screw mounting in cored deck

[dhays]

Hi folks, haven't been on a long time. Been busy.


I am going to mount some chocks on my boat deck for the RIB. The area of the deck is reinforced I believe with a plywood sandwich for strength. The issue is that aft chocks will be in an area that my wife will not allow me to through bolt them to the deck as that would be visible since it overhangs the aft cockpit.



So that means I will need to screw the chock mounts into the deck. My concern is both strength and water intrusion. The mounts can take up to a 1/4" bolt/screw.


I'll have the same issue with pad eyes that I'll need to install.


I know what I would do if I was through-bolting, but I've never screw mounted something like this before. Looking for suggestions.

 

[Comodave]

Go to marinehowto.com. He has an excellent article on sealing penetrations in cored decks. It would be waaay better to through bolt since there may be lots of stress on the fittings when the boat is working through heavy seas. Personally I always go for function over form, it must work first and look good second. I would use some polished 316 washers on the bottom and live with the look and feel good knowing that it will not break loose. But then again you have to live with your wife.

 

[DavidM]

Drill your holes oversize, like 7/16" for a 1/4" screw. Then fill with epoxy and let set. Then drill the hole an appropriate size for a tap. Tap it and then screw it down. Machine screws hold well in tapped epoxy.

David

 

[dhays]

Yes, I've read Rod's excellent article a number of times over the years and did it again this morning. I will see what I can get by my wife.


I was thinking that I could use a similar technique that I've used when through bolting. Over drill the hole, remove some core, fill with thickened epoxy, then drill the proper size for the screw. Two concerns; the first is I'm not sure how well a screw will hold in the epoxy. Secondly, I'm not sure that I'd be able to remove all the ground core out of the hole since there isn't a bottom hole.


Even if I can get away from through bolting the chock mounts, I'm won't be able to do that with the pad eyes, so still have to solve the screw problem.

 

[dhays]

Drill your holes oversize, like 7/16" for a 1/4" screw. Then fill with epoxy and let set. Then drill the hole an appropriate size for a tap. Tap it and then screw it down. Machine screws hold well in tapped epoxy.

David

I wouldn't have thought about tapping the epoxy. Thank you. That may be the ticket.

 

[Frosty]

[Edit: Typed this while DavidM was replying similar - so I had not seen his when I posted]

If you "can't" through bolt, than rather than using pointy screws, I would do some variation of machine screw/tapping. All three of these will also seal off the core (although for all three I would first seal with unthickened epoxy before using any thickened).

Options:

1) Overdrill (remove core), fill with structurally thickened epoxy, then tap for your machine screw (bottoming tap so screw goes into blind hole, not through bolting).

2) Same idea but set in a block of something like G10 pre-made fiberglass board (with thickened epoxy), then tap that. Or bond in a short piece of G10 pre-made tubing that you have tapped the inside of (requires much smaller hole).

3) Bond in a machine screw directly (also using thickened epoxy). If you use a waxed screw, you can then remove it, so it ends up similar to #1. I sometimes wax an identical sacrifical screw and then use a never-waxed one to put back. You can also not wax but then heat up the screw with say the tip of a soldering iron and it will back out.

I have done #1 and #3. May try the tube version of #2 next time I have a situation that calls for something like this (this idea came from member DDW).

 

[trawlercap]

Chuck a 90 deg. Bent nail in your drill. Use it to dig out the core.

 

[SoWhat]

Chuck a 90 deg. Bent nail in your drill. Use it to dig out the core.

Allen wrench or set of Harbor Freight picks also work.
The set epoxy will be stronger than the original core.
I wrap the threads with a string of butyl so there is zero possibility of water intrusion. I use self tapping course threads. Never had a problem.

 

[dhays]

Frosty,


Thanks. I appreciate the specific information. A quick check showed that finding a premade tube that had the correct inside diameter for the tap, could be an issue. Could a pre-made tube be epoxied in place and then drilled out to the proper diameter for the tap? The other idea was to use a G10 rod that was epoxied in place and then drill it out for the tap. FWIW, I had never heard of G10 products until you mentioned it. Always more stuff I don't know than what I know.


