BTW, that submarine does not become useless once the Germans lose the ability to win against the Allied navy/navies.
I like to send it to Southern Africa to ‘persuade’ Japan not to make a Brazil run, at least, force Japan to commit more to a Brazil run then what is normal and optimal. It also keeps Japan honest by not leaving transports out there undefended.
And, finally, I’ve had a situation where Japan FORGOT the Russian submarine was down there by Madagascar and been able to attack his battleship and 3 transports with 3 fighters and a submarine. Odds are, he only gets one hit, you get 2, that’s a good trade! Japanese transport which is useful for Russian submarine which is not so useful. (I actually got 3 hits to his 1 that time, but I was lucky.)