Propulse propellers are made of 4 single blades in a material similar to hard composite. These blades brakes easy so that the gear house (hopefully) don’t get any serious and expensive damages. These blades are easily replaced and costs 300 SEK/blade. If we would use aluminium propellers the risk of getting expensive damages increases. It’s also difficult to see on a aluminium propeller 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.