Tores mailbox VIIC and VIIC/41 operation and technical details

[VIC20]

Currently my calculation gives a thrust force of 20,001.42 lbs (88,970.75 N) for a single propeller of the Type VII at 470 RPM, 1400HP and 17.7kn - does that sound OK?

Each main thrust bearing can transfer up to 10,000 kg (22046 lbs) of thrust. So it looks ok.


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Regards
Maciek

Thanks, this whole ship propulsion stuff is a lot like the thing with the chicken and the egg. Each time you find a way to calculate something you find out that you need another thing that might need the other etc. 

[SnakeDoc]

Currently my calculation gives a thrust force of 20,001.42 lbs (88,970.75 N) for a single propeller of the Type VII at 470 RPM, 1400HP and 17.7kn - does that sound OK?

Each main thrust bearing can transfer up to 10,000 kg (22046 lbs) of thrust. So it looks ok.

Thanks, this whole ship propulsion stuff is a lot like the thing with the chicken and the egg. Each time you find a way to calculate something you find out that you need another thing that might need the other etc. 

Can you briefly describe, how you calculated thrust force?


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Regards
Maciek

[VIC20]

Can you briefly describe, how you calculated thrust force?

There are several ways that I've found how to do it… and I am not happy with any of them    - I'm searching since weeks for this stuff but haven't found anything better so far.


The most primitive is just a rule of thumb used by normal boat users:
For a good propeller the rule of thumb is:
HP * 20 = Thrust in LBS
would be:
1400 * 20 = 28000 lbs


well… I guess it's clear that this is nonsense


Next thing is the "pullard boll" (boat/propeller is fixed in the water and does not move)


thrust = 11.87 * (SHP * DiameterOfThePropellerInInches)^0.67 (power of 0.67)
would be:
11.87 * (1400 * 63.779527559124)^0.67 = 24631 lbs


Next estimation is (SI units):
(BrakePower * 0.5) / BoatSpeed*0.95    (0.5 is estimation of Propulsive Efficiency which is a bit low as it can be usually 0.5-0.7)
would be:
(1400*745.6998 * 0.5)  / (17.7 * 0.51444) = 57326 N or 12887 lbs


This is also nonsense because it uses the speed of the boat which is not correct, the correct calculation would use the speed of advance of the propeller


What I currently use is:

LBP (Lenght between perpendiculars at the water line) = 62.27 meters
B (Beam - I ignore the saddle tanks) = 4.7 meters
T (draugth) = 4.74 meters
Displacement = 769
Density (of Water) = 1025
StandardTon =  1016
RPM = 470
HP = 1400
VS = 17.7 kn (speed of the boat)


Volume = Displacement * (StandardTon/1000)*(1000/Density)
Blockcoefficient = Volume / (LBP*B*T)
PropulsiveEfficiency = 0.836 - (0.000165 * RPM *  Volume^1/6)      (An estimation by Henschke, 1965)
WaveFraction = (BlockCoefficient / 2) - 0.05        (Taylor)
VA = VS - (VS * WaveFraction)      (speed of advance of the propeller)
Watt = HP*745.69987158227
VA = VA * 0.5144444  (convert to meter per second)


Thrust Force = (Watt*PropulsiveEfficiency) / VA = 88970.76 N = 20001.42 lbs


usually you would also add the Coefficient of hull efficiency (close to 1) to the calculation…


HullEfficiency = 0.98


Thrust Force = (Watt*PropulsiveEfficiency) / HullEfficiency*VA = 90786.48 N = 20409.61 lbs

[Don Prince]

Gentlemen,


I'm have been looking at the valves for a few hours and I see the following:


D1 = HP hull valve + non-adjustable check valve
D2 = HP hull valve + non-adjustable check valve
D3 = HP hull valve
D4 = HP hull valve + non-adjustable check valve (per Mr. Tore's photo)
D5 = HP hull valve + non-adjustable check valve


My understanding of a non-adjustable check valve is the device will allow the medium (in this instance air) to flow in one direction.


My understanding of a hull valve is it is generally used in the full on or the full off state.


Then I reviewed the symbol section of the Skizzenbuch and seen a little ball on the valve wheel handle???  Could the drawing simply be indicating when there is a line with a ball on the end of it that this is a HP hull valve.  This is different form the valve handle on the Q Tanks...


See the drawing:


Regards,
Don_

[SnakeDoc]

Hi Don,

Then I reviewed the symbol section of the Skizzenbuch and seen a little ball on the valve wheel handle???  Could the drawing simply be indicating when there is a line with a ball on the end of it that this is a HP hull valve.  This is different form the valve handle on the Q Tanks...

You are on good track. In fact, all hull valves were fitted with the ball - I have marked them on the bottom part of attached drawing. But the symbol for such valve is quite different - see top part of attached drawing.

