Wannabe is right ES....as wide as your boat is you need less pitch.......but your turning 5800 now and less pitch is going to bring those rpms up a little higher so you need to see what Mariner reccomendations are for the max rpm's on your motor. .......I have a feeling that a 19 will do the job for ya. Still best to.... Check with a boat shop or prop shop and see if they do not agree