Tores mailbox VIIC and VIIC/41 operation and technical details

[Don Prince]

Hi Maciek,


Newton and Bernoulle and a bunch of other dead people would be ashamed of me because I ignored what my High School physics teacher (Marist Brother Matthew) taught me 55 years ago...  I was just in such a hurry to get that darn Q tank to burst and ignored the basic fluid mechanics that I had known for years...


Again, thanks for the physics re-fresher course...


Don_


PS - If anybody is doing he the math, I will be 73 on June 3rd

[tore]

Mark.
Reverting to my notes on waterleakages in the exhaust system I would emphasize filling the cylinders was really exceptional and has happen to me once, nevertheless the blow thru with open indicators was a routine after being submerged.
 Tore

[VIC20]

Mark.
Reverting to my notes on waterleakages in the exhaust system I would emphasize filling the cylinders was really exceptional and has happen to me once, nevertheless the blow thru with open indicators was a routine after being submerged.
 Tore

OK 


I've just found a website which says bhp and shp is identical? Is that true?

[tore]

Maciek, Mark.
Fuelconsumption, output.
The BHP is referring to the output measured at the enginemanufacturers testbed under controlled environment. BHP stand for Brake Horse Power and is referring to the testbed calibrated waterbrake which brakes the engine at various rpm in order to put up an output graph for the buyers approval according to the contract output. The other output designations like shp, is less accurate and I assume could be measured by a torque meter on the shaft at various places the output will then vary fi. if you have a liquid coupling after the engine and take readings after the thrustbearing, gearboxes etc. thus less specific designation.
Fuelconsumption is likewise measured on the testbed in close connection with the waterbrake for accurate output readings, temperature, barometric pressure etc. Then the most important detail, the reference fuel. Dieselfuel is not an accurate specification as you have many variations and I guess during WW2 it was a difference between the German diesel fuel and to days prime diesel quality called gasoil. For consumption measurement the lower calorific value is the most important, it can vary considerably like 42.000 MJ/kg to over 44.000 MJ/kg. So if you want to compare apples don't`introduce pears. So I assume without knowing the lower calorific value of the reference fuel you cannot compare the various fuel consumption figures.

[tore]

Mark.
Deviation on the engine telegraphs rpm.
We never used the engine telegraph designation and corresponding rpm which are put up in the lists you are referring to. I believe it is strange to have stages as 396 and 471 rpm. It looks to me as composed by somebody behind a writingdesk. Whatever order was given, you adjusted the engines to an approximate rpm level and the governor kept the engine at that level plus minus 3-5 rpm. We did not use these accurate figures as this was not a science issue. The CO passed on messages verbally if some unusual outputs would be necessary.
Tore   

[SnakeDoc]

Hi Mark,

full load, 1400 HP, 470 RPM, 190 g/bhp

hehe, my confusion rises…

The 1400 HP at full load should be shaft horsepower (manual says 1400 SHP, 470 RPM at full load) but the fuel consumption is gramm per brake horsepower.
The manual says fuel consumption of the G.W. is 254 kg/h at full load (470 RPM, 1400 SHP) which is 0.181 gramm per SHP
I think consumption per bhp should be lower than per shp which makes the difference even worse. But actually I have no idea.

it seems, that in the manual (that is U-Bootskunde fur U-Boote Bauart VIIC available on the uboatarchive.net) are given rounded values of the power
(1600 against 11612, 1515 against 1500 and 1400). I think, that rounded values are given for practical reasons, as the accurate value were measured
using waterbrake under test conditions (as Tore explained).

I have one note - in the original German documents, the power is nominated in the PSe units, that is equivalent of bhp. I was mistaken, translating it to the SHP as on the uboatarchive.net.

In hurry, I have given fuel consumption in the g/bhp, while in the original documents is given as g/PSe/h. I have attached the scan of the data table.
As a bonus - chart of the power and fuel consumption in the relation with the RPM.

--
Regards
Maciek

[SnakeDoc]

There are different battery chargingloads, starting by not cruising, only charging at very high loads when your batteries were low and very light, just topping up the batteries. As far as I remember the rpm varied with the load I can`t remember the revs. When cruising you put the chargingload according to the requirement of the speed and the timeneed for getting batteries fully charged ,up to the decision of the CO.

