Speed sensor under the floorboards
One of the instruments that we rely on is our speed log, which measures how quickly the boat is moving through the water. There is a small paddle wheel that protrudes from the bottom of the boat – as we move, the flow of the water causes the paddle wheel to spin. The speed at which the wheel spins is then converted to nautical miles per hour, or knots.
We can also measure our speed using GPS, but that actually gives us the speed at which the boat is moving relative to the ground. This is often different from the speed we’re moving through the water – if we have a current behind us, for instance, our GPS speed will be faster than what is shown on our speed log. The difference between the GPS speed and the speed log reading gives us the speed of the current, which can become important when we are maneuvering in close quarters.
Once a year or so, the speed log stops working. This is typically caused by something getting wedged between the paddle wheel and the hull. When this happens, the paddle wheel can’t spin, and as a result our speed log shows zero.
It’s usually a pretty simple fix – all that’s required is pulling the paddle wheel assembly out and then cleaning out the obstruction. The challenge is that the paddle wheel is about an inch and a half in diameter and sticks through the bottom of the boat a couple of feet below the waterline – when we pull it out, there’s a lot of water that gets forced in through the hole. This makes it a bit exciting!
Installing the safety wire
To help prevent a huge gush of water into the boat, our speed log is built with a spring-loaded flapper valve. When the paddle wheel is pulled out of the hull for cleaning, the flapper valve pops closed and limits the amount of water geysering into the boat. There’s a momentary spray of water, but then it backs off to a slower flow once the valve snaps shut. Even with this valve, we still don’t like to see the ocean trying to force its way into the inside of our boat. So, we try to move pretty quickly when removing and replacing the paddle wheel.
The culprit, with a penny for scale
Once the sensor is back in the hull, the final step is to add a safety wire to make sure that it can’t somehow vibrate out on its own.
This time, the culprit was a small pine needle wedged between the wheel and the housing. We’ve only passed about 3.5 quazillion evergreen trees on our way north so far, so it’s actually surprising that it hasn’t happened already.
