From U-5 onwards German u-boats had RADIO TELEGRAPHY (RT) , also from U-5 on they had UNDERWATER TELEGRAPHY (UT). For RT 2 tall aerial masts were fitted to the deck and could be lowered from inside the boat when it was time to dive. Ship to u-boat range was 50-62 nautical miles and between u-boats about 30 nautical miles. The UT was a bell and clanger operated by compressed air,but this did not work very well and was very seldom ever used.