Complex sentence: This also made for less stress and wear, and hence reliability problems, for the engines; if Merlins had been used they would have been needed to run at comparatively high power settings for hours at a time once a submarine contact had been detected.
Split into simpler sentences: This also made for less stress and wear, and hence reliability problems, for the engines. If Merlins had been used they would have been needed to run at comparatively high power settings for hours at a time once a submarine contact had been detected.