Propeller question please advise

[TeamJoseph]

Hi All

we are based in South Africa and we boat at a small lake about 4300 Ft above sea level.

we have a Moomba Mobius LSV 2005 model
I am struggling a bit to get a few more miles out of my boat as my kids are now barefoot waterskiing.
When towing a barfoot waterskier I am getting a max of 38 miles.

I am currently running a 13.5 x 17.5 propeller ( 4 blade)
What prop size / pitch would be ideal or what would be the std fitted propeller that should be used

When we got the boat it was fitted with a 4 blade propeller size 13 x 15.5 which I still have but it had more torque but less speed

I would really appreciate if you could direct me in the right direction.

regards

TJ