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TAMD 41B problem

Z

zetec7

Member
Boat is a 1991 Prowler 26' cruiser with a single 200hp TAMD 41B, Volvo Penta 260DP drive. Total of 850 hours on the boat from new.

Scenario:

- boat was running great 3 weeks ago when last used. Cruised easily at 20 kts @ 3450 RPM.

- Today, started fine, ran smooth.
- Took on fuel, headed out and throttled up.
- Engine running smooth, no vibration, but huge amounts of black sooty smoke from exhaust, and a thick trail of oily sludge floating in our wake. Due to the volume of smoke, my wife thought we were on fire.
- Smoke and sludge disappeared at lower throttle settings, but re-appeared again if throttle (& RPM) increased
- Unable to exceed 2500 RPM, so no turbo spooling.
- Considered lack of air, so removed filthy air filter (which was not, apparently, replaced at last service, although I paid for it)
- No improvement on removal of air filter

History -
- Fuel pump removed & fitted with all new parts approximately 4 years ago (probably 40 hours of running time ago), due to poor servicing (at fuel filter replacement, mechanic put Racor filter together upside down - seawater, previously filtered out of fuel, sucked into pump). Boat returned to us with new (not required - only 2 hours on existing new clutch) cone clutch, improperly installed, jammed in gear. *Three months to complete repairs.*
- Leg completely rebuilt 15 hours ago (new shafts, seals, gears, bellows, u-joints, clutch, etc. - only the housing re-used - due to sudden seal failure, allowing all gear oil to be pumped into the sea), plus new inner transom plate & exhaust. Again, due to poor service, boat was returned to us without shifting capability (shift cable jammed & kinked during engine removal/replacement). Another *three months to complete repairs*

At this point, I have no idea what could be causing my current problem. Unfortunately, the ONLY Volvo Penta service center around is the one that's done the poor service recently (in the last 3 years, I've spent about $15,000 in repairs there, with only 15 hours running time before another major problem).

I REALLY need to have a good idea what might be wrong before I have it taken back there again, as I need to search for another mechanic (any mechanic), and need to have some idea what is wrong!!

HELP!! Please!!
 
By the way - while searching (desperately) I just found another thread by someone with a TAMD 41 who says his turbo has stuck from lack of use a couple of times. Apparently, he gets the heavy sooty smoke, no turbo spooling, and heavy oil residue on the water. He says he can just remove the air filter, reach in & free up the turbine with his fingertips, and all is well for another season. Another post in response said the same thing occurs on his turbo diesel truck that is used infrequently.

I suppose a stuck turbo could cause these problems, right? - very little air getting in for the amount of fuel used at higher throttle settings.

By the way, my engine runs fine (smooth, no smoke etc.) at low throttle settings (say, up to 1500 RPM or so - more than that, and it starts to smoke, increasing with the throttle setting).

Does it seem likely that this could be my problem? I'm clutching at straws here!!
 
Obviously dont stick your hand into turbo while engine is running! With engine off see if you can turn the turbo blades on compressor side from behind the air filter. If its stuck, use a socket on the wheel nut to turn it gently back and forth to free it up.

Black smoke is unburnt fuel, so its either dirty bottom, fouled/damaged/spun/wrong prop, stuck/bad turbo, or injector(s) stuck open.

You need to learn how to work on your own motor, local dealership sounds like bunch of clowns. I am sure you would have to try extra hard to mess up more then they did. Doing your own work is very rewarding, and way cheaper. And it will give you confidence out on the water knowing the engine/drive.
 
Yeah, it sucks, that's for sure. Funny, I rebuild motorcycle engines & car engines, am fabricating a car from scratch, but working on my boat is difficult for me - partly because it takes a month's lead time to get it hauled out, but more because I'm just not familiar with diesels...so it seems like a black art.

You're so right, though - I need to make the "leap of faith".

I WILL try your suggestion re: the gently rocking with a socket, and thanks! The more I reason this out, the more likely it seems that this is the problem. I've tried to eliminate all of the other issues you mentioned, and as it still runs great at lower RPM's (smooth, good power, no smoke) I don't think the injectors are a problem at this point. The props (DuoProp leg) etc. are all the same as when I used the boat about 3 weeks ago - I don't think anything's changed there. My first instinct was that it was running massively rich - even though it was smoking like crazy, it was still running smooth - it just wouldn't climb to boost range (2600+ RPM). It makes sense to me that if the only air getting into the engine had to pass by a stalled turbine, this would happen.

