A Little Help Please

[Doolar]

I was reading on another thread about Bouyancy. In one of the Mod Release threads, bouyancy was in question , or lack off negitive bouyancy. I had used it in one of my installs but removed it, because of reading, U-Boats were trimed or designed with a positive bouyancy. Basiclly the boat would not sink at rest. Now even after this was explained, " U-boats were trimmed for positive buoyancy", a request for a negitive bouyancy mod was made. "In AlexOne there is a mod called ‘Negative buoyancy’ which at least adds a bit of realism to the diving behaviour of the submarine". Is my confusion caused by my lack of knowledge on this subect or because of not understanding the difference between real life boats and the game boats behaviors. In SHIII is Negitive Bouyancy needed or not.

[Fifi]

Whatever people may think or say, the fact is that a submarine not moving IS sinking. It’s well explained in few books written by real U-Boat commanders.
In SH3 the first to introduce this realistic feature was Stiebler.

:Kaleun_Salute:

[Raf1394]

If i recall correctly.

Uboats had to be trimmed all the time. Under supervision of LI Chief Engineer

A uboat always loses or gains small level of water. Its never 100% waterproof.

[VikingGrandad]

From my discussions with DerStosstrup:

U-boats were not trimmed for negative buoyancy at periscope depth. Instead, they were meticulously balanced to achieve neutral buoyancy at periscope depth at a slow speed with the bow angled slightly down.

Before a patrol, a U-boat's weight was adjusted in a test called the Trimmversuch ("trim attempt"). This established an amount of reserve buoyancy and established the starting point for the dive logbook (side note:- an example of this logbook is shown on the crew/boat management screen [F7] in GWX KC). The boat was brought down in a static dive in the harbour until the top of its periscope shears (the housing for the periscope) were sticking above the sea surface. This establishes an amount of reserve buoyancy to the tune of the amount of pressure hull protruding. The trim attempt is generally stopped at just under 2 tonnes of reserve buoyancy, but > 0.

To maintain periscope depth (11-13m), the crew would need to set a slight downward angle on the boat. As the boat moves forward, water flows over the dive planes, creating a downward force to counteract the positive buoyancy. Without forward motion, the dive planes lose effectiveness, and the boat would very gradually rise to its condition established in the trim attempt, with scope shears just poking out.

As the boat dives deeper, water pressure compresses the hull, reducing displacement and decreasing buoyancy. To compensate, a Type VII U-boat would need to drain approximately 1 tonne (1,000 litres) of water from its regulating tanks per 100 metres of depth. If we assume a loss of buoyancy of about 5% of that, which would be about the loss going just to periscope depth, that’s the equivalent of about 50 kg of weight. If, during the trim attempt, the boat had established say 0.5 tonnes of reserve buoyancy, and it loses only 50 kg due to the constriction of the hull at periscope depth, then it stands to reason that, in order to maintain depth, we need some speed and we need to angle down a bit. And this is exactly what we see in the diving regulations. Without forward motion and dive plane adjustments, the boat would gradually rise back to its trim attempt state.


https://i.postimg.cc/gkCLvVzZ/image.png (https://postimg.cc/RNT0mnnS)

^ The left-hand scale of the Papenberg gauge in the control room (U-995) is marked with reserve buoyancy graduations; the blue water level added to the photo indicates 0.5 tonnes of reserve buoyancy against the outline of the scope shears.



Gameplay context:
Positive buoyancy was a feature of GWX3 and is retained in GWX KC. The mod doesn't perfectly simulate every aspect of U-boat buoyancy — after all, SH3 isn't a full physics simulator — but it captures the essence of positive buoyancy and dive plane dynamics. And so it's fair to say that what the team achieved leans in the right direction for players who seek to add more elements of realism to their gameplay.

[derstosstrupp]

If the boat were trimmed and balanced at patrol start according to the provisions in the MDv 381 “Tauchvorschrift für U-Boote”, then it has established a certain measure of reserve buoyancy to the tune of something above zero and less than 2 tons. The MDvs were working documents, and not procedures that weren’t followed. They were made based on best practices and not in spite of them.

Once the boat has provisioned, and before it leaves port, it heads out to a spot in the harbor and performs a static dive as part of what is called the “Trimmversuch”, or trim attempt. The purpose of this is to establish a small amount of reserve buoyancy, and also serve as the starting point for the dive logbook, which is a running calculation maintained through the patrol and validated every day with trim dives.

The commander orders the trim attempt started, and the chief engineer has the ballast tank vents opened. The regulating tanks will be mostly dry, but they may have a little water in them. In a perfect world, with just the right provisioning and regulating tanks empty, and the ballast tanks flooded, in the right seawater density (about 1.003), the boat would hypothetically sink by design down to about 0 tons of reserve buoyancy, which means the top of the periscope shears are just at the waterline. Because the boat likely isn’t perfectly provisioned, and the density of the seawater is probably higher because 1.003 is very low (the Baltic), the boat will probably have to take a little water into the regulating tanks in order to get down further.

