So, Cal 28' weighs about 8000# (4 tons)
Volvo Penta 2002 is an 18HP diesel
Prop is probably an Indigo A4 optimized prop which is a 3 blade 10" x 7.4" pitch Boat Seem Underpowered?
Other factors to consider: your prop is not in an aperture and thus should get good bite out of the prop flukes.
If you feel under powered (what rpms do you run your engine at?) it is not so much the pitch (~7") as the diameter that I'd question as being inadequate.
I know that this is not entirely comparable but folks with an Atomic 4 in their Catalina 30's use a 2 blade 12"x6" (or 7") prop and report that it works fine.
I use a 13" x 8" on my 7200# boat with our A4 but our prop is in an aperture between the keel and rudder.
The wider diameter of the prop gives it much more surface area to push against the water. The pitch will always be in the 6" - 8" range most likely.
Try to find a used 12"x6" (or7") 2 blade prop and try it out.
If you want to stay with a 3 blade prop I'm not sure what dimensions would work best for you as I'm more familiar with 2 blade fixed props. You can have a good prop shop take your existing 3 blade 10" prop and they can add material to the flukes to make it 11" or 12". They can also modify the pitch if you want.
First you need to find a good prop shop though.