I will respond with the results! I seriously hope I don't have to replace the turbo, as I'm about broke now.

By the way - any idea what size socket fits the nut on the turbine shaft?

Thanks!
 
I am also having turbo issues right now. I would suggest finding a turbo shop to install new seals and bearings in you turbo. Volvo does not offer the turbo shaft, and that's when things get expensive. The kit is less than $100 and takes less than an hour to install. However, it must be done by a shop that can balance the shaft. I think the nut on the turbo is a 10mm. I took apart two turbos and both shafts were worn, causing oil to leak into the engine and exhaust.
 
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Thanks, Glen. If the freeing-up fix doesn't do it, I'll start shopping for a kit. It's hard finding part numbers for some of this stuff - it doesn't help when Volvo doesn't even supply numbers!
 
At lower RPM engine always runs good. Even if your raw water cooling was 90% blocked it would not overheat at lower RPM. However, when diesel engine gets loaded, every blemish in the system(s) will show up very quickly. Same with black smoke, there is just not enough cylinder pressure for flaw to manifest itself, until you try to plane the boat out (placing load on the engine).

Buy VOLVO Service Manual for your motor RIGHT NOW, for 95% of the service and checkups you do NOT need boat out of the water. In the meantime download the User Manual (http://www.softrocker.ca/manual.pdf) and learn how to use it and what to look out for.

Report back what you find. Photos always help as well...
 
Sounds a lot like a stuck turbo. It is a very common issue in a marine environment due to soot/corrosion inside the exhaust housing. Often you can just free the blades with your fingers on the intake (compressor) side, but sometimes you might need a socket on the end of the shaft. After you do this, it may stick again next time it is left idle for a while. I would suggest once it is free you give the boat a good run at WOT to try and help burn out any build up. If it doesn't move pretty easily with light finger pressure, it is probably best to remove the turbo and clean the housing rather to try and force it. If you do this, check the vane tip to housing clearances as with 850hrs on that engine you might find that it it best to get the housing replaced if you have the turbo off. They generally do not have a wonderfully long life span in recreational use. Because this is such a common problem there are people around who make a business out of putting improved quality (CNC shaped) inserts into the old Volvo exhaust housings. This gives you a fix for a fraction of what Volvo will want to charge you for the new turbo that they will usually tell you is needed.

Final comment is that it may not be a stuck turbo at all, but a blown hose connection. Check any of the rubber connector hoses that are between the compressor side of the turbo and the intake manifold. You might need to remove them to check them properly. A blown hose might give similar symptoms, and this is another very common Volvo issue (just replaced 4 of these hoses last week). If they are blown, try your local auto parts store for compressor hose (not standard rubber water hose). Usually around 1/3rd of the Volvo cost and if you get the good stuff it will probably last longer as well.

Your problem is probably the turbo being stuck from what you describe, but if it is already free, check the hoses next.
 
I was researching turbo information on the net and was puzzled until I figured something out. My engines started out as TAMD40As or TMD40s. Somewhere along the line they were up graded with better aftercoolers etc. Now they are up to TAMD40Bs. I just bought a used TMD40 because I lost oil pressure and scored a few things. Anyhow, I had two turbos that did not work. The part number from Volvo is not clear, but information from www.invasionautoproducts.com shed a little light on the subject. The engines with the aftercoolers should have the 3k K26-2664GA/12.71 (VP#845494). My issue that I have two 3K K26-2470/10.71w (VP#829368). The question is, What is the major difference in the two turbos? Will it affect my engine?
 
aliboy said:
Sounds a lot like a stuck turbo. It is a very common issue in a marine environment due to soot/corrosion inside the exhaust housing. Often you can just free the blades with your fingers on the intake (compressor) side, but sometimes you might need a socket on the end of the shaft. After you do this, it may stick again next time it is left idle for a while. I would suggest once it is free you give the boat a good run at WOT to try and help burn out any build up. If it doesn't move pretty easily with light finger pressure, it is probably best to remove the turbo and clean the housing rather to try and force it. If you do this, check the vane tip to housing clearances as with 850hrs on that engine you might find that it it best to get the housing replaced if you have the turbo off. They generally do not have a wonderfully long life span in recreational use. Because this is such a common problem there are people around who make a business out of putting improved quality (CNC shaped) inserts into the old Volvo exhaust housings. This gives you a fix for a fraction of what Volvo will want to charge you for the new turbo that they will usually tell you is needed.

