine RPM

I have a 44' Ocean Alexander with the Caterpillar 3208's turbocharged engines. The build sheet that came from OA show the 25x17(?) props I have came from the factory.

When I go to the Caterpillar website they indicate the maximum engine RPM's for the 3208 should be 2800, but I only get 2400 at WOT. I've been told that means I'm over propped.

I've read and heard that cruising RPM's should be 75% of WOT. So here is my question, should I cruise at 1800 RPM, 75% of 2400, my engines maximum or should I bump it up to 2100 RPM which is 75% of what the engines should turn?

If it was me I would change the props so the 3208s reach the specified rpm at wot. Then cruising at 80% of that rpm shouldn't be a problem. I would also install egt and manifold boost gauges if they aren't already in use.

You can leave it overpropped if you only run hull speed. Running above that will be hard on the engines. There is no need to run them at 75% power, ignore that advice.

First make sure you are getting the data for the exact model engines you have. Look on the metal tag for engine model number and it might also show MAX rated RPM
The next step is to verify the tachometers are correct. A photo tach is cheap or a mechanic can check it for you. If both engines read the same it is unlikely the tachs are wrong.
Another quick test is with the transmission out of gear and engines warm briefly run up to max rpm. The rpm you hit will be the engines rpm setting. If you hit 2800 out of gear then the problem is probably in overpropping.


If indeed you are not able to hit MAX rated rpm then you do need to have the props repitched. At 6 knots it may not make any difference but if you are running faster they are overloaded significantly.

Is the 2800 rpm the WOT no load rpm or the rated HP rpm under load. It is the under load rpm that counts for adjusting prop to match load curve of the engine. The no load rpm is usually higher than rated rpm under load. There may be a label on the motor with this information. follow Bayviews and ski's instructions and advise.

Most 3208 tags give both rated power, as in "XXX hp at XXXX rpm". Also gives "high idle XXXX rpm"

The rated power rpm is full load rpm, usually 2800, best to prop a little above like 2850 if you are going to run planing speed.

High idle is the no load rpm, full speed in neutral. Usually around 3000, but I think some are around 3100.

Tags are usually on the front of aftercooler on TA versions, or on rocker cover if a T version.

The worry about RPM is from 2 problems , overloading and underloading.

If you record the max RPM of each engine , and it is not pumping huge black smoke (overloaded) simply reducing by AT LEAST 10% solves any overloading problem.

If thats where you wish to cruise , your fine.I like to put a line on the tach 300RPM down from WOT., and use that as the max RPM.

Most folks want economy esp for Long Range Cruising (LRC) and that is usually about at the SQ RT of the LWL . Forget so called hull speed unless you have your own oil well.

Under loading comes from running the engine at minor power 10% or 15% of rated power for long periods of time.

A bigger cruise prop is one cure but see overloading above.

A run of 15 min at near full power will help engine life, as will oil sampling and more frequent oil changes.

For most folks its not a problem as reducing the life of a 8000 hour engine to 4000 hours is still many many years of cruising.

You will know the end is near when the engine white smokes during warm up under load until 140F -160F or more is required to clear the exhaust.

There are many reasons a boat does not perform. Heres a few to consider:
Are the tachs true?
Are the props true?
How's the bottom paint? Smooth or fouled? It doesnt take much to drag you down in RPM.
Dirty fuel and air filters
How much extra weight added since new boat delivery?
Are the engines making rated HP?
Trim tab position? Too much or too little can have a big effect.

Our 44 OA has twin 250 HP engines and is propped with 24X20s. It achieves rated rpm at a speed of about 17 kts. The rule of thumb I've seen says 2 inches of pitch roughly equates to one inch of diameter. Don't know the power rating on your turbo 3208s, but intuitively, it seems they should be able to swing those props to rated rpm.

Skid,
Two " to one " dosn't seem right. That would be a 10" dia & a 20" pitch .... or the other way around a 20" dia w a 10" pitch would be OK for a bollard pull but not for a boat in motion.
From what I've read and heard there is a fairly narrow range of appropriate pitch for a given dia. For a trawler pitch should be about 75% of dia. Like a 24"dia w an 18" pitch. Some to quite a few trawlers IMO have too little pitch. Too little pitch means too much power wasted turning blades too large through the water. Get the blade big enough and all the engine's power may be required to turn up to rated rpm and w zoro pitch and absolutely no thrust will result. Extreme example of course but leaning in that direction results in power and efficiency loss .... like a 20" dia and 10" pitch. However if blade area is handled right a 24" dia w a 10" pitch can be very optimal for efficiency. It would take longer skinnier blades than usual and most all the props commonly available favor a limited prop clearance w wider "fatter" blades.
Too much pitch and too much tip loss ..... water tumbling around the blade ends but not enough moving aft creating thrust.

