Hi wetdoc23,

Here is some more information that I found that might be of interest to you in your quest about signaling aboard U-Boats. Let me know if this is any help to your. I am assume that it is for both U-Boats and surface craft.

World War I was also the first major war that was able to draw upon electrical technologies that had been in development at the turn of the century. Radio, for example, became essential for communications. The most important advance in radio was the transmission of voice rather than code, something the electron tube, as oscillator and amplifier, made possible. Electricity also made a huge impact on the war. Battleships, for example, might have electric signaling lamps, an electric helm indicator, electric fire alarms, remote control—from the bridge—of bulkhead doors, electrically controlled whistles, and remote reading of water level in the boilers. Electric power turned guns and turrets and raised ammunition from the magazines up to the guns. Searchlights—both incandescent and carbon-arc—became vital for nighttime navigation, for long-range daytime signaling, and for illuminating enemy ships in night engagements.

Scott Sorenson