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I have a Garmin 478 and want to add on sonar. According to the manual I need the GSD 21 or 22 which is digital instead of analog. I am guessing there is no need for digital. I have not had bottom finder for work or recreation for 8 years. I have several questions.

1. There are GSD 21 on ebay for about 130, they are refurbed from Garmin. What do you guys think about refurbed electronics?

2. I have a old ducer in the boat mounted between engines. It is in a fairing block that has a paddle wheel. I have no knowledge with this stuff. The electronics that were in the boat when I bought it are long gone. Do all thru hull ducers fit the same hole in the fairing block?

Based on appearance of the block it looks like pull some pins and unscrew the large nut and the ducer would pop out the bottom thru the fairing block. I would rather not have to cut another hole in the hull. Any ideas would be appreciated.

I have the Garmin with the GSD21 and have used the Garmin supplied transducer that is installed on the inside of the hull on a B28. It shoots thru the hull and performs very well and eliminates having to re drill the hull. From your description of the transducer you have installed, the paddle wheel is for measuring speed. I would guess that that transducer would not work with the Garmin.

I too have the 21 hooked to my Garmin 2010C plotter and it works great with the shoot thru the hull hockey puck ducer. The ducer has to be frequency matched to the sounder unit....200 hz to 200 hz, etc. I do not know of a way to easily test the ducer to determine the frequency. Hook any sonar of a known frequency up to your existing ducer and see what happens......other than frequency (and power rating) matches, they are all pretty much the same.

UV

If the fairing block and transducer were installed correctly you will have a difficult time getting the xducer to "pop" out.

Most installers use 5200 on xducer mounts. Ive seen them ripped off the bottom with no hint of anything being wrong inside the boat.

You can always get a shoot through the hull

When I haul the boat out to swap props I will see if I can get the ducer out. If not I guess I will just get a shoot thru. Vic, do you know the model number of what you are using. The only requirement I can see is dual freq of 200 and 50. Do you guys just epoxy them to bilge?

The easiest way is to find a place you think it will fit/work (no lifting strakes directly underneath or in front of the spot) you'll need clean water flow under it. The other guys can maybe shoot you a pic of where theirs are to save you some time.

It sounds silly but silicone works just fine. There is no pressure on the ducer you just want no air between the face and the inside of the hull.

The transducer I have is installed in a container of mineral oil. It came from Garmin with everything that was needed as well as the instructions. Be sure to install it away from running gear but far enough toward the stern so that the area of installation remains in contact with water and does not come out of water when going at speeds. It can also be epoxied to the hull. I prefer the mineral oil so that it can be moved to get the ideal position. I have had mine installed for 5 years without problems.

You have a wet box, I believe UV is talking about the glue in place puck. Big difference.