Skidgear I must not be reading your post correctly.

Ski in NC said:

You can leave it overpropped if you only run hull speed. Running above that will be hard on the engines. There is no need to run them at 75% power, ignore that advice.

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Ski is far more knowledgeable than me on this topic but I seem to recall a discussion on boatdiesel that ended with the conclusion that over propped/overloaded is still the case regardless of the RPM.

timjet,
Not at any rpm. Most over propped boats re not overloaded at 1/2 rpm.

manyboats said:

Skid,
Two " to one " dosn't seem right. That would be a 10" dia & a 20" pitch .... or the other way around a 20" dia w a 10" pitch would be OK for a bollard pull but not for a boat in motion.
From what I've read and heard there is a fairly narrow range of appropriate pitch for a given dia. For a trawler pitch should be about 75% of dia. Like a 24"dia w an 18" pitch. Some to quite a few trawlers IMO have too little pitch. Too little pitch means too much power wasted turning blades too large through the water. Get the blade big enough and all the engine's power may be required to turn up to rated rpm and w zoro pitch and absolutely no thrust will result. Extreme example of course but leaning in that direction results in power and efficiency loss .... like a 20" dia and 10" pitch. However if blade area is handled right a 24" dia w a 10" pitch can be very optimal for efficiency. It would take longer skinnier blades than usual and most all the props commonly available favor a limited prop clearance w wider "fatter" blades.
Too much pitch and too much tip loss ..... water tumbling around the blade ends but not enough moving aft creating thrust.

Skidgear I must not be reading your post correctly.

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Yes, I have a copy of Dave Gerr's prop handbook where the initial sizing factors are discussed in detail. Can't recall where I saw the 2:1 ratio...maybe Gerr's book, but it was in the context of a very rough thumbnail guide to estimate impact on power required when making tweaks to existing props. Nothing to do with initial sizing.

I just checked the specs on CAT marine engines and the lowest rating I could find for a turbo 3208 was 320 HP. Would be interesting to know the power rating on the engines and the transmission gear ratio....then run it through a prop calculator.

skidgear said:

I just checked the specs on CAT marine engines and the lowest rating I could find for a turbo 3208 was 320 HP. .

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Cat offered a marine 3208TA category A rated at 235HP and 2400 RPM. The HPs go up from there with the next at 275 HP. As you correctly note, the engine plate data is needed to go any further on prop sizing.

timjet said:

Ski is far more knowledgeable than me on this topic but I seem to recall a discussion on boatdiesel that ended with the conclusion that over propped/overloaded is still the case regardless of the RPM.

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I agree with this. The govenor is going to add more fuel to maintain rpm between a correctly propped vessel and one that is over propped. Just because it's not against the pins doesn't mean it's not overloaded. And unlike a engine in a dozer or scraper that is intermittently overloaded during its life, a engine is continuously overloaded in a over propped boat. There is less harm at RPMs below WOT but the effects of overloading do not disappear. A exhaust gas temperature gauge I'am sure would show higher temps at the same rpm between the correct prop and being over propped because of the increase of fuel to maintain rpm. I doubt any of us will kill our engines by being over propped by a small amount because we're not going to put that many hours on them, but a owner who ignores being over propped is probably ignoring other items in his engines maintenance and the cumulative effect may substantial shorten engine life.

sunchaser said:

Cat offered a marine 3208TA category A rated at 235HP and 2400 RPM. The HPs go up from there with the next at 275 HP. As you correctly note, the engine plate data is needed to go any further on prop sizing.

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Hmmmm. If the engine is the 235 @ 2400, then it's propped exactly right.

A exhaust gas temperature gauge I'am sure would show higher temps at the same rpm between the correct prop and being over propped because of the increase of fuel to maintain rpm.

Overloading (to not reach max rated RPM at WOT) and over proping are two different conditions.

Overloading is what most plaining boats do as they climb up on to the plane , you can see smell the black exhaust as they accelerate.

A displacement boat that has a >to large< prop to reach rated WOT would be overloaded IF it were operated at WOT all the time.

Most cruisers are far from WOT so the larger prop will show higher EGT , as it is a sign of higher efficiency.
The better heavier loading gets more HP from the fuel at that RPM , just what is needed.

