Check Yamaha sites to find the range of props they recommend for that motor, then try to guess which might be best for your application. I had a Zodiac MkIIC with a 25 OMC on it. Of the recommended props I tried a 10X15, which was OK when just me in the boat but bogged down when loaded with dive buddy and gear. 10X13 was best compromise for me. You might be able to up to the next pitch since your prop seems to be a little small.
Do you play harmonica? We had a great player up here who went by your name.
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