Mast Fittings

[Trailer_sailor]

Hi All, I just removed my spreader brackets to drill out the spreader holes. They look like aluminium castings. I was surprised to find some kind of grey bedding sealant between the fitting and mast. It feels like silicon. When I come to refit them, I shall dip the rivets in some duralac. But I assumed that I would not need to apply anything between an aluminium fitting and mast before riveting. Is that correct?

 

[KevinV]

If it worked well before, then why not reinstate it? Not much hassle, and there's a really good chance the mast builder knew what they were doing - perhaps worried about corrosion in the gap, and they are almost certainly slightly different alloys.

Put it another way - if you are wrong you'll trash the mast, if I'm wrong you've wasted a tiny bit of sealant.

It won't be silicone BTW, more likely sikaflex (for which cheaper alternatives exist)

 

[vyv_cox]

Even if their composition was identical there is a strong likelihood of crevice corrosion without sealant. Don't use silicone anywhere on a boat, apart from possibly the heads sink.

 

[Boathook]

Hi All, I just removed my spreader brackets to drill out the spreader holes. They look like aluminium castings. I was surprised to find some kind of grey bedding sealant between the fitting and mast. It feels like silicon. When I come to refit them, I shall dip the rivets in some duralac. But I assumed that I would not need to apply anything between an aluminium fitting and mast before riveting. Is that correct?

The description makes it seem like arbomast br, which is a butyl sealant/ bedding compound.

 

[Trailer_sailor]

Thanks all.

Boathook, you may be right. I've just googled arbomast, which comes in grey, the same colour as my mystery sealant. Whatever it was, it did well for 37 years. I cleaned it off this afternoon and underneath all is looking good. I have a tube of sikaflex 291i (if it hasn't gone solid since I last opened it), but the joint doesn't need its adhesive properties so I think I might see if my local chandler has a tube of arbomast.

 

[Boathook]

Thanks all.

Boathook, you may be right. I've just googled arbomast, which comes in grey, the same colour as my mystery sealant. Whatever it was, it did well for 37 years. I cleaned it off this afternoon and underneath all is looking good. I have a tube of sikaflex 291i (if it hasn't gone solid since I last opened it), but the joint doesn't need its adhesive properties so I think I might see if my local chandler has a tube of arbomast.

I use it for bedding all fittings above and below the waterline plus fittings on the mast. Be warned, It does stick to everything !
Some prefer to use butyl tape but it would be too thick for a mast fitting imho.

 

[ProDave]

I always use Duralac for joints between dissimilar metals, I see no reason it could not be used for similar metals?

 

[Tranona]

Thanks all.

Boathook, you may be right. I've just googled arbomast, which comes in grey, the same colour as my mystery sealant. Whatever it was, it did well for 37 years. I cleaned it off this afternoon and underneath all is looking good. I have a tube of sikaflex 291i (if it hasn't gone solid since I last opened it), but the joint doesn't need its adhesive properties so I think I might see if my local chandler has a tube of arbomast.

Butyl sealants like Arbomast are commonly used in this sort of application as they are non setting

 

[doug748]

Hi All, I just removed my spreader brackets to drill out the spreader holes. They look like aluminium castings. I was surprised to find some kind of grey bedding sealant between the fitting and mast. It feels like silicon. When I come to refit them, I shall dip the rivets in some duralac. But I assumed that I would not need to apply anything between an aluminium fitting and mast before riveting. Is that correct?

This came up recently. I would use Arbomast as well, it can be difficult to get locally but it is worth the additional delivery charge because it remains usable in the tube, effectively, forever.

The sealing compound keeps moisture out but electrolysis can take place between what are basically the same materials. In this case between the alloy extrusion and casting - even if they were identical alloys which they won't be. Even two pieces of identical material can promote corrosion if they have been worked differently or have a different heat treatment history.


.

 

[vyv_cox]

The sealing compound keeps moisture out but electrolysis can take place between what are basically the same materials. In this case between the alloy extrusion and casting - even if they were identical alloys which they won't be. Even two pieces of identical material can promote corrosion if they have been worked differently or have a different heat treatment history.

I think you mean galvanic corrosion, not electrolysis. There is no applied voltage involved. However, the galvanic voltage involved between different aluminium alloys is tiny, and most masts are not immersed!

As I posted earlier, crevice corrosion is by far the most likely cause. This occurs in metals that rely on the formation of an oxide film for corrosion resistance, of which aluminium is one. Small amounts of water, such as rain, are sufficient to cause it.

 

[wonkywinch]

Don't use silicone anywhere on a boat, apart from possibly the heads sink.

Basin, sir, basin. Sinks are found in a galley or kitchen

 

[vyv_cox]

Have you seen the receptacle provided in the heads of a Sadler 34? The word basin does not spring to.mind, more like an open tank!

 

[doug748]

I think you mean galvanic corrosion, not electrolysis. There is no applied voltage involved. However, the galvanic voltage involved between different aluminium alloys is tiny, and most masts are not immersed!

As I posted earlier, crevice corrosion is by far the most likely cause. This occurs in metals that rely on the formation of an oxide film for corrosion resistance, of which aluminium is one. Small amounts of water, such as rain, are sufficient to cause it.

Indeed.

Still surprising how many masts you see which seem to have been professionally made but have signs of missing sealant. I had a spinnaker mount (granted stainless steel) that had blown a 1 inch hole through the mast.

.

 

[vyv_cox]

Indeed.

Still surprising how many masts you see which seem to have been professionally made but have signs of missing sealant. I had a spinnaker mount (granted stainless steel) that had blown a 1 inch hole through the mast.

.

I most definitely did not say that sealant was not necessary, just the opposite. Read post #3 again.

Many years ago we raced Hobie 16s. The builders went through a difficult time of mast losses due to the chain plate pop riveted to the trampoline frame corroding the aluminium frame. Their response was simple but effective. In build they put a small piece of 'sticky back plastic' between the two. Problem solved.

 

[doug748]

Yes, we are both saying the same thing, I would not suggest anything else