Knotmeter and Depth Sounder Replacements

[shank]

It was -17F here in Vermont last night so of course I'm thinking about the upcoming boating season.

I have an 82 Catalina 27 with the original knotmeter and depth sounder. They both work very sporadically and I want to replace them with some instruments that are more modern and reliable. I'm not sure how old or the brand of the transducers and not sure if the problem is with the gages or the transducers but still want to replace the gages.

Could someone recommend some replacements for these gages preferably ones that are the same size and could fit in the same holes?

Can I just twist the white threaded cap around the transducer and pull it out with the little handles? If I need to replace the transducers as well, I'd really like to avoid cutting new holes in the hull and just slide new ones in the existing holes if that's how they work.

Thanks

 

[dlochner]

Those are venerable SR Mariner instruments once made in Buffalo, NY or at least the company was based in Buffalo. This is really old technology and the output is not compatible with current electronics, meaning the transducers will need to be replaced and they are probably smaller than current transducers. Just to make things more interesting, you will only need 1 transducer as the most popular are now Speed/Depth/Tempature sensors and made by Airmar.

When faced with a similar issue a few years ago, I removed the speed transducer and the through hull, enlarged the hole and installed the new transducer. The old depth transducer was left in place, it was not leaking and sealing the hole would entail a lot of extra fiberglass work in a very thick section of the hull.

I prefer B&G products over the competition, Raymarine and Garmin. Both B&G and Raymarine have been around along time in the sailing instruments business, Garmin is relatively new to the field (although they have a long history with GPS and aircraft instruments). B&G has a long history of supplying sailing oriented instrumentation.

Aside from its history in sailing, B&G for the most part uses standard NMEA 2000 connectors and cables, I think Raymarine uses proprietary connectors and I'm not sure about Garmin.

 

[Davidasailor26]

B&G for the most part uses standard NMEA 2000 connectors and cables,

Navico (Lowrance, B&G, and Simrad) use SimNet cables. It’s a standard NMEA 2000 protocol but their own cable form factor that’s a little easier to work with. Since it’s still regular NMEA 2000 you can use any other NMEA 2000 equipment with pretty inexpensive SimNet adapters. I think RayMarine’s SeaTalk NG is basically the same arrangement but with their own cables and connectors. I think Garmin is standard NMEA 2000.

+1 for the recommendation for B&G in general. You could probably get Triton instruments the same as those holes. They’re square, but at least you wouldn’t have to fill anything in. Since they’re SimNet you could easily add any other Navico or NMEA equipment you might want in the future.

 

[dlochner]

Navico (Lowrance, B&G, and Simrad) use SimNet cables. It’s a standard NMEA 2000 protocol but their own cable form factor that’s a little easier to work with. Since it’s still regular NMEA 2000 you can use any other NMEA 2000 equipment with pretty inexpensive SimNet adapters. I think RayMarine’s SeaTalk NG is basically the same arrangement but with their own cables and connectors. I think Garmin is standard NMEA 2000.

+1 for the recommendation for B&G in general. You could probably get Triton instruments the same as those holes. They’re square, but at least you wouldn’t have to fill anything in. Since they’re SimNet you could easily add any other Navico or NMEA equipment you might want in the future.

The current crop of B&G equipment use standard NMEA 2000 connectors almost exclusively. And where they are used, one end is Simnet, the other NMEA. The wind sensors have a NMEA 2000 at one end and a special connector at the instrument end, a terminated Simnet. The 4g radar has similar set up, but the Simnet connector is not a terminator.

Their ethernet connectors are a different story, proprietary connectors and expensive RJ45 adapters.

 

[Davidasailor26]

The current crop of B&G equipment use standard NMEA 2000 connectors almost exclusively. And where they are used, one end is Simnet, the other NMEA. The wind sensors have a NMEA 2000 at one end and a special connector at the instrument end, a terminated Simnet. The 4g radar has similar set up, but the Simnet connector is not a terminator.

Their ethernet connectors are a different story, proprietary connectors and expensive RJ45 adapters.

I stand corrected. My whole backbone is SimNet cables, but you’re right that the instruments themselves are NMEA connectors.

 

[TimFromLI]

In addition to the transducers, you will need to replace the gauges. I went with a B&G MFD. I ended up replacing everything including wind. The new speed transducers incorporate temperature, heel, speed and depth all in one.

