My boat has a ST60 paddlewheel-type speed gauge that I've never used. The blanking plug was in place when I bought the boat last summer, and it stayed there until today. The paddlewheel needed cleaning up, and I also wanted to practice removal/replacement while on the hard to make sure nothing catastrophic happened when I first tried it.
So today, while on the hard, I cleaned up the paddlewheel and also removed the blanking plug and cleaned it up too (see attached pic). Upon inspection, it looks like there are two O-rings missing from the plug's shaft (though there are still two O-rings at the top of the plug). Based on the dirt that was present (before I cleaned it up) there is definitely water that entered the annulus around the plug's shaft, but no sign that any of it leaked past the other O-rings at the top.
The paddlewheel insert also has the two O-rings at the top, and one lower O-ring in the shaft near where the ones are missing from the plug.
I'm curious whether others have all 4 O-rings on their ST60 plugs, or have the two lower ones missing like on mine. I don't want to add them if they're not supposed to be there. In fact, it looks like they could cause more harm than good. If you read the link below, you'll see that water can get between the O-rings and freeze, causing the through-hull casing to crack:
So maybe Raymarine/Airmar opted to remove the lower O-rings from the plug's design, which would allow water to drain from the annulus upon haulout, preventing ice damage if someone were to forget to remove the plug while on the hard.
Just curious to hear all of your experiences with this, and whether you think I should get some additional O-rings for the plug.