Beyond a certain point more power just represents extra weight and reduced fuel economy (it takes more fuel to turn a larger engine over than a smaller engine). Larry's boat needs about 18 hp to run at 8 knots on flat water. In rough conditions (30 kt head wind and worst case seas), he will need about 100% more power. That works out to about 36 hp. He has 59, which is more than he can use in the worst conditions. Hence he is over powered. Most sailboats are set up to reach nominal hull speed plus about 10% at maximum engine output on flat water. That power is based on the idea that a sailboat will normally cruise at about 75-80% of hull speed where the power requirement is on the order of 25-30% of that required to run at hull speed. For example on a boat with a 32' waterline, hull speed is about 7.5 knots. 6 knots is 80% of hull speed and requires only about 30% as much power as 7.5 knots. So for a 32' LWL sailboat having three times the power needed to cruise at 6 knots provides an more than ample reserve. In addition, fuel consumption is pretty much directly proportional to the power output of the engine. Using three times as much power takes three times as much fuel. Finally a given prop setup (gear ratio, prop diameter/pitch/blade count/DAR) can only put out a certain amount of power. Having more than that available serves no purpose since you can't transmit it to the water