Quote "Ok, So I know the Diameter now 35mm and I know the pitch 1.4. What does this number tell me? This is what I need to know in order to select a suitable prop for the boat.
Prop shop said I need RPM's as Im using smallish brushless twin motors. Not power." Quote
It tells me that you haven't given enough information for me to comment much. The important thing about a power system is to think of it as a whole and not as a series of separate parts. To give significant thoughts we need to know the type, size & weight of the model together with the size & kv of the motor and the batteries used together with the prop details that you have given except that you haven't told us about the number of blades nor the type of prop surface piercing v sub surface etc. We need to know what level of performance you are looking for. Slow progress around a lake is wholly different to planing nicely and again wholly different to blasting around like a possessed demon.
In general a 35mm diameter 1.4in pitch prop is, as Propshop have said, suitable for a planing hull type of model which requires a small column of water moving quickly rather that a slower model which might require a larger column of water moving much slower.
If you are looking for a prop recommendation we need as much of the rest of the information as possible. If you want a recommendation for a complete power train then we need more detail of the model that it will power. I am afraid that a 41in long seaplane is not a description that works in my brain.
This is not me being awkward but rather needing as much information as possible in order to make sensible comment.
