World War 2 U- boat tonnage query

[Danny McG]

I'm currently reading a fictional book about a Royal Navy signalman in 1942.
His ship is being stalked by a U-boat and I'm confused by the given tonnage, are submarines heavier when they're underwater?

Excerpt:-
U-157 was a large, ocean-going submarine of the Type IX/C, of 1051 tons surfaced and 1232 tons submerged, carrying a crew of five officers and forty-six ratings.

 

[Foxbat]

I'm currently reading a fictional book about a Royal Navy signalman in 1942.
His ship is being stalked by a U-boat and I'm confused by the given tonnage, are submarines heavier when they're underwater?

Excerpt:-
U-157 was a large, ocean-going submarine of the Type IX/C, of 1051 tons surfaced and 1232 tons submerged, carrying a crew of five officers and forty-six ratings.

Probably because they fill their tanks with seawater to reduce bouyancy (and therefore sink below the surface). The sub will be lighter when the tanks are filled with air (sub on the surface).

 

[Danny McG]

Probably because they fill their tanks with seawater to reduce bouyancy (and therefore sink below the surface). The sub will be lighter when the tanks are filled with air (sub on the surface).

Ah! I never thought of the tanks, cheers.
I actually thought maybe somebody had calculated the weight of water in all the external nooks and crannies

 

[Swank]

The surface weight would be useful to understand the size relative to surface ships, while the submerged weight is best to describe the size relative to other subs.

All subs submerged weigh exactly the same as the volume of water they replace (or displace). Which is kind of an interesting design limitation given that the sub is mostly air - you have to have heavier than water materials to get the average weight close enough to seawater for the dive tanks to be useful. Otherwise you'd have to fill all the air space with water to dive.

 

[Pyan]

As Swank says, when the ballast tanks are full of water, and the buoyancy of the sub is less than seawater, the sub sinks - and positive buoyancy is restored by pushing the water out of the tanks by compressed air stored in pressure cylinders, which take up less space and are heavier than water.

 

[KiraAnn]

It's not exactly a matter of weight but the displacement of water. Any ship/submarine that is on the surface will have their upper works out of the water, therefore not displacing any water.

And submarines do not necessarily take on more ballast than their displacement, but also use dynamic forces in altering depth. Watch Das Boot; when they do an emergency dive, the crew not actively involved rush to the front of the boat. This forces the bow down, and the diving planes "bite" more increasing the downward velocity, and additionally utilizing the flat weather deck to function as a plane as well.

Note: While in the Navy, I was involved in training submarine crews, particularly fast attack boats.