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jakeetaylor
Member Posts: 3 ✭
I have a 1991 rinker 260 fiesta vee and wanted to upgrade my prop and am looking for advice. It has the 7.4 mpi with a bravo one stern drive. I'm looking to improve on fuel economy first and performance second. Any advice or recommendations or experiences would be appreciated.
Tks
Jake
Comments
PC BYC, Holland, MI
too much bite, as represented by a higher pitch, puts too much stress on the engine and it can't spin it to speed.. too little a bite allows the engine to spin far too fast and above both the balance rating of the engine as well as spin outside the most effective range of the camshaft.
imagine trying to drive a screw into wood with a low pitched (high thread count) machine screw.. imagine trying to turn a wood screw into a block of metal... the wood screw (if you could pilot the hole) would be pitched far too much and you wouldn't be able to turn it... the machine screw you could turn as fast as you want, but it won't bite.. you, (the constant, here) being the motor, would be ineffective.. if you made the screw the constant you could find the material that allows you to apply full force and bite that doesn't slip too much or resist too much.. if the material is the constant (water, in this case) the load shifts back to both the engine (power/torque) and the leverage- and the screw becomes the final leverage device... the balance for the effort should be evenly spread using the outdrive's reduction and the engines power- too low a pitch (low pitch number, say 15 forward inches for every revolution) and the engine overpowers the bite (traction), and you'll spin with more slip than bite.. too high a pitch (say 24 inches for every revolution) and the engine doesn't have the power to turn it up to speed.
the right pitch allows the engine to take the best advantage of the reduction and maintain bite and a good effort (RPM) to reward (forward movement) ratio.
assuming your current prop allows the engine to achieve it's proper RPM's, dropping a pitch while adding a blade is an attempt to retain that operational range mathematically or on paper, and is translating what 'should' happen when altering props.
because you will have greater 'grip' on the water and theoretically less slippage with adding a blade, you will make more out of the RPM's you produce (translated to forward speed). If you drop too much pitch, you'll be able to turn the prop faster than it can bite and MAKE slippage.. If you add pitch, you'll have TOO MUCH bite, and stress the engines ability to twist it to the top of it's intended range.
PC BYC, Holland, MI