I think it would be pretty easy to epoxy a rod into a drilled hole. Maybe harder to drill a perfect hole the the tap however.


Your #3 idea took me a while to understand. Let me see if I do.... Prep the hole first by predrilling and removing core. Sealing it with epoxy (thanks for reminding me of that step). Then filling with thickened epoxy but put a machine screw in the epoxy plug and allow it to set. Once set, remove the screw and you have now a tapped plug to use. Waxing would make removing it easier. Is that basic idea?


You have done #1 and #3. Which did you find easier?

 

[dhays]

Allen wrench or set of Harbor Freight picks also work.
The set epoxy will be stronger than the original core.
I wrap the threads with a string of butyl so there is zero possibility of water intrusion. I use self tapping course threads. Never had a problem.

I would think that if the fill epoxy was done well, then there wouldn't be a risk of water intrusion. The self tapping screws sound easier. I wonder about the epoxy being two brittle for that...? I have a life-time supply of butyl tape I got from Rod back two sailboats ago.


What screws have you found that work well?

 

[Comodave]

I would do it this way. Buy some G10 from Jamestown Distributers. Drill holes for bolts in the pattern you need for your brackets. Countersink the holes and put in 316 flathead bolts and put nuts on them and tighten. Lay the G10 pieces out with the bolts sticking up. Mark the hardtop around the G10. Grind the glass to get some tooth in the hardtop. Then epoxy thickened with some structural filler and epoxy the G10 blocks to the hardtop with the bolts sticking up. Use thickened epoxy to make a fillet around the G10 blocks. Paint the G10. Now you have studs sticking up to bolt the brackets down and no holes in the hardtop and no possible leaks into the core.

 

[koliver]

Hi folks, haven't been on a long time. Been busy.

Dave

Good to see you back! We have missed your contributions. Always thoughtful and respectful.

I hope this means you are back for a while and will be enjoying your boat this season.

 

[Frosty]

Frosty,


Thanks. I appreciate the specific information. A quick check showed that finding a premade tube that had the correct inside diameter for the tap, could be an issue.

I pretty much always do it way #1. I've had good success with it. This is where I remove some core (ideally leaving the top fiberglass skin intact except for final fastener diameter hole), seal with plain epoxy, then fill the hole with structurally thickened epoxy (I mix WEST 406 - colloidal silica; and 404 structural thickener). I chisel it flat (with the existing deck surface) in the green stage (firm Parmesan texture). Then once it's fully cured I drill and tap for machine screws (blind hole). I find that the drill bit size can be slightly smaller than you'd use for metal. It's useful to do a few "test holes" in a scrap board or something to be sure of everything (drill hole in board, fill with thickened epoxy, drill, tap, test). Then you know you have the drill size correct, can practice the tapping -- and if you leave the test screw long, you can also tug test the head to satisfy yourself it's good.

Your #3 idea took me a while to understand. Let me see if I do.... Prep the hole first by predrilling and removing core. Sealing it with epoxy (thanks for reminding me of that step). Then filling with thickened epoxy but put a machine screw in the epoxy plug and allow it to set. Once set, remove the screw and you have now a tapped plug to use. Waxing would make removing it easier. Is that basic idea?

[Note, see below; way #3 is probably not ideal for machine screws] This is a WEST System method, I think they call it bonded fasteners? If you download their free epoxy book (very useful, IMO), they discuss it. I refer to these manuals even after years - either something I haven't done, or if I'm feeling rusty.

https://www.westsystem.com/wp-content/uploads/1116-User-Manual-2016.pdf

https://www.westsystem.com/wp-content/uploads/Fiberglass-Manual-2015.pdf

You have done #1 and #3. Which did you find easier?

I would say #1. In fact as I think about it, I've used the #3 concept mostly for other things (meaning something I want to conform to with epoxy, but then remove and put back, or sometimes pointy screws although I avoid those when I can). Of course I can't think of any examples off hand Also as I refresh my memory in the second linked manual (they show it on page 50), I see they do show it for pointy screws. So perhaps it would be difficult for blind machine screws.

