SI transducer mount issue

OK, I got a new 899 ci hd si unit and have done the exchange for the High Def transducer. My issue is that my aluminum boat has 6 holes drilled thru the transom from the current and a previous transducer mounting exactly in the place that I need to mount the new transducer. This prevents me from mounting the new transducer bracket without the current screw heads from being in the way of letting the new bracket from laying flat.I am concerned that if I don't leave the stainless steel screws in the current holes,I will have leaks in the boat. I am considering using the plastic transducer mounting deal from Bass Pro, but it will still have to be drilled or routered to accomodate the existing screw heads so it will lay flat on the transom. I guess I could JB weld the holes, but I am concerned that they would would not hold up over time. I am thinking of routering out a place in the plastic mount to let the current screw heads fit in behind the plastic, then silicon caulking the crap out of it, holding the plastic with a couple of screws and - finally - mounting the new SI transducer to the plastic may be my best bet. There will be a gap of an inch or so from the bottom of the plastic mounting plate to the bottom of the transducer. Will that create a "turbulance" issue?

I could get the current holes welded but there would still be a "hump" on the outside of the transom from the welding which would not allow the plastic mounting plate to lay flat against the transom. I don't think it is possible to reach the area of the holes from the inside of the transom for it to be welded from that side.

Any ideas to help with my concerns would be appreciated..... maybe I'm over thinking it !! Thanks.

I mounted the Plastic board over existing holes. I took out screws doped it with marine grade silicone and installed transducer mounting board. No leaks. ��

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Originally Posted by SlabPappi

I mounted the Plastic board over existing holes. I took out screws doped it with marine grade silicone and installed transducer mounting board. No leaks. ��

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Same here, but I took a hammer and flattened any bulges the screw holes had made, filled them with JB Weld and then put a bead of silicone around the plastic transducer mounting board and screwed it down tight hoping that between the JB Weld and the silicone, it wont let any water in the transom.

Have had the same issue several times. In most cases the old holes are covered by the new mount. Smear silicon on the entire new mount and screw into place. Very little water pressure. You will have no problem unless the new mount comes loose. Check mounting positioning before adding the silicon mess.

I can promise you that if you have a good man weld those holes up, he will grind down the outside of the transom and there won't be any humps.

Just not sure if they can weld in that area...would the heat melt any flotation material?

I think this is a good thing to do Making a Transducer Mounting Plate - Smith Mountain Striper Club

No new holes in the boat every time you change a transcuer mount.....The starboard can be replaced if it gets too full of holes.

You could use existing screws to mount the plastic board - counterbore the screw holes so screw head are flush - assuming the screws for the new mount aren't on top of counterbored screws. I did this method and mounted an aluminum plate to mount new transducer, and silcone everywhere.