Since the naval architects of yore tried to figure out how fast a big fat merchant ship or a warship could sail and developed a formula for the theoretical hull speed of a vessel, much knowledge has been gained and insight into the factors that limit the speed of displacement hulls. And, make no mistake about it, most of our boats are displacement hulls although under certain conditions they can surf down waves, they do not plane in the same way a sport boat or a speedboat plane.
The age old formula of 1.34 times the square root of the waterline length is for the most part a reasonably accurate estimate. If the formula yields a hull speed of 6 knots, the boat in question might indeed exceed 6 knots for a variety of reasons, chief among them is wave making resistance. However, it will only exceed the theoretical hull speed by a relatively small amount, at best a knot or so.
To dismiss the notion of hull speed and adovacate for excessively large, powerful, and heavy motors does no one a service. A 20 hp motor will not power a boat significantly above that theoretical hull speed (however you calculate it) than a 6 or 8 HP motor. My last boat, a 30 ft 8,000lb boat had a 13 horse motor and did just fine pushing the boat pushing it to 6.5 knots. My current boat a 36 ft 15,000 lb boat does just fine with a 33 hp motor.
The idea that more horsepower is needed to counter an adverse current is simply erroneous. Speed through water is all the boat knows and it doesn't matter which way the current is moving. If the current is 3 knots and adverse and the boat can only make 6 knots through the water, then speed over ground will be 3 knots regardless of how much horsepower the boat has.
As sea states get larger, horsepower becomes somewhat less important. Once the seas are greater than about 2 feet, driving into them is sort of like driving into a brick wall, by the time they are 4 or 5 feet hitting them head on with lots of power will result in a bone jarring experience. That's why we slow down and quarter waves. If the waves are big and there is breeze, it will be much faster, safer, and more comfortable to sail.
So,
@mohi700 save some money, get a 8 hp long shaft outboard. Put the rest of the money in your rigging and sails to make sure they are up to the task of sailing in or crossing the Gulf Stream.