The RPM loss while charging you can roughly estimate using these documents. They are valid for cruising submerged (while snorkeling), but I do not have nothing better.

http://www.ubootwaffe.pl/images/okrety/instrukcje/chrapy/preliminary_snorkel_description1.jpg
http://www.ubootwaffe.pl/images/okrety/instrukcje/chrapy/preliminary_snorkel_description2.jpg

The starting procedure after surfacing was a bit cumbersome, you wanted to get the engine ready as soon as possible in order to exhaustblow the ballasttanks as you was semisurfaced and the  boat was in an unstable position. However you had always some seawater leakages in the exhaust system and some time these leakages could even fill the cylinders. In order to prevent waterstroke, you opened up all the indicator cocks and blow air through the cylinders to get all the water out. Then came the normal starting after shutting the indicatorcocks ,run the engine to some 100 rpm. by air before admitting fuel to the cylinders and ignition. It could take at least 3 minutes.

After surfacing in a dangerous war situation would they have taken the risk to start the engines without blowing possible water out of the engines first?
(OK, I just have to think what kind of situation this could be… possibly makes no sense at all, the only reason to surface while being in range of enemy vessels would be to show the white flag)
I guess the U-Boat can run on E-Machines during this procedure? (or isn't that a good idea without blowing ballast tanks first?)


Then I'll will allow usage of reversed diesel in the game only if the E-Machines are damaged.

I guess you could engaged the clutch at 350 revs, but the wear and tear increases with the speed and inertia forces so we usually started with the clutch engaged.

This makes sense with the information of the seawater leakage in the exhaust system after surfacing. And I think that also means usually E-Machines were shut off until the ballast tanks were blown? Otherwise the shaft would still run at something close to 290 RPM when the diesel is clutched in which is basically the same stress on the clutch just flipped.
So you switched from diesel to electric but usually avoided to switch from electric to diesel while running?

For the surfacing procedure (including exhaust diesel blowing) you can read here:
http://uboatarchive.net/DivingRegulations.htm
The corresponding paragraphs - 201 - 221 and 306 - 312.

--
Regards
Maciek

[VIC20]

Mark.
Deviation on the engine telegraphs rpm.
We never used the engine telegraph designation and corresponding rpm which are put up in the lists you are referring to. I believe it is strange to have stages as 396 and 471 rpm. It looks to me as composed by somebody behind a writingdesk. Whatever order was given, you adjusted the engines to an approximate rpm level and the governor kept the engine at that level plus minus 3-5 rpm. We did not use these accurate figures as this was not a science issue. The CO passed on messages verbally if some unusual outputs would be necessary.
Tore

That's extremely important info! 

[VIC20]

I have one note - in the original German documents, the power is nominated in the PSe units, that is equivalent of bhp. I was mistaken, translating it to the SHP as on the uboatarchive.net.

In hurry, I have given fuel consumption in the g/bhp, while in the original documents is given as g/PSe/h. I have attached the scan of the data table.
As a bonus - chart of the power and fuel consumption in the relation with the RPM.

That's exactly the kind of data that I need :-) I've made a crude interpolation from some data of the 9 cylinder machine of the IX before (which has same bore & stroke)


PSe (effektive Pferdestärke) is the power measured at the shaft of an engine (before the gears of a car) - wikipedia says PSe is like WPS (Wellen-PS) which is equal to SHP. After seeing this website I believe SHP and bhp are really the same http://www.engineeringtoolbox.com/horsepower-d_472.html
So I think you've made no mistake in your translation (but I gave up to study machine engineering after 3 days so I could be wrong and anything from the wiki is… well, we all know what I mean) also the whole PS thing is completely outdated so it's hard to google for it.


Your document with the graph seems to show PSe with auxiliary engines clutched in (angekuppelte Hilfsmaschinen) - the other document that is just a table is different and shows PSe without auxiliary engines.
You can also see that from the results - best to see at 375 RPM which is at 701 PSe on the table and at something like 715 PSe on the graph (both with supercharger).
It's great to have both because this could possibly make it easier for me to calculate the engine.
Where do you got those documents from?

[VIC20]

Tore, do you remember the impact of good and bad speeds (like hump and hollow in regard to the Froude number)? The slender shape of the hull combined with saddle tanks is pretty unusual. Do you remember if the resistance was clearly higher at bad Froude numbers so that you avoided those speeds?