So long as the cruise RPM is 10% or so less than whatever is observed WOT RPM , the engine is NOT overloaded by the larger more efficient prop.

It will have a longer service life and use less fuel when operating with the highest efficiency.

A glance at any engines prop chart will show how drastically the HP required goes down with a slight RPM decrease .

I'd overprop our 44 OA in a New York minute, and those 25x17's would be close to what I'd use... although 25x18 would be better. The only time the semi-planning boat will ever see maximum power under my ownership is for a survey. I'd slap on the old props if a buyer insisted. If the OPs boat is in fact overpropped I cut him a deal.

Need to clarify this a bit:

If engine is overloaded at full power, that is, incapable of reaching rated rpm or above, then it will be overloaded also when power is reduced slightly. Say from 100% down to say 70%, or even below if boat is still trying to plane.

But if power is reduced significantly, such as dropping down to hull speed, then the overload condition will disappear.

The above applies to planing or semiplaning type boats with high output engines. Often they are overpropped to optimize hull speed running. No harm in that, but it will make it hard on the engines if they are run at high power.

A 3208TA rated at 320hp or whatever is not going to be harmed by running at say 50hp at 1200rpm, even if it can't make rated rpm at full. Just don't try to plane out.

Ski: True enough but I don't see the benefit of not propping to make full power use an option. Fuel use at 6 knots will probably be the same despite different RPMs.

Ski in NC said:

A 3208TA rated at 320hp or whatever is not going to be harmed by running at say 50hp at 1200rpm, even if it can't make rated rpm at full. Just don't try to plane out.

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But what if the PO never knew or cared that the vessel was over propped and ran it on the pins frequently. The new owner quite possibly would have then bought a soon to be relic. Is this not a buyer beware issue Ski and why during survey a full load test is done?

bayview said:

Ski: True enough but I don't see the benefit of not propping to make full power use an option. Fuel use at 6 knots will probably be the same despite different RPMs.

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I agree with that. I would not intentionally overprop a boat.

Where it becomes an issue is when previous owner did it. Then is it worth several grand to prop it correctly? If it is only run at hull speed, it can be run as is.

sunchaser said:

But what if the PO never knew or cared that the vessel was over propped and ran it on the pins frequently. The new owner quite possibly would have then bought a soon to be relic. Is this not a buyer beware issue Ski and why during survey a full load test is done?

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That is possible. A good engine guy can tell if engines were run hard, and hints are also available from trip logs and owner interview.

Full power test is crucial to test for cooling system performance, engine performance, blowby, etc. If engine can't make revs, test is cut short.

If I were in the market to buy a used boat and the sea trial demonstrated significant over propping I would have to pass. That the owner or broker might tell me they never went above a certain rpm would be little solace and an owner who switched back to a acceptable prop before sale would be fraud if discovered. That owner would be sending a message to me that they think they know more about the engine than the manufacturer, and I would be thinking where else on the boat have they applied that philosophy? The engine manufacturers often specify how they want their engines propped and I for one see no long term practical reason particularly with modern light high out put engines to ignore their advise in the direction of over propping unless the process is well documented and also includes mechanical or electronic safe guards that prevent serious overloading. Even then as Ski points out a motor has to be run out now and then.

My $.02 .....
Most of the time most boats are heavy and dirty. Assuming one would want to be correctly propped under these "normal" conditions and considering that frequently a boat is propped just after launch when it would be light and clean under propping 100rpm would seem the best route to being spot on in the future. Then w all tanks full, a slightly fouled bottom, lots of extra "stuff" and several guests aboard the boat run nice and easily for being properly propped.

Ski in NC said:

Need to clarify this a bit:

If engine is overloaded at full power, that is, incapable of reaching rated rpm or above, then it will be overloaded also when power is reduced slightly. Say from 100% down to say 70%, or even below if boat is still trying to plane.

But if power is reduced significantly, such as dropping down to hull speed, then the overload condition will disappear.

The above applies to planing or semiplaning type boats with high output engines. Often they are overpropped to optimize hull speed running. No harm in that, but it will make it hard on the engines if they are run at high power.

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This is a pretty large generalization. As FF said earlier, anyone contemplating an overprop needs to look at the prop curves (power required) and the engine max power available curve for the engine(s) in question. Our 44 OA will stay nicely on "semi-plane" at about 13 knots. The prop charts say that equates to about 210 HP (105 per engine) power required. The engine power available chart says the engines are capable of producing 420 HP total (210 each) at that same rpm. That's a cushion of 100% with the boat comfortably on plane. Now, the "cushion" gets smaller as the semi-planing speed goes up, but even at rpm for 70% of max rated power (175 HP/engine), the cushion (for each engines) is about 60 HP (34%). That's plenty.


