the thing i was worried about with like a 60" is that my boat is only a 48 and might be like 56 or so at the top it dont have but 16 inch sides so it dont get much wider at the top, the reason i was wanting a 23hp is for the cost, when you get up in the 35hp i could get a harley motor off ebay for the same price and have a lot more hp, who could build me a custom reduction
look up warp drive or hazcon props they will have a tip speed calculator based on prop dia. vrs RPM. one of the reasons to use a reduction is to keep prop tip speed down to reduce noise, another reason is to increase torque so you can use a larger prop or one with more pich.
Harley motor has power pulse issues that props do not like. I run a 48" 3 blade powerfin on a direct drive 24hp so your gonna need more blades, much wider blades, or longer blades. Reliable reductions are not easy to build unless you have a machine shop at your disposal. I would go the same route as seaurchin on the 503 Rotax. There are reduction built specifically for rotax and with a 48 bottom you need to reduce any unnecessary weight.
This is an old one but im sure you guys appreciate a good search and find. I've been debating a fan build and had the craziest idea to put a motorcycle engine one, and sure enough here it is. Anyone tried this, id like to see one if so. I know theres supposed to be problems with it but im hardheaded and would have to see it with my own two eyes. I cant help but think about one of these babies pushing my boat.