Mounting SI/DI transducer, filling holes, and sealant

convergent

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I am replacing an outdated fishfinder and transducer with a new Garmin EchoMap with SI/DI. I have removed the old transducer and speed/temp sensor from the hull. There are remaining 4 holes that need to be filled from the two old units. First question is if it is OK to just fill these holes with 3M 5200 sealant. I bought 4200 to use on the new installation, but thinking 5200 would be better for these holes. This is below water so I am not really that concerned with how it looks, which is one reason given when people recommend resin and gelcoat. The holes for the new mount will be a bit higher so should go near these holes.

Next question is exactly how to mount the new transducer which I'm a bit nervous about. The new unit is a lot bigger than the old so would be much more prone to bumps. Its hard to tell from the picture, but the shape of the hull starts to curve quite a bit the further you get from the center. If the transducer is angled a slight bit in, vs. being exactly parallel with the boat travel, is that a problem?

I don't believe that the roller there is a concern because the boat is quite a bit up above it, but mentally it seems like it would be a problem even though I'm telling myself it won't hit it. The old transducer was mounted so that it stuck down below the bottom plane of the hull just a bit. Is that the correct placement, or should it be so that the bottom of the transducer is even with the bottom plane of the boat? I'm thinking that I'll mount it kind of between where the other two units were mounted and seal with 3M 4200.

Any thoughts on the placement and sealant for holes and mounting?
 

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dingbat

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5200 and 4200 are sealers, not fillers. Both will fail in that application sooner or later and the first inclination of a problem is a rotten transom.

Counter sink each screw hole twice the diameter of flat head screw. Fill hole and coat screw with epoxy and screw into place. Fill remaining countersink hole with White MarineTex and let dry. Let dry then sand for a permanent repair
 

convergent

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dingbat, thanks for the suggestion. Why not just use straight Marine Tex and not countersink the screws? Seems like that would be a better end result in case I ever drilled into that area again in the future. I wouldn't want to hit a hidden screw.

The boat is a 96 ProCraft 200 Combo, which I was told a while back was no-wood construction, but I can't now find that anywhere documented. Also, the transducer mount is up under the boat on the area that sticks down... not sure the correct name for that. Its not the main transom where the motor is mounted... so not sure if the inner core is different down there than on the main part of the stern.
 
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dingbat

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Nov 20, 2001
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dingbat, thanks for the suggestion. Why not just use straight Marine Tex and not countersink the screws? Seems like that would be a better end result in case I ever drilled into that area again in the future. I wouldn't want to hit a hidden screw.

The boat is a 96 ProCraft 200 Combo, which I was told a while back was no-wood construction, but I can't now find that anywhere documented. Also, the transducer mount is up under the boat on the area that sticks down... not sure the correct name for that. Its not the main transom where the motor is mounted... so not sure if the inner core is different down there than on the main part of the stern.

I guess you could try using a syringe with Marintex but good luck filling a blind hole....

I went to a thru- hull transducer and installed a transducer mounting plate to mount my speed sensors. I have no interest to drill holes in the transom ever again.

Don?t fool yourself. No-wood doesn't mean the hull is impervious to water damage. A no-wood build presents a different set of problems and failure points. Soaked foam, delamination etc.
 

convergent

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OK, so if I understand what you are saying, since I can't get to the backside to cover the hole, by countersinking the old screws, I create a base to apply the MarineTex against, vs. it potentially running out the hole on the other side and thus thinning/weakening the repair? That makes sense... just feels wrong to make the holes bigger before I repair them! :)

I was thinking about getting one of the transducer mounting plates to permanently avoid this in the future, but the hull has a slight curve to it so not sure that would work well.

I have figured out that for the SI to work it needs to have clear line of site to both sides, and its going to be close because of the curved shape of things down there. I think that if its in exactly the right spot it will work for SI and give me at speed readings... but I don't have a lot of room for error.
 

convergent

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dingbat, I took a shot at filling the holes last night and can confidently now say that I should have listened to your advice about the screws! I get it now... although its too late to go back and take that approach. I was also too cheap to buy a syringe. I did use a countersink bit to open the holes a little and then used a combination of a toothpick and popsicle stick to jam the MarineTex into the holes and then smooth it out. I believe that I got enough of it in there to have a good result, but as you said its somewhat of a "blind" repair because I can't tell for sure how much air was trapped in there. Where its at on the boat, it should be OK but burying sealed screws would have been better. A syringe would have been easier to force it in deeper with. I'll try to sand it down after work today to see how the finish looks.

While I was under there I also notice some very deep gouges on the hull bottom in a couple of places, most likely caused by rocks (from the prior owner). They were deep enough to expose the material below the gelcoat pretty deeply. That material is a black really hard material.. looks like hard plastic or some kind of composite. That appeared to be also what was behind the gelcoat where the screw holes were. Any idea what that material is? Doesn't look like any kind of wood or foam. I filled the gouges with the MarineTex since I had a lot of it left over from my screw holes.
 
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