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Discussion Starter · #1 ·
I'm also getting ready to purchase a Raymarine ST40 Bidata and need to install the paddle wheel speed sensor in an existing through-hull that is currently plugged. The old paddlewheel has been laying in the bottom of the hull next to it since I bought the boat. I was wondering if I can quickly pull the plug that is threaded into the thru-hull and replace the new one? Or... will I have too much water pressure coming in, to install the new one?
 

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Don Radcliffe
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People do it all the time to clear fouled paddle wheels. Be prepared for a fountain of water and move quickly, but you should have no problems pushing in the sensor against the water pressure. Compare the new paddle wheel with the old first to make sure that the new one will fit, and lube the o-rings with silicon grease or vaseline.
 

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Be sure to compare the paddlewheel dimensions AND the threaded cap... it's quite unlikely that they will be interchangeable, esp if the brands are different....

Changing the thruhull itself cannot be done wet... you'll need at least a half lift or a tide grid to get that job done.
 

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If your thru-hull is like mine (old B&G), it has a little flap that pops open to block most of the incoming water whenever the plug or sensor is removed ... makes it even easier.
 

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Here is a video I took when I replaced the paddlewheel on my sensor. this is taking the plug out and putting the sensor back in. Its a pretty good stream of water. And yes I know I did not have the wheel aligned right. I fixed that later. :)

 

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Airmar makes most of the transducers for the major marine instrument manufacturer's. Most times for the paddle wheel transducers it's plug that fits in to the unit it's self that changes. What I trying to say is the transducers are the same they make different plug ends. Chances are that if the thru hull is an Airmar you should have no problem.
Airmar Marine Products Home Page
 

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So how high exactly will the water shoot? Most people are surprised to learn that it will be slightly LESS than the water line! So while it seems like there is lots of pressure it really is very little, and there is no chance of the water hitting the overhead as many people expect.

Gary H. Lucas
 

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Discussion Starter · #8 ·
Here is a video I took when I replaced the paddlewheel on my sensor. this is taking the plug out and putting the sensor back in. Its a pretty good stream of water. And yes I know I did not have the wheel aligned right. I fixed that later. :)

They say a picture is worth a thousand words.

Thanks
 

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The water doesn't shoot out, but more just low-pressure-gushes.

The mistake a lot of people make is trying to snag-out the one and slap in the other in one fell swoop. Thing to do is take out the one, then slap a hand over the top of the thru-hull, grab the other, lift your hand off the thru-hull and quickly put the other in-place. Done right, you'll get maybe 2-3 cups of water in the boat. (Tho I usually end-up with more than that :p.)

Jim
 

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I just did that underway last weekend. Even though the bottom was clean, something got stuck on the paddlewheel while under sail out a few miles. When I sponged the water up afterward I squeezed the sponge into a water glass. I measured about 10 ounces of water that came in the boat, even though it looks like more when you're doing it. (I wasn't as fast or smooth as the video, either).
 
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