Knee length grass and only 3.5HP? Are you serious?
Give yourself a fighting chance by removing the blade and putting a REALLY SHARP edge on it. Most people think that a rotary mower cuts forever, with no attention to the blade at all. This is true up to a point, but a sharp blade will HALVE the power required.
Forget about the self-drive. Take it SLOW and EASY - cutting the grass is only half the story. The mower needs to be able to mulch/clear out the cuttings too.
If it's really that long. you'd be better off using an industrial strimmer first (hire one if necessary). It will be much kinder to the mower, too!
phil_saunders(a)bigfoot.com
Phil Saunders, May 2012