So, the useful power, if we had a ship, the ship would have taken this thrust power T into V A, and the input power on the other hand is what we have calculated. Do you get it? That is, T into, what have we calculated? We have calculated the input power know V a plus V 1. This is the power of the, this is the thrust power generated by the, is inherent in water and this is the power we are using. This is the useful power; this is the input power.