Fixing of thruhull question

[Metabarca]

I'll be gluing in a thruhull for the depthsounder this weekend, but since I have no part of the hull that is flat and parallel to the seabed, in order to have the depthsounder point straight down, a part of the lip of the thruhull will not be flush with the hull (standing about 4-6mm proud). Is it ok to have a Sikaflex seal this thick, or I am going to have to build up a GRP lip? (Surely I would get innaccurate readings if the depthsounder points downwards at and angle?). Inside the hull, of course, the whole thing will be encased in matting and resin.

 

[oldsaltoz]

G'day Lucian,

You don't have to cut a hole at all, if you have cut one or have an existing hole consider filling it and mounting the sounder inside the hull.

Sounder signals pass thru' fibreglass with problems provided no air is trapped in the glass or the mounting.

You have a couple of alternatives for internal mounting. direct stick and swinging. the direct stick can be done with Sikaflex or resin, just make sure no air is trapped under the transponder. The swinging method gives the correct depth even when heeled, just suspend the transducer so it move in a container of oil: fixing a section of suitably sized PVC pipe with and end cap to enclose the whole thing.

To check that the fibreglass has no air voids in it, just drop the transducer into a plastic bag and wet the area, then move the bag around till you get a good reading, then move it a few inches to port and st'b to ensure it will read well when at an angle.

I hope this helps.

Avagoodweekend......

 

[Metabarca]

Hi Oldsaltoz,

I tried the direct stick method last year but it didn't work satisfactorily. Hence the hole. Which I've already cut!

 

[snowleopard]

OK, here's a method that will give a good seal:

Sand the outside of the hull thoroughly for about 1" all round the hole. apply a ring of filler, e.g Plastic Padding around the hole. Get a piece of melamine-faced chipboard e.g. Contiboard, give it a quick rub with furniture polish, then press into the filler until it makes contact with the hull at its highest point. Hold in place with e.g. masking tape while it sets.

As soon as the filler has set, remove the board and use a rasp or coarse sandpaper to fair the excess filler. Allow to fully cure then sand smooth.

You now have a flat surface to bed the flange of the through-hull onto. You can do the same thing on the inside to give the retaining nut a flat surface to screw down on - you have to be a bit more careful with this as the two flat surfaces need to be parallel.

 

[pvb]

Sealant choice...

You mentioned using Sikaflex. There's some concern that polysulphide sealants such as Sikaflex can cause longterm damage to plastics, and so some transducer suppliers are now specifying that a silicone sealant should be used, not Sikaflex. Ordinary silicone sealant would do, or you can buy "marine grade" silicone sealant. It's also much nicer stuff to use than Sikaflex!

Edit: Sorry, Sikaflex is polyurethane, so is safe on plastic. Polysulphide sealants such as 3M 101, LifeCaulk, etc, shouldn't be used on plastic fittings.

 

[michael_w]

Re: Sealant choice...

I would build it up with epoxy rather than Sikaflex. Have done this very job on three boats now.

Internal instalation never satisfactory IMHO.

 

[fireball]

[ QUOTE ]
(Surely I would get innaccurate readings if the depthsounder points downwards at and angle?)

[/ QUOTE ]

Up to a certain degree it is not an issue - the sound waves are emitted from the bottom of the device and radiate out (like throwing a stone in a pond except the stone tends to damage the fish more!) ... so if your only a little out of kilter then I wouldn't worry ... certainly 4-6mm off horizontal isn't (IMO) an issue ...
but then - if you follow the other advice on this thread and mount it inside you can get it true and level /forums/images/graemlins/smile.gif

 

[simonfraser]

i've fixed two on inside with silicone, works fine

 

[PCUK]

Sikaflex is polyurethane and a 'marine sealent'. LIfe caulk is polysulphide.
Never use silicone below the water-line.

 

[pvb]

You're right - Sikaflex is polyurethane (not polysulphide as I posted earlier), so would be safe on plastic. But there's no reason not to use a good silicone sealant below the waterline.