what would you reccomend mate as im looking on ebay for decent size engines and there aint many about ive looked for 60hp but the price is a joke
how much you want to spend!
a decent second hand 40 to 50hp motor especially if its a 4 stroke is going to be well over a grand at the very least, probably nearer 2
which makes a brand new 15hp 4 stroke seem very attractive
the suzukis are belting motors IMHO, will run all day on half a pint (or so it seems) and are so quiet you've have to look to see if they are running.
PA Lynch in Morpeth are doing them with a 5 year warranty for £1800 at the minute
all boats have a theoretically maximum hull speed, anything over that and the boat needs to 'climb out of the hole' it creates in the water and get on to the plane, so the motor needs sufficient grunt to not only shove the boat forward, but to lift the weight of it as well, so hulls are designed to 'plane' so over a certain speed they climb out of the hole and skim across the top of the water, if yours is a displacement hull, it will be designed to grip the water as it shoves it out of the way, which is what makes them very seaworthy and stable, especially if it gets lumpy, but would need a massive shove to get it on the plane - probably more than the boat was built to take
there's usually a bit of leeway either way though, roughly speaking the maximum hull speed can be worked out as 1.5 times the square root of the length, 20 the square root of 16 is 4, times 1.5 gives around 6 knots
15 horses might push you up to 7 or so maybe tops, but to get it into the low teens would be a huge leap into 60 70 or 80 hp territory, and the boat is probably just not strong enough to take that kind of power ( or weight)