You can forget 1/4" drives and your 115mm RC props. All a little too small for a cracker box that size (basically 1/2 scale).
A Blata 40 or G62 Zenoah are not going to do much with a 115mm prop. And they definately wont move what will likely be a 60+ pound boat. I would strongly consider picking up a 4 or 5 hp outboard motor and adapting it to fit. I started doing this with a 4hp out board (Comes with tank, prop, and most other parts you will need). You can pick up a quality used late model one for around $350 (I recently got a 3,4,and 5hp Mercury so have been shopping the prices well).
You are going to need torque to get that boat moving and I am thinking a Mercury will likely do it best for the $$
If your really after an RC prop - I have a 11013/4 factory stock - I paid $135 for it. I could probably ship it your way for a little less
[link=http://groups.msn.com/_Secure/0UQA0Ds8YLvm!hA4FjiZ8388xDBLwGVi5Ig7LefMDmyJpQSDyb RZa*YZFI4XiZ1em7dfOJkBXB2BnAyoKfeYrc9eG2N4XlFguEMA IMsBXdKR3ke1bphq5mzLGpetM1srL/Props%20023.jpg?dc=4675531577083026980]Pic of prop here[/link]