Final comment is that it may not be a stuck turbo at all, but a blown hose connection. Check any of the rubber connector hoses that are between the compressor side of the turbo and the intake manifold. You might need to remove them to check them properly. A blown hose might give similar symptoms, and this is another very common Volvo issue (just replaced 4 of these hoses last week). If they are blown, try your local auto parts store for compressor hose (not standard rubber water hose). Usually around 1/3rd of the Volvo cost and if you get the good stuff it will probably last longer as well.

Your problem is probably the turbo being stuck from what you describe, but if it is already free, check the hoses next.
Click to expand...

THANK YOU!! That's a whole lot of valuable information. I'm going to print it off & take it out to the boat with me when I go to work on it. Sometimes it's hard to find stuff like this out, and working in the dark is kind of counterproductive. Now I feel like I'm armed with more than guesswork!

I'll reply again once I find out what's going on - I'm sure there will be others in the...same boat...(sorry, couldn't resist!).
 
Well, I just got back from the boat (it's moored a couple of hours away from my home - a little awkward to work on that way). I removed the air filter assembly and had a look at the turbo impeller. Looks good, nice & clean. I tried moving it - it spun free, no issues at all. No endplay, no radial play, just smooth rotation with only air resistance slowing it down. I can spin it with my fingertip, just by touching it.

Apparently, the turbo isn't the problem after all. It spins nicely when the engine is idling, too - smooth, no noise, well-oiled bearings. All fittings to & from the turbo are tight, no corrosion (still have original Volvo Penta green paint), clamps are tight, and there is no leakage at all (I was leaning against the intake side of the engine while it was running), so pipes & hoses aren't part of the problem.

It's still putting through too much fuel. At anything much above idle (in neutral) it starts to produce diesel-smelling smoke in the exhaust. The higher the RPMs, the more smoke it makes.

My buddy suspects fuel injectors, but it seems strange that all 6 injectors would fail simultaneously & let way too much fuel through. Is it possible that the new (around 30 hours on it) fuel pump is pushing huge amounts of extra fuel through??? If that were the case, would that be enough to stop the engine from reaching turbo spool-up speed (2600 RPM - I can only get 2500 RPM at the moment) & put lots of fuel sludge & oil into the exhaust??

I did consider the possibility of a failed seal in the turbo, allowing it to suck out engine oil, but the engine oil is still good, and still right at the full mark, so I guess that's no it.

The fuel quality looks good in the clear Racor filter bowl - no water, and very little crud in the bottom of the bowl. I DID fuel up immediately before all this happened, and added Racor Biocide fuel additive at fuel-up, as I always do. It's hard to see how this could be involved, but at this point I'm stymied. The only V-P service place in the area (the one that did all the shoddy work) doesn't even have a mechanic on today, and couldn't tell me if one would be in this week.

Now I still have this problem, and NO answers at all. I'm suspecting either injectors or fuel pump (I hope not - the last pump rebuild cost me $5K) at this point, but I'm at a loss as to how either of these could cause this.

One other point - the fuel return line from the injectors - if it was plugged, could it cause over-fueling at the injectors, maybe?

I'm clutching at straws here, guys...help!!
 
I feel its a fuel quality issue. If the engine was over supplied with fuel, you would have a run away diesel. Low RPM is a symptom of fuel starvation. Start at the tank and check to see if the pickup tube is blocked. Go from there to the racor and then the secondary fuel filter and check for flow. Also double check air intake. Good luck!
 
If you have had water through the fuel pump the injectors could be quite damaged, but if it is starting and idling OK I would not think this the most likely cause. At 850hrs though it is probably not a bad idea to get them checked anyway.
Your symptoms do sound a lot like either lack of boost or lack of fuel delivery. The most common cause would normally be the stuck turbo (ruled out now) or a split in one of the rubber hoses on the intake. Did you physically inspect these for splits or just tighten the clamps? The splits are often right at the edge of the clamps and hard to detect without removing the hoses. Running the engine out of gear will not often produce enough boost to find the splits by feel/sound either. Can you pick up a boost gauge at your local auto parts store and check the boost?
If you are sure that the turbo is OK the next step is probably the injector pump, but you will probably need Volvo experience for that step. I assume that you have checked/changed all the fuel filters?
 
Firstly, thank you very much for sticking with me on this problem!! I'm determined to get it fixed, and not spend another summer with the boat on the hard (this would be the 4th summer in a row).

