3208T 320hp overpropped

[mpstan]

Our OA42 twin screw makes 2400 rpm WOT. This is fully loaded. We have 26x24 props. Most but not all advice is that despite running at 1300-1400 rpm, we are nonetheless lugging the engines and not doing them any favors. The prop shop I’m talking to is guessing we should be running approx 24x22 props. Because of the huge difference, repitching will not be practical. So looking at new or used props. I plan to clean and repaint the bottom before a sea trial to nail down my WOT, prior to changing props.

Anyone else with a 35,000 lb boat with these engines: do you know your prop specs?

Suggestions on where to look for used props?

 

[rslifkin]

What's rated RPM for a 320hp 3208T? A quick search turned up that it's 2800 rpm for the 375 and 435hp 3208T as well as the 210hp 3208 NA, but I'm not sure if the 320hp rating may have been at a different RPM.

You're definitely over-propped right now, but I'm not sure I'd want to go down in diameter unless necessary, especially if you're mostly running at low speeds. You'd likely loose some low speed prop bite with the reduced blade area.

Props can usually be re-pitched up to about 2 inches, so I'd be inclined to get the current props knocked down to a 26x22 and see where that puts you RPM-wise. There's a good chance it'll get you in the ballpark.

What speed does the boat run at WOT currently? Assuming your trans reduction is somewhere around 2:1 and the boat should do ~20 kts at WOT, the Michigan Wheel calculator points to a 26x22 prop as being adequate to get you to 2800+ RPM at WOT.

 

[tiltrider1]

CAT recommends operating at 1500 RPMs at a minimum. This is probably why you are being advised there is an issue. WOT of 2400 RPMs on a fully loaded boat might not be an issue.

I really can’t give any advice due to a lack of enough information on your situation.

Based off the info I do have it sounds like a little change in pitch would get your operating window into the 1500 RPM range. However, I wouldn’t do anything until I was positive that the boat wasn’t meeting my actual needs.

 

[Comodave]

They should be able to get 2” difference in repitching your current props. I would try that first and see what happens in the RPMs. Since you never come close to WOT you may be ok with the repitched props.

 

[Alaskan Sea-Duction]

I have 3208T/A 375hp. I run 26x25 three bladed props. WOT for me is 2800rpms. Doing open oceans runs I am running 2100rpms

 

[mpstan]

WOT for the 3208T is the same as the others: 2800 rpm. 1.98:1 ratio.
Top speed fully loaded at current WOT is 16-17 kts but the bottom is dirty

My understanding is that being overpropped is harmful to some extent regardless of the RPMs. Akin to lugging a car— one of my engines has already had a head gasket issue, and I want to rectify this. The Cats should be happier with proper pitching, and I’ll probably end up at 1500 rpm to do 9-10 kts.

Taking 2” of pitch off is certainly worth trying from a economics standpoint. Still curious to know where one might shop for used props

Thanks for the thoughts so far

 

[DavidM]

I agree with the advice given above. Repitch the current props down 2” and see how it goes.

Routinely running at 1,000 rpm off of top after repitching shouldn’t be a problem even if you don’t reach 2,800 at wot after repitching. Just don’t run above 2,000 for any length of time.

David

 

[Jeff F]

Routinely running at 1,000 rpm off of top after repitching shouldn’t be a problem even if you don’t reach 2,800 at wot after repitching. Just don’t run above 2,000 for any length of time.

Follow those guidelines with existing props and you'd be fine too. Anyone who claims you're lugging the engines at 1400 rpm needs to go back and study up on prop curves.

If you never plan to run at more than 2000 rpm the existing props are fine. Maybe a previous owner deliberately overpropped. It's not always bad.

 

[mpstan]

My credentials for marine diesel: no fewer than 50 hours on the internet, including boat diesel. 70% of the opinions suggest that overpropping should be corrected. At 2800 rpm WOT, 3 things are balanced: load, amount of fuel injected, and air added. When the throttle is pulled back, the balance is maintained. If a 3208 only makes 2500 rpm, then excess fuel is being dumped into the cylinders, and the turbo is not contributing the air it’s supposed to. This imbalance happens across all RPMs, which is why most people submit that even at low RPMs, overpropping should be avoided.

