Re: prop question
I'm no prop guy for sure, but I would say if you've lost boat speed with the same rpm and same prop dimensions (I assume also same trim and same boat!), either you are wrong about the old prop dimensions, the new prop design is different or your throttle isn't adjusted to go to full w-o-t. Possibly your old prop may have been repitched or repaired at some time so the markings are wrong. However, it's hard to tell what the problem is and if you just buy a smaller pitch prop you may well just go even slower or over-rev your motor.<br /><br />I suggest you figure out the recommended operating w-o-t range for your motor, and then work backwards from your current w-o-t rpm and prop to figure out what pitch/diam props will run in the right range (see below for how to adjust expected rpm for changes in pitch and diameter). Then pick a type of prop that suits your uses, and get the biggest pitch prop of that model you can that you think will work in the range if all you want is speed. I don't think anyone can know for sure what pitch will work best based on what you've told us -- we don't know what speed you get, what rpm you are running or what kind of boat you have. <br /><br />On your diameter/pitch questions, in my attempts to reprop my own boat I've been told more than once that every increase or decrease of a quarter inch of prop diameter changes w-o-t down or up by about 100 rpm, while every upward or downward inch of pitch changes w-o-t down or up by between 200 and 250 rpm. If you go to a smaller pitch propellor with a larger diameter you probably will get slightly higher revs, slightly slower speed and slightly better control (unless you are currently overpropped so your engine is under-revving).<br /><br />I say probably because there are a ton of other prop variables including blade area and shape, cupping and material (aluminum/steel/composite) that I can't begin to account for. Maybe someone with experience on the site can. Bottom line is that your new prop may not be the design for you if it doesn't give you the speed you want, so look at the old prop and see what differences there are (is one cupped?).<br /><br />One more caution, although I'm working on it I haven't managed to figure out the right prop size for my own boat yet, so anyone reading this feel free to correct me on any of this...