[SG]

So if you want to compare apples don't`introduce pears.

 

[Don Prince]

Ruptured Negative Buoyancy Tanks (Q tank)
Let me try and make a fool of myself again...




http://www.uboatarchive.net/DivingRegulations.htm


33.  The pressure difference meters are to be monitored, particularly during expressing (danger of rupture with closed flood valve) and when the boat rises.  More than 2 atmospheres higher than outboard pressure is prohibited and must be equalized.[size=78%] [/size]
Equalization takes place after previous request to the Engineering Officer (for passing on to the conning tower) by complete opening of the "Vent negative buoyancy tank" valve, appropriately up to an internal pressure of 1.5 atmospheres (external pressure at periscope depth).


I can see two ways to rupture the Q tanks and the key element is the flood valve:
1.  Fail to close the Flood Valve until at depth
2.  Expressing at depth and then close the flood valve


Instance #1 - After expressing at 10 meters and failing to close the flood valve (bottom mushroom valve) will result in the Q tank's air being compressed.  This is not a problem for the tank at this point because the internal and external pressure are the same.  However, If the crewman closes the flood valve at 100 meters, now we have a problem!  As the boat rise with the closed flood valve, the outside pressure will decrease and the high internal pressure will stress the tank structure which was braced for external pressure.


Instance #2 - expressing the Q Tanks at a depth of 100 meters instead of 10 meters.  Again this is not a problem (same reason as above).  However, if the crewman closes the flood valve after expressing at depth, now we have same scenario with the compressed air in the Q tanks.  As the boat rises with the flood valve closed and decreasing external pressure, it is likely that the Q tanks will be ruptured because of the high internal pressure.


I sure hope I got it right this time...  If not, then I'll need another post high school lesson from Professor Maciek.


The manual says to monitor the pressure difference meter.  I assume that is the meter very close to the Q tank top vent valve (black and red wheel-handle).  Does this meter have a input line going to the inside top area of the Q tank, and a second input could be just a hole in the back of the meter to sample the internal hull pressure.


Regards,
Don_

[tore]

 Mark.
I cannot remember we had any problem which could be related to the Froude number. We had a small problem more related to revs of the shafting than the speed of the vessel. When running on E-motors we had no problem. Diesel running could create a minor problem as you had a complex shaftingsystem having masses at different length from the engine, like in/out of propellers and in and out of E-motors, thus you got different torsional vibration patterns creating stresses in the shafting, further I assume we got certain free moments which could excite hullvibrations. I wouldn`t say they created problems. The latter could anyhow be avoided by passsing the revs area quickly. The torsional vibrations in the shafting were diffcult to discover as no torsional meter was installed, however the engines were equipped with torsional vibrationdampers up front, but I doubt if that took care of all situations. I have seen a few photos of VIIC shafts breakages which clearly was due to torsional vibration stresses, but if these engines had been equipped with a torsional vibrationdamper I cannot tell. I personally believe the most serious rpm related problem would be pitting of the propeller. The late war VIICs were equipped with  special cast steelpropellers. At the root of the propellerblades we had serious pittings due to cavitation. Cavitation is a noise problem as well , however we never figured out which revs should be avoided. Sorry this may be a bit off the topics.
Tore

[tore]

Don.
I am not sure if your man violating every rule for operating theQ and common knowledge for a submariner would have a long career in the submarine service, using the Q as a compressor . However somehow the designers have taken such people into consideration. As we discussed previously each Q is fitted with a reliefvalve as well as differential manometers both are not shown on the pipediagram nor mention in the manuals I believe. The diff man is most probably connected to a common seapipe having direct connection to the sea on port side, serving the diff.manometers for the regulating tanks (4) and the Qs (2). The other pipes to the diff man. goes to each different tank, the pressure inside the submarine is irrelevant in this case it is  the  difference in pressure inside the Q and the sea which should be shown on the diffmanometer. When the inside pressure of the Q tanks exceed a safe level, the reliefvalve opens and the Q tank pressure is released, see photo below. This has happened not on my boat, but on another in my time. I guess your man would probably be offered a job as potatopeeler on the shore  submarinestation.
Tore

[VIC20]

BTW:

Tore, I know it's not perfect (and by far not as detailed as Simon's awesome drawings),
also it's still untextured and some stuff still missing…
and remember it's just for a mobile game.

What do you think of my command room model?