The last time my top sprocket was larger than the bottom my boat and motor were both upside down. Mine has the smaller sprocket on the engine. Spinning the prop slower than the engine using different size sprockets increases torque at the prop, therefore a larger and more efficient prop can be used.
One reason most longtails are slower than surface drives is they run the prop at the same rpm as the engine. That limits the prop rpm and size by the amount of torque the engine can develop. If we use a hypothetical example it may be clearer. Suppose the top sprocket (on the engine) is a 20 tooth and the driven sprocket is a 40 tooth. That gives a ratio of 2:1. (I know, no one uses a 2:1 ratio, as I said this is hypothetical). At 2000 engine rpm the prop turns 1000 rpm and while cutting the rpm in half it effectively doubles the torque at the prop. There are all kinds of variables that enter in like slip and friction, but lets keep it simple. All those variables just confuse me. When torque is increased the prop can be more aggressive in diameter and pitch. A larger prop is generally more efficient at providing thrust than a smaller prop. Look at the recommended prop diameters and pitch for a longtail versus a surface drive of equal engine size. On my EFI turbo Briggs the ratio is 1.4:1. At 1400 engine rpm my prop turns 1000 rpm. On my 24' X 60" boat with EFI and no turbo I turn 3400 to 3600 engine rpm depending on the load. If I can significantly increase my rpm, say 4200/4300, with the turbo boost I will have, obviously, increased the torque. That tells me I can go to a bigger prop or change my ratio. I don't know of any larger props available for my rig so I will change the ratio with incrementally larger upper sprockets and see how high I can get the rooster tail. I sure wish the rivers would open up!!!!! I hope this answers your question.
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