Then there are the other 30% that suggest that “putting” at 1400 RPMs in an overpropped boat is not necessarily harmful. As Jeff E has stated. Our engines run cool, I see no problems when we putt, so maybe I’m overly concerned. Previous owner putted also. It’s a 30 year old boat with 2600 hrs on the mains. The starboard engine has had 2 head gasket replacements by the way. If my mechanic and prop guy are correct, taking 2” pitch away won’t get me to 2800, so i may be looking at $3-4k for new props to get this done. Pretty frustrating not having a clear vision here!

 

[Comodave]

One thing to consider when checking RPMs is the dash tacks may or may not be accurate. Pick up a photo tack to be sure you are actually getting the RPMs you think you are getting by reading the dash tacks.

 

[Southern Boater]

One thing to consider when checking RPMs is the dash tacks may or may not be accurate. Pick up a photo tack to be sure you are actually getting the RPMs you think you are getting by reading the dash tacks.

Good point Dave, my original analogue gauges of a boat the same age as OP’s are suffering from resistance issues and inaccuracy’s are across the helm dash.

 

[Jeff F]

If a 3208 only makes 2500 rpm, then excess fuel is being dumped into the cylinders, and the turbo is not contributing the air it’s supposed to. This imbalance happens across all RPMs, which is why most people submit that even at low RPMs, overpropping should be avoided.

The notion that this is a problem across the range of operation is nonsense. It's only a problem as you near WOT. If you never go there it's never a problem.

Over fueling happens when the demand on the engine approaches its maximum HP/torque available at a given rpm. That's not good. But the prop curve is not linear. At 1500 rpm when properly propped you're at ~20% of available power. Being over propped to the degree you are might bump that up to 25-30%. Still well within operating specs. In fact probably better for the engines to be loaded up a bit more at that speed.

Have a look at the propeller demand curve and HP curve and this should be clear.

 

[Comodave]

Good point Dave, my original analogue gauges of a boat the same age as OP’s are suffering from resistance issues and inaccuracy’s are across the helm dash.

Yes, photo tacks are easy and cheap. If you have diesels I think you should have one in your tool box.

 

[Southern Boater]

The notion that this is a problem across the range of operation is nonsense. It's only a problem as you near WOT. If you never go there it's never a problem.

Over fueling happens when the demand on the engine approaches its maximum HP/torque available at a given rpm. That's not good. But the prop curve is not linear. At 1500 rpm when properly propped you're at ~20% of available power. Being over propped to the degree you are might bump that up to 25-30%. Still well within operating specs. In fact probably better for the engines to be loaded up a bit more at that speed.

Have a look at the propeller demand curve and HP curve and this should be clear.

This has made me feel better about my own arrangement, the 3208NA, non turbo-ed, WOT on my (potentially) resistance effected helm gauges indicate 2300 rpm. Never cruised below 1500, and up to 1800 while in beam seas to reduce roll.
Original owner over propped apparently to load the engines at the lower revs.

I will check with the device comodave recommends though.

 

[tiltrider1]

I share much of Jeff F’s opinions.

As to 3208 head gaskets. They generally fail due to begin overheated. I don’t believe the Starboard engines head gasket issue is related to being over propped. At this point we don’t even know if your throttles are adjusted to give you WOT. You might not be over propped. You might just be dirty bottom and over weight.

 

[rslifkin]

If the bottom is significantly dirty, that's costing both speed and rpm. Get it cleaned up, try again and see how much rpm you gain. I'd bet that a 2 inch repitch would get you plenty once the bottom is cleaned. Between getting the engines to full rpm (giving a bit more power) and reduced bottom gunk, you'll likely gain a could kts of speed.

 

[Delfin]

While I don't think slight over-propping of a trawler that typically runs at cruising speeds is a problem for the engine, there are other benefits of correct propping. Including lower idle speed, lower exhaust gas temps, and optimal fuel economy. I've been through 2 pitch changes on my single 44" wheel and verified that the above is true, at least for my boat.

 

[rslifkin]

While I don't think slight over-propping of a trawler that typically runs at cruising speeds is a problem for the engine, there are other benefits of correct propping. Including lower idle speed, lower exhaust gas temps, and optimal fuel economy. I've been through 2 pitch changes on my single 44" wheel and verified that the above is true, at least for my boat.