Generally - this ball is the part of identification system, which should allow distinction of various type of valves in poor light conditions.


Short description:

Further identification is provided at handwheels on valves.  Wheels for hull closures all have a large ball cast on the rim.  Valves for seawater lines are balanced cranks with straight handles.  For fuel oil, balanced cranks with tapered handles are provided.  Lub oil handles are five-sided.  Fresh water and hydraulic system valves are five-clover type.  Compressed air system valves are circular, with the rim slightly raised above the hub, with three spokes, and with an oval rim section.  The exhaust gas blow system valves also have three spokes, but the rim is greatly offset from the hub, and the rim is a flat oval.  Emergency vent valves have three spokes, a U-section rim and a hand grip.  Hand gear for vent valves has similar wheels with five spokes.  The main H.P. blow valve handwheel is a large flat wheel with five spokes.  The main H.P. blow valve handwheel is a large flat wheel with five spokes, and the main L.P. blow valve is similar, with three spokes.  Positions of cocks are indicated by grooves on the end of the square stern head.

http://uboatarchive.net/DesignStudiesTypeIXC.htm


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Regards
Maciek

[SnakeDoc]

Mark,
thank you for your explanaintion.


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Thanks, regards
Maciek

[tore]

Mark.
Sorry I have been traveling for a couple of days and have not been able to respond. I believe you have to get the modelbasin towing graphs in order to get an accurate input you for your game, it could exist some place in Germany. However the speed/ output relations have so many variables and would f.i. change according to the condition of the hull (fouling) often related to days at sea, weather and sea. If you want to vary the prevailing circumstances in your game, you need a lot of info which I doubt exist for a VIIC. I assume the designed max load of  the thrustbearing could give a clue if all conditions were ideal, calm sea, clean hull, draught and trim etc. which hardly exist,  as you mention thrust cannot generally be empirically  transferred into speed.
In the Uboat information effective July 15, 1940, translated by Maciek there is a table put up at page P 14, Section I General. where outputs related to dieselengine / E-motor revs are put up under two conditions A and B; I cannot find the definition for these conditions, but if we assume A is the optimal and B is the a realistic unfavourable condition, you should have the same rule of the thumb input as the crew would have, may be that would be the easiest way, remembering that the revs is not necessarily identical to the output of the engines, which f.i. could be overloaded at the revs stated.
Tore
   

[SnakeDoc]

Hi Tore,

In the Uboat information effective July 15, 1940, translated by Maciek there is a table put up at page P 14, Section I General. where outputs related to dieselengine / E-motor revs are put up under two conditions A and B; I cannot find the definition for these conditions, but if we assume A is the optimal and B is the a realistic unfavourable condition, you should have the same rule of the thumb input as the crew would have, may be that would be the easiest way, remembering that the revs is not necessarily identical to the output of the engines, which f.i. could be overloaded at the revs stated.

Trim condition A - Tauchbunker 2, Tauchbunker 4 and Regelbunker 1 empty.
Trim condition B - Tauchbunker 2, Tauchbunker 4 and Regelbunker 1 filled with fuel.

--
Regards
Maciek

[tore]

Maciek.
Thanks, you always are able to put up written facts.
Tore

[tore]

Don.
Hullvalves. Generally, as Maciek explained, the valve handle or wheel has a cast ball and is usually indicated as a dot or dots on the valvesign handle. The valve sign having a line and a circle I guess, as Maciek found out, is a "plombiert" valve, not necessarily a hullvalve, which I believe can be secured in a certain position. Technically this can be done in several ways f.i. like a counternut or counterhandle. The individual blowing valves are vital, but not always easy accessible and should be in a fixed secured position according to the CO order. Normally for the MBTs this would be in an open position. When MBT 2 and 4 are used as fuel tanks the valves should be secured shut. You shall find this arrangement on all MBTks including 1,3and 5 even if these tanks cannot carry fuel. In addition you shall find the same arrangement on the hullvalves for emergency airsupply and divers airconnection shown on the systemsketch but not allways installed on the VIICs.
Tore

[VIC20]

However the speed/ output relations have so many variables and would f.i. change according to the condition of the hull (fouling) often related to days at sea, weather and sea.

There is a rule of thumb for fouling and different sea routes. I need to get a convincing ideal state with calculations first. But even if my thrust calculation seems to be correct (all publications I've found use it) it seems to work for a static condition only (boat has reached maximum possible speed for this engine setting) and not while accelerating. 

[Don Prince]

Hello Mr. Tore,


Trim Condition B (main ballast tanks and main ballast and reserve fuel oil tanks filled with fuel oil increased load).

I have a lot of assumptions before the questions...  If my assumptions are totally wrong, then I need to be corrected.  I am in the learning process and definitely need guidance and will not take any offense...

With this additional weight, the boat is setting lower in the water.  This is verified by the Information Manual for the Type VIIC.  The turning radius is wider and slower because of the increased drag introduced by lowering the hull.