 

[shank]

Thank you for the responses and helpful info. I found an installation manual for the old SR Mariner instruments which I have now. It indicates that the transducers operate at a 160KHZ frequency and a 2 1/8" hole for the thru hull fitting. Most of the instruments and transducers I'm looking at on Defender Marine operate at 200 KHZ and require a 2" thru hull fitting.

I'm guessing that the differences in the frequency will require me to replace the transducers?

Will the 1/8" difference in the hole size require me to fill in the hole and redrill it to 2" or is there some kind of bushing I can use to fill the gap?

Thanks

 

[jssailem]

I'm guessing that the differences in the frequency will require me to replace the transducers?

Correct. New Transducer if you want the instrument to work properly.

Will the 1/8" difference in the hole size require me to fill in the hole and redrill it to 2" or is there some kind of bushing I can use to fill the gap?

Bad news. You need to have a 2 inch hole not one with an extra 1/8 inch.
No way to strengthen the bottom, or fill in the 1/8 inch. You would run the risk of what ever way you tried to patch it, the patch breaking loose and flooding the boat.

Patch it correctly and then drill a new hole to properly support your new transducer.

You could try to find a transducer that uses a 2 1/8 in hole.

 

[dlochner]

Thank you for the responses and helpful info. I found an installation manual for the old SR Mariner instruments which I have now. It indicates that the transducers operate at a 160KHZ frequency and a 2 1/8" hole for the thru hull fitting. Most of the instruments and transducers I'm looking at on Defender Marine operate at 200 KHZ and require a 2" thru hull fitting.

I'm guessing that the differences in the frequency will require me to replace the transducers?

Will the 1/8" difference in the hole size require me to fill in the hole and redrill it to 2" or is there some kind of bushing I can use to fill the gap?

Thanks

There will be a 1/16" gap around the thru hull. This is not a large amount. As with most boat projects there are lots of ways to deal with this. This is not really a structural repair.

On a Catalina 27 the hull thickness at this point is likely to be fairly thin, maybe a ½" to ¾" thick. You will know for sure when you remove the old through hull. The easiest way is to remove the old through hull and clean all the old sealant from the hole and surrounding areas. Seal the bottom of the hole with some plastic wrap backed up by some cardboard and taped securely to the hull. From the inside fill the hole with thickened epoxy, leaving the upper surface as smooth as possible. Use an easy to sand thickener.

Once the epoxy has cured, remove the plastic wrap and sand to match the bottom contour. On the inside, sand the epoxy smooth. This is important so you get a good seal. Now drill a new hole centered in the old hole with a hole saw. Follow the manufacturer's recommendations for installation and sealant.

Why is this not a structural repair? Because it is a very small amount of missing hull and the filled area will be captured between the mushroom on the outside of the hull and the flange nut on the inside. There will also be a lot of sealant on both sides of the hull. All you are trying to do is stop any movement by the through hull.

The above instruction are only applicable to reducing the size of the hole not to permanently sealing the hole. In that case there is a lot of fiberglass work to be done.

More important than the frequency of the transducer is the connection to the instrument display. The SR Mariner is probably not an NMEA 0183 or 2000 device. It probably uses a shielded coaxial cable to connect the transducer to the display. These will not work with contemporary displays.

You will have to replace both the transducer, the cabling, and the display.

 

[shank]

I really appreciate the detailed instructions on this. It doesn't sound too difficult. In your opinion, since the gap is relatively small I can just fill the hole with epoxy and redrill it to the correct size without grinding it all out and reglassing the whole thing?

 

[dlochner]

I really appreciate the detailed instructions on this. It doesn't sound too difficult. In your opinion, since the gap is relatively small I can just fill the hole with epoxy and redrill it to the correct size without grinding it all out and reglassing the whole thing?

Yes, because the area is small and because it is captured by the thru hull fitting. The only grinding sanding is to make sure all the old sealant is gone and after the new epoxy cures to make certain there is a smooth flat surface for the thru hull to mate against.

The instructions will probably tell you to apply sealant (not 5200) on both the outside the hull and inside on the threads.

Since you have 2 thru hulls, just leave the other one in place. If you remove it, you will be in form lots of grinding and glass work.

 

[shank]

Thanks again. Sorry about the Bills :-(