What do you mean by "self powered"?
Smaller hydro steering systems normally just have a helm pump
driven by turning the wheel
and require no electrical
power. Hydro autohelms have a small elctrically powered pump
Are you asking about a primary steering system or just an auto pilot?
As a primary steering system on a 28 foot mono(?) It should work, but why? Most boats that size are tiller steered anyway. I would stick with that...dead simple, no hydraulic system complexities and maintenance
issues, WAY better feel at the helm (versus none)...and thats part of the fun of sailing a small mono anyway.
A hydro AP should work, but again, why? There are simpler solutions available, like a tiller AP.