One would think that the boat would have no problem diving since it weigh was increased.  However, since D2, D3, and D4 are filled with fuel oil which weighs 13 percent less than water.  It looks almost impossible to get the boat to submerge.

D1             30.7 - M3              Trim Condition B
D2             22.6 - M3                      22.6 - M3
D3             47.75 - M3                   47.75 - M3
D4             26.6  - M3                     26.6  - M3
D5             25.15 - M3

Lets say 1 M3 = 1 metric ton

Total     =  152.8 tons                    96.95  - tons    (96. 8 *  13% less weight FO  =  12.6 - tons)
                  -12.6 tons
Total      =  140.2 - tons

They add about 5 tons of additional supplies, which even lowers the boat further and decreases surface performance even more.  With the supplies The total weight increases.

140.2 tons + 5 tons = 145.2 tons

In normal Trim Condition A the ballast weight would be 152.8 tons and it achieves neutral buoyancy and dives using the Regulating Tank 2 for adjustment.

Regulating Tank 2 has a total capacity of 15.2 M3 /tons
[/size]to be continued....

[Don Prince]

In Trim Condition B the weight is 145.2 tons,  or 7.6 tons less; my assumption is that the Regulating Tank 2 has the capacity to achieve neutral buoyancy and dive under these conditions.


[/size]If I haven't got things totally screwed up...

[/size]Q1.  Did the U-Boat you served on run in a Trim Condition B (fully loaded)?
[/size]Q2.  How did the weight effect the draft, generally how much lower was the boat in the water?
[/size]Q3.  With the fuel oil in the ballast tanks, did that change the handling characteristics when submerged?
[/size]Q4.  A basic fuel oil question - as fuel oil is consumed by the diesel engines is the depleted tank area pumped   with water at 87% to keep the boats weight in balance?  Tanks with compressed air would affect the boats weight and neutral buoyancy?
[/size]Q5.  Fuel oil floats on water; are the internal fuel oil storage tanks pumped with water as the oil is being taken from the tank?

[/size]Please feel free to add any additional wisdom,

[/size]Regards,
[/size]Don_

[/size]I don't know why i have the [size] [/size]

[SnakeDoc]

Don,

One would think that the boat would have no problem diving since it weigh was increased.  However, since D2, D3, and D4 are filled with fuel oil which weighs 13 percent less than water.  It looks almost impossible to get the boat to submerge.

Only D2 and D4 were used as fuel oil tanks. So weight difference between A and B trim condition would be only 6,4 ton

Q2.  How did the weight effect the draft, generally how much lower was the boat in the water?

The KTB for U-604's 1st patrol:

28 August, 1942, 04.20:
"During the pursuit, at the suggestion of the Engineering Officer, converted
main ballast and reserve fuel oil tanks 2 and 4 for use as ballast tanks. The
remaining fuel oil, 1 ton, was transferred from the ballast tanks to the regulating
and reserve fuel oil tank. At the same time the negative buoyancy tanks were blown.
Thereby gaining about 1 knot. With the high enemy speed of 16 knots these
measures were decisive."

http://uboatarchive.net/KTB604-1.htm

There is a book "U-Boat 977 The U-Boat that Escaped to Argentina", by
Heinz Schäffer, where is described a pursuit for a tanker, then it is said:

"All our blowers and compressors are screeching for all they're worth—we keep
on blowing our diving tanks every five minutes, for we have to keep as high
above the water-line as we can, since the higher we are the faster we go.
True, the increase in our speed's only fractional, but it is an increase."

(this description is not technically strict, but it shows the idea)

Q3.  With the fuel oil in the ballast tanks, did that change the handling characteristics when submerged?

There should be no difference.

Q4.  A basic fuel oil question - as fuel oil is consumed by the diesel engines is the depleted tank area pumped   with water at 87% to keep the boats weight in balance?  Tanks with compressed air would affect the boats weight and neutral buoyancy?

All fuel oil tanks (except regulating tank 1) are water-compensated. Regulating tank 1 - as filled with the fuel oil and air - would affect weight of the boat.

Q5.  Fuel oil floats on water; are the internal fuel oil storage tanks pumped with water as the oil is being taken from the tank?

Yes, internal tanks are also water-compensated. See the plate:
http://uboatarchive.net/U-570Plate10.htm

I don't know why i have the 

You are probably using some external editor, which uses its own tags, and then you copy and paste the text into the web browser.

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Regards
Maciek

[SnakeDoc]

Q2.  How did the weight effect the draft, generally how much lower was the boat in the water?

It is generally hard to say without knowing the construction displacement and exact shape of the hull. But when I approximated the U-Boat hull by a cylinder and assuming the surface displacement of 769 ton, the change of draft between the condition A and B would be few centimetres.

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Regards
Maciek