Hi Jonas Grumpy, you are right concerning the effect of sea pressure at various depth or the salinity of the sea on buoyancy. They are not modeled in the game. Maybe one day...
Now let's take an example: The initial trim at the start of the game is 8.6 m. With MBTs full and Trim tank empty, I get a base depth of 7.9 m. I trim the boat for neutral buoyancy with 9.15 m³ for a depth of 11.6 m. There is 6% error between the initial and actual trim value.
This is the standard procedure (more or less) for trimming the boat according to the "Diving Regulations for U-boats" document, Section V.
Your method is equivalent and was actually used later during the war when the threat of surprised air attack was increased. You fill up the Negative tank with 4.5 m³ and remove water from the Trim tank to reach Neutral buoyancy. The volume left is 4.65 m³. The total amount of "trim" is the same, ie 9.15 m³.
Crash Dive is meant to go from the surface to below the surface (depth of 15 m) quickly.
Test 1:
- Negative tank empty.
- Trim tank set for Neutral Buoyancy (9.15 m³)
- Electric Engines at Full Speed, Dive Planes Down. Start timer.
- Fill MBTs (100 m³)
- Fill Negative Tank (4.5 m³)
- Total Ballast = 113.65 m³
- Depth of 15 m reached. Stop timer: 40 sec.
Test 2:
- Negative tank full (4.5 m³)
- Trim Tank set for Neutral Buoyancy (4.65 m³)
- Total "Trim" = 9.15 m³
- Electric Engines at Full Speed, Dive Planes Down. Start timer.
- Fill MTBs (100 m³)
- Fill Trim tank (15 m³)
- Total Ballast = 119.5 m³
- Depth of 15 m reached. Stop timer: 30 sec.
There is a gain of 10 seconds, which could be a matter of life or death in the case of an air attack.
On the other hand, the rate of descent if much higher, which could be difficult to stop before hitting the bottom or reaching the crush depth. More compressed air is used.
Conclusion: Both methods are correct and specific to the level of threat from an air attack.
Good point Jonas Grumpy!
