My first attempt at adding a second gauge was a Smiths-styled capillary gauge mounted on the rear of the pigeon box. It worked OK but it was difficult to make out the readings and it had white numerals on a black background so it didn't match the excellent large brass oil pressure gauge mounted alongside. The plumbing inside was also a bit Heath Robinson, but it worked. After sometime the gauge began to corrode; they aren't meant to be used outside a vehicle, so I looked for a better solution.
Engine with twin water gauge sensors - first attempt
The my second attempt was to use a brass capillary gauge calibrated in degrees Fahrenheit - matching the PSI of the oil gauge. I bought this from a supplier to vintage vehicles.
Brass gauges on the pigeon box
The new gauge required an interesting location for the capillary tube - since nearly all capillary gauges are designed for automobiles (~2 m long) and the sensor pipes aren't usually long enough for boats. The solution was use of a 22 mm diameter copper pipe positioned alongside the engine control rods to carry the sensor pipe internally. That made a collection of 5 engine "rods" - one brass speed wheel rod, one gearbox rod, one brass engine stop rod, one new copper pipe carrying the water temperature gauge capillary tube, and one sturdy 28 mm copper tube to hang onto and to "protect" the engine.
Cluster of "engine" rods
I retained the original twin sensor connection on the water manifold which was constructed from a series of connectors and adapters. However, I recently began to think that this collection of connectors looked untidy. It also had a habit of dripping.
So this week I took the plunge and I updated the sensor system with some "proper" brass connectors. The job wasn't as easy as I first thought since the newer capillary sensor doesn't pass through some standard 3/8" fittings despite being fitted with a 3/8 "BSP nut. However, some boring and tapping later I managed to get a much neater system. And it appears not to leak - perhaps neater plumbing means less leaks.
Latest plumbing for water gauge sensors
I also managed to fix a split water pump hose (outer only) - before it failed.
