I have been checking out different props and have noticed on some props next to the hub but on the trailing edge they have filed a 1/2 round .
Q :- whats the purpose behind these 1/2 rounds and are there hits and misses ?
many thanks for your help

The cut you are referring to is called a Barr cut and is named after the gent who came up with it, Stu Barr ( not Bar ).

The purpose is to reduce the blade area ( therefore load ) of the prop when it's lower in the water. IE; accelerating and going through corners. This frees the motor somewhat, and pulls harder. Once the hull is up and hauling, the working surface of the blades is not so close to the hub, and the cut has little if any effect. The size of the cut varies greatly between props as applied by the prop modder. A cleaver prop ( pointed tips ) usually surfaces higher than a chopper ( rounded tips ) prop, so they can use a larger cut if need be. Some prop modders put such a small Barr cut on the blade, that it does little more than "sell" props.

Reading this short section should help you understand props - everyone has their own views - stick with a reputable prop guy and they will help you a lot.

www.modelgasboats.com/resource...wing-your-prop/