If you are looking for an estimate of the HP required to overcome your extra windage, lw395 has the right approach. I would do the calculation for your expected speed through the water when motoring - say 6 kts.

As lw395 says 1HP expended at 1kt requires a "force" of 86lbs (he points out that referring to force, in pounds is not strictly correct)
So 1HP expended at 6kts requires a force of 86/6 = 14 pounds.
That is every additional 14 lbs of extra windage will need another 1 HP delivered to the boat to keep you at 6 kts.
But if you want to know the extra engine power needed to provide this 1HP, you must take into account prop (and transmission) efficiency say 50% to 70%.

To maintain speed you would need 1/0.5 to 1/0.7 = between 2HP and 1.4HP from the engine for every extra 14lbs of windage.