Sealant? : Stainless Chainplate, Aluminum Deck

[KelseyBohanon]

Hello!
We are replacing stainless chainplates on our aluminum ketch. I'm looking for any advice or experience with the sealant to use where the chainplates go through the deck. Below deck the chainplates are bolted to thick aluminum members, from which they will obviously be galvanically isolated. What I'm looking for a is a product to seal the slot in the deck around the chainplate. Considering Sika 291 and Gflex epoxy (but not sure if it will be flexible enough). 3m products don't get good reviews for sticking to aluminum or other metals, though I wouldn't think that the adhesive quality itself would be under much stress. Previous owner appeared to have used silicone, which failed. I can attach pictures if it helps.

[team karst]

How exactly did the silicone fail? adhesion or fracture or? If fracture, then that indicates quite a bit of movement, the dreaded RTV stuff is pretty stretchy (low durometer).

[KelseyBohanon]

Originally Posted by team karst

How exactly did the silicone fail? adhesion or fracture or? If fracture, then that indicates quite a bit of movement, the dreaded RTV stuff is pretty stretchy (low durometer).

Good question. The silicone failed in adhesion from both the aluminum and the stainless.

I guess another question would be how much of a gap we should allow between the chainplate and the deck eg. how big of a bead of sealant.

[barnakiel]

Do you really know it was silicone ?


Very often different materials are used (on steel, wood or grp hulls).



I may be wrong on alloy hulls though.


A fail of sealant round the plate is promoted when the gap is too narrow and the mastic material is too fine a layer (not enough thickness).


When the deck round the plate is raised with a lip (as it should be) the mastic / sealant can be built up the plate. This practically eliminates any water ingress from the top down. This also promotes proper thickness of the sealing material.


Still, unlikely a silicone, I think.



b.

[Bowdrie]

O please, please let it not be silicone.
If it is/was you need to do a thorough cleaning, any residue of silicone will cause adhesion failure of any other product introduced.
The best products are probably the Polysulfides, they give great flexibility and exhibit better adhesion to stainless than the Polyurethanes.
The old standby was 3M 101, but it's now gone, Boatlife has a polysulfide.

[markxengineerin]

I have yet to have any long term success with anything sticking to stainless, especially polished stainless, in this environment. Curious if anyone has good adhesion over multiple years, especially for boats that see hot summers and cold winters.

[dlj]

Just throwing it out there - why not go to aluminum chain plates, weld them in and be done with it?

dj

[Orion Jim]

Use Bed-It butyl tape and chainplate covers if not already equipped.

[markxengineerin]

Butyl tape seems good, but ugly as it oozes out the edges over time and collects dirt. I guess tidying that up every so often is better than leaks, though.

[Jim Cate]

^^^^^
Many chainplates move relative to the deck under load, some quite a lot. I don't think that butyl tape does so well under such conditions, and I would use one of the elastomer sealants instead. I've had success with both Sikaflex 291 and Fixtec (don't remember the exact number, but it is their general purpose sealant). This has been on both FRP and timber decks...

Jim

[Benz]

What about Spartite, designed to seal mast partners?

[GordMay]

Originally Posted by Benz

What about Spartite, designed to seal mast partners?

Spartite is not meant to be a waterproofing agent - it doesn’t bond that well to polished stainless steel[1], and after installation, it shrinks slightly [0.001" over 1.0"], and will allow water to run down the annular cavity*.

*This gap can be filled with BoatLIFE’s “LIFE-CALK”, or other polysulfide, or polyurethane[2] caulk.
Do not use silicone.

Spartite Instructions ➥ http://www.transatmarine.com/acrobat...structions.pdf

[1] You can improve a sealant/bedding’s adherence to chainplates, by dewaxing the metal, then giving it some tooth, by sanding it with 220-grit emery cloth. Afterward, clean the chainplate, and the surrounding deck bedding surface, with solvent, then liberally apply bedding material.

[2] For bedding deck hardware, I prefer polysulfide, over polyurethane, but, I've successfully used both.

See also: “Use Spartite to seal chainplates” ➥ https://www.cruisersforum.com/forums...es-232230.html

[EmeraldCoastSailor]

I am a big fan of butyl. Worked well on my ss chain plate arrangement where is passed through the deck. I rebedded twice with butyl. First time replaced factory silicone sealant. Second time (after 5 years) replaced butyl and took less than half as long to complete.

[barnakiel]

I have used Bostk MSP


https://www.bostik.com/files/live/si...rbo%20bond.pdf


I think it did an excellent job. Will use it again, if I still can buy it.


Our boat is grp (gelcoat where the plates exit the deck, our plates are polished and chemically passivized SS.


The bond is about 7 years old now. It looks prime. It weeps small amount of white over time (I think from UV degradation maybe, or maybe to extreme heat here - we are in Africa)



b.

[KelseyBohanon]

Originally Posted by barnakiel

Do you really know it was silicone ?


Very often different materials are used (on steel, wood or grp hulls).



I may be wrong on alloy hulls though.


A fail of sealant round the plate is promoted when the gap is too narrow and the mastic material is too fine a layer (not enough thickness).


When the deck round the plate is raised with a lip (as it should be) the mastic / sealant can be built up the plate. This practically eliminates any water ingress from the top down. This also promotes proper thickness of the sealing material.


Still, unlikely a silicone, I think.



b.

Yes, we are quite sure it is silicone, being carpenters and having applied and removed lots of the stuff. transparent and very rubbery. Though I can't say I would know of what other products might have been used 40 years ago, or the last time they were sealed.

The gap being too narrow makes a lot of sense, though of course you don't want to big of a gap either. I will double check what the clearance was on the old ones.

I have noticed that fiberglass boats with chainplates going through the deck have cover plates, which allow for a gasket type seal at the deck level, and raise the level of the adhesive sealant above deck level. Is this what you mean by a "lip"? Ours are just a slot straight through the deck. I'm thinking perhaps welding an aluminum coupon around the slot, and then sealing between that the chainplate would be a good way to keep the sealed seam out of most of the water.

A couple of pics from above deck, below deck (you can see the bead of old sealant), and a sample of the old sealant.