The chief engineer watches the Papenberg, which shows tons of reserve buoyancy on its left side, and slowly brings the boat down until the ordered amount of reserve buoyancy is achieved. This was less than 2 tons (when bridge floor is at the waterline) but more than zero (when scope shears are at the waterline).. Once the ordered amount of reserve buoyancy was achieved, let’s call it 0.5 tons, and the boat stops settling, the chief engineer has the ballast tank vents closed (oscillates the boat a few degrees up and down using the trim tanks before doing so), and then the control room mate jots down the fillings of the regulating tanks. This becomes his starting point for his dive logbook.

Reserve buoyancy can be thought of as those parts of the pressure hull that stick above the water. It’s a reserve of buoyancy that will create additional positive buoyancy should it go under. Remember, buoyancy is a function of the relationship of an object’s weight to the weight of the water it displaces. A ship floats because it displaces exactly the weight of water equal to its own weight. The amount of its reserve buoyancy, therefore is the amount of its watertight hull above the water.

The same is the case here except we are talking about a meter or so of the scope shears sticking out of the water. This too is reserve buoyancy, but only a minimal amount.

So we’ve established the procedure per the diving regulations of achieving somewhere over zero but less than 2 tons of reserve buoyancy. We know that the pressure hull constricts as a submarine goes deeper. In the case of the type VII, this created a loss in buoyancy of about a ton per 100 m of depth. When going that deep, they would shed this weight by removing it from the regulating tanks. Periscope depth is an increase in keel depth of about 5 m from the depth during the trim attempt. So that’s about 5% of that is 50 kg of buoyancy loss. If the boat were ballasted during the trim attempt to have 0.5 tons of reserve buoyancy, and it only loses 50 kg at periscope depth due to hull constriction, then the boat needs speed, and it needs a slight down angle to maintain periscope depth. And sure enough, this is exactly what is prescribed in the diving regulations (1/2 to 1° down angle on the boat, at slow speed, at periscope depth to be considered properly in trim).

And so a slight rising behavior at periscope depth is appropriate if the motors are stopped. At depth other than periscope depth in reality, it could go either way. As I mentioned, the hull constricts as you go deep, and you must shed weight. But at what depth are you, and how much weight have you shed? It could really go either way. The above is only speaking about periscope depth and the stopping of the motors at that depth. SH3 is not a physics simulator. I don’t think in game buoyancy can be changed based on depth.

Hope this helps provide context. It’s important to remember that submarines don’t behave like airplanes do. Airplanes maintain altitude by virtue of the lift generated by their wings as a direct function of their speed. Strip them of speed and this decreases lift, and it must go down. But that assumption can’t always be made with a submarine.

[derstosstrupp]

Thanks, VikingGrandad, you hit the nail on the head!

The only thing I will stress again is the fact that the game is not a physics simulator. The positive buoyancy behavior in real life is when stopping motors at periscope depth. The boat was trimmed for this depth and the trim was validated at this depth. At deeper depths, it could go either way because you can never shed exactly the amount of weight you need to offset the hull constriction perfectly.

[derstosstrupp]

Here’s a real world example, in this case U 44. This is the result of a trim attempt. The target was reserve buoyancy (Restauftrieb) of 750 kg or 0.75 tons. During the attempt it was discovered she was just under 600 kg too light and needed to take on weight to achieve 0.75 tons. So the reserve buoyancy of this boat established at the beginning of its patrol was 0.75 tons. She is positively buoyant at shallow depths unless she has speed and angles down. Once she gets below 20 m or so then the constriction of the hull begins to be enough to offset this positive buoyancy and she will indeed sink unless she sheds weight

Negative buoyancy in German is Untertrieb. There is no provision for that on the form. Only positive reserve buoyancy, or Restauftrieb.

https://www.subsim.com/radioroom/picture.php?albumid=1435&pictureid=13753

[derstosstrupp]

Explanation of the numbers:

The boat was returned from refit 11736 kg (11.74 tons) too light. It wants to achieve reserve buoyancy for operations of 750 kg (0.75 t). They add 11736 and 750 to get 12486 kg of total reserve buoyancy.

The attempt was conducted, and 0.75 tons achieved on the Papenberg. Seawater density was not the 1.003 hypothetical but rather 1.016, for a difference of 0.013. This multiplied by the displacement of the pressure tight spaces (fully provisioned) of 916 t gives 11910 kg, and that is entered on the form.

The difference is the 576 kg, which represents the amount of weight the boat needed to take on to achieve 750 kg of reserve buoyancy. It’s a function of a higher seawater density as well as its underweight condition when delivered.

[Doolar]

Thanx, for the detailed answers. Pretty interesting stuff. Not only is SHIII fun to play, but I learn something new almost every day.

[Shadowblade]

yeah, there is a lot of stuff to learn. It is a very complex theme.

[Raf1394]

Great explanation guys !