IanD

If you know the engine/gearbox details (power/rpm/ratio) you could do worse than seeing what the Vicprop calculator suggests as a starting point, and whether this will physically fit on your boat... https://vicprop.com/displacement_size_new.php (for all the Beta engines I looked at it suggests prop sizes pretty close to what Beta recommend, or maybe a bit smaller) If the recommended size won't fit (they're usually about P/D=0.7 which is optimum) because the diameter is too big, then you can decrease diameter and increase pitch by twice the amount (e.g. diameter -10% pitch +20%) and get similar performance. It's preferable not to go oversquare (pitch>diameter) but you might have no choice if there's not enough space from prop shaft to skeg/uxter plate...