HS93 has done a brushless motor set up , hopefully he will see this and advise accordingly, at least you are then getting advice from someone who has actually done it. I helped him set the speed controllers up, we had a very hard time doing this, they where both exactly the same, but just would not come in at the same time. I think the motors where mystery outrunners, I know the boat was smaller than yours, and it was WAY over powered!, extreemly quick
You might consider single screw, I have done this on a 42" Huntsman, Align brushless 600 motor, castle creations 120amp esc, 6 lipos cells, 5000mah, 3:1 gear ratio.I did a load of testing, 2:1 ratio was giving very fast acceleration, 3:1 is better top speed, 52.5mm "X" prop on a small lake, 55mm on a big lake.
I ran the boat tonight, its ballistic, and topped 99.9amps
, If I hit 100amps is that a record for a scale boat
I also ran freshly charged 19.2v nims, 4600mah, the speed was noticeable slower, unfortunately I didnt have any means of recording it, but I guess it was 2/3rds the speed of the lipos
Hope that helps