Thank you both very much for your replies, not least for the recommendation of Dodgy Geezer's excellent website, which I have consulted extensively during the build process so far.

I am minded to agree that careful programming of the ESC is likely to prove the most effective means of guarding against untoward overpowering.

I shall endeavour to upload photos/videos of the boat in due course. if something goes spectacularly wrong as a result of the motor setup, let's hope that it is at least caught on camera!

I am building a 14" Keil Kraft Mermaid. I have included a 9.7:1 gearbox connected to the prop shaft, cannibalised from an aeroplane gearbox, taking a 12mm diameter motor (i.e. large enough for a small brushed motor or a small inrunner).

I should be grateful for views on which of two motors two use:

1. Brushless, 11,000kv, max 7V, max 49W
2. Brushed, max 7.2V, max 20-25W

Both are at my disposal. I had planned to use the brushless motor, but I am experiencing misgivings that it might overpower the boat or cause some catastrophic marine conflagration (despite the heat sink). The brushed motor came with the original gearbox, and I now wonder whether it might be better suited.

In case it is relevant, I plan to use a 20mm three-bladed propeller. I have a 30mm three-bladed propeller available should that be advisable.

Many thanks.