Depending on the engine you can also gain better responsiveness to throttle changes in tough situations like a bar crossing. An overpropped engine often won't spin up as fast if you give a sudden burst of throttle.

 

[Jack2427]

On my old boat, an Atlantic 44, I had 3208 NAs. They were spec'd to run WOT at 2800. The props that came with the boat were 25 X 22. They were perfect. I suspect your boat will be perfect at 26 X22, 2" pitch change is absolutely doable. the new pitch will also give you much better handling at low speed as you will rev up faster.
Your OA 42 and the 44 are about the same weight(give or take loading) and have similar hull forms. I would bet that the same prop pitch will work just fine.
As for used props there are prop stores that carry them, and sometimes on line prop shops will list inventory. If you are going to cruise, you should have an extra pair. This will ensure that you will never need them, but if you don't have an extra set, you will most assuredly need them, this is an immutable law of the sea.......

 

[mpstan]

The notion that this is a problem across the range of operation is nonsense. It's only a problem as you near WOT. If you never go there it's never a problem.

Over fueling happens when the demand on the engine approaches its maximum HP/torque available at a given rpm. That's not good. But the prop curve is not linear. At 1500 rpm when properly propped you're at ~20% of available power. Being over propped to the degree you are might bump that up to 25-30%. Still well within operating specs. In fact probably better for the engines to be loaded up a bit more at that speed.

Have a look at the propeller demand curve and HP curve and this should be clear.

Jeff F, thank you for introducing prop demand curve to the discussion. No one has brought this up in all the times I’ve tried to discuss this. I’ve read quite a bit about this and have made some conclusions and observations:

1. I don’t know my prop demand curve, but I know that it decreases much faster than the hp curve as I reduce RPMs.

2. The hp and prop demand curves intersect at WOT; in my case 2400 rpm.

3. At 1400 rpm, my engines are delivering a relatively small amount of the total available horsepower, but using the correct amount of fuel to do so. BIG REVELATION FOR ME: If I add weight, or let my bottom continue to fowl, at 1400 rpm the engines would simply use more fuel to deliver more horsepower—- using more fuel, but maintaining speed, without any harm to the engines

4. Reducing the pitch in my props as much as is practical is still a laudable goal, but not for the reasons I originally stated— will provide negligible benefit at lower RPMs regarding engine health. Will allow me to run faster

5. Assume I run at 9 knots at 1400 rpm, and after repitching I run at 9 kts at 1500 rpm….. the engines would deliver identical horsepower, and burn the same fuel….. correct?

6. Because my Cats have quite a lot of reserve HP at low RPMs, I would not expect to see any overheating;l (and I’m not); furthermore (as people have noted) I need to look elsewhere for head gasket issues with previous owner

I don’t think I need a sea trial to know what to do here. My boat survey documented WOT of 2500-2600 half loaded. I think what I’ll do is pull the props when I haul out this Spring, and have them reduced as much as they can— but probably not from 26x24 to 24x22. I’ll. Keep putting at 1400 or so and run them up to 80% of whatever my new WOT throttle is to clean them out routinely.

Do I have this pretty much right?

 

[Jeff F]

Do I have this pretty much right?

Bingo. When cleaning out, 400 rpm below max achievable rpm is safe, and should get things hot enough.

 

[sunchaser]

What does the Cat expert, David, on boat diesel say? That would be the go to advice I'd recommend listening to. There is zero harm to being propped correctly. There will always be a question if significantly over propped.

By the way, in addition to checking tach accuracy, insure your EGTs are in working order. Doing a run up to max RPM would provide some helpful temperature data.

 

[motion30]

If you are running your boat at trawler speeds, 7 to 9 mph 100% of the time, maybe the props Were purposely selected To load the motor at low RPM and getting the Water the temperature To operating temperature. I have the same motors and this is the case on my boat. The downside is you cannot run full RPM for any length of time

 

[SteveK]

If you are running your boat at trawler speeds, 7 to 9 mph 100% of the time, maybe the props Were purposely selected To load the motor at low RPM and getting the Water the temperature To operating temperature. I have the same motors and this is the case on my boat. The downside is you cannot run full RPM for any length of time

I agree with most of this and only ask for clarification of the italics. You do not run at max RPM whatever that is because that is not the purpose of WOT.