On my particular engine, there are no flexible hoses connected to the turbo except for the oil return. All pressure connections between the turbo and manifolds are cast metal (cast iron on the exhaust side, and cast aluminum, I think, on the pressure side) and appear to be tight. The engine is clean, with no signs of leakage of fuel, oil, water, etc. anywhere. The water bubble test did not reveal any leaks, although since I couldn't get enough RPMs to get any real boost, I'm not sure this would have revealed anything. I can't do this test on the underside of the intake manifold/cylinder head joint, though, as it's pretty much inaccessible, especially with the engine in the boat.

I do have a boost gauge, but again, since I can't put a real load on it, I'm not showing any boost. Mind you, it has always needed sustained boost to show up on the gauge. The engine does rev strong & quickly in neutral, but starts making smoke once the RPM's are up - the higher the engine speed, the more smoke, even with no load. With a load, it makes even more smoke, a lot of it, and leaves a heavy trail of oily, bubbly fuel on the water.

It actually looks a lot like when the bottom leg shaft seal went (last year), and it was pumping the leg oil out into the water, although there was no smoke and there was lots of vibration on that occasion. No vibration this time.

From my limited knowledge, it looks like I'm getting either a) way too little air, or b) way too much fuel delivery into the engine at higher RPM's.

My first instinct, of course, was a stuck turbo. I'm assuming that it's unlikely it would spin freely at lower RPM's but not be able to spool up at higher RPM's to produce boost. With manual finger turning, it moves as easily as a ball bearing rolling across glass.

If that is the case, it would appear I'm getting a large amount of over-fuelling at larger throttle settings. Is there any chance that a failure of the "smoke limiter" on the fuel pump could cause this? Or, could one (or more) injectors fail in such a manner as to allow more fuel through than required?

I've found another V-P mechanic, locally, although he's too far away from the boat for him to work on it, but I'll see if I can talk to him today to see if he has seen anything like this before.

Thanks again for sticking with me!
 
Bearing in mind glen_adolph's post above, I hadn't thought of the runaway-engine scenario. If over-fueling could lead to this, it would seem I'd be back to too little air going in, i.e., a turbo problem, with the lack of sufficient air (boost) acting like a brake on the engine, relative to the amount of fuel going in.

Since the intercooler ("charge air cooler", as V-P calls it) is next in line, between the turbo and the engine, has anyone ever come across a blockage in one of those? It's hard to imagine that anything other than seawater could be getting into the air passage, but even if that happened it would probably kill the engine immediately. It still does resemble a major air intake blockage of some kind, though...
 
Hard to imagine that the intercooler could get that blocked. I guess it is possible, but I haven't come across it myself. I would put the boat in the water and check the boost under load if possible. Whilst not so common, you could have a blown manifold gasket etc. If you seem to be getting plenty of fuel, then lack of air flow seems the best place check until the boost etc can be confirmed. After that you need someone who can check injectors and fuel pump. You can also do a compression check whilst the injectors are out in case there is a valve or cylinder problem.
 
The mechanic I spoke to today seems to suspect one or more bad injectors, as it's something that could happen suddenly, like this problem did. As I don't have the "special tool" to remove the injectors without leaking coolant into the head, and the local mechanic won't travel the 60 miles to my boat, it looks like I'm going to be stuck with the service place that's done all the really bad work so far. They don't even service injectors there - they will just remove them and mail them off for testing for me.

The cost of replacing the injectors is likely beyond my means now, as I've pretty much gone broke trying to keep the boat going. This is very frustrating!
 
Just today my staff replaced a Volvo injector with a cracked housing. It was caused by some water in the fuel over-pressurising the injector according to our injector guy. You may have similar issues, but I would expect the engine to idle badly if the injectors were bad. This is when you normally first notice that they are not right any more, and you say that it idles fine? Our one affected the idle, but had little effect on the running at cruise etc. I am really struggling to believe that your engine has suddenly (overnight) got so many bad injectors that it performs like you say unless it has dropped a tip or two, but that should be very obvious at idle.
If it is injectors, get on line and find some used ones. Get them pressure and spray tested and you will save plenty over used ones. The testing procedure is not that expensive unless you need new parts. Our last set of tests worked out around $60 - $70 per injector including some new tips from memory.

