It is not possible to achieve hull speed with only 40hp. The best you are likely to see is perhaps 7.8 knots. This is primarily a function of weight - 10 tonnes+ needs closer to 50hp. The optional D2 55 is a much better choice for this boat and would give a speed of around 8.75 knots.
However, you have to work with what you have. First thing to check is your engine rpm and actual speed at full throttle in flat water. The engine is rated at 3200 maximum and you would expect a correct prop size to achieve at least 3000, hopefully higher. As I explained in reply to Winga below, 1" of pitch change leads to a change in engine revs of 250-300 so given propellers come normally in 1" increments it is difficult to get exactly right, but you need to be able to access maximum power so aim to get as close to 3200 as possible. Normally this engine/drive combination would have a 17" propeller, but going up an inch in diameter is a good thing when you are marginal on power. I suspect that 12" pitch is already on the coarse side and certainly moving to 13" would not be a good idea as it would limit engine revs and lead to a lower maximum speed, although the trade off would be lower cruising revs for a given speed.
Carry out your speed test (make sure the hull is clean) and if you get over 3000 rpm and around 7.8 knots then you have got the best you can. If the maximum revs are below 3000 and speed below 7.5 knots then there might be value in going down 1" in pitch. You can buy just a blade set which should be much cheaper than a complete propeller.
