I downloaded from
http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm
I have heard that it gives "optimistic" numbers but it at least is a simple comparison tool.
Edited to add
The MVVS web site
http://www.mvvs.nl has a Prop Power Calculator spreadsheet in the "Utilities" section. It was developed by Pe Reivers.