Tores mailbox VIIC and VIIC/41 operation and technical details

[tore]

Don.
Waterstroke. Starting air pressure at 30kg/cm2 is normal for a medium speed engine. The GW engine starting system allows the engine to run on the air (without fuel injection) for a manually controlled time. By allowing the engine to run at full air pressure it picks up speed very quickly.  At a massive waterintrusion filling the engine exhaust manifold, water might enter some of the cylinders and cylindercover inlet and exhaustvalve ducts draining down on the piston top. The compression clearance in the cylinder is 6,6 mm leaving only a very small compression volume in the cylinders during the compression stroke, ( piston at TDC. and all valves shut). Hence at the wrong quick start, the rotating massforces might create an impact between the cylindercover under side and the water on the top of the piston with collateral damages. Some of the engineers were experts in manually controlling the airstart, managing to reduce the starting air and blowing the water out of the open indicatorcocks before the engine picked up speed.
Tore

[Don Prince]

Hello Mr. Tore.

"Hence at a wrong quick start, the rotating mass force might create an impact between the cylinder cover under side and the water on top of the piston with collateral damages."

1. I agree, the cylinder/piston was driving upward in an exhaust stroke with an open exhaust valve, but the water is not pushed out through the exhaust valve port in the head cover because of back pressure; and water is not compressible.

2. I agree, If the start-up is very slow with the indicator cocks open, then this process may clear the flooded cylinders.

3. Was the water stroke primary damage done to the piston or the cylinder cover?

4. What was the function of the Pressure Relief Valve?

Regards,
Don_

[tore]

Don.
At the beginning of the compression stroke a piston moves from BDC to TDC and the exhaust and inlet valves are shut. Prior to this stroke is the inletstroke and the air inletvalve is open creating a suction when the piston moves to the BDC. At this point some of the trapped flooded seawater left in the cylindercover airducts drains into the cylinder and piston top. This seawater reduces or even might eliminate the smallcompression volume (compression clearance 6,6 mm) which might create an impact force on the cylindercover bottom plate by the rotating masses at the air start and you have a waterstroke.
We never experienced a waterstroke so I personally have not seen the damage, but assume the cylindercover bottom and overstressing the cylindercover bolts could a damage.
As on every high pressure vessel a relief valve is installed in the cylindercover. The function is to protect same for excessive high pressure stresses which might occur, like misfire of a cylinder.


Tore

[Don Prince]

Hello Mr. Tore,

"At the beginning of the compression stroke a piston moves from BDC to TDC and the exhaust and the inlet valves are shut."

I understand this part...

"Prior to this stroke is the inlet stroke and the air inlet valve is open creating suction when the piston moves to the BDC."

I understand this part...

"At this point some of the trapped flooded sea water left in the cylinder cover air ducts drains into the cylinder and the piston top."

I don't understand this part???

The sea water intrusion was on the exhaust side of the diesel engine. Therefore, the exhaust inlets in the top cover are flooded, and water may enter through the open exhaust valve and drain into that cylinder on top of the piston.

How does the sea water intrusion get from the EXHAUST INLET side of the top cover to the AIR INLET side of the top cover to flood the cylinder chamber during the intake stroke? I thought the intake and the exhaust chambers in the top cover are physically separated?

As I pointed out previously... If a cylinder piston was neat TDC in an exhaust stroke, then both the inlet valve and the exhaust valve are open (valve overlap with 20 - 60 degrees). Therefore, with this rare occasion you could have sea water entering the exhaust top cover chamber and then flooding into the cylinder chamber and over flowing into the air inlet chamber in the top cover.  With this scenario, I could see a compression stroke where a solid piston stop would happen "DEAD HEAD"...

I don't know of any other way to get sea water into the air intake head top cover...

Am I missing something else?

Regards,
Don_

[tore]

Don.
 I don`t think you are missing anything and agree with your conclusion of filling the inlet ducts by the exhaust flooding water and am sorry to have mentioned the waterdrain from the valve ducts as it is confusing. I once experienced filling water in the cylinder from the exhaust valve having a waterleakage into the cylinder from one of the exhaustvalves water cooled cages having a corrosion hole while the engine was running, but did not experienced any waterstroke as the water evaporated prior to reach the pistontop. We better stop mentioning the exceptions and stick to the basics.
Tore

[Katuna]

Hello Mr. Tore,

Then if your spares included Intake and Exhaust valves, then your crew used the lifting bar to pull the head casting to replace the valves. That seems like a very time consuming Job to me, along with replacing the crankshaft bearings - Wow! It's a darn fortunate thing that you were not being hassled by Allied aircraft and British Destroyers when attempting to repair an ailing diesel engine...

Regards,
Don_

I remember reading that the Germans packed as many crates of spares as they could on board, both below decks and around the Engine Room. I know when U-534 was cut into sections, they found parts crates including a piston and I think a con rod and a liner. they were stored around the engines. I'm sure a lot of smaller parts are stored under deck on the fuel/oil tanks.


Are the racks between the frames, behind the engines original to that time period or were they added after the boat was turned over?

[tore]

I guess I have mentioned it before,  but we found a barrel where a piston was stored in conservation oil onboard KNM Kaura ex. U 995 at that time I was very young and engaged or newly married. In those days we used engagement rings of gold. To get a ring we had to trade in gold at the goldsmith and my wife at that time working for WHO in new Dehli bought som gold to trade trade in for the rings. Opening the barrel I wanted to check the piston submerged in the slippery oil and all of a sudden my ring slipped off and disappered in the barrel. It was of course a disaster and I must confess I used my engineroom crew to retrieve the ring with some work and my marriage was saved. We are still married, this year for 65 years.
Tore

[Don Prince]

Hello Katuna,


As far as I know the metal storage bins in the diesel engine room are original items.  It would make no sense for them to create storage bins during the retrofit and not use them?  Here is an attached photo from the Skizzenbuch Addendum Booklet...


Regards,
Don_

[VIC20]

Yes Tore, you must have mentioned it before, I remember the story about the ring.

[Katuna]

I had forgotten your ring episode! Thanks for sharing again.

[Don Prince]

Hello Mr. Tore and All...

[tore]

j

[Katuna]

A very Merry Christmas to all and hopes for a much better New Year.

[VIC20]

Merry Christmas https://www.youtube.com/watch?v=J_Ucuu-doao

[Don Prince]

Hello Mr. Tore and All,


I updated Skizzenbuch page 433 with new photo of the Schnorchel Indicator...


Regards,
Don_