Overpropped

Rain Man

Super Anarchist
7,754
2,486
Wet coast.
Yes.

Yes to 2" pitch reduction.

Are you positive about gear ratio (looked at the plate on the gearbox?) 2.62 is also available on those transmissions

Ask Propspeed or other supplier for appropriate solvent to remove. If it's silicone based there are silicone removers. Turns it sort of chalky and you can brush off.
Thanks for the help, you can tell I haven't owned a diesel before.

Good question about the ratios.  This is the plate on the transmission: 

20200921_141445.jpg

Unless they changed the ratio internally (is that possible?) it should be 2.21.  I am still mystified how they got this wrong. 

 
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Zonker

Super Anarchist
10,915
7,491
Canada
Yeah you could change the gears internally but very unlikely. 

Far more likely the guy inputting the data into the computer form to predict the prop diameter/pitch just entered one item wrong. Not enough to give a crazy result, but enough to give wrong results.

 

longy

Overlord of Anarchy
7,447
1,571
San Diego
Meant to copy BobJ's comments
If you're over loaded in normal mode, overdrive would be way over. Gori does not state how much pitch one gains spinning the blades backwards. So if the prop is correctly sized to let the engine just hit max RPM, then O/D must be used with an understanding of engine loading/fuel supply. All Gori says is "same speed at lower RPM". No hint of possible engine damage (of course). So a operator could easily engage O/D mode & firewall the throttle expecting 4th mode speeds, and if they do not know/don't care/can't see exhaust (& don't know how to read it) can damage engine. RTFM doesn't work, it's not in the manual. Even the Yanmar manual doesn't talk about smoke colors, so how does the average boat owner learn? the hard way, after paying the mechanics a few $$$$. Yanmar dealers/mechanics are stressing this quite a lot, lately. Must have had some expensive warrantee claims.

 

slug zitski

Super Anarchist
7,495
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worldwide
Just tried out my new folding prop in the last two days.  I am seeing classic signs of overpropping with a 2-blade 16x12 which was the prop manufacturer's recommendation.  Engine is rated max continuous 3400, max 3600, and I am getting 3000 rpm in flat water with a bit of wind behind us.  Above 2700 rpm the black smoke starts, and by 3000 unburnt fuel is puking out the exhaust.  The boat is moving a bit fast for docking at idle as well.

The question is, do I drop an inch, or two inches to fix that?  Try it and see?  Sadly, the blades are fixed pitch, but I think the manufacturer has a blade exchange program for this situation.   Any help appreciated.  
Prop pitch is a related to exhaust temp  

too much pitch , high exhaust temp

with a variable pitch prop it’s simple to constantly adjust prop pitch to optimize exhaust temp as load conditions change .... upwind , seaway , heavy load  , Towing ....

with a fixed prop you must compromise and choose a prop that allows  the engine to run at a lower exhaust temp ..full rpm and cruise rpm ..to compensate for adverse conditions , overload condition with higher exhaust temp

Borrow a pyrometer from your engine shop and do some test 

Plugged Pyrometer insert shown poking thru insulation on the hot  engine exhaust leg

1FAA9C78-722D-460E-A9FE-A36B0C75F2BA.jpeg

 
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Rain Man

Super Anarchist
7,754
2,486
Wet coast.
Just realized today that the fixed prop we took off was a 16 x 10.  It was pretty messy when we took it off the boat and didn't clean it up until well after the new prop was installed.  Shit.  If we had done things in the right order the alarm bells should have gone off a lot earlier.  The old prop is a nice shade of de-zinced pink in the right light.... 20201113_155947.jpg

 

Rain Man

Super Anarchist
7,754
2,486
Wet coast.
Update:  the prop manufacturer has accepted responsibility, so all is good. It was a data entry error.

 
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batkinmok

New member
34
3
The Ocean
You should be able to get Max RPM tied in the slip...not with a favorable breeze.
^This. Indefinitely, without a smokeshow. Anything else and youre gonna start breaking stuff right when its most inconvenient, while trying to make those last couple miles to harbour against 30 knots and a steep sea..

