SAILING

Cleaning Your Boat’s Speed Impeller

On February 8, 2022
By Kimberly

Servicing the speed impeller sounds terrifying; opening a hole in the boat. But, it’s actually easy once you get over the idea of purposely allowing water to gush in while viewing the bottom of the seabed!

Find the thru-hull fittings

Go below decks, locate the hatch to the bilge where the hull speed and depth sounder thru-hull fittings are located.

One of these two black circles with an arrow on it will be for depth. The other one is for speed. They usually are labeled.

First, locate the blanking plug or plugs

Sounds like salty language, doesn’t it? The “blanking plug” is a blank – it’s a plug, but without the device on the end. It just stuffs the hole while you work with the one that does have the device.

If there are two blanking plugs, the one that is smaller in diameter is probably the correct one. If there are two, be ready to try them both. Get a few paper towels ready; some water will enter the boat.

Next, unscrew the collar around the plug

Much like a boat electrical power cord, this will have a separate threaded collar. You’re going to unscrew the collar, lefty Lucy, righty, tighty, until it spins freely. This is the equivalent of removing that little wire cage from the cork of a champagne bottle.

Next, you’re going to pop the cork.

Firmly grasp the plug, and remove it. Some of these devices will have a flapper on the outside, which will slow down water entering the boat. Others don’t, and you can see blue water below the hull. In your other hand, have the blanking plug ready.

Once you have removed the speed indicator plus, push the blanking plug into the opening, and turn the collar counter clockwise until the threads are seated, then give it a few spins clockwise to keep it in place. It does not need to be terribly tight, since you are going to replace the speed log momentarily.

Gently remove any crusty sea life growth from the impeller

At the end of the plug, that you have removed, there is a small four-prong device that freewheels. Or at least, it is supposed to freewheel.

Gently clean it up until it is able to spin without catching. It’s constructed of plastic, so carefully remove sea growth with a rag or wooden chop stick.

See the wheelie-deal in the end of this Raymarine DST800 Nylon Thru Hull Transducer with Depth, Temp, and Speed? That’s what you will clean when it is out of the shaft.

Replace the cleaned device

Once it is cleaned, you are going to remove the temporary blanking plug, and insert the actual speed indicator plug back in the thru hull fitting. Once again, very carefully, twist the plug counter clockwise until you feel the threads drop into place. These are plastic devices, and you definitely do not want to accidentally cross-thread them.

How often you will need to clean the impeller depends on the temperature of the water in which you sit – sea growth occurs more rapidly in warmer water. Racers will clean it daily, charter boats weekly or monthly.

Reading forward motion

When the collar is almost tight, you want to align the arrow slightly to port of center, facing forward. You can use the teak and holly lines of the cabin sole to help you lined up the two.

If the arrow is pointing toward the stern, you will only get speed readings when you are motoring in reverse. So, point the arrow toward the bow.

Clean up and check bilges

Wipe up any water, and look for seepage. This is a great time to listen and make sure the bilge pump noticed that there was an influx of water and runs a little bit.

Is it working?

Replace the hatch board cover, and you are all set. If you get underway and do not have a speed read out, double check that you lined the arrow up. facing forward.