Thanks for the reply Desmond.
Yes your are 100% correct with us too there is no normal load anything from myself to 5 up .
25l fuel tank and battery are mounted underneath the rear seat.
I too found the best posistion for the trim pin was 2nd from bottom.
In the handbook Mariner have the WOT at 5150rpm - 5850rpm.
They have list 6 props
3 x light duty
3 x 252 x 360
3 x 252 x 332
3 x 250 x 288
3 x Heavy duty
3 x 250 x 260
3 x 249 x 236
3 x 260 x 210
Non of these seem to match my current prop 9 7/8" x 10.5" (250.82 x 266.7).
Do agrre the best way to go is get the tiny tach fitted (when it arrives) and go from there.
Its just a PITA as i can only try it when im on holiday as im miles from the sea