Dug you have been given good advice from the other members...
The simple explanation is you Can't fight length of water line... You boat's overall length is 24', but its water line is 20'. Theorectical Hull Speed
won't change unless you increase the water line.
Since the prop was replaced in 2006, make sure it was the proper pitch
for your engine/transmission. If it is, the only other thing you can do is keep your bottom and prop and shaft clean... You can lose over a knot
of boat speed with a dirty bottom or towing a dinghy
My first keel
boat was a Catalina
27, I was in San Francisco
Bay and most times had stiff 20 knot
breezes throughout the summer months, directly into the mouth of my marina. It had a 6 hp Pettit Deisel Engine
, which made about 5 kts in calm conditions. The only way I could get out of the marina at a reasonable speed during the summer was to motor
with full sails
I ended up replacing the engine later on, after the orginal gave up the ghost. The new engine was 14 hp and I re-pitched the two-blade prop to accomodate the higher horsepower/transmission ratio. I got about 3/4 of a knot more speed and burned a little more more fuel
So with all that said, if you want more boat spend, don't speed a lot of money
trying to squeeze a little more speed of your boat.... Play with this one, get more experience, save your money
and in a couple of years buy a bigger boat!