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Prop Science- Diameter effect?

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Let's say we have 2 identical props, being... Mercury ET & Lab cleaver 28 pitch props and the only difference is the diameter. What would the change in performance and handling be for both style props on the same STV w/same engine? Let's compare them as 14inch and 14.5inch diameter.

Who got the answers?

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Diameter has a large affect on useable blade area. Reduced diameter changes the lever arm length for each blade. A 14.5 inch diameter has a blade with slightly more leverage than one with a 14 inch diameter. This gives a lower amount of slip for the slightly longer blades.

Also, since a surfacing prop is only partly exposed to water at speed, and since the inner-most part of the blades are shrouded by the gearcase, the slight change in length is actually larger than it would appear.

let's say you have a 14.5 inch diameter prop that you like, but cannot pull well to a given rpm. By trimming the tips of the blades a very small amount and reworking the blade tip shape, you can reduce the torque required to turn the prop at the expense of a slight increase in slip. This can get this prop an increase in acceleration by allowing the engine to get into a better area of its power band than was available before.

Then by adjusting the propshaft height and/or the prop's setback on the propshaft, you can tailor the prop's bite for your usage.

Example: I have a very good Yamaha 24p prop that has been trimmed very slightly. On a boat with near stock power, this prop accelerates better than it did with its full diameter. But once it gets to a given speed it hits a wall and will not go any faster. On the same boat with more power, it slips too much and still has the same top speed.

I hope this helps.

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