Hull speed potential is a good (calculated!) starting point. After that you assess your particular boat's resistance to moving thru the water. Basically, the parts of the design that contribute to more interior "living" room slow the boat down and will require more HP to power it.
It's best to remember that some hull designs are intrinsically more slippery than others. While a few Hunter hulls, like the original 25, were transitioned from faster-performing hulls, most of the shapes have been more dedicated to interior comfort.
Your H-33 is long enough to maintain 6.5 continuously in smooth water. Your engine was probably a bit marginal when new, but still adequate. A new sailboat that size would have an aux. engine rated from 23 to 28 hp.
Part of that is also because of evolving owner desires... for amenities... like a horsepower-consuming large alternator.
For one data point, I have done a multi day delivery on a 33 foot offshore performance boat with the Yanmar YSM-12. That particular design (Yamaha 33) is a very slick hull form with a very efficient engine installation. We could do 6 kts for 24 hours in the ocean with that engine. It's a real good motor, even tho the 12 HP is likely closer to 10 HP at the shaft, as has been pointed out. It was also unexpectedly quiet considering that it's a big single cylinder. Parts are harder to find nowadays, but that's a challenge for most engines of that vintage.
Further, and no, you did not ask... but... be Really sure that your boat has a correct size and pitch propeller. Most new owners take for granted that whatever piece of cast bronze is down there (out of sight) under the water must be OK, because it came from the factory or maybe some prior owner. This is seldom true.
And getting whatever amount of HP you have delivered efficiently to the water is more than 50% of the problem.
Does your boat have a "little bit" of slime or growth on the bottom? There goes another half a knot.
Good luck in finding some more knots!