I have an employee with a 21' or 23' boat, and his mechanic recommends a 17/23 variable pitch.

I have not heard of these?? What is the concept behind them, and do they work?

I did tell him that no matter what he does, try it on the boat first.

I had a customer one time that ran composite (controllable pitch??) props on his fishing boats. Said you could change the pitch of each blade individually when on the trailer to offset the amount of passengers/fuel/gear etc without changing the entire prop out. Also the blades could be removed and replaced for about $30 each if he was to tear one up.

I may be wrong but the 17/23 would be a proggresive pitch, not a variable pitch.
. A 17/23 progressive pitch propeller has a low pitch leading edge (17) and a high pitch trailing edge (23). The blade progresses from 17 to 23. At one time, someone (incorrectly) called it a variable pitch and it stuck, so now, everybody calls it that.

thats according to a forum over at pwctoday

Mercury used to make a variable pitch propeller. They are now history. The spring operated cam system was a problem when it got dirty or corroded. Torque loads determined the pitch automatically. They had a place in family boating for skiing and handling a load. They were expensive and costly to repair. They really didn't work that great.

By the way 17-23 inches of pitch would be a very large pitch change and not used in progressive pitching.

Land and Sea use to make one also