Is the boat on the water or on a trailer etc? If it is on the water, have you checked that the hull and prop are completely clean? Are you sure that there isn't heaps of water sitting in the hull somewhere etc etc? Your symptoms sound much more like an overload or lack of air issue than an injector issue, so I would really try to eliminate these things before spending one your fuel system. It may still be the fuel system, but you need expensive (questionable quality?) help for that so really eliminate what you can yourself first. Even though the turbo spins freely, I would really suggest running the boat on the water and checking the boost under load. The most common cause of what you are getting would be a boost problem of some sort. Eliminate that and dirty/damaged hull/props and then you can be pretty sure it is either in the fuel system or the engine has lost compression. At that stage you need help. If you do get the injectors pulled, do a compression test once they are out and before the service guy takes them away i.e. tell him to bring his compression tester when he comes to pull the injectors. Also get him to check the fuel pump timing before he leaves. If the boost is OK, compressions are OK, timing is OK, and the hull/props are good, then you know it is in the fuel pump internals or injectors. Tackle it logically and do the self/cheap tests first. Once you get your head around them these engines are not overly complex, and with your history of poor service experiences I would be getting ready to take on my own servicing asap.
 
I'll try to answer a few questions here, to clarify what's going on.

The boat is in the water all the time (it's a 1991 26' Cooper Yachts "Prowler", a heavy deep-V cruiser with a command bridge, a single TAMD 41B, and a Volvo Penta 260 Duoprop leg). I don't have a trailer, or a truck big enough to haul it (boat supposedly displaces 11,500 lbs).

The boat ran great 3 weeks ago - even though there is a bit of growth on the hull, it still ran up to a shade over 24 kts., which is at (or even 0.1 kts. above) its originally-advertised top speed. As usual, it accelerated okay up to 2500 RPM, paused for a moment, then you could hear the turbo whistle as it spooled up & the boat surged forward, quickly reaching 24 kts. I usually pull it back to around 3450 RPM, which gives a comfortable 20 kt. cruise - smooth & quiet. The boat ran for the whole weekend like that.

The only thing that changed from that trip to the next one (3 week later) was that we took on fuel at our regular fuel dock. It's a high-end place, and very busy, with lots of mega-yachts fueling up there daily.

The next time we ran it (a week ago), all was the same initially, with about a mile run out of the marina area (at 5 kts), stopping for fuel on the way out. I added Racor Diesel Biocide to the fuel, as I always do. When we got out into the channel and started to accelerate this time, however, we noticed the exhaust was unusually smokey, and by the time it got to 2500 RPM, there was a huge could of diesel smoke behind us, and a scummy, oily discharge in the water. Even at that, the engine was still running smooth, but it just wouldn't break through the 2500 RPM barrier where the turbo normally starts to produce boost. We were very worried about the engine, so we turned around and headed back to our slip immediately.

Before returning to the slip, I did consider airflow, and pulled the air filter. It was a little oily (the crankcase breather is right beside the air filter housing), but okay other than that. We tried running it again, with no air filter element, and it was still the same (a virtual Exxon Valdez in the water behind us, and we were trailing so much smoke we looked like a giant comet).

The bilge has about a gallon of water in the bottom (too little to activate the bilge pump), which is normal. There is almost no oily film in the bilge.

I did notice a lot of gurgling noise from the leg area for several seconds when I shut the engine down (sounded like water working its way back into the exhaust elbow), but then, I'm normally not down in the bilge & against the transom when I shut it off, so maybe this is normal. The entire exhaust system was replaced last year, from the turbo out, including the alloy elbow & the exhaust bellows (inner transom plate, too).

As of now, it still starts effortlessly and runs normally, with little or no smoke (at low RPM's and no load). It idles perfectly - smooth, quiet, no vibration at all. It revs well (and very rapidly) in neutral, although even without load, there is more exhaust smoke than normal at higher RPM's.

Temperature is normal (this boat has never had a cooling problem...knock on wood!) no matter whether under heavy load or no load at all.

Revving it in neutral does not produce visible boost on the gauge, although I understand that without load, it won't produce measurable boost anyway. The turbo does spin nicely, even at idle. There is no end play or axial play in the turbo's shaft, and no noise or vibration.

It still seems like a boost (or, at least, air) problem to me, but with the turbo spinning freely I just can't understand what could cause it.
 
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I installed a used engine in my boat recently and was puzzled about the amount of smoke it was producing. I realized the injector pump timing was a bit off. I loosened the three bolts that hold it and turned it counter clockwise. Smoke gone! Check to see if your injector pump inadvertently came loose and turned. It is like a distributer on a gasoline engine.
 
