Is a glob of silicone sealent enough to put on a screw that will be holing a transom mount transducer? I coated the screw and screwed it in tightly.

In almost every type of use for silicone, you will have better results if you do not screw it down tightly when you first mount an object, or use it to fill/seal screw holes. When you do, you squeeze out most of the sealant.

You should just snug the screws down slightly, then let the silicone set up overnight or for a couple of days if you have the time. Once set, the silicone will not squeeze out. It will then compress, making a watertight seal, rather than squeezing out. When you squeeze it out, rather than letting it set, all you have is a thin film of silicone, which is just slightly better than not using it at all.

First, it's best to use 3M 5200, it's made to be used under water.

It's always best to drill your hole, put a glob of sealant on the hole and push a little in the hole, then mount the bracket on top of the sealant and and add a little more to cover the screw when done.

Just putting it on the head of the screw after the fact may let water still seep in, especially if there is something under the screw like a mounting bracket.

First, it's best to use 3M 5200, it's made to be used under water.

It's always best to drill your hole, put a glob of sealant on the hole and push a little in the hole, then mount the bracket on top of the sealant and and add a little more to cover the screw when done.

Just putting it on the head of the screw after the fact may let water still seep in, especially if there is something under the screw like a mounting bracket.

He coated the screw with silicone. In any case, I would never use 3M 5200 for this project. It's an adhesive, and a darn strong one at that. It would be just about impossible to remove the screw, or the bracket without causing unwanted damage. See exerpt from article below.

Read the portion with the subtitle, "Demolition Begins" on the right hand page. Sorry, the program wouldn't allow me to copy and paste.

http://www.nxtbook.com/nxtbooks/boatus/diyboatowner_issue108/index.php?startid=55

Just putting silicone on a screw, I've seen it lift/turn loose too many times, but then it's not my boat. I've been using 5200 for years and will continue to do so. Yes, it's an adhesive so it's much less likely to turn loose and let water get into the screw/bolt holes. Only on one occasion have I had to cause me any kind of a problem. Removing a jack plate that had 5200 on and in the mounting holes, it pulled a spot of the clear coat off under the plate about the size of a quarter. This was on a boat that had been re-clear coated so it was not that strong of a bond to the original coat to start with, and was easily covered by the motor/jackplate when installed anyway. Yes, it makes things a little more work to take back apart, but I can handle that little bit of extra work over what's involved in replacing a rotted transome.

Even with silicone, I still fell it should go on first and not last. Putting it on after mounting leaves too many chances for voids that could let the water get in. Again, though, that's why the make chocolate and vanilla ice cream, everybody likes things a little different.

Just putting silicone on a screw, I've seen it lift/turn loose too many times, but then it's not my boat. I've been using 5200 for years and will continue to do so. Yes, it's an adhesive so it's much less likely to turn loose and let water get into the screw/bolt holes. Only on one occasion have I had to cause me any kind of a problem. Removing a jack plate that had 5200 on and in the mounting holes, it pulled a spot of the clear coat off under the plate about the size of a quarter. This was on a boat that had been re-clear coated so it was not that strong of a bond to the original coat to start with, and was easily covered by the motor/jackplate when installed anyway. Yes, it makes things a little more work to take back apart, but I can handle that little bit of extra work over what's involved in replacing a rotted transome.

Even with silicone, I still fell it should go on first and not last. Putting it on after mounting leaves too many chances for voids that could let the water get in. Again, though, that's why the make chocolate and vanilla ice cream, everybody likes things a little different.

As you say, to each his own. But a marine grade silicone sealant will do the job just fine. The trick is to fill the holes, and put an eighth inch or so coat on the bracket. Put the bracket in place, and tighten the screws until a bead starts to ooze out between the bracket and hull. Leave it like this for 24 hours or whatever cure time is on the container.

Once it has set, finish tightening the screws. It will compress, making a watertight seal, but will not ooze out. I guarantee it will not leak, but it will allow the part to be easily removed. I put all sorts of through hull fittings on my commercial lobsterboat for water intakes, transducers, keel coolers, wheelhouse windows, etc., to know this stuff holds up under everything the ocean and the skies can throw at it. I made the mistake of using 3M 5200 for one of those jobs. When it came time to change a below the waterline fitting it was an all day job. Had to literally drill, cut and grind the fitting away to avoid damaging the wood hull. I took a belt sander to the 5200.

One of the boatyard workers showed me the trick to using marine silicone sealant mentioned above. 3M 5200 is great stuff, for permanent jobs.

Years ago, I installed two vanities in our home with Corian sink tops and a back splash. When I purchased them, I asked what they used to fasten the backsplash to the top. They said to use a good grade of bathroom silicone. It should last at least ten years. I got a tube of 3M 5200, glued it to the counter top and the back wall. That was over twenty years ago. The backsplashes haven't budged since. They'll still be stuck fast when I'm planted six feet under.

You've used it. You know what you're dealing with, and you like it. I've recommended it to a lot of people, but I also inform them that once on, it's there to stay.