Complex sentence: Not only could he use it for sextant astro-navigation, but it provided a sheltered place from where he could steer his yacht during a stormy race, quite a useful help as his wind-vane autopilot (also of aeronautical technology) had broken down.
Split into simpler sentences: Not only could he use it for sextant astro-navigation, but it provided a sheltered place from which he could steer his yacht during a stormy race. This was quite useful, as his wind-vane autopilot (also of aeronautical technology) had broken down.