What is This? It has Mercury on it.

[Berdink]

The line going to it appears to be a small hose line.

 

[Mc Tool]

Its a speedo pick up ....water is forced up that tube and goes to a pressure gauge on the dash that has mph on the dial .....pretty old tech and notoriously innaccurate but better than nothing ...last century

 

[Berdink]

Okay, yep, the boat has a speedometer.
Thanks

 

[boscoe99]

Its a speedo pick up ....water is forced up that tube and goes to a pressure gauge on the dash that has mph on the dial .....pretty old tech and notoriously innaccurate but better than nothing ...last century

All modern aircraft still use a pitot tube for speed.

Only way to know their airspeed. Speed through the air.

Pretty much the same except for a boat which has an alternative. That being a paddle wheel for speed.

 

[99yam40]

You can always use GPS speed if you are not too particular on the difference in water speed and GPS, land speed.
currents do affect water speeds, going with or against the current