Replacing a knot meter transducer

[Rex Noel]

The knotmeter on my 1993 Legend 43 has become an ongoing problem. This is an Autohelm instrument, vintage 1992 manufacture. It frequently doesn't register speed after sitting for a while. Originally, this would only happen if it sat for a couple weeks. Early this season, it would happen after just a week. We always attributed this to a build up of marine growth, generally just slime. Cleaning the transducer would take care of the problem. Not a fun project, as it spews a great deal of water all over the forward cabin, and inspires dramatic fears of the boat sinking if I miss something!In the middle of this season, cleaning the unit no longer seemed to work. I pulled it out, cleaned and replaced, and still not speed indication.This went on for weeks, when it suddenly came to life, when going through some rough waves. Since then, with cooler weather and minimal marine growth, it has generally worked. However, last weekend, we got extremely eratic readings. Speed would occasionally drop slowly to zero, then come back up after a bit. It also went up to as much as 12 knots... I dream of this, but really don't believe it happened.Is my transducer failing? Can it be replaced? If so, how do I get a replacement? Can I use the old cables, attached at the transducer, or do I need to pull new cable back to the readout (nearly impossible)? Can I use a new model transducer without replacing the thru hull?

 

[Mike]

Is it worth it?

I have the same problem with my Kenyon knotmeter. It just stopped working. I have thought about fixing or replacing the unit, but every time, I just shudder at the thought of the work involved in testing the display, troubleshooting the transducer, etc. So I stopped using it and have relied solely on my handheld GPS since. I do not miss the knotmeter at all. The GPS is more flexible, more accurate, and doesn't require a through hull.

 

[Tom Payne]

Hey Rex, I noticed that Raymarine was avaliable

in the HOW's guest expert forum. Ask the experts!!! I use both my tridata and my GPS to determine speed and current. GOOD LUCK!!!

 

[Bill Boudreau]

Replacement Transducers

The ST50 Speed transducers had two generations. Pre 1993 had 6 paddles and no check valve in the through hull housing. Post 93 had 4 paddles and a check valve to slow the flow of water into the boat. The replacement part number for the 6 paddle wheel is D138. The replacement for the 4 paddle wheel is D234. You can remove the speed insert in the boat. You should have a blanking valve, but Speed transducers manufactured after 1993 have a small check valve to slow the flow of water into the boat. The D234 and D138 come with the paddle wheel, spindle shaft and replacement O-rings.The entire transducers can also be purchased. If you have a pre 1993 then order a Z092 if you have a post 1993, or want to replace the through hull housing also, then order a Z092P.

 

[Jim]

Bill is correct

I replaced my knot transducer with a new one that has the check valve.. what a difference in terms of water coming in the boat.. much easier.. suggest you try it.. was not very expensive... and besides my constant cleaning of the old one resulted in a broken paddle.. which made the old one not work..