OP stated 2500-2600 max RPM and specs call for 2800, when the boat is moderately loaded and bottom clean, then it is over propped.

But, is there anything wrong with that? Diesels like to be loaded/worked. If the prop is tuned for WOT then the lower end is not loaded perhaps as much as it should be. If temp and oil pressure are not lower than suggested, what is the issue if 1400 is where the sweet spot is for this boat running most of the time.
We all talk about travelling a trawler speed in boats designed for speed.

 

[motion30]

I agree with most of this and only ask for clarification of the italics. You do not run at max RPM whatever that is because that is not the purpose of WOT.

OP stated 2500-2600 max RPM and specs call for 2800, when the boat is moderately loaded and bottom clean, then it is over propped.

But, is there anything wrong with that? Diesels like to be loaded/worked. If the prop is tuned for WOT then the lower end is not loaded perhaps as much as it should be. If temp and oil pressure are not lower than suggested, what is the issue if 1400 is where the sweet spot is for this boat running most of the time.
We all talk about travelling a trawler speed in boats designed for speed.

I believe those 320 hp motors Have a recommended cruise speed of 2200 rpm. On my boat that consumes about 18 gallons per hour per motor at 13 miles an hour . Back of to 1400rpm. Speed around 8 or 9 mph. Burning 5 gph total. . For me it is not enough increase in speed to justify the additional fuel consumption

 

[Greg QS]

over propping is a cooling problem

Guys there is tons of info out on this. The boat is close but needs fine tuning. Generally 1" of pitch for 100 rpm decrease. maybe shave a 1" in diameter and 2" change will get you there.

OVERPROPPING IS A COOLING PROBLEM

OVERPROPPING = OVERHEATING = Engine failure. Its simple!


Engines turn props and props move boats. On mechanical engines the only way to know your of the engine is by the full rpm test. If you do not reach wot+ you are under cooling your engine through the full power curve. Even at idle.

Lets try a different approach to simplify the explanation of this. no charts or graphs , No power curves not math.


This is a big deal. When you are over propped you are INTENIONALLY making the engine to do what it is not capable of doing and it will Break! IT CANNOT COOL IT SELF.


1 you need to match the prop to the engine and the boat to get max rpm + ( The reason for + is to compensate for extra loads and bottom fouling) Say a cooling safety factor. Always better to be under propped than over propped.

2 When you are "over Propped" you are running the engine with an improper fuel/air /cooling mix.

3 Running and engine with this wrong COOLING mix, will at any rpm raise the exhaust gas temptress at every rpm (HEAT) . At lower rpm not a big deal but, in the mid to upper rpms things get out of control fast.

4 Heat Kills! Your engine is designed to run at a certain rpm, on a certain fuel COOLING mix to get a specific HP. The engine is very specifically engineered to remove the heat of the exhaust through, sea water cooling, oil, and Airflow through the engine. Yes turbo input and exhaust are part of the COOLING system. Over propping changes the air flow COOLING, water cooling, and oil cooling. You are now outside the cooling ability of the engine.

5. If you run your boat over propped you are destroying your engine. You are not COOLING your engine properly. Not maybe! You are! You are asking all the parts in the engine to run hotter than designed for., all the time. Your are melting the internal parts of the engine. your are cooking and hardening gaskets that will fail, You are thinning and breaking down you oil. Your engine will fail. not maybe but when.





I'm sure you head gasket failure was 100% the result over propping. Your engine got too hot.

A quick word on UNDER Propping . EXTRA COOLING

When and engine is under propped say 1-200 rpm you have a cooler running Engine at all rpms. You have a turbo that kicks in earlier and generally matches the hump better. You have extra cooling capacity of the engine at any rpm.

Extra COOLING ? What does this mean? (for the post 200 rpm below max rpm)
It takes a certain amount of fuel to make a certain hp. Lets say you make 300 hp at 1500 rpm. You have HP and heat. You have the cooling volume of the air extraction ( wind through the engine), certain volume of oil cooling, a certain volume of water cooling. If you make the same 300 hp at 18oo rpm you simply have more (200 rpms more) of air, oil, and water flow to cool the engine for same heat of 300 hp.

Think of Under propping is a safety factor. to combat a ding a prop , bottom gets dirty, you add weight to the boat you will still be withing the design cooling capabilities of the engine.

