I ran a 13 3/4 x 15P OMC SST on a 115 my Seafari for years and it was a perfect match. However that was a 1975 motor rated at the powerhead, so it was probably only about 90-100 hp at the prop, and your boat is lighter so you should be able to turn something like a 13 1/2 X 17p prop. I'm not that familiar with Yamahas and don't know what gear ratio they run, which would affect prop selection. It was 2:1 on my old Evinrude. Snookerd ran a 130 yami 2S on his 18, so could probably give you a better recommendation.
There are at least 3 things you need to do before you start changing props: 1. Make sure your tach is accurate. 2. Make sure motor mounting height is correct (AV plate out of water when on plane). 3. Do all your testing with full fuel and the maximum load you intend to carry. Add some water ballast if necessary.
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