Don. You are a true U boat enthusiast having the stamina to spend almost every week end and vacation on a model project for two years.
Seeing the shiny brass propellers reminds me of the VIIC propellers which I guess up 1942 were of a propeller brass alloy, afterwards the VIIC propellers were made of a special steel. Submarine propellers are a special science, the VIIC propellers were not of a particular advanced design, they were susceptible to cavitation on the tip of the blades as well as towards the boss. The pittings were bad, but cavitation is noisy due to the implosions of the vapour bubbles.
Tore