Max Speed in knots =1.4 x (the square root of the waterline length of boat) in feet. Max Speed in knots = 2.43 x (the square root of the waterline length of boat) in metres. Max Speed in km/h = 4.5 x (the square root of the waterline length of boat) in metres. Many owners of light glass fibre cruisers may be surprised when they apply these formulas to their boat. They may well find that their engines are grossly over-powered for river cruising.