Purpose of the tri-sensor
The ‘Airmar smart multi tri-sensor’ measures three things: boat speed, water temperature and depth. We notice that we need to clean it when the apparent wind and true wind readings were the same value when seen at the helm wind display. Although this device reads the depth and water temperature, we are most concerned with the boat speed detector to calculate apparent wind speed. We need apparent wind speed to decide which sails to use.
Other names people use are transducer and speed impeller. I’m sticking to the name written on the device I’m cleaning.
Preparation- find the correct bilge cavity
First, we have to locate the bilge where the through-hull sensor is installed. In our Helia 44 Evolution, it is in the forward bilge at the base of the bed in the port hull. In the photos below, it is labeled 7.
Gather the right tools
We need a suction handle to lift the piece of floor, sponge and towel, vaseline and a pair of scissors. You may also need knee pads (if you have dodgy knees), wire brush, scourer and a snake spanner. Note that the area to work in is very narrow, due to the bulkhead protruding across the throughway.
I tried to video this without success. You need another to do the filming- the other was busy, so I just continued doing the job and snapping photos from time to time.
I’ve noted others have made excellent videos ( In 2015, Starry Horizons https://www.youtube.com/watch?v=1M4AWIQVriE, and Paul Briant from Southcoast Yachts https://www.youtube.com/watch?v=myxKf9eS_vY in 2017. These are good to view too.
First, lift the bilge cover. Place the towel between the tri-sensor and the back of the bilge, so it blocks the through bilge opening. Use the scissors to cut the cable ties holding the blanking plug. (Knee pads are handy at this point). You now need to bend over double to reach the tri-sensor. Locate the tri-sensor at the front of the bilge, note the wire twisted around the screw mount and the arrow facing forward. It’s important to replace the tri-sensor in the same position after it’s cleaned. The boat needs the sensor to record the forward movement of the boat. It won’t send correct information if you have it facing another direction.
Be cautious, be steady, be quick!
Be ready for a rush of water. It is quite surprising. I didn’t get a photo of that of course- I was too busy trying to switch the tri-sensor for the blanking plug. There is a trick to it. It’s my opinion that it is easier to describe than take a photograph or a video.
The sensor incorporates a self-closing valve which minimizes the flow of water into the vessel when the ultrasonic insert is removed. The curved flap valve is activated by both a spring and water pressure. Water pushes the flap valve upward to block the opening, so there will be a little gush of water into the boat when you remove the tri-sensor.
So, first, place vaseline around the O ring of the blanking plug. Then untwist the wire that is attached to the ring at the bottom of the tri-sensor.
Once the wire is free from the ring, unscrew the locking nut to maneuver the sensor upward, to free it from the through-hull position with one hand. With the other hand, quickly place the blanking plug into the vacated space. Try to ignore the rushing upward water.
What to expect? Well, your heavy boat now has a hole through to the sea below. Its’ weight pushes downward so the water moves upward into the vacated space. Don’t worry – the flap-valve is activated, which minimizes the water volume rushing into the vacated space.
Once the blanking plug is in the vicinity of the upward moving water, push down on it firmly, to engage it back into the hole. A little twist at the same time is needed. The twist is to engage the locking key built into the design.
Clean up the water at this point to ensure there is no new water coming into the bilge, now the blanking plug is in place.
Inspect the state of the pinwheel
The pinwheel measures the speed of the boat. The smooth side of the base of the device measures the depth. Now we can see why the sensor has stopped working. Ours was jammed with 7 small mussels and calcified sea matter. I was able to pop the mussels out using a small flat screwdriver to reach the hollow underneath, where the speed sensor is. The calcified matter took more than the wipe of a scourer.
The owner’s guide recommends using a “Scotch brite and household detergent” and to avoid scratching the depth sensor. I choose the soft wire brush to remove all the debris to make quick work of the task. I left the vaseline/silicone lubricant that was smeared on the long tube and only focused on the end piece.
Paul Briant mentions that if there is calcified white worm coral, you can just soak the device in a cup of plain white vinegar for 10- 15 minutes to soften it. It would then be like wiping off toothpaste. Although he says you don’t need to clean the depth sensor when you do the speed pinwheel, the owner’s guide does recommend scourer and detergent here if it is fouled. The owner’s guide also advises us to remove the paddle wheel to lightly use wet sand-paper if it is very fouled.
