In at least one case I know of, a cat owner had Michigan Wheel make a 4 blade unit to replace the original 3 blade that came standard on PDQ 34 catamarans with twin 75 or 100 HP Yanmars. According to the owners on the PDQ forum, the efficiency and vibration was improved markedly, especially the "singing" the three blade units were known for. At least one boat claimed to gain two knots on top end but all seemed ecstatic over the sound levels...some claimed 50 percent improvement. PDQ was known for their design prowess, but this was apparently something they missed. Rare case, but sometimes it works out.