NASA through hull depth transducer - fitting question.

[maxcampbell]

I've got the Clipper depth sounder and intend to fit the transducer through-hull (i.e. through a hole, not in-hull) because the boat's wooden. I've got the plastic fairing kit. The question is whether the watertight seal is between the fairing and the hull, in which case I guess I need to bed it into Sikaflex or similar, or does the fairing fill with water, with the watertight seal around the shaft of the transducer itself - in which case I guess that's where I wodge the sikaflex. I would have assumed it was between the fairing and the hull, but the fairing kit has two small holes, about 2mm diameter, between the bolt holes and the transducer shaft, that seem to be designed to allow water to fill & drain the space between the fairing & the hull. If that isn't their function, what is?

 

[2nd_apprentice]

Been there last year. The fairing kit does indeed fill with water but you're NOT supposed to use Sikaflex. Something to do with sulphur turning the plastic brittle IIRC. The kit comes with a small squeeze bottle of silicone. The idea is to drill the holes slightly oversize, apply a generous dose of silicone and then wait until tightening.
Unfortunately it didn't really work for me with water weeping in along the threads. Had to get the boat out again
I'm not sure what the point of letting water into the fairing block is and decided to just fill everything with epoxy. Actually I drilled two holes from inside my boat to fill the damn thing. Seems to have worked: no more leaks and the transducer works perfectly.
Mine's a plywood hull however, traditional timber might not work as well.

Edit: you should have these fitting instructions

 

[VicS]

I'd have thought that all holes through the hull should be sealed. Namely to two holes for the fairing fixing screws and the hole for the stem of the transducer.


I would have assumed that a modified copolymer sealant like CT1 would be a good choice. I d not have though there would be any objection to a polyurethane sealant like Skaflex but presuambly there is some objection to the use of a polysulphide if what 2nd apprentice says is correct about sulphur affecting the plastic.

 

[maxcampbell]

Thank you 2nd apprentice - I had considered doing as you suggested - i.e. filling the block with epoxy, and may do so. My hull is plywood too - only 6mm - which if anything is more challenging to plug effectively than an inch of timber. Thanks for the steer re silicone / sikaflex, too. I bought all this at the start of last season and never got a chance to fit it, and the instructions have evaporated.

 

[grafozz]

why not fit it inside the hull in a 50mm hose filled with oil .
, I have and it works fine (grp) so one less below waterline hole in the boat , epoxy the poly hose directly onto the hull ,
fill with oil ( I use olive oil )and gently push in the t/ducer until oil oozes out from the top , this expels the air .

 

[ghostlymoron]

That works on moderate thickness grp and may do on plywood but unlikely on planked construction.

why not fit it inside the hull in a 50mm hose filled with oil .
, I have and it works fine (grp) so one less below waterline hole in the boat , epoxy the poly hose directly onto the hull ,
fill with oil ( I use olive oil )and gently push in the t/ducer until oil oozes out from the top , this expels the air .

 

[maxcampbell]

That works on moderate thickness grp and may do on plywood but unlikely on planked construction.

Doesn't work in our plywood / epoxy-microballoon faired / glass sheathed / copper coated hull either - tried the chewing gum / bags of water things - no reading - dangle over side - fine. Needs a hole, wouldn't make one without need.

 

[vyv_cox]

I believe it is the solvent in the usual sealants that can cause stress corrosion cracking in some plastics, most notably in polycarbonate. They provide water-based silicone to avoid it.

 

[ghostlymoron]

Perhaps the ply isn't sufficiently hard and absorbs the sonic energy.

Doesn't work in our plywood / epoxy-microballoon faired / glass sheathed / copper coated hull either - tried the chewing gum / bags of water things - no reading - dangle over side - fine. Needs a hole, wouldn't make one without need.