 

longy

Overlord of Anarchy
7,447
1,571
San Diego
^This. Indefinitely, without a smokeshow. Anything else and youre gonna start breaking stuff right when its most inconvenient, while trying to make those last couple miles to harbour against 30 knots and a steep sea..
Um, no. A tied down boat will put more load on the system than a moving boat. If you can hit max RPM  tied up you are underpitched a bit

 

El Borracho

Barkeeper’s Friend
7,220
3,149
Pacific Rim
Um, no. A tied down boat will put more load on the system than a moving boat. If you can hit max RPM  tied up you are underpitched a bit
Yes, on a 26 knot powerboat. However the difference between being tied in a slip and grinding into a gale and monster seas is minuscule for a sailboat. The tach error is likely a bigger difference.

Prop slip in the slip is not 100% as the water circulates around the prop. The load may well be identical to slogging at 2-3 knots. When a gale promises a several hour slog it is the wrong time to dive down to change the prop pitch, or dial back the engine speed.

Besides, what is the harm in foregoing one or two horsepower? Most boats these days are overpowered. Longer life will be the prize.

 
Um, no. A tied down boat will put more load on the system than a moving boat. If you can hit max RPM  tied up you are underpitched a bit
Not getting this, maybe a little slow.  If you can hit full rpm at the dock then it would seem you are within a decent load curve. As you state it puts more load on the system or am I missing something. Unless you are referring to being right at the line at full RPM on a seaway being less load so at the so at the dock you should not be able to get full rpm?  Like I said a little slow.

 

IStream

Super Anarchist
11,023
3,207
FWIW, I'm in the process of re-pitching my Max Prop so I've done this experiment. My engine can hit 3400 rpm making about 8 knots but when I'm tied up in the slip, I have trouble getting past 2500 kts.

 

longy

Overlord of Anarchy
7,447
1,571
San Diego
Not getting this, maybe a little slow.  If you can hit full rpm at the dock then it would seem you are within a decent load curve. As you state it puts more load on the system or am I missing something. Unless you are referring to being right at the line at full RPM on a seaway being less load so at the so at the dock you should not be able to get full rpm?  Like I said a little slow.
^^Note the RPM discrepancy Istream has measured. Now, I don't know enuff about propeller physics to competently explain it, I just know it happens. And different engine/trans/prop will yeild different deltas. Bit when testing a prop, all prop companies want the final test to be flat water, no breeze, clean hull. Engine loading under foul weather will depend on some other things: how much is prop being thrown about due to hull movement? Size/steepness/angle of waves hitting the boat will all change prop loading.

Knock on wood, I've never been in a situation that required the utmost out of my power train. The few times I might have had that scenario I've been able to: stay home/go real slow/go somewhere else

 

Rain Man

Super Anarchist
7,754
2,486
Wet coast.
One of the many reasons we switched from fixed to two-blade folding was the annoying noise the rotating fixed prop made while sailing.  We had been told to leave the transmission in neutral when sailing or risk damage to it.  The transmission had been recently replaced before we bought it.  

Since we have to to haul again anyway to change the prop blades (at least) we decided to upgrade to a three-blade folder instead.   We noted some vibration from the two-blade folder and research suggested that is common and going to a three-blade will generally fix it.  Note that we will also replace the cutlass bearing when we haul to put the three-blade on. 

The question is, with the three-blade, will we still be able to keep the transmission in gear to stop the rotation and annoying noise?  I was pretty confident we could do that with the two-blade, on the assumption that the torque on the shaft from water flow around the closed blades would be minimal, but I am a bit less confident with the three-blade.  My understanding is that most three-blade folders do try to rotate on an angled shaft.

Any help from the collective expertise here is appreciated.

 
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IStream

Super Anarchist
11,023
3,207
Once the folder is folded, whether 2 blades or 3, it shouldn't be exerting much if any torque on the shaft. It therefore shouldn't matter if you keep it in neutral, it shouldn't spin the shaft.

 

Ishmael

Granfalloon
58,549
16,367
Fuctifino
Our Max Prop instructions say to leave it in reverse to keep it fully feathered. Seems it would keep a folder tucked up better as well.

 


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