Unfortunately, Volvo Penta TAMD 41B pumps are not so easily adjusted, as I recall. They require several specialized tools to adjust, such as a special dial gauge, flywheel insert/locking tool, etc. All timing adjustments are made inside the fuel pump - there are no timing adjustments externally. But ALL ideas are welcome - keep 'em coming! At least one of the answers I'm getting is going to help me to fix the problem! I just don't know which one yet - but I'm going to try them all, sooner or later, until it's fixed.

I'm going to make the 60 mile trip to the boat again tomorrow to check & clean the props, change both the fuel filters, and introduce some aggressive injection cleaner into the Racor filter housing. I'm thinking that if I can get it as far as the injectors, then leave it there for a while, it may help clean them - on the off chance that I have a dirty injector or two.

When the injector pump was rebuilt a couple of years ago, because the engine was running so well afterward, we didn't service the injectors. The reason for the pump failure was seawater in the fuel, due, again, to both bad fuel and bad servicing - the "mechanic's assistant" who actually tinkered with my boat didn't even read the instruction sheet that came with the Racor filter kit - he assembled the filter upside down & omitted the mandatory spacer, causing the fuel to not only bypass the filter, but to pick up all the seawater and sediment from the bowl (which he didn't clean out while replacing the element) and pass it through to the pump. I'm thinking now that some seawater must have gone through the pump to the injectors, and may have caused some corrosion. It's hard to imagine how the engine could still idle perfectly yet have damaged injectors, but who knows?
 
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Like glen said, pump timing is adjusted by turning the body of the pump, a little at a time. In order to set initial TDC timing, you need dial indicator inserted into the back of the pump, and a drop pin installed into the flywheel housing. However, if the engine runs, its safe to presume TDC timing is very close to being right, and slight adjustment of the pump itself (rotating the body) will change few degrees, which might be enough to clear up smoke.

Injectors (tips) are wear items, and if they get bad fuel they will easily clog or get stuck open. If they are stuck open, they will leak continuous fuel into the cylinder washing the wall lubrication away, and eventually you will burn up a liner. So, get a slide hammer with correct thread adapter, pull the injectors out, and send them to a Bosch shop to get pop tested. That will eliminate or isolate them as a problem.
 
Oh, okay - I understand that now. I'll check today to see if the pump body is loose enough to have moved a bit & gone out of adjustment.

It definitely looks like having the injectors tested will be one of the next tasks on the list.

Thanks again! I'll keep you posted...
 
I wasn't able to get out to the boat today after all. My big beloved dog Toby is dying of spleen cancer, and has been for months. He has good days and bad days, but today was one of the worst in quite a while. He will most likely recover some over the next few days.

He is not in any pain, but today he is exhausted just by the effort of going outside to pee, and I couldn't leave him alone. The end is coming soon, and I need to be with him. My apologies for burdening you all with this, but it helps to talk about it. :(

Wish him luck.
 
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An update - fuel pump & timing, injectors, fuel delivery, filters, compression, etc. all checked & 100% correct.

Brand new turbo fitted, intercooler rebuilt & reinstalled, almost instant boost now, but engine hits a wall at 3400 RPM. Now narrowed down to a restriction in the exhaust inside the leg (previous V-P service dealer used several incorrect parts on the last rebuild, so we have no idea what we'll find inside the leg). Sea trial sheared the prop hubs, naturally, so now I need props too.

My dog is still with me - a little weaker yet, but still happy. It's not time yet.
 
Pretty good, actually. A tiny bit of slime on this occasion, but that's all. Last time the boat ran well (25 kts) it had almost a foot of growth - although that buffs off pretty quickly at 25 knots! Props were in pretty good shape.

I've had them spin hubs before (there's a lot of torque in these motors), and never had one successfully rebuilt. All the rebuilt ones (5, so far, over the last 10 years) have failed within an hour's running time. New seems to be the only answer, but these Volvo-Penta A4 prop sets are getting hard to find, and tend to be breathtakingly expensive. I wish there was a composite set available, but there isn't. I've spoken to V-P spe******ts, who advise that a 3-blade/3-blade will not work well - the original 4-blade/3-blade combo is necessary for proper operation.
 
That's what I thought, too. Seems like a pretty straightforward pathway for the exhaust. The only other thing I can think of is something in the leg's shafts, bearings & gears that causes massive drag internally, only at higher RPM's but, again, I can't think of anything that could do that. Mind you, the same place that did the leg also did the exhaust...

I guess we'll find out soon enough.
 
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