Using RTV sealant on a suction cup to adhere better to my fiberglass transom

[Pain'n'da'boat]

Long story short...I got a Lowrance X-4Pro fish/depth finder for my birthday, don't want to cut out part of the hull to do a shoot-thru-hull setup but I also don't want to drill into the transom for the external transducer mount...

I purchased some large suction cups from Academy, these exact ones:

and fabricated a brace that bolts the transducer to the suction cup while allowing 1.5" of gap between the bottom of the suction cup and the hull (center point for the transducer to prevent cavitation)...

so while the suction cup says it will hold up to 60lbs, I feel that 40 mph might rip it off the back if applied with ordinary "spit".....

Im thinking that if I prep the area with a grease remover, clean the suction cup thoroughly and squeeze some RTV SEALANT on before I stick it in place.

will using RTV sealant ruin the gel coat upon removal at a later time or will it be okay?

any ideas on a better way to semi-permanently apply a suction cup to a fiberglass boat transom?

 

[oops!]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

a trick the guys are using.....is to throw the transducer in the bilge......then add a few cups of water in the bilge.....they say it works just fine.

then no suction cups.....i can really see them staying in at 40 mph even with sealant unless its 5200. but 5200 will make it a permanent fixture

 

[Pain'n'da'boat]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

a trick the guys are using.....is to throw the transducer in the bilge......then add a few cups of water in the bilge.....they say it works just fine.

then no suction cups.....i can really see them staying in at 40 mph even with sealant unless its 5200. but 5200 will make it a permanent fixture

do it as if I am installing a shoot-thru-hull transducer and wedge it securely and it will work ok, so long as there is water inbetween the transducer and the hull? (no air obviously)

 

[RogersJetboat454]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

Lot's of RTV's contain a mild acid that etches the metal it's bonding to for a better hold. Whether or not this acid will do anything to the gel coat is debatable. In essence, you are more or less gluing the suction cup on which defeats the whole idea of a suction cup (atmosphere on the outside being greater than the atmosphere on the inside, causing the outside atmosphere to push down on the outer surface of the suction cup... bla..bla..bla). If you feel as though a suction cup won't be up to snuff to do the job as it's designed to do it, I would be looking for another way to mount the transducer.

 

[smokeonthewater]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

no silicone won't hurt the boat.... that said epoxy it in the bilge and be done with it.... you don't have to cut anything.... If you DO choose to glue it to the stern, ditch the suction cups.... mount the 'ducer to a piece of aluminum plate and glue THAT to the transom

 

[Pain'n'da'boat]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

Lowrance X-4Pro...you think the transducer signal is strong enough to go through the hull? It says it does a shoot-thru-hull application but that I would have to cut down through the different layers until I reached the final exterior layer of fiberglass...but you're saying that I can just epoxy it inside the bilge and it will be fine??

Where in the bilge is the best place? more towards the stern or bow?

Thanks for the info guys

 

[zopperman]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

maybe if you use 5200 or something that will stick to gelcoat

 

[smokeonthewater]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

You only need to sand down layers if you have a hull with a core.... such as balsa or foam sandwiched between layers of glass.... If it is solid then just epoxy it in and ur gtg

 

[chriscraft254]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

I would mount it on the stern or buy a thru hull. The shoot thru supposedly doesn't read as well. If your concerned with mounting on the stern, you can take a peice of 3/4 inch starboard, 5200 the starboard, even or a little above the planing surface, then mount the ducer to the starboard with 3/4 inch screws.

 

[mpdive]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

5200. And NO it is not permanent. Have been using it for years and it comes off if you want it too. It will be perfect for your application. And YES I have used and removed plenty of times.

 

[smokeonthewater]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

starboard works too if the 5200 will stick to it (i'd use 4200 or 8200 tho as 5200 is RUNNY) .... I have had several transom mount ducers mounted to shoot through the hull on several different boats and the all worked perfectly

 

[ssobol]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

You may be able to get a removable mount for the transducer either from the manufacturer of the depth sounder or the actual manufacturer of the transducer (usually Airmar).

I got one for my P23 transducer so I could try locations without drilling the hull until I was sure the depth finder would work where I intended to install it. The removable mount consists of a suction cup, metal bracket for mounting the xducer, and misc bit parts.

