Clarke and the Lord Dunravan (the Earl of Dunraven) entered yacht race pursuant to Yacht Racing Association rules which provided that if a yacht was damaged as a result of negligence, the negligent party must pay damages.  Lord Dunraven’s yacht (the Valkyrie II) was damaged by Clarke's yacht (the Satanita) as a result of the latter's negligence (the collision also killed a crewmember). 

By entering the race on the YRA terms the competitors entered into a contract with each other on those terms – consequently Dunraven had to pay.