Yamaha’s “logical boat speed” formula
Posted: Fri Aug 03, 2018 2:49 pm
I have recently repowered my 1985 17-foot Newport with a new Yamaha F70LA, using the excellent advice given on this forum, and am now testing the initial prop selection – an aluminum Yamaha K-series 13-5/8” X 14” – and, of course, comparing speeds and RPMs to those posted on this site for that motor on other 1980s 17-foot hulls with other props.
In the process of trying to understand how all of this works and whatever else I need to consider, I came across an online version of Yamaha’s 2012 Rigging Guide which identifies a “logical boat speed” formula in the portion of the Guide which addresses the different types of Yamaha propellers then available. After applying it to the results I got with my current prop, it seems to me to make sense, and since I have not seen this formula expressed this way previously on this web site, I am posting it in the hope that others will comment or find it useful. So, at the risk of presenting something that is so obvious that everyone here, except me, already knew it, here is the formula stated in the Yamaha Rigging Guide for calculating “logical boat speed”:
“Boat speed (km/h) = propeller pitch (inch) X engine speed (r/min) X .001524 X propeller efficiency / gear ratio.”
The Guide also states “Propeller efficiency = Actual advancing distance when propeller rotated one time / Logical advancing distance when propeller rotated one time.”
The online Mercury propeller slip caIculator asks for the same information - your propeller pitch, RPMs, speed, and gear case ratio to determine slip. So, I understand the Yamaha formula to mean that if a propeller is 100% efficient (which would be expressed in the formula as an efficiency of 1.00), the maximum possible boat speed under ideal conditions for any prop of any pitch at any engine speed will be determined by the formula, and that any loss in propeller efficiency – i.e., slip – will be reflected in a reduced top speed.
I think I have correctly determined that the formula constant of .001524 is used to convert one inch per engine revolution per minute to kilometers per hour, and therefore that a constant of .00094697 would be used to convert one inch per engine revolution per minute to miles per hour.
Applying this formula to my results with my new outboard and prop yields:
14” pitch X 6200 RPMs = 86,800 X .001524 = 132.28 / 2.33 = 56.77 kilometers per hour, which converts to 35.27 MPH top speed, assuming 100% propeller efficiency.
Or, using the MPH constant:
14” pitch X 6200 RPMs = 86,800 X .00094697 = 82.197 / 2.33 = 35.27 MPH top speed at 100% efficiency.
Dividing my GPS-determined top speed of 32.2 MPH with two people and a dog on board my Newport, and my 33.4 MPH top speed with one person, by the theoretical top speed of 35.27 MPH at 6200 RPMs yields a propeller efficiency of 91.3% and 94.7%, respectively. I expect the difference is slip and other factors. I would be interested to hear other thoughts on this.
Thanks to Jim and others for the terrific reference materials and advice offered on this site.
Ken
In the process of trying to understand how all of this works and whatever else I need to consider, I came across an online version of Yamaha’s 2012 Rigging Guide which identifies a “logical boat speed” formula in the portion of the Guide which addresses the different types of Yamaha propellers then available. After applying it to the results I got with my current prop, it seems to me to make sense, and since I have not seen this formula expressed this way previously on this web site, I am posting it in the hope that others will comment or find it useful. So, at the risk of presenting something that is so obvious that everyone here, except me, already knew it, here is the formula stated in the Yamaha Rigging Guide for calculating “logical boat speed”:
“Boat speed (km/h) = propeller pitch (inch) X engine speed (r/min) X .001524 X propeller efficiency / gear ratio.”
The Guide also states “Propeller efficiency = Actual advancing distance when propeller rotated one time / Logical advancing distance when propeller rotated one time.”
The online Mercury propeller slip caIculator asks for the same information - your propeller pitch, RPMs, speed, and gear case ratio to determine slip. So, I understand the Yamaha formula to mean that if a propeller is 100% efficient (which would be expressed in the formula as an efficiency of 1.00), the maximum possible boat speed under ideal conditions for any prop of any pitch at any engine speed will be determined by the formula, and that any loss in propeller efficiency – i.e., slip – will be reflected in a reduced top speed.
I think I have correctly determined that the formula constant of .001524 is used to convert one inch per engine revolution per minute to kilometers per hour, and therefore that a constant of .00094697 would be used to convert one inch per engine revolution per minute to miles per hour.
Applying this formula to my results with my new outboard and prop yields:
14” pitch X 6200 RPMs = 86,800 X .001524 = 132.28 / 2.33 = 56.77 kilometers per hour, which converts to 35.27 MPH top speed, assuming 100% propeller efficiency.
Or, using the MPH constant:
14” pitch X 6200 RPMs = 86,800 X .00094697 = 82.197 / 2.33 = 35.27 MPH top speed at 100% efficiency.
Dividing my GPS-determined top speed of 32.2 MPH with two people and a dog on board my Newport, and my 33.4 MPH top speed with one person, by the theoretical top speed of 35.27 MPH at 6200 RPMs yields a propeller efficiency of 91.3% and 94.7%, respectively. I expect the difference is slip and other factors. I would be interested to hear other thoughts on this.
Thanks to Jim and others for the terrific reference materials and advice offered on this site.
Ken