Join Date: Aug 2011
Location: Central New Jersey, sailing on the Navesink River and Sandy Hook Bay mostly.
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I had thought you were kidding. I did not mean to offend. To drive a bigger prop, you need more low end torque, not high end horsepower. Or, as has been mentioned, you could change the gearing of the transmission. I just picked up Don Street's "Ocean Sailing Yacht" and he talks about the British Sea Gull outboard, which has a really low gear ration and swings a big prop and generates a lot of thrust. In short you want to increase displacement (bore and stroke) to increase torque. Props are most effective, roughly speaking, below 2,000 RPM. Increasing horsepower by the means you discussed, will allow the engine to breath better, rev higher and produce horsepower at the top end, exactly where you don't want it for a propeller. Typically when you increase horsepower, you have less at lower rpms and more at higher RPMs, leading to the "peaky" nature of racing engines. The best torque producers are big single cylinder thumpers. That's why many boats had an optional 10 HP diesels like the Yanmar YSM 12 (1 cyl), vs. the 30 HP Atomic (4 Cyl). Less HP, more torque and a lower gear ratio. This is why tractors have four cylinder engines and Formula 1 cars have v12s and the like. For planing hulls, you can make use of high horsepower, as drag is reduces when on plane and out of the water, but for a displacement hull once you are at hull speed, drag rises dramatically and the bow wave gets bigger, but speed doesn't increase. Both the Macgregor 26M and the Hunter Edge have a planing hull so they can use the 90 hp outboards. My 28 displacement hull will get to hull speed (6.4 kts) with 10 HP. No matter how much additional horse power it won't go faster than 6.4 kts. The YSM in my boat swings a 13 inch propeller, vs. the 9 inch you quoted for the Atomic. I hope this makes sense.