Propulse propellers are made of 4 single blades in a material similar with hard composite. These brakes easy so that gear house (hopefully) don’t get any serious and expensive damages. These blades are easily replaced and costs 275 SEK/blade. If we would use aluminium propellers the chance of getting serious damages increases a lot. It’s also hard to see on a aluminium propeller for which guests has caused which marks/damages if you don’t replace the aluminium propeller every single time these have been touched by stones etc.