I mount the xducer on the mount and figured out where it should go and then outlined the suction cup on the hull so I knew where to put it when the boat is in the water.

With the boat on the trailer the suction cup would come off after awhile and the xducer would hang down. However, when I put it on the when the boat was in the water, the suction cup held the xducer in place at speeds up to ~29 mph (couldn't go faster than that) and stayed on all day long.

If you are using the P23 transducer I could sell you my mounting kit at a fair price. It did what I wanted and now I don't need it anymore.

 

[mpdive]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

Long story short...I got a Lowrance X-4Pro fish/depth finder for my birthday, don't want to cut out part of the hull to do a shoot-thru-hull setup but I also don't want to drill into the transom for the external transducer mount...

I purchased some large suction cups from Academy, these exact ones:

and fabricated a brace that bolts the transducer to the suction cup while allowing 1.5" of gap between the bottom of the suction cup and the hull (center point for the transducer to prevent cavitation)...

so while the suction cup says it will hold up to 60lbs, I feel that 40 mph might rip it off the back if applied with ordinary "spit".....

Im thinking that if I prep the area with a grease remover, clean the suction cup thoroughly and squeeze some RTV SEALANT on before I stick it in place.

will using RTV sealant ruin the gel coat upon removal at a later time or will it be okay?

any ideas on a better way to semi-permanently apply a suction cup to a fiberglass boat transom?

I would make the inlaws hang over the side and hold it in the water....

 

[Pain'n'da'boat]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

my suction cup bracket is actually quite strong, I stuck it to my dinosaur 36" TV front and it lifted if forward without breaking loose but the instruction manual says I can mount it shoot thru style and it will work....

but I don't want to epoxy it inside the bilge in case it doesn't work that well....can I use hot glue to keep from having an air gap between the ducer and the hull?

Also, I have a 1989 Ebbtide Dynatrak 160 fiberglass....does anybody know if that hull is solid or if it has styrofoam?

You guys are awesome, thanks for the advice!

 

[Home Cookin']

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

seems like a lot of work to avoid two screw holes for the transducer to the transom. There isn't a down-side to doing that if you seal the holes with 5200--well at least for the next 25 years. And if you keep the boat out of the water when not in use, it will never be a problem.
And if you remove it, it's nothing to fill the screw holes back and sand them down. You seem concerned about "damage" to the boat but 3 little holes is not as bad a as a glued-on and removed plate. After all, it's a 22 year old hull already.

If you do stick with the suction cups, I'd drill a 2" SS screw through the center of it and into the hull to be sure it stays there.

 

[ufm82]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

Your hull is solid and will be fine. Look for a spot in the center of the hull in the bilge area. Mark a spot where the transducer will go and mix enough 2 part epoxy to give you a 1/2 thick pad. Be careful and take your time to ensure you get no bubbles in the epoxy. The epoxy should be thick enough to not spread too much when you apply it. Take the transducer which you have also cleaned thoroughly and press it into the epoxy. You may not even need to weight it down if you situate it so there cable doesn't move it. Let it dry overnight and then try it. You'll lose a tad of sensitivity but most people don't even notice it. I did mine this exact way in my boat and it reads at all speeds- no more losing bottom at 30 mph. No holes in the transom, no spraying water, no hitting stuff with the transducer, nothing hanging off your boat. And if it doesn't work as you wish, a sharp rap with a hammer will pop the transducer right off the hull.

 

[smokeonthewater]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

no you can't use hot glue.... you HAVE to use epoxy because it is hard and transfers acoustic energy... hot glue or silicone will absorb the energy

 

[haulnazz15]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

I really can't understand why you'd be wary of 2-3 tiny screws into the transom, especially if you used sealent on the threads. Seems like an over-engineered fix to a problem that isn't really a problem.

 

[Thalasso]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

ufm82 is right on target.

 

[Faztbullet]

Re: Using RTV sealant on a suction cup to adhere better to my fiberglass transom

If you still want to use the suction cup ,apply a thin film of Karo syrup to cup and install. This is how we use to install the headlights on the early tri hull bass boats as they had 3 small cups.