A word about the tri-sensor.
The tri-sensor attaches to the electronics of the boat. Once the pinwheel spins freely, you need to leave the tri-sensor in the bilge until you want to use it again. The Blanking plug will stop the egress of water while it is in position. If you are on the move again, first replace the tri-sensor by removing the blanking plug and swapping it over, before moving on.
Re-inserting the tri-sensor
Again, prepare the bilge with a towel at the back, to block the flow-through drain hole. Put extra petroleum jelly or silicone lubricant around the O- rings of the sensor. Lift out the blanking plug slowly, to allow the insert to jack up, activating the flap-valve.
Although I had only used finger tightness when using the screw-down nut on the blanking plug, I could not lift the blanking plug out in the same way as I had removed the tri-sensor originally. I used a snake wrench, mainly for a better grip, remembering to twist slightly, before pulling slowly upward.
Place the tri-sensor into the through-hull housing, ensuring the yellow arrow is directly forward. You will need to push down on it and slightly twist then, screw up the locking nut, maintaining downward pressure of the insert until the nut engages. Re-tie the wire of the tri-sensor to the locking nut. This is a precaution to stop the nut from working loose with the movement of the boat (or as the owner’s guide advises, in case you don’t screw it down correctly).
Again, clean up the escaped water and then check for leaks.
Tie the blanking plug with some wire back to the top of the bilge cavity, so it is handy for future use.
After one week in the water at the marina, I removed the recently cleaned tri-sensor. Already the sensor was covered in seaweed strands and something calcified that would not come off with a scourer. Although the pinwheel spun, it was clear that the calcified growth and seaweed would increase with continued lack of movement, and quickly block it up again. This time I cleaned it by soaking it in vinegar, as recommended by Paul Briant (above). I think this short time frame for calcified fouling, is the best evidence that the sensor does require anti-fouling.
Winterising and other maintenance
The little orange tag on the blanking plug advises the method for winterising. After haul out, the plug needs to be removed to drain any water, then repositioned. This is to prevent any freezing causing cracking in the plastic.
The Yellow O ring at the top of the sensor and the smaller O ring needs to be checked for wear and replaced if necessary. The same inspection is required for the two O-rings at the bottom of the blanking plug. Check the pinwheel for cracking, or its shaft for bending. Replace if required.
Yearly maintenance: Anti-fouling paint
Airmar recommends that the sensor parts protruding outside the hull should be included at the yearly haul out time when the rest of the hull is done.
Use water-based anti-fouling paint only, as ketone-based paint can attack plastics and may damage the sensor. The following surfaces must be painted: outside the wall of the insert, below the lower O-ring, the paddlewheel cavity, the paddlewheel, the exposed end of the insert, exterior flange of the housing, the bore of the housing up to 3 cm. Additionally, the blanking plug below the lower O-ring, including the exposed end should also be anti-fouled. (Images below ©Airmar Technology Corp 2006).
Thank -you/ Acknowledgements
Fontaine Pajot do not list this maintenance in their new owners’ boat manual. There is an “Owner’s guide” hidden between the thicker manuals in the new boat owner’s blue packs for the Airmar device, which I missed finding until after I wrote this. Ours is model DST800V, ‘ Triducer® Multisensor’, despite what is written on the device head. The information in this piece doesn’t change, whatever it is called.
I gleaned this from the Airmar website (http://www.airmar.com ). I also want to thank Amy & David Alton from their Out Chasing Stars blog (https://www.youtube.com/watch?v=1M4AWIQVriE,) and Paul Briant from South Coast Yachts (https://www.youtube.com/watch?v=myxKf9eS_vY )for their instructive videos.
I must thank Anthony Gain from SV Boomerang for encouraging me in the first place. I had a few visits to his hull in the first place to see what he was doing. No wonder ours was so fouled the first time. We knew we had a wind reading problem a long time ago, but didnt know how to fix it. Reading our new owner’s user guide did not point us in the right direction. Marina’s are great places to pick up tips! Thank you, Anthony.