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Old 05-03-2017 Thread Starter
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Tightening hull to deck joint

How snug should joint nuts be tightened?
How tight is too tight?
Any thoughts on separating the joint enough to insert more butyl tape?
The joint appears to have pop rivets and sheet metal screws under the tow rail (which can be seen inside the cabin) which may have been used to hold the joint together before installing the toe rail.
Does anyone have information on assembly procedure from the factory?
-CH

My boat is a 1977 Mirage 24.

Last edited by Chas H; 05-05-2017 at 09:13 AM.
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Old 05-04-2017
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Re: Tightening hull to deck joint

You don't name the manufacture but typical hull to deck joint on a production boat was through bolts on a fairly wide spacing like a foot or so and sheet metal screws in between at 4" or so spacing. They could have used pop rivets in place of the sheet metal screws. A fairly easy repair is pull the cap rail, remove and rebed the fasteners which are probably the cause of most of your leaks. Might replace the pop rivets or sheet metal screws with machine screws and nuts for a more secure fix. If your leaks are extensive and not limited to the fasteners you could take off the cap rail, remove all the fasteners, pry the deck apart, recaulk and refasten. Because I had other issues with the hull to deck joint, I glassed the hull and deck together on the outside. That was expensive as it was way above my skill level to get an acceptable cosmetic appearance when done. Do have my first ever fiberglass boat with no hull to deck leaks.
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Old 05-05-2017
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Re: Tightening hull to deck joint

I tried to reduce the leaking on my hull to deck joint. Was tons of work, and wasn't very effective (probably my fault). If I was going to do that again (especially on a boat I was going to paint), I would glass over the joint. Sorry, that probably doesn't answer your question at all.

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Re: Tightening hull to deck joint

My son and I tightened the screws and nuts along the rails. Some nuts were quite loose.
If I see leaks this year I think I'll remove each screw, wrap butyl tape around the screw threads and try to get some butyl in the hole before tightening. I agree with roverhi that most of the leaking is probably from around the screws. In the main saloon I have seen drops of water on the ends of some screw threads.
I don't like the thought of removing the rail completely, but will if the pumps can't keep up with the amount of water coming in.
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Re: Tightening hull to deck joint

Quote:
Originally Posted by roverhi View Post
You don't name the manufacture but typical hull to deck joint on a production boat was through bolts on a fairly wide spacing like a foot or so and sheet metal screws in between at 4" or so spacing. They could have used pop rivets in place of the sheet metal screws. A fairly easy repair is pull the cap rail, remove and rebed the fasteners which are probably the cause of most of your leaks. Might replace the pop rivets or sheet metal screws with machine screws and nuts for a more secure fix. If your leaks are extensive and not limited to the fasteners you could take off the cap rail, remove all the fasteners, pry the deck apart, recaulk and refasten. Because I had other issues with the hull to deck joint, I glassed the hull and deck together on the outside. That was expensive as it was way above my skill level to get an acceptable cosmetic appearance when done. Do have my first ever fiberglass boat with no hull to deck leaks.
My boat (35') has 1/4-20 machine bolts every 4 inches at the hull-deck joint. It has never leaked since it was built 27 years ago. If you are going to refasten, invest in a through-bolted approach and proper caulking/butyl tape by the bolts. If the joint is leaking, you need to open it and use a proper adhesive caulk. You might observe the hull-deck joint while sailing with seas to see if it is working--in which case an adhesive caulk would certainly be in order.

The fiberglassing approach might work, but seems like a more expensive alternative, if done correctly.
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Old 05-06-2017 Thread Starter
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Re: Tightening hull to deck joint

My Mirage does have slotted drive oval head machine screws through the toerail with hex nuts below decks. The sheet metal screws and pop rivets I described in an earlier post I think were used to keep the alignment of the deck and hull until holes could be drilled for the machine screws to hold the rail and make it fast.
-CH
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Old 05-06-2017
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Re: Tightening hull to deck joint

I had one leak where AFAIK (when the Boat was built) a block was attached to the under side of the deck by drilling a hole through both Fiberglass sections up to the aluminum toe rail. When I removed the screw it really leaked. I tightened a small section maybe 3' and could see the butyl squeezing out and that stopped the leak I checked a few others and none were loose so I let it be.

I would go maybe 1/2 to one turn past snug all you're doing is squeezing it in a bit not trying to push it all out. I think an aluminum toe adds to the security. Getting to all the bolts will be a challenge and separating and inserting some new sealant or butyl would be an unimaginable chore, I wouldn't even consider that as an option but if someone has done it successfully Ill get out of that argument.

I don't think butyl under the bolt head is as big an issue a bolting stuff to the deck tightening squeezes it around the bolt. Just removing the bolts does more damage than you make up for by adding some on top.
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Re: Tightening hull to deck joint

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Originally Posted by albrazzi View Post
Just removing the bolts does more damage than you make up for by adding some on top.
Do you think I might pull more butyl out of the hole than I might benefit by trying to push more in around the screw threads?
-CH
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Re: Tightening hull to deck joint

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Do you think I might pull more butyl out of the hole than I might benefit by trying to push more in around the screw threads?
-CH
Yes, that's why I would try tightening a little first.
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Old 06-20-2017
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Re: Tightening hull to deck joint

I think you have figured out correctly how boat was put together. The toe rails would have gone on well after the deck and hull were joined. The butyl tape never sets up, so that can be "squeezed" later when rails are added.

One problem boat builders had, was that it took two guys to install all those bolts. One outside, one inside. Later Mirage switched to an external flange and then one guy could put in and tighten all bolts.

If you find some bolts leaking and loose, take them out, add a sealant that remains pliable, like Lifecaulk or ??? and push bolts back in. Then have one person hold head while other tightens nuts underneath.

Water probably enters from deck, so check along edge of toerail to see if there are gaps in places. If there are, one option is to mask the deck about 1/16" from edge of rail, then try to run in some thin silicone. Regular marine silicone may be OK with caulking gun, but you can get a product for sealing car window glass from auto suppliers. It is quite thin and will run in. Tolleys Creeping Crack cure is similar, but probably too thin to work in this case. After it has set, cut along edge down to deck, then remove masking tape. This should hopefully prevent water from entering and getting to the bolts.

Last edited by FreeAgent; 06-20-2017 at 12:20 AM.
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