I'm pleased to report that, of course, this forum is right again! In trials with 4 different sizes and pitches of props on the Fairmile, I can confidently say that in-board turning props do give better manouverability (I'm sure that's not spelled right!).
The Fairmile is 55" LWL, 4 shafts, handed props. With in-board turning props, low speed turning was much improved, High speed turns didn't seem any different. I couldn't really see much in the way of an overall speed difference between in-board and out-board rotation.
The props used were:
Rivabo 35mm brass, 3 blade
Graupner 40mm, plastic, 3 blade
Rivabo 45mm brass, 3 blade
Prop Shop, 38mm, scale type 3 blade
The last set, the Prop Shop items give the best performance, not only a nice, realistic scale speed, but also cool running motors (540 type) and a long run-time too.
Mike