[Delphinus]

You'd almost think it wouldn't matter if the prop was upsidedown on the shaft though, since if its sucking water instead of pushing, it will pull out and hit the tab.

If it's happening every few minutes and the shaft is OK .. hmmm. I've never used the big props, mine are older and from the time they didn't give you the two. So I don't know if it's just something to do with the bigger one .. but it seems to me if it's hitting the tab and switching directions ... then hitting the tab again a few minutes later, then it must have switched directions in that time. Dodgy electricals? It's not running on a wavemaker is it (seems like an obvious question but I'll ask it anyhow).

(And if so, I found they really don't last when on wavemakers. Everytime they startup, there's a little vibration in the whole unit. This eventually distresses the little holder-arm thing that attaches the powerhead to the glass magnet, and it breaks at the socket point. I tried all manners of fixing it, but basically once this is gone, it's gone, nothing I could think of would keep it fixed longer than a week or so.)