LittleSister
Well-known member
As Fr J Hackett says you only need sealant between the fitting and the hull. You do not need sealant between the skin fitting's threads inside the hull and the securing nut.
Personally, I see no advantage, and only disadvantage, in using a high adhesive sealant in this application. A low adhesive sealant is preferable in this (and almost all boat related sealing situations) in my view. The fitting's flange and internal nut are plenty strong enough to hold the fitting to the hull. Why on earth would you need strong adhesive here? Why the manufacturers recommend a high adhesive sealant is beyond me. It just needs to be slightly adhesive (just sticky) to ensure it makes a close bond with the fitting and hull.
The sealant is only there to fill - 'seal' (the clue's in the name) - the tiny gaps between the skin fitting flange and the hull to prevent water ingress. This does not take a lot of sealant. If your hull surface and skin fitting flange were perfectly smooth and flat (they're not) you would not need any sealant. It's just there to fill the surface imperfections and slight differences in shape (e.g. flat flange vs. slightly curved hull) Any significant thickness of sealant beyond this remaining after fitting is undesirable as it weakens the rigidity of the fitting in the hull.
You do need to apply enough sealant so that you have a slight surplus - put on, say, equivalent to an eighth of an inch or 3mm across the whole surface of the flange. The surplus is 'only' needed so you can see that there is sealant coverage around the entire circumference of the flange when you tighten it down. Tighten the nut progressively as much as you can and do so BEFORE THE SEALANT SETS! (Assuming its a setting sealant.) Note that because of the high viscosity of the sealant it will take a little time for all surplus to be squeezed out. So progressively tighten the nut, first by hand then by spanner, leave a few minutes and tighten again, as hard as you can without damaging the fitting or hull material.
Ignore those who say leave it slack and tighten later 'so you don't squeeze all the sealant out' - you won't. You want the sealant squeezed into the all voids and imperfections resulting from machining and slight differences in shape, and you don't need sealant (but anyway won't be able to avoid having some) where there is a perfect mating between the two faces. Any that can be squeezed out should be - it is not helping but only hindering. (Note that in other applications different technique is required - e.g. when using non-setting sealant (such as butyl) to seal windows in a coachroof, where you do need to leave a significant continuous layer of sealant to take up movement and flexing, but in those situations you never fully tighten the clamping screws.) Wipe away the sealant that has squeezed out from under the fitting - it's now doing nothing but looking bad.
As for the thread between the skin fitting and a seacock, hose tail or whatever, these are normally sealed by the thread itself and/or the tapered faces at the ends of the thread. Like Fr J Hackett I've only ever used PTFE tape or nothing. I think something like a well chosen Loctite thread lock/sealant (again, avoid high strength adhesives - one day you or someone else will need to undo it) might be appropriate to hold a fitting at the desired point of rotation where this can't be achieved by a locking nut or whatever.
Personally, I see no advantage, and only disadvantage, in using a high adhesive sealant in this application. A low adhesive sealant is preferable in this (and almost all boat related sealing situations) in my view. The fitting's flange and internal nut are plenty strong enough to hold the fitting to the hull. Why on earth would you need strong adhesive here? Why the manufacturers recommend a high adhesive sealant is beyond me. It just needs to be slightly adhesive (just sticky) to ensure it makes a close bond with the fitting and hull.
The sealant is only there to fill - 'seal' (the clue's in the name) - the tiny gaps between the skin fitting flange and the hull to prevent water ingress. This does not take a lot of sealant. If your hull surface and skin fitting flange were perfectly smooth and flat (they're not) you would not need any sealant. It's just there to fill the surface imperfections and slight differences in shape (e.g. flat flange vs. slightly curved hull) Any significant thickness of sealant beyond this remaining after fitting is undesirable as it weakens the rigidity of the fitting in the hull.
You do need to apply enough sealant so that you have a slight surplus - put on, say, equivalent to an eighth of an inch or 3mm across the whole surface of the flange. The surplus is 'only' needed so you can see that there is sealant coverage around the entire circumference of the flange when you tighten it down. Tighten the nut progressively as much as you can and do so BEFORE THE SEALANT SETS! (Assuming its a setting sealant.) Note that because of the high viscosity of the sealant it will take a little time for all surplus to be squeezed out. So progressively tighten the nut, first by hand then by spanner, leave a few minutes and tighten again, as hard as you can without damaging the fitting or hull material.
Ignore those who say leave it slack and tighten later 'so you don't squeeze all the sealant out' - you won't. You want the sealant squeezed into the all voids and imperfections resulting from machining and slight differences in shape, and you don't need sealant (but anyway won't be able to avoid having some) where there is a perfect mating between the two faces. Any that can be squeezed out should be - it is not helping but only hindering. (Note that in other applications different technique is required - e.g. when using non-setting sealant (such as butyl) to seal windows in a coachroof, where you do need to leave a significant continuous layer of sealant to take up movement and flexing, but in those situations you never fully tighten the clamping screws.) Wipe away the sealant that has squeezed out from under the fitting - it's now doing nothing but looking bad.
As for the thread between the skin fitting and a seacock, hose tail or whatever, these are normally sealed by the thread itself and/or the tapered faces at the ends of the thread. Like Fr J Hackett I've only ever used PTFE tape or nothing. I think something like a well chosen Loctite thread lock/sealant (again, avoid high strength adhesives - one day you or someone else will need to undo it) might be appropriate to hold a fitting at the desired point of rotation where this can't be achieved by a locking nut or whatever.
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