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Author Topic:   Mounting SONAR Transducer to Block on Transom
diverdave posted 05-27-2006 06:44 PM ET (US)   Profile for diverdave   Send Email to diverdave  
I saw a boat recently that had a butcher block white plastic plate secured to the transom with 3M-5200 marine grade permanent adhesive. A transducer and speedometer sender were mounted to that piece of plastic instead of screwing into the transom. Tell me about this mounting method. It seems like a pretty good method of mounting to avoid putting holes in the boat below the waterline.
Dick posted 05-27-2006 07:07 PM ET (US)     Profile for Dick  Send Email to Dick     
The most widely used procuct is 1/2-inch King StarBoard. It is available through many marine stores or if you have a plastic shop in your area give them a try.


highanddry posted 05-27-2006 10:22 PM ET (US)     Profile for highanddry  Send Email to highanddry     
What you may have seen is a block made by Rig Rite Manufacturing Inc, part number 920, marine grade polymer, 12 X 3.5 inches. There may be similar products or make your own. The idea is that holes and adjustments are made to the block and it only has to screw into your transom one time at two screw hole locations. If the block needs to be replaced it can be removed and another installed using the old screw holes. Some people might use marine grade sealant to also help adhere the block and I have heard tell of some who use only sealant and no screws at all.
jimh posted 05-28-2006 12:24 AM ET (US)     Profile for jimh  Send Email to jimh     
This topic was recently discussed in the REPAIRS/MODS discussion and an image showing a mounting plate was provided. See:

Mounting SONAR Transducer to Block on Transom

jimh posted 05-28-2006 08:56 AM ET (US)     Profile for jimh  Send Email to jimh     
While attending a boat show this winter, I noticed that boat manufacturer EDGEWATER BOATS was providing a SONAR transducer mounting plate already attached to and faired into the hull and transom of their boat that was on display. That seems like a very good idea. Not only does it make installation of a SONAR transducer easier for the customer, but you would assume that they have located the mounting block in a proper position.

The block also tends to eliminate the chance a customer will drill holes into the hull below the waterline in an improper manner. Having poorly sealed holes below the waterline could lead to problems with water ingress into the hull. Since the manufacturer is covering the hull with a long warranty, it probably makes sense to reduce the chances for problems by giving the customer a block for installing a SONAR transducer.

On the other hand, it does not take a neurosurgeon and a rocket scientist to drill two holes in the transom, install a transducer mounting bracket, and seal the holes properly. There are hundreds of thousands of Boston Whaler boats with transducers mounted without an intermediate block.

diverdave posted 05-28-2006 09:07 AM ET (US)     Profile for diverdave  Send Email to diverdave     
When you mount the transducer to the King Starboard, do you use screws or do you need to bolt it all the way through the board. I wasn't sure if that board would hold the screws or whether they would back out. I guess 5200 would hold the screws in?
bsmotril posted 05-28-2006 09:40 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Screws work fine. Just don't overdrill the size of the pilot holle. BillS
highanddry posted 05-28-2006 01:52 PM ET (US)     Profile for highanddry  Send Email to highanddry     
Mine has stainless screws holding the block to the boat with the 3M 5200 sealant. I routed the back to provide extra tooth. The transducer holes are counter sunk on the back side so I can use stainless screws to through bolt the transducer. To allow adjustments without the screw spinning I installed the screws with epoxy. This allows me to loosen the stainless nylock nuts and adjust the transducer position as needed.
A faired in block would be nice but what they really need to do is to mold in at least two transducer wells into the hull. This would eliminate possible leakage nd provide much better performance.
When I get around to getting an old 13 or 15 I plan to do just that. I will core out through the foam down to the outer hull. I will construct a glass sleeve/tube and lay it in with flocked epoxy or other suitable resin. Before hand I will add several layers of glass or glass matting to the hull at that location--inside--to stiffen it. When completed it will be a part of the boat and fully waterproof--even if water gets into the well, it will not matter because the well will be water tight and solid glass.
Why don't I do this to the boat I have now, well, because one of the huge downsides to owning a new boat is the trepadation that comes with drilling holes into it or modifying it and of course speculating about the warranty coverage as a result.
When you drill the holes for the screws for the tranducer or the block, chamfer the edges of the gell coat to prevent splintering and cracking. Pump the holes full of the sealant and install the screws "goobed" with the sealant and put the sealant under the block at least around the holes. In this manner you can be positive no water will enter your boat--there anyway.
Royboy posted 05-30-2006 08:57 AM ET (US)     Profile for Royboy  Send Email to Royboy     
A small dab of 5200 on the screws into the Starboard material will hold them in. Works great for keeping screws put in fiberglass too. No problem getting them back out later. Think of it as locktite for plastic.


highanddry posted 06-05-2006 02:23 AM ET (US)     Profile for highanddry  Send Email to highanddry     
It turns out I have an extra Rig Rite board. If somebody wants it for free let me know. It is a light gray color.
17 bodega posted 06-07-2006 11:43 PM ET (US)     Profile for 17 bodega  Send Email to 17 bodega     
As long as the holes in the starboard are small enough to create some resistance on the screw it should hold with the 5200 as insurance, but some dealers use standard stainless lock washers and a flat washer too. It probably wouldn't hurt anyway.

Thanks for the link on the transom mount. I think I will go with one of these for my new install coming up. It makes a whole lot of sense.

Another thing I was told about starboard is that it's composite nature doesn't allow it to adhere to any adhesives alone, so it should be attached by screws. The 5200 of course is used also for screw holes and water sealing.

17 bodega posted 06-07-2006 11:50 PM ET (US)     Profile for 17 bodega  Send Email to 17 bodega     
After reading that thread posted above it appears some have been able to use only 5200 adhesive to hold a starboard block to the transom by scoring the two surfaces to allow better adhesion. I will probably use two screws like the photo and tons of goop and it should be good to go. This thread is just in time for my new depth sounder/gps aquisition.

Thanks Jim!

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