Not sure how accurate you're trying for... most of the time I'd use something like
that merely for comparing differences between props or mufflers, myself.
They don't sell fishing scales down under? If not, it should be trivial to order one online
Like this one
, if you want digital, for about $25.
Or this one
, if you don't need digital, for $5 or so.
(Just a couple I ran across on a quick search)