Sorry but I think we need to know what type and size of boat are you looking at, and of course, what you are intending to do with it. There's considerable difference in terms of power requirements between...say...a tailerable sailboat verses like..a 50' Cat or Tri...or a even 45' mono set up for offshore
I'm not an expert on this, but its pretty much a given for cruising sailboats that they have a designed hull speed
(a formula having to do with length and beam on the water
size etc. etc)...and most cruising boats...in general...appear to have an engine
that should push the boat close to that calculated speed. I say most, because there's always lots of exceptions. We have a 41 foot sailboat that the iron-Jenny will push at hull speed
of about 7.5 to 8 knots...which is the design calculated hull
(sailing) speed. (Of course it's slower if we're going up hill but that's another topic)...I don't know how fast we can really go because with the sails
up and engine running we jump to Warp Speed and our gauges are not set up for it...although I have seen Marlin trying to keep up with a plug
I was trolling once and I figured it was pretty fast because he burned off two fins just trying to catch up to the bait... but I digress....there are people out cruising who don't even have an engine which tend to make the rest of us look really bad sometimes.
On a more serious note...In the channels and docking
I think you only need enough power to overcome wind
...our motto on Pure Joy is slow is good...slower is better and if you're able to dock
with no engine running at all...that's a Royal Flush! OTOH I've seen people who can't make it into the dock
or down a channel without a thousand horsepower...go figure...in sailing...like life...it's not what you got...it's how you use it.
Ray and Sandy
S/V Pure Joy