I have an Airmar DST 810 multi sensor for speed, depth and temp. The actual sensor has a yellow o ring but the dummy had none. There was a yellow one and a black one as spares. Does anyone know which one goes on the dummy? Black or yellow. thanks

They both fit and they both came in the same packet, but the black one is slightly smaller in thickness.

I used the yellow one and it worked but had to be done up very tight, otherwise it leaked.Im wondering why there is a black one, and maybe it's for the dummy and the yellow is a spare?

the directions are not much help.

O rings don't last forever and they are cheap. Looking at those photos there is a lot of grit around the area that can tear the O ring.

They also look flattened, ie lost their shape.

Take them off and find an O ring shop where they will measure and try to match it. It could be metric or imperial so its worth buying a few to see which one fits the best.

Clean the area well and apply some vaseline or similar to the plug and through hull then check the insertion fit with the O rings you bought noting the sizes for future reference. Carry and label spares.

The different colour could be a different material like nitrile. Material is not that important if you replace the O rings every few years

Hope that helps