Hull/Deck Fastenings - Al v S/S

Richard10002

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Just looking for some wider clarification of a query I have asked on the Moody Owners Forum:

My deck and hull are joined together by overlapping GRP, glued, and mechanically fastened through the aluminium toe rail, with stainless steel machine bolts.

About 3 or 4 have sheared somehow, so I need to replace them.

Given that this is an Aluminium/Stainless Steel situation, I am thinking that Duralac is appropriate. However, the holes will also need to be sealed with something like Sikaflex 291.

I am also thinking that Duralac and Sikaflex may not be compatible, so I will either not get a seal, or I will get a reaction between the Al & S/S, or both.

Any thoughts and ideas on how to make this work.

Many Thanks

Richard
 

PetiteFleur

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What I have done is partially insert the new screw, wipe sikaflex or equivalent around the thread & head then fully insert it, put sikaflex on the penny washer internally and tighten up. No leaks on my Moody 33. Used when replacing the central screw holding the stanchion base to the deck & toerail. Also used sikaflex between the stanchion base and toerail which squeezed out.
 

Richard10002

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[ QUOTE ]
What I have done is partially insert the new screw, wipe sikaflex or equivalent around the thread & head then fully insert it, put sikaflex on the penny washer internally and tighten up. No leaks on my Moody 33. Used when replacing the central screw holding the stanchion base to the deck & toerail. Also used sikaflex between the stanchion base and toerail which squeezed out.

[/ QUOTE ]

Do you think the sikaflex is enough to prevent the Al and S/S touching, and therefore corroding and welding together over the years? - I would hate to have to get corroded bolts out of the toe rail - it's bad enough with the stanchion bases, which can be removed for working on.

Cheers

Richard
 

PetiteFleur

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Not sure if it will prevent the ss and Ali. welding together. The original ss bolts on my boat don't seem to have had anything used and they seem fine after 30 yrs.(noy had to remove any yet.....) I'm sure it will help. Alternatively put Duralac on the underside of the countersunk screw and first bit thread that's in the Ali and sikaflex through the GRP? Messy to actually do but you could use a small brush for the Duralac maybe? If the seal between the Ali and grp is still ok the just Duralac on the screw and sikaflex on the penny washer - on reflection this may be the best way.
 

charles_reed

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Begging the most important question: "Why have the bolts sheared". There really shouldn't be any movement in that area of a boat.

As a rule of thumb, use polyurethane sealant (Sika produce a range of sealants, I assume reference is to 291 the polyurethane version) when you never expect to dismantle the parts. So using or not using Duralac is probably irrelevant if you are going to use polyurethane. Alternative sealants are polysulphide (stays flexible so used where movement is likely) or silicone (which you know all about) or acrylic, which probably has no place in a marine environment.

Is "Rogue" still out of the water in Manoel Island? If so it must be costing a bomb, this is high season for hard-standing places.
 

Strathglass

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When fitting a toerail I tend to (very lightly) coat the top part of the thread and under the head of the bolt with duralec and let it set. I then fit the torail / bolts in place with Sikaflex.

Iain
 

Richard10002

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[ QUOTE ]
When fitting a toerail I tend to (very lightly) coat the top part of the thread and under the head of the bolt with duralec and let it set. I then fit the torail / bolts in place with Sikaflex.

Iain

[/ QUOTE ]

That looks like the best way to cover all the bases!

Charles - agree with the question, but cant think of an answer in the short term. Back in the water - paid for 4 weeks at about €70 per week. Only 2 of the weeks were truly productive.

Many Thanks to all

Richard
 

Richard10002

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This is the answer from the surveyor:

"Dear Richard,

Liberal application of Duralac will both seal the joint and inhibit corrosion between the dissimilar materials. I suggest you assemble everything hand-tight for a few hours before finally tightening the bolts up hard - this will allow the Duralac to slightly set and you will get a better seal.

Best regards,

John"

I certainly didnt realise that Duralac will seal as well as separate.

Cheers

Richard
 
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