Ok ... let us know , but for a prop to be the most efficient , it need to be a bit away from anything - front or rear .
Do a simple test ...
Take ANY prop , on ANY boat, plane . ( use a piece of wood here folks , don;t get stupid and try your hand )
Place the wood in FRONT of the prop near the center ( hub ) . Now , increase the size of the wood . And increase again ... notice any difference ? ( you bet you do )
Now , do the same with it in BACK of the engine ... seems like you LOST a lot , huh ?
see the pic below .
In the circled area , this is MOST vital area for air to get to for the props efficiency . The less area that is covered/blocked , the better the prop does its thing .