Been boating all my life and have recently become confused as to how to convert knots into MPH. All of this has come from a discussion with a bud about speeds of boats and all and I'm thinkin our speedo on the depth sounder might be out of wack. GPS and radar is claiming about 23-24 knots at 3400rpm while the MPH is reading about 20MPH. Is this about right??This bud tells me that a guy in his boat club tells him he cruises at 30mph at 3400rpm with an identical boat, although a year older but same engines. What gives? Props give ya that much?