S195 Fuel Injection Timing Proceedures

[rcavictim]

Bob,

That was really good to learn about 'critical speed'.  I experience it on my VW plant at very low RPM.  Now I know what that is.  It manifests itself as a very horrible vibrational shaking of the entire machine like the engine is gonna come apart.

[veggie]

Bob,

That was really good to learn about 'critical speed'.  I experience it on my VW plant at very low RPM.  Now I know what that is.  It manifests itself as a very horrible vibrational shaking of the entire machine like the engine is gonna come apart.

Wrong ! That's just how VW's run. It;s normal. 
Kidding.... ( I have a VW diesel)

veggie

[mobile_bob]

RCA:

thats exactly how it manifests itself,

i remember studying about it in the engineering text years ago, but never gave it a lot of thought, until i experienced it on my first
ferry boat ride out here in seattle 20 odd years ago, the whole ship shakes and vibrates like it is going to come apart for a second or so
but feels like minutes to someone that is accustomed to what a diesel should sound like.

it wasn't till today writing about it here, i realized i have heard that going way back in locomotives when they startup from idle
they too sound like all hell is coming lose.

trucks do it as well, usually at very low rpms, if you miss a gear and overshift into to high a gear, the engine lugs down and you get
that familiar heavy vibration that tries to shake your teeth out of their sockets.

when i feel that i am reminded of the text books warning about, "if you must operate an engine within its first critical speed, get through it
to a higher rpm as quickly as possible and do not operate under load at this critical speed if it can be helped"

the ferry boat has to as it accelerates the prop as it moves away from the dock, but
a good captain will accelerate quickly through the critical speed as evidenced by the first puffs of black smoke and once
clear of the vibration he will slow the acceleration somewhat to limit more heavy smoking as he continues to increase rpm.

thinking back on this i believe the locomotive engineers do the same thing, you get the initial puff of black as they push it through
critical speed as quickly as possible and then slow the acceleration rate to limit the amount of smoke.

that is those operators that are concerned with putting out clouds of smoke anyway

bob g

[quinnf]

With the ferries (are we still allowed to call them that?) there are a couple of other issues in play.  I wish Shipchief would show up; he knows all about this stuff and can talk about it more intelligently than can I.  When power is first applied to a propeller, until smooth water flow is established past the hull in front of the propeller, the propeller tips cavitate, momentarily spinning in voids of vacuum as the water is physically ripped apart, then the voids collapse and water slams back into the propeller with enormous force, which is felt as vibration.  The force of these impacts is so great it can actually erode pits into the leading edges of propellers, which only makes the problem worse.  I suspect the auto ferries are in service for so long, their props probably get pretty beaten up before they're reworked or replaced.

There is also a bearing called a cutlass bearing that supports the prop shaft just ahead of the propeller.  All of the radial loads the heavy propeller generates from imbalance, or while the ship is maneuvering, and from every submerged deadhead a propeller blade bonks into is transmitted to the hull through this bearing, causing it to wear.  After a while the shaft is said to "shiver" in the bearing, and that is what I think you're feeling on the ferries when running at constant speed.  So I bet in the case of ferries, it's actually the propeller shafting, or drivetrain, that is generating the noticeable vibration, rather than the engine.

That said, the engine close-coupled transmission will likelly have its own critical speed, the power train components their own critical speed, so when starting up from the dock there are probably only a few "sweet spots" between critical speeds that the ferry skipper can choose.

Quinn

[mobile_bob]

Quinn:

you may very well be right, but i seem to remember the heavy vibration before the outdrive is engaged while they are spooling up the
engine.

however i do recall the vibration from the screw cavitating as well.

guess it is time to go take another ride on a ferry,, boat that is,, thought i better clear that up before someone made a crack,, er joke

awe forget it!



now i know i have heard similar when a locomotive comes up off idle, it goes through an incredible rough rpm where it shakes the tracks, ties and surrounding ground heavily.

according to the engineering texts, anything that rotates has a critical speed, obviously some rotating assemblies are worse than others.

bob g

[quinnf]

Bob,

I know I'm right.     That's why I said the engine/transmission likely have a critical speed, then you engage all the shafting with that heavy propeller hanging off the end and you've got at least one other critical speed.  Couple them together, and there's a whole lot o' shakin' going on!

And come to think of it, don't the ferries up your way have props on both ends so that one pulls, while the other pushes?  If so, then there is an awful lot of rotating mass available to vibrate.

Sure wish Shipchief would decloak.

q.

[Henry W]

Hello Guys,

quinnf,
After thinking about what you wrote it makes sence. Just recently I purchased a small Evinrude 4 Deluxe outboard and I made an emergency turn to miss a log at full speed on my jon boat and the boat slowed down and I heard the engine load up and then the prop cavitated. It was awfull!! I thought I broke a shear pin or shreaded a gear in the gearcase. I lowered the rpm's and the cavitation dissapared. The vibration from cavitating was unreal.

The prop I use is a pusher prop for heavy loads like displacement hulls that sailboats use. Even though it is only a 4hp outboard it can push more than the 6 hp yamaha outboard I had. I was surprised the little 4 hp had enough power to cavitate the pusher prop. I tried the standard pitch prop on my boat and the engine had problems moving the boat to the speeds it can go with the pusher prop.

I guess if you have enough hp, torque and the gearing is right you can cavitate any prop.

Henry

[vdubnut62]

Uh.. Ahem.. Easy on the VW stuff please!

Reference the moniker of the present poster.

Just kidding, the things WILL just about shake your teeth out, and mine had silicone filled engine mounts!
Some of the old Rabbits had a big lead weight inside the front bumper that was mounted on springs to try and soak up some vibes.
Ron

[rcavictim]

Uh.. Ahem.. Easy on the VW stuff please!

Reference the moniker of the present poster.

Just kidding, the things WILL just about shake your teeth out, and mine had silicone filled engine mounts!
Some of the old Rabbits had a big lead weight inside the front bumper that was mounted on springs to try and soak up some vibes.
Ron

Ron,

My genset is using a small 1.5 litre NA diesel from a 1980 Rabbit.

[vdubnut62]

Well rcavictim, have you got a 10lb lead weight mounted to the frame on leaf springs so it can flop around?
Also, do you have the hermetically sealed, lead lined, time lock equipped vault ready for my @#$%# little sticker?
I would really hate for the thing to get damaged!