• On September 1st, Maine Sail suffered a major hemorrhagic stroke. One of the most generous members of our sailing community, he has helped thousands. Now it's our turn. Click here to learn more

Replacing an old AutoHelm BIDATA depth and speed electronic

Sep 14, 2015
Hunter 23.5 Montross
I bought my 1993 Hunter 23.5 used two years ago. It came with an AutoHelm BIDATA device for measuring depth and speed. It's connected to a in-hull transducer (no hole in the boat). The depth measurement works fine, but the display is worn out and very hard to read which is driving my desire to upgrade it. The speed function does not currently work.

I'm looking for suggestions for a new display to connect to the existing transducer or a whole new unit+transducer to install. The best I've found so far is the Raymarine i40 BiData instrument. My existing unit has a SeaTalk connector so I expect I could reuse that. I'm not sure if I could reuse the existing transducer since I can't see model info on it to check. This article makes me think it's possible.

Also wondering if the speed measurement on the original unit ever worked because I have an in-hull transducer, not a thru-hull transducer. (Seems like it was wired to - see pictures.) Measuring speed isn't important enough to be to go through putting a hole in the boat so if it's not possible to measure speed via an in-hull transducer, I'm guessing the best move is to just buy the Raymarine i40 depth instrument that just measures depth. All advice and suggestions appreciated! Evan
old autohelm.JPG
back of unit unplugged.JPG


May 21, 2004
Jeanneau Sun Odyssey 409 Mt. Sinai, NY

Why do you think you have an inhull speed transducer and not a thru hull? The second picture clearly shows the thru hull speed transducer on the left. You can see the pin that hold the transducer in. I'm 99% sure that if you unscrew the cap, remove the pin and give a tug, the speed transducer, with paddlewheel attached, will come right out. If the boat is in the water when you do this, a geyser of water will shoot into the boat.
Have you ever dove under the boat and looked? The paddle wheel would be clearly be visible.

Good luck,
Sep 14, 2015
Hunter 23.5 Montross
Hi Barry,

Great insight! Looks like I made a wrong assumption about my transducers! Thanks for educating me on the difference. I'll dive under and take a look later this week.

Aug 1, 2011
Catalina 270 255 Wabamun. Welcome to the marina
If virtually anything gets in the paddle wheel it will stop spinning with expected results. Hence, the ability to pull it from the inside to get the blockage out.
Jun 10, 2014
Catalina 250 WK Lake Texoma
Evan, I also have a broken autohelm bidata device on my Catalina 250k. I have exactly the same wires com into the back from the boat (seatalk plus 3 speed and 3 depth wires.). Did you get the i40 Raymarine device, and did it work?


Feb 11, 2017
J/Boat J/160 Annapolis
Somewhere around the boat there (used to be and/or) is a "plug" that fits in that speed sensor's "hole". If you find it, it's designed to allow you to simply pull the transducer and replace it with the plug. The it's a pull, put other hand over the hole, push the new plug in, etc. exercise with a little water involved.

Some of us always pull the transducers to keep gunk from blocking the paddle wheels or interfering with their relative accuracy and consistency.

I you pull the transducer, you can have someone spin the wheel while another person looks at the display. If you get no speed, then it's either the transducer, the cable, or the cpu/display unit.

Evan, the Autohelm unit owes you nothing after all these years (as you know) ;^)))))