Diagram of a <a href="https://en.wikipedia.org/wiki/Combined_diesel_or_gas" class="extiw" title="en:Combined diesel or gas">CODOG</a> propulsion system for one shaft (there are usually two).
Not that in reality the clutches and gearboxes would be arranged differently, it's just to show the principle.
<a href="//commons.wikimedia.org/w/index.php?title=User:Alureiter&amp;action=edit&amp;redlink=1" class="new" title="User:Alureiter (page does not exist)">Alureiter</a>
2005-12-02
<span class="int-own-work" lang="en">Own work</span>