Don't know exactly for outboards. Most are designed for high speed, if you get a high thrust version it would probably be close to the values below.
Skene's calcs show about a 20 shp engine
for your boat, this includes adding 1/3 more S.H.P. than needed to make hull
speed. I used 30% for prop efficiency (outboard probably should use lower eff.).
Rules of thumb vary.
In a different time Hiscock said all you need is 1 hp per ton (long), that's 5 hp for you. This is only good for getting in and out of marinas
at less than hull
speed. Hiscock thought 3 hp per ton was the biggest engine you would ever need, that's under 15 hp.
For sailboats, Ted Brewer states that "3-4 bhp per ton is adequate, 5 bhp per ton is ample and 6 bhp per ton is often too much."
So about 23 BHP using 5 BHP/ton, assuming he means long tons.