I think I may have a change of plan regarding the prop shafts.
From what I can see, a pair of 10" prop shafts ( with 11" stainless inner shafts ) should work out about right for the twin brushless motors I have in mind to power the finished boat (maybe a bit optimistic about the finished bit), but I am tempted to buy a pair of 6" outer shafts with the two 11" stainless inner shafts and a pair of P brackets from Modelboatbits.
I intend setting the motor drive so both props turn inwards towards each other, but I have not made a twin shaft model boat before and so I am not too sure about the availability of left and right handed propellers of the size I may need, and if I need to have one of the inner shafts cut with a left hand thread to stop it "unwinding" when under power?
The other option I considered was making the shafts with a dog-drive collar fitted, to ensure both props remain secure irrespective of the rotational direction.
Then I have the problem of finding a "pair" of props to suite the brushless motors. With a single shaft drive it is possible to use inexpensive plastic props to use when testing to find the best size that works, but from what I can see I would need to buy a pair of brass props, and this could make testing a little expensive unless I happened to "guess right" first time!
I am using a pair of Turnigy 3648-1450 outdrive motors. If anyone has used these motors and can advise what prop sizes work the best in a large hull it could save me taking a "stab in the dark" when buying a pair of brass props and it could save me some time (and cash!) too.