It's time to seal my deck joint

[rfrance0718]

As earlier reported, I've been working on the deck joint on my Oday 240. Thanks to some great guidance from several of you I have removed all of the old sealant, sanded in the resulting gap with 80 grit, and cleaned up with Acetone, as suggested on the 4200 tube. As I worked my way around the boat I would remove all of the screws for 6 feet or so, work on the gap, and then put every third screw back. Charles had reported that removing all of the screws could result in the deck shifting around and making re-alignment difficult. I'm noticing that driving the screws as far as they will go actually pulls the hull to the deck and deforms the hull a bit. It seems like I want the gap to be as uniform as possible, and filled with the 4200. I'm thinking that after I fill a length of the gap I will want to drive the screws in just far enough to put some pressure on the 4200, but not go so far as to squeeze it all out. My thought is to drive the screws just far enough, leave them until the 4200 cures and then tighten them down against the bed created by the 4200. (1/8 to 3/16s I believe) A previous owner had already tried to repair the joint and it looked like several mistakes had been made. There were many places where the hull and the deck had been squeezed so tight that there was little or no sealant at those spots. I also could see tell that he hadn't removed all of the original sealant as I was removing 2 different types of material.

The directions indicate that the 4200 can be used in temps down to 40 degrees. We are going to pull the boat into the pole barn. I'm thinking that we can set up a few heaters on the floor under, our working area. We can probably do one side at a time and keep the work warmer than 50. I have noted that blue tape will be needed to reduce or eliminate sag. I'm thinking that a strip of wooden molding pressed against the tape, and held in place, might help with that.

I welcome any critique of these plans, and any further ideas. Thanks for all of the help!

P.S. We've avoided getting new cabin cushions for a couple of years, waiting until the joint leaking was solved. Our new cushions are now in the works! I'm looking forward to cleaning up the stains on the liner and then doing some bright work on the interior woodwork. Also planning on ordering a new jib, and maybe a main as well, depending on price.

[Spot]

3M does have tech support. I learned a couple things from them not included in the standard 'folklore' or online narrative for 5200, including the correct thickness range to insure product cures and performs as tested at 3M. I used removeable aluminum spacers on my keel, all went well except for the general mess/sagging and that one spacer did not want to come out at the end. I probably should have made the spacers from G10/FR4 board and just left them in.

Your plan sounds good. I would keep the tubes 70 to 80 F, heat up building as you described, and would also use IR bulbs to heat the joint area as you work.

Exciting stuff,  all the best!

[Riley Smith]

I think I'd keep the workspace warm enough during the cure too. I know cure time for the 5200 is several days but haven't used 4200. The plan sounds good; hope it goes smoothly!

[sesmith]

For that application, I'd use 5200 rather than 4200. I'm aware of the reputation, but 5200 has more than twice the tensile strength of 4200. A hull to deck joint is exactly the place where this makes sense.  Either way, use the fast curing version.  I believe it cures in 1 or 2 days vs a week or so.

Maybe another product that might be a better choice is Sikaflex 292.  It's Sika's version of 5200.  It's not as strong of an adhesive as 5200 but still around twice the strength of 4200. I've used their Sika 291 for thru hulls and other sealing jobs and I like it better than the 4200 that it is equivalent to.

[rfrance0718]

Interesting, thanks. I've had recommendations on both sides of that coin. Someone did say that the screws were more important for providing the strength, while the sealant provides the barrier, suggesting that its adhesive ability was needed to keep it in place.  Any other thoughts?

[Wyb2]

I think the argument behind 5200 or 292 isn't so much that it's required structurally, but more just the job is a pain in the butt, so do whatever you can to make sure the joint is firmly stuck together so hopefully you never have to do this again.

The flips side being that if you do have to do it again, you may have made it an even bigger pain in the butt.

About closing the gap, I found that the hull and deck flange on my MacGregor 25 were not parallel.  What I did was tighten the hardware until the top edge of the hull made contact with the deck flange.  When it did, the torque required to keep tightening would start to climb quickly, and there was still about 1/8" (3mm) of gap at the bottom of the flange.  This seemed like a good stopping point to me.  Made a bad MS paint image to try to show this more clearly.  I would tighten until the gap at the red arrow was just about 0.  I had good visual access to the joint on the inside, which is how I knew what was happening.

This works until you get close to the bow or stern.  Around the bow and stern the shape makes the hull and deck very strong, so the gap is what it is

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[rfrance0718]

Nice diagram. My hall is pretty vertical at the top, and the gap is more parallel. without the screws the gap is about an 1/8 inch. It's interesting that Oday's diagram shows the screws tightened down so that there is no room for sealant. If I do that the hull actually ends up being "scalloped, with bulges where the screws pull the hull out further.

In my top picture, taken before I pulled any screws,  you can see that the the gap is narrow, or non-existent, where the screws had been tightened all the way. It's also interesting that the diagram shows the screws higher than they were actually installed. I'm wondering if they put them right at the bottom of the deck overlap for a reason? I'm still scratching my head, but I think that my plan of not tightening down the screws all the way down until the sealant has cured is still a good plan. Seems like that is essentially what was done in the MacGregor example.
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[rfrance0718]

I have lunch with a group of old salts on Tuesdays and we talked about this project. One friend, who's last boat was a Tripp 40 that was campaigned in some serious ocean races, texted a trusted boatwright, who offered the following. You cannot view this attachment.You cannot view this attachment.

His recommendations are right in line with what you all have suggested except for using a different sealant, Sikaflex 291. I'm going to look into that. The group also talked about my fasteners. I have some screws that aren't biting, and some that have been driven in so hard that the hole in the deck is compromised. With those it was suggested that new holes should be drilled, which makes sense. I'm not sure that I can effectively fill the existing holes since I won't be able to see if I get gunk into the inside holes. I'll probably go ahead put screws back in those old holes just to be sure that they are filled. We also talked about thru bolting, in some cases, around the shroud chain plates and the stanchions. (not at the bow and stern where there is really no issue)

Thanks for all who have contributed. I think that I have a plan now!

[Wyb2]

For what it's worth: Rod, who runs the Marine How To website, recommends against the two step method with sealant (re-tighten after cure).  I don't have the personal experience to say one way or the other, but I do kind of agree with the logic.  The sealant is supposed to stick to everything when you assemble it, then cure to stay in place and keep the water out.  If you re-tighten the screw, you just broke the bond that the sealant had with the screw.  People say it's to form a 'gasket', but if that were goal, why not use actual gasket material?

https://marinehowto.com/bed-it-tape/

Macgregor, much to my surprise, actually did use a foam gasket for the deck-to-hull joint.  Not sure if it worked well in the 80's, but by 2020 it was so bad that rainwater was following the screws into the boat.  I can't imagine how bad it would have leaked if I actually sailed it like that with the joint getting dunked with every wave.

But I know lots of people swear by this two step method, so I suppose it must work well enough most of the time.

[Peter S]

For what it's worth: Rod, who runs the Marine How To website, recommends against the two step method with sealant (re-tighten after cure).  I don't have the personal experience to say one way or the other, but I do kind of agree with the logic.  The sealant is supposed to stick to everything when you assemble it, then cure to stay in place and keep the water out.  If you re-tighten the screw, you just broke the bond that the sealant had with the screw.  People say it's to form a 'gasket', but if that were goal, why not use actual gasket material?

Re-tightening after cure works with through bolted fittings where you can tighten the nut while ensuring the bolt does not move. With a screw I'd agree that it is counter productive to turn the screw once cured.