Saving Old Seagulls

can anyone tell me how you work out the knot speed of a displacement boat .
someone told me a long while ago but have forgot.
say i have an 18ft x 7ft cobble boat with a draft of 2ft .
i want to put an outboard on but dont want to buy an engine that is going to be more powerfull than i need.as once the boat gets up to speed it dose not get any faster no matter what size engine gos on it.
sounds like a bit of a quiz. i know but there you go .
i am shaw someone will have the ansure.

Only rule I know for max. hull speed of a displacement boat is square root of hull length x 1.4 - eg a 16 foot boat = sq. root 4 x 1.4 = 5.6 knots, but this doesn't relate to engine size which is de[pendent on a lot of other factors - eg will the boat be used in areas of strong current, tidal flow etc, do you want the boat to stop in the shortest distance etc.
I visualise cobles as heavy boats but often double-ended? Rick

I learnt at school: square root of waterline ( length of the hull which is in the water) in meters times 2.45 is your speed in knots.

But: School was a long time ago!

cheers lads .but i am not that good at sums .
and a BIG thanks will see if i can work it out .
will just be useing it for a bit of fishing.
i knew some of you old seadogs would help.

Yes its the waterline length that matters.

So the max speed in knots is 2.45 times square root of LWL in metres or 1.35 times square root of LWL in feet.

here is a link to a site that you may find usefull
Stephen
http://www.sailingusa.info/cal__hull_speed.htm

nice one my old mate .and i thankyou.