first I find it hard to believe the welds themselves are that much raised/rough to cause cavitation in a liquid at say 35? knots. Second, the vapor pressure mentioned above in reference to cavitation, is counteracted and somewhat negated by the significant underwater pressure, depending on the depth of the sub. Now if we are talking welds on the props themselves, which have a much faster velocity in the water, that is another matter. What little knowledge I have on the matter, I suspect exterior hull welds would be ground more for fatigue in resisting repeated dive cycles, and improved coating ability with a smooth surface in regards to corrosion, algae build up, etc. Regardless, we are just chatting here, and to make sure I get everyone's attention, "Not for Nothing"