That LOOKS like a short shaft on a 15" transom to me, hard to tell in that pic though.

What width is that boat, 48"? If it is, my advice would be to sell that 15 and go with a 25. In all honesty, that's gonna be the only way to safely give you the performance your looking for. The motor you have could sell for 1000-1200 dollars realistically, and you could use that towards a 25. There's a guy on here selling a 25 2-stroke yamaha that would be ideal on your boat if its a 1548. That would be the best thing to do in my opinion. May not be what you want to hear, but it's the truth.

If it's not a 1548, and is maxed at 15hp rated, your stuck with what you have, but just a warning, those clamps in your pic are not safely secured. That motor will come off in some point, you better bolt it through the transom