My digital fish scale my be off some, but if it is it is not off by much. Here is a excell speedsheet you can use to calculate prop thrust on many different type of props. It is from Pe Rivers who works with MVVS engines in europe. It is always very close to the true thrust results. Have a look, I think you'll like it.
According to this spreadsheet a APC 17x8 prop at 7500RPM is creating 13+ lbs of thurst. So my digital fish scale is off almost 1 lb[&o]
http://www.mvvs.nl/prop-power-calculator.xls