I do see the sense and logic in all of that, and sorting the wheat from the chaff I shall take your advice accordingly.All this stuff about tapers and splines is irrelevant to your issue of what prop you should have. As I suggested earlier, pretty sure a 2 blade was fitted as standard. The aperture is more than big enough to swing a 16" and probably a 17" while still leaving enough tip clearance. It is true that some are fitted with a 3 blade, but the ones I have seen are a feathering propeller like this one harbouryachts.co.uk/Boat-Details/SEKP%201641/Freedom%2035/ which also used to be in our club. This has a Featherstream the same as Dragonfly, the boat berthed next to mine. Certainly worth considering fitting one - far superior to a Kiwi but of course much more expensive.
While it is clearly important to fit the right prop, there is nothing magic about it. Once you have the basic data about the boat and engine the calculations are easy for a standard fixed blade prop. A little more complex once you get into feathering and folding propellers as designs vary and the size may well be different from a fixed blade, and different between designs. So always if buying any prop take the supplier's recommendation. That way if it is wrong the responsibility is theirs.
Before you do any of that, though you need to establish what prop you have fitted currently and how it performs.
Sorry I have assumed somehow that there is a sail drive involved need to read betterNone at all of course, I am under the impression the prop fits on a taper with a key in it, no mention has been made of a spline, other than by your good self, quite a different proposition as both prop and shaft can only fit if both using the same measurement system, either imperial or metric.
Splines are much more critical in that respect.
This is in reply to #40