The engine in my boat is a Stuart Swan, designed in 1906. It has 2-1/4 in. bore, 2 in. stroke and is a twin cylinder double acting engine. I built it from a kit in the 1980s. I've wanted to upgrade to something a bit more modern for 30 years but when nothing was offered I decided to make my own.

My engine is based on  one designed by Mr Anton Bohaboy in the 1950s. It was originally built as a single cylinder engine for tethered model hydroplanes. I've scaled it up and made it a 3 cylinder engine to give it the same swept volume as my old engine to make performance comparisons more fair.  A version of the engine which can be printed is now on Thingiverse: https://www.thingiverse.com/thing:3877037

I've designed everything except the cams; I need a little help with those! Meanwhile I'm now redrawing all the components, making them suitable for lost PLA casting in aluminum. Stay tuned: as soon as I learn how to do it I'll upload an animated rendering of the thing.