This is a series of calculations of the power required to drive the hull at the required speed, and is based upon calculations that deal with the dimensions of the hull and the weight of water that it displaces.

The Speed / Length ratio is the required speed in knots divided by the square root of the waterline length in feet. In our example vessel this is 7 / (sq root 27) = 1.347 S/L ratio.

It is imprtant to remember that every displacement hull has a maximum speed that cannot be practically exceeded no matter how much power you throw at it, a quick approximation of this speed can be obtained by 1.5 times the square root of waterline length in feet, for our 27 foot LWL example hull this gives a maximum hull speed of 7.79 knots. Our required speed of 7 knots is thus a practical and economical target speed.

If we divide the 10,000 lbs displacement of our example vessel by the 42 SHP available we can determine our available power to weight ratio as 236 lbs (of vessel) per shaft HP.

We can calculate a minimum required power to weight ratio by doing a different calculation of horsepower required at the propeller based upon the S/L Ratio of 1.347. We divide the displacement of 10,000 lbs by a complex little equation to give a power required at propeller of 20 SHP, and a minimum required power to weight ratio of 496 lbs (of vessel) per shaft HP. This is easily lower than the 236 lb/SHP available. Since this is just another cross check I haven’t gone into too great detail here, examine the spreadsheet if you want the exact formula.