That is a very tough question to answer with the limited information provided. Haha. First,a 40hp motor on a 24 foot pontoon is not going to break any speed records.
With that being said, you need to see what your rpm's are at WOT(wide open throttle) while out on the water. Then you would need to check your manual for that motor and see what the recommended max RPM's are for that specific motor. If you are below the max recommended RPM's at WOT, for every inch you drop the pitch of your current prop, you will gain about 200 rpm's.
So, you are currently running a 15 inch pitched prop (that is the 15P on the end of the number you provided). If you are only running at say 5000 RPM's at WOT, and your motor is rated up to 5600 RPM's for WOT, you could drop down to a 13P, or 12P and would gain about 400-600 RPM's. Just make sure you get the same diameter prop that you currently have on the boat. You should be able to find out the diameter by looking up that part number through Quicksilver, or most props will state the pitch and diameter on the end of the prop (under the nut). It should say something like 14 1/4 X 15. Is it a stainless steel prop on that motor? Also make sure you check WOT RPM's with same load on boat you will be using it with.
Depending on your current WOT RPM's, you may not be able to do much to get more speed from your boat. Hope this helps!
