Grenå <a href="https://en.wikipedia.org/wiki/hot-bulb_engine" class="extiw" title="en:hot-bulb engine">hot-bulb</a> or <a href="https://en.wikipedia.org/wiki/semidiesel" class="extiw" title="en:semidiesel">semidiesel</a> single-cylinder boat engine, on the dockside at Suomenlinna. Maybe (unconfirmed) a 27T, which would be about 45bhp and maybe 1960s build.
<a href="//commons.wikimedia.org/wiki/User:Andy_Dingley" title="User:Andy Dingley">Andy Dingley</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>