I understand the temptation, but I`m told Propspeed only works on things that turn, spinning any growth off before it takes hold. I was ready to Propspeed shafts, skegs, and props, until the shipwrights told me doing the skegs, which foul badly, would not work and would be a waste of $. So we did the props and shafts,I won`t know until the boat comes up how it is going, unless I take a swim inspection over the hols.
While things that turn may stay cleaner, I don't think the shipwrights who told you Propspeed only works on things that turn are correct. Obviously there's only one way to prove it. Now we've used it on things that don't turn and found it effective.