I have a 25 active thunder with a stellings box and 1" drive ext,spacer, currently spinning a B1 4b 28p.
The prop blows out, slips, when getting on plane, turning and acceleration while under way.
Is all of this to be expected with the ext box or is there another prop I should be using?
est. 500hp
73mph (gps) @5000rpm

If the propeller has ever been repaired, there could be issues with the propeller itself. It's stock condition should be varified.

Lowering the box, if possible, may prevent the ventilation. I like this prop choice but there are other brands and styles of propellers also available. Some worth testing would be the Hydro P5X, and the Turbo OXP. These are 5 blade props and would exhaust the current set up variations first.

Brett

What pitch 5 blade should I try.

I also have a porpoise problem that begins a 60 mph, only stopped by a lot of tab, would the 5 blade cure this or should i try something else.