A critical requirement of Spec. R.24/31 was the ability of the aeroplane to maintain height with one engine shut down, and unfortunately, the Stranraer was unable to do this.
It wasn't a lack of power that was the problem, more powerful engines were available, and indeed tried, but some unknown characteristic of the Stranraer's configuration meant that it was impossible to trim out the yaw and resulting wing drop that developed with one engine shut down. Various changes to the engines' position were tried, both fore-and-aft, up and down, and sideways, along with toe-in and out, to no avail. Likewise, various combinations of enlarged and re-positioned fins and rudders were also evaluated, with the same negative results.
Finally, under pressure of both finances ( nothing had been noted officially, but a 'gentleman's agreement' had been put in place for a production contract to follow the successful completion of the Stranraer's trials ), time ( if the aeroplane did not prove satisfactory before the end of 1935, the production contract would be withdrawn ), and resources (development of the Type 300 fighter, the Spitfire-to-be, was ramping up, and Design and Technical Office staff were needed for this, rather than the 'already designed' Stranraer ), in semi-desperation, RJ Mitchell authorised a limited redesign to fit tandem engines . . .
While obviously advantageous, as of course, the wing drop problem disappeared, and with the engines mounted above the hull, the engines and propellers were shielded from the spray when taking off and alighting ( surprisingly, the metal Fairey-Reed propellers then in use proved to be much more prone to spray damage than the wooden propellers used previously).
In addition, a ready designed pusher engine installation was available, in the form of that used on the Seagull V / Walrus, although in this case the engine would be fitted with a cowling.
However, there were downsides. The rear engine later showed a tendency to overheat, and with the engines now on the centre line, the twin fins and rudders were robbed of their slipstream. However, a redesign of the tail to incorporate a centre line fin and rudder was deemed impractical ( the tail gun position would need to be revised, meaning alteration to the rear of the hull, and the tailplane structure would need to be redesigned and re-stressed ). Also, the new rear propeller was uncomfortably close to the dorsal gun position, and moving this aft would involve too much work ( production jigs and tooling for the hull had already been constructed, upon receipt of the aforementioned 'gentlemen's agreement'. )
Nevertheless, the go-ahead was given to reconstruct the prototype Stranraer into the new Mark 2 form.
cheers,
Robin.