As suggested by gkamysz I was interested enough and did indeed do some looking up of nylon and water absorption.

First - there are different types of nylon and thus the effects can be different.
Not being of a chemical bent I didn't delve into this and suspect that the type of nylon used in props may not be identified in their description - I could be wrong

As far as I can tell.... the original purpose of boiling nylon props was to relieve stresses induced in them during manufacture. These stresses would normally be relieved by ageing of the propeller, but because you don't know how old the prop is when you buy it the trend was to boil it and thus ensure the stresses were relieved.

That was OK for nylon props but now some props are made from composite materials i.e. fibreglass and nylon or carbon fibres and nylon.

For composite materials APC state that excess moisture will weaken the prop... other manufacturers I don't know about... and it was advice about an APC prop that I was querying.

I might be wrong but I will put forward an idea anyway........
Props made with composite material if subjected to excess moisture could exhibit hydroscopic behaviour whereby the fibres, either fibreglass or carbon, draw in water because thay are in effect a tube. In a normal fibreglass layup, such as used in boat hulls, this would lead to de-lamination and weakness.

I assume this is why APC state NOT to subject composite props to excess moisture.

I have had a look at Graupner's site and it is far less informative and I couldn't be bothered delving about for info that may/may not be there.So does the advice to boil Graupner props only apply to their nylon ones or the composite ones as well ?

Now... if only a leaflet was included with a new prop explaining best/safe practice how nice that would be. I haven't had such a leaflet with any prop I have bought so far [&o]

From a very quick scan around I see props made from a variety of materials, which may require different ways of treating them, and for some makes the information is not as visible as it could be.