What prop size for my narrowboat?

Hull: 70ft long 6ft 10 inches wide.
Weight: 10 tons
Engine: 48 BHP at 2800 (54BHP at 3000)

There is 11 inches between the centre of the shaft and the counter

I currently have a prop with an 18 inch dia and 13 inch pitch. However the
engine never seems to struggle to turn the prop even when slamming in
reverse from full forwards.

Adrian Smith