 

[SteveK]

But what if an over propped boat does not overheat. Each engine has specs for operating temp range. How does that fall into your lengthy explanation?

 

[rslifkin]

But what if an over propped boat does not overheat. Each engine has specs for operating temp range. How does that fall into your lengthy explanation?

Even if the coolant temps aren't high, you can't tell for sure if you're causing thermal stress and running it hard enough to be a concern unless you have EGT gauges.

 

[SteveK]

Even if the coolant temps aren't high, you can't tell for sure if you're causing thermal stress and running it hard enough to be a concern unless you have EGT gauges.

if the EGT is too hot then the cooling system is not doing the job. But really, throw in another monkey wrench because of a belief that you should not over prop.

Over propping in itself is not the problem. Boaters have been doing it for decades. But there are two schools like many things, those that have done it with success and those that have not done it at all opposed to it.

 

[mpstan]

Guys there is tons of info out on this. The boat is close but needs fine tuning. Generally 1" of pitch for 100 rpm decrease. maybe shave a 1" in diameter and 2" change will get you there.

OVERPROPPING IS A COOLING PROBLEM

OVERPROPPING = OVERHEATING = Engine failure. Its simple!


Engines turn props and props move boats. On mechanical engines the only way to know your of the engine is by the full rpm test. If you do not reach wot+ you are under cooling your engine through the full power curve. Even at idle.

Lets try a different approach to simplify the explanation of this. no charts or graphs , No power curves not math.


This is a big deal. When you are over propped you are INTENIONALLY making the engine to do what it is not capable of doing and it will Break! IT CANNOT COOL IT SELF.


1 you need to match the prop to the engine and the boat to get max rpm + ( The reason for + is to compensate for extra loads and bottom fouling) Say a cooling safety factor. Always better to be under propped than over propped.

2 When you are "over Propped" you are running the engine with an improper fuel/air /cooling mix.

3 Running and engine with this wrong COOLING mix, will at any rpm raise the exhaust gas temptress at every rpm (HEAT) . At lower rpm not a big deal but, in the mid to upper rpms things get out of control fast.

4 Heat Kills! Your engine is designed to run at a certain rpm, on a certain fuel COOLING mix to get a specific HP. The engine is very specifically engineered to remove the heat of the exhaust through, sea water cooling, oil, and Airflow through the engine. Yes turbo input and exhaust are part of the COOLING system. Over propping changes the air flow COOLING, water cooling, and oil cooling. You are now outside the cooling ability of the engine.

5. If you run your boat over propped you are destroying your engine. You are not COOLING your engine properly. Not maybe! You are! You are asking all the parts in the engine to run hotter than designed for., all the time. Your are melting the internal parts of the engine. your are cooking and hardening gaskets that will fail, You are thinning and breaking down you oil. Your engine will fail. not maybe but when.





I'm sure you head gasket failure was 100% the result over propping. Your engine got too hot.

.

Greg Q I’ve heard all of this before. Not sure if you read my comments mid thread but your statements represent the 80% I referred to. I was right there with you until I started reading about prop demand. Math and graphs are useful tools and don’t scare me, and there is a lot of math and physics going on here. So we are going to have to use them

Comments/questions:

Out of 640 bhp available from my Cats, what would you guess they are transferring to my props at 8 kts, which for me is 1350 RPM? 150 hp? 200 hp? Do you really believe my EGTs or water temperatures are being stressed at this RPM? if you do, how can you reconcile a prop demand curve, plotted over a horsepower/Rpm curve? Put another way: it’s been suggested on this thread that engine stress and overheating doesn’t occur until prop demand approaches available power (WOT). Further, if my prop demand for hp at 1350 rpm is 30% of what my mains are capable, it’s been suggested that all the talk about this causes “lugging” or overheating is simply unfounded. Thoughts?

I used to believe putting caused the gasket issues on my stb engine, but not anymore.

We all most definitely talk a lot about running our big engine boats at 7-8 kts. Why do we run them up to 80% load—- to escape storms, tidal currents, or because we’re running late? Nope. We do it only to clean them out.

I’ve heard from folks suggesting I reprop to make 2800rpm on one extreme, to leaving them alone on the other. At this point I’m torn between just leaving things be, and taking just 2” of pitch off. Thanks everyone for your input