Senior Member
It matters not. It will make the boat heavier. If the tanks would float on their own, because they are attached to the boat they act like life preservers. If the tanks would sink on their own, because it is attached to the boat it would pull it down like weighted shoes.As these tanks were on the outside of the pressure hull, the heavier seawater replacing the diesel would not make the boat heavier.
The tanks are open on the bottom to allow the fuel in so that it greatly reduces the change of weight as fuel is used. The water is heavier and so the tanks are heavier, but the difference is still much less then if the tank was filled with air instead. This means that instead of having to pump in a lot of water to keep the same weight, they only had to pump out a little bit of water.
Also as you go deeper the pressure goes way up. They would have to make the tanks their own pressure hull if they were closed. Because they have the hole on the bottom, the pressure in the tank increases as the boat goes down. This means that you could have paper-thin tank walls. This is of course theory not fact, if you had paper thin walls the tank would not be able to survive a handful of knots on the surface or rough seas etc, etc... you get the idea. But making a tank to take rough weather is much easier then making a pressure hull tank to go down to 250 Meters. (And it saves weight too.)
Subs can control depth just with the trim tanks. But this is slow and clumsy compared to dive planes. Because I have changed time compression near land, I sometimes sit on the bottom instead of hover. Real u-boats sometimes did this.
Senior Member
Member
That is true, but the AMOUNT of difference is less than if that fuel were burned out of a tank, leaving VERY BOUYANT AIR inside it!
In addition, since the ballast tanks are to some extent open to the sea, as the air inside them compresses at depth, a U-boat gets heavier as it gets deeper, adding ANOTHER level of complexity to depth keeping.
I've never heard a story of a U-boat or any other submarine "hovering" in combat. The closest I've heard, is bringing the sub to 1-2kts making very slow turns on the propellers, just enough to keep steerage. The slight movement of the water over the planes allows a sub to keep accurate depth and heading.
Senior Member
Not all the tanks are open to the sea. The main dive tanks are, but they are normally full of water when diving so this is not an issue. The trim tanks are not open to the water, and it is the trim tanks that are used to fine tune everything.In addition, since the ballast tanks are to some extent open to the sea, as the air inside them compresses at depth, a U-boat gets heavier as it gets deeper, adding ANOTHER level of complexity to depth keeping.
The boat doesn't get heavier when diving deeper, the problem is the boat displaces less water for the same weight. This is because the boat itself contracts a bit as it sinks.
Subs can hover but it is very clumsy. It would be very difficult to maintain a useful periscope depth for example. Especially in rough weather. You have to make small changes and wait to see the difference because of lag in effects. In rough weather the difference gets masked by the wave action so it gets much more difficult.I've never heard a story of a U-boat or any other submarine "hovering" in combat. The closest I've heard, is bringing the sub to 1-2kts making very slow turns on the propellers, just enough to keep steerage. The slight movement of the water over the planes allows a sub to keep accurate depth and heading.
In the game I noticed that you don't seem to be able to pump out water when at depth. If this is acurate to the real sub, then you couldn't maintain extended hovering at depth. The sub would almost certainly slowly sink or rise. The change in pressure as the sub sinks or rises would exagerate this.
However I think the effect of depth on displacement of the sub is not in the game. Though I have never tried to hover at depth.
