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Author Topic:   Wind Resistance Measurement

chopbuster posted 01-18-2009 12:08 PM ET (US)
How would one measure wind resistance on given boat surfaces above the waterline ?
Jerry Townsend posted 01-18-2009 01:59 PM ET (US)
You can't. One can measure the total (wind and water) resistance using a test tank or canal. Evaluating the resistance componenets above or below the water line can only be done by analyses and calculations - which are subject to a few uncertainities - including the wetted perimeter, et al. --- Jerry/Idaho
jimh posted 01-18-2009 05:01 PM ET (US)
You can estimate the wind resistance of a boat by measuring the surface area exposed to the wind. Usually a correction factor is applied depending of the nature of the surface. There are several standards in use for calculating the force of the wind on a surface. Here is a good review of several of them:
http://www.arraysolutions.com/Products/windloads.htm
Notice that the pressure of the wind increases with the square of the velocity. The implication is that as a boat goes faster, its wind resistance becomes more influential.
Jerry Townsend posted 01-19-2009 01:17 PM ET (US)
Jim - realize that the reference you cited addresses the wind loads on antennas - which do not look like boats - but which allows one to make reasonable assumptions. But, I have read the whole thing in detail.
And as I mentioned above - you can't measure the drag force for boat surfaces above the waterline. Calculate and guess-ta-mate, take a WAG - but not measure. ---- Jerry/Idaho
jimh posted 01-19-2009 01:20 PM ET (US)
I don't imagine there is any simple model for the wind resistance of complex surfaces such as a boat superstructure or an automobile. This probably accounts for the existence of wind tunnels to get measurements from scale models.
Jerry Townsend posted 01-19-2009 06:05 PM ET (US)
Jim - and everyone else - I apologize as I slipped a cog - as I omitted the word NOT - and my statement should have read "... But, I have NOT read the whole thing in detail. ..." . As - I only looked at the formulations in two of the sections to see what the author was doing/assuming. Again - my apologies. -- Jerry/Idaho
Jefecinco posted 01-19-2009 07:19 PM ET (US)
First let me say that I am no matmatician, nor can I spell it. What follows will undoubtedly prove the truth of this statement.
In a prior life my work sometimes required me to calculate numbers from data that was not appreciably solid. I did that work for five years and surprisingly the predictions made from soft data almost always proved to be accurate to withing a couple of percentage points.
So, having said that it seems to me that some reasonable atmospheric drag coefficient can be derived from what we know. I'm not sure how to get there but a reasonable starting point would be to measure speed over the groung on a body of water that has no current. This should be done on a day with a steady wind from any direction. It would be necessary to measure speed over the ground on first a down wind and then an upwind course. Engine RPM should remain constant. Having the two speeds over the ground on both an upwind and a downwind course with a known wind speed should provide a well qualified engineer with sufficient data to determine wind resistance of the tested vessel within a fair degree of accuracy.
Or not.
Butch

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Author	Topic: Wind Resistance Measurement
chopbuster	posted 01-18-2009 12:08 PM ET (US) How would one measure wind resistance on given boat surfaces above the waterline ?
Jerry Townsend	posted 01-18-2009 01:59 PM ET (US) You can't. One can measure the total (wind and water) resistance using a test tank or canal. Evaluating the resistance componenets above or below the water line can only be done by analyses and calculations - which are subject to a few uncertainities - including the wetted perimeter, et al. --- Jerry/Idaho
jimh	posted 01-18-2009 05:01 PM ET (US) You can estimate the wind resistance of a boat by measuring the surface area exposed to the wind. Usually a correction factor is applied depending of the nature of the surface. There are several standards in use for calculating the force of the wind on a surface. Here is a good review of several of them: http://www.arraysolutions.com/Products/windloads.htm Notice that the pressure of the wind increases with the square of the velocity. The implication is that as a boat goes faster, its wind resistance becomes more influential.
Jerry Townsend	posted 01-19-2009 01:17 PM ET (US) Jim - realize that the reference you cited addresses the wind loads on antennas - which do not look like boats - but which allows one to make reasonable assumptions. But, I have read the whole thing in detail. And as I mentioned above - you can't measure the drag force for boat surfaces above the waterline. Calculate and guess-ta-mate, take a WAG - but not measure. ---- Jerry/Idaho
jimh	posted 01-19-2009 01:20 PM ET (US) I don't imagine there is any simple model for the wind resistance of complex surfaces such as a boat superstructure or an automobile. This probably accounts for the existence of wind tunnels to get measurements from scale models.
Jerry Townsend	posted 01-19-2009 06:05 PM ET (US) Jim - and everyone else - I apologize as I slipped a cog - as I omitted the word NOT - and my statement should have read "... But, I have NOT read the whole thing in detail. ..." . As - I only looked at the formulations in two of the sections to see what the author was doing/assuming. Again - my apologies. -- Jerry/Idaho
Jefecinco	posted 01-19-2009 07:19 PM ET (US) First let me say that I am no matmatician, nor can I spell it. What follows will undoubtedly prove the truth of this statement. In a prior life my work sometimes required me to calculate numbers from data that was not appreciably solid. I did that work for five years and surprisingly the predictions made from soft data almost always proved to be accurate to withing a couple of percentage points. So, having said that it seems to me that some reasonable atmospheric drag coefficient can be derived from what we know. I'm not sure how to get there but a reasonable starting point would be to measure speed over the groung on a body of water that has no current. This should be done on a day with a steady wind from any direction. It would be necessary to measure speed over the ground on first a down wind and then an upwind course. Engine RPM should remain constant. Having the two speeds over the ground on both an upwind and a downwind course with a known wind speed should provide a well qualified engineer with sufficient data to determine wind resistance of the tested vessel within a fair degree of accuracy. Or not. Butch