Well, ask yourself this question...How many boats of your size/type/power have props of 28x24 going at 12+knots?
Whilst we can debate maybe the pitch of diameter should be slighter higher or lower, accordingly to the calc's....but, is it a realistic meaningful result? Ergo, how many boats do you see with a 28x24 prop?
Naval Architecture (designing boats) is not an exact science. There are disciplines within, which are such as: mechanical, electrical and structural, some aspects of hydrodynamics too. However, as a complete package, the "exactness" often falls down in reality. ..ie a boat is greater than the sum of its individual parts. Only experience can tell you this, not numbers spued from a program. In the absence of experience, look around you and your marina/magazines etc..what do you see, hence the question about how many props of 28x24 do you see? So, some self analysis can be done to obtain a better idea.
So, a 28" prop is very very large for a boat your size and power, just go to your local marina and check.
There are only 3 things to consider when selecting a prop
1) The speed of water in way of the prop (different for different hulls)
2) Horse Power at the prop
3) Prop RPM.
The hull, can only go so fast with so much power given and also the shape of the hull shall dictate the speed too...etc etc...so, many things to consider, which a prop DIY program wont tell you.
PS..forgot to add.
If your engine has an even number of cylinders, ie 2,4, or 6 cylinders, for example, best to select a prop with odd number of blades. And vice versa.
