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This is a fun kind of prop and hull calculator that uses a math formula and your gps to calculate expected speed from props, I, dont know who wrote it, but its works pretty good actually, I, call it a "Guestimator"
Simple way to calculate a "Theoretical" speed, With using a said 5000 rpms and using the average 19 pitch prop the theoretical top speed should be 38 MPH. the assumption is based on the RPMs and the lower unit gear ratio that is about a 2 to 1 let me explain how this works, use this calculation below.
if you have 5000 rpms divided by 2 equals 2500. 2500 times 19 inches is 47,500,then divided by 1056 equals 44.981060606 and multiplied by .85 or %15 equals about 38 MPH theoretically speaking.
"you have to account for hull drag, wind resistance and tide flow of course" but 1056 is a constant factor and is the result of 12 inches per foot and times 88 feet per second at 60 MPH, when its divided into the product of propeller pitch the RPM times prop pitch in inches it gives up a direct (theoretical) top speed in MPH result for the "perfect world". prop slips of 15% for aluminium and 10% for SS is an average, but then there is the hull drag, by using your GPS real speed reading you can determine that by doing this, after you find your theoretical speed first, and If your GPS result shows an actual 34 MPH from the 38 MPH theoretical speed you would have a hull drag and prop slip of about 10-12% keep in mind that hull drag changes constantly with the speed and load on board so use it as an average this is why its impossible to find a one size fits all approach.
prop slips at 10-15% is about the norm for most props. this formula also does "not" take into account for horse power drop curves or gains and number of blades on the prop. its only a theoretical calculation but it does come pretty close to what you should see within about a 5 mph range when thinking about switching up props
Simple way to calculate a "Theoretical" speed, With using a said 5000 rpms and using the average 19 pitch prop the theoretical top speed should be 38 MPH. the assumption is based on the RPMs and the lower unit gear ratio that is about a 2 to 1 let me explain how this works, use this calculation below.
if you have 5000 rpms divided by 2 equals 2500. 2500 times 19 inches is 47,500,then divided by 1056 equals 44.981060606 and multiplied by .85 or %15 equals about 38 MPH theoretically speaking.
"you have to account for hull drag, wind resistance and tide flow of course" but 1056 is a constant factor and is the result of 12 inches per foot and times 88 feet per second at 60 MPH, when its divided into the product of propeller pitch the RPM times prop pitch in inches it gives up a direct (theoretical) top speed in MPH result for the "perfect world". prop slips of 15% for aluminium and 10% for SS is an average, but then there is the hull drag, by using your GPS real speed reading you can determine that by doing this, after you find your theoretical speed first, and If your GPS result shows an actual 34 MPH from the 38 MPH theoretical speed you would have a hull drag and prop slip of about 10-12% keep in mind that hull drag changes constantly with the speed and load on board so use it as an average this is why its impossible to find a one size fits all approach.
prop slips at 10-15% is about the norm for most props. this formula also does "not" take into account for horse power drop curves or gains and number of blades on the prop. its only a theoretical calculation but it does come pretty close to what you should see within about a 5 mph range when thinking about switching up props
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