Power required for a given hull speed should be less with a more efficient (larger diameter) prop, but at the same time, the engines could be making the power required at a slightly higher torque than for the smaller prop...again the bigger prop is more efficient and producing thrust required at a slightly lower rpm. The interplay between the various parameters would depend on the engine design and how it makes its power. In any case the changes would be small. There's nothing but goodness in overpropping a boat with slobbering pig engines ...as long as the operator respects the new limits. This is something aircraft operators have done for ages. Surely boat herders are equally as capable.

So overprop, put a new red line on the tachs, and move on. Enjoy the benefits of improved prop efficiency...depending on where in the speed range you elect to operate your slobbering overpowered turbo charged beast...or even your overpowered NA twin Lehman, for that matter.

Regarding a previous remark that it would be dishonest to overprop and then survey with the original rpm rated props...I have to scratch my head. If the boat was operated within the cushion, and the owner explained overprop operation to a prospective buyer...what's the problem. I wouldn't even change out the props unless the prospective buyer couldn't understand the concept and insisted on a demonstration of max rated rpm. In fact, if a prop curve was available for the new bigger prop, the new (lower) max rpm could be demonstrated with the overprop props. By the way, I don't know more than the manufacturer....but I believe I know pretty nearly as much. And as FF says, they recommend propping the boat for max rated power to protect against stupid drivers. Make the boat error proof and you protect against warranty claims. Probably one reason (among others) that variable pitch props don't show up on pleasure boats...if you can't automate them, you're stuck with the least common denominator operator who would rip the guts out of the engines on the first day.

eyschulman said:

If I were in the market to buy a used boat and the sea trial demonstrated significant over propping I would have to pass.

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Couldn't agree more. There are plenty of good boats out there that are correctly propped as compared to accepting the risk of a wrong propped vessel.

Having been through the details of what a good manufacturer does for a new build for prop and engine optimizing, I'm confident the knowledge and science is very well utilized in this regard by most if not all reputable builders.

If over propping is not such a big deal why is it then that JD would not give me an extended warranty until there technician did a two hour sea trial including verification of correct propping? This was on a one off build. I asked and was told that if over propped no warranty until prop was changed. If people on the used boat market want to buy a over propped boat that a broker or owner tells them the motor was never used above a certain rpm so all is good-good luck to them probably will need it.

eyschulman said:

If over propping is not such a big deal why is it then that JD would not give me an extended warranty until there technician did a two hour sea trial including verification of correct propping? This was on a one off build. I asked and was told that if over propped no warranty until prop was changed. If people on the used boat market want to buy a over propped boat that a broker or owner tells them the motor was never used above a certain rpm so all is good-good luck to them probably will need it.

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Because John Deere was covering their warranty butts...precisely as I said earlier. Nothing to do with the technical merit of overpropping. Likewise your last sentence has nothing to with technical viability, engineering logic, or adhering to operating limits. I can't understand the logic of assuming that an owner who carefully repropped using manufacturer data, established safe new operating limits, and understood the consequences of exceeding those limits....would purposely damage an expensive piece of hardware? Why would anyone do that?

skidgear said:

Because John Deere was covering their warranty butts...precisely as I said earlier. Nothing to do with the technical merit of overpropping. Likewise your last sentence has nothing to with technical viability, engineering logic, or adhering to operating limits. I can't understand the logic of assuming that an owner who carefully repropped using manufacturer data, established safe new operating limits, and understood the consequences of exceeding those limits....would purposely damage an expensive piece of hardware? Why would anyone do that?

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Because I am not a expert mechanic or qualified engine specialist I prefer to follow the advise of the engine manufacturer. Your presentation of how over propping affects engines may be right but how would I or other boaters know for sure. I have heard both sides of the argument presented with various technical support. I have lived through a 50 year career that was highly technically based and there were many disagreements on how things work. If one sets up their propping so that it can overload the engine, my question is why? The answer usually is to save fuel. Since most pleasure boating is a low hour per year use situation, fuel is in my opinion a low priority on the total boat cost list. I encourage boaters to follow the engine manufacturers advise on loading. Regarding buying used mechanical equipment it was not I who coined the concept of buyer be ware.