advice on new prop

[mlbm79]

Hi

I own a quintrex 455 coast runner runabout it has a 50hp 4 stroke johnson on the back .I can only get 5500 rpm flat out at about 45kmhr when 4 people aboard it has a hard time getting on the plane.I cant find any measurements on the prop the only thing it has is s1400 .The manual says it makes 50hp at 6200 rpm.

Thanks for the help mlbm