 

[Cigatoo]

Drill your holes oversize, like 7/16" for a 1/4" screw. Then fill with epoxy and let set. Then drill the hole an appropriate size for a tap. Tap it and then screw it down. Machine screws hold well in tapped epoxy.

David

 

[FF]

A dab of paint , or a quick shot from a rattle can can make very load sharing large washers acceptable to "she who must be obeyed".

 

[angus99]

 

[SoWhat]

I would think that if the fill epoxy was done well, then there wouldn't be a risk of water intrusion. The self tapping screws sound easier. I wonder about the epoxy being two brittle for that...? I have a life-time supply of butyl tape I got from Rod back two sailboats ago.What screws have you found that work well?

You are probably right but I am OCD about potential seepage paths. I usually put some butyl under all screw heads. I drill appropriate size hole prior to inserting screw so I suppose I am deviating from self tapping a bit.
I buy all SS screws from West. More expensive but high quality.

 

[localboy]

I've also installed a davit base by drilling over-sized (into balsa core) and filling with thickened West Systems with their hard filler (don't remember the #). I then took the machine screws and covered the thread etc with a very thin coating of silicone grease. I "mounted" the mount with the machine screws into the un-set epoxy and let if go off. When hard the threads looked like they were tapped. Worked like a charm.

 

[Frosty]

One potential issue when using pointy screws with butyl (or even other caulking) can be that when you apply the necessary force to compress the sealant, the screw can strip out. There doesn't have to be anything "wrong" with the material you are screwing into for this to happen. Drilling and tapping is one way to get around the issue. It will also be favorable when the time comes to re-bed the item. (That's not to say that pointy screws can't possibly work.)

 

[RonT]

Hi dhays Guru, I am a life-long sailor on the Australian Coast. Along with over 50 years of sailing I have built many vessels. I see you have received numerous good ideas. But the following is by far the best way to fix fittings of any type to a deck. Particularly when the deck is cored. Even with foam coring this works well. I suggest you do a trial run using a bit of scrap timber; pine-board etc. Instructions: This involves casting in female threads for the chosen male screws or bolts. In this case we choose threaded bolts (or machine screws) Firstly chose the appropriate fixing bolts. Lets say 1 inch or 1 1/2 inch X 5/16" threaded bolts (Plus further length for the thickness of the chocks. choose ones with a reasonably course thread) in 316 stainless steel. The length depends on the thickness of available deck. At the same time buy the appropriate stainless washers to suite. But also buy another 316 stainless washer for each bolt. These need to have an OD or 1 inch and an ID of 5/16". Such washers will be available at specialist stainless fitting suppliers. Next center-punch where each bolt is to go. Then take a speed drill bit of 1 inch diameter (you know the flat drill bits) and drill out the holes to a suitable depth for the bolts. 1 inch or 1 1/2 inch deep. Take each of the bolts and grease their threads with any automotive grease or just Vaseline. Don't overfill the threads. You only need a smear on them - so wipe away any excess with your finger. Mix some good quality epoxy glue and 3/4 fill each hole with the glue.
Place the 1 inch diameter (1" OD) washers into the hole on top of the glue. Push the greased bolt into the washer making sure to keep the bolt vertical. You can use you fingers or small screw drivers to push the large washer further down into the glue. These washers ensure the bolt is kept center in the hole. Use a chisel or putty knife to neatly fill the hole around the remainder of the threaded bolt, to bring the top of the glue to the level of the deck. You should have sufficient length of bolt protruding from the glue to accommodate the thickness of your RIB chocks. Let the whole lot set for 24 hours. Then the bolts will unscrew quite easily. Later bolt down the RIB chocks with the bolts. Place the small stainless washers under the head of the bolts. These bolts will never come free. The surface tension between the epoxy glue plug and the deck material is enormous. For instance with the 1 1/2 inch depth of glue the surface area is about 5 1/2 square inches of surface tension. I have done this hundreds of time always with success. The threads in the glue are enormously strong.
Any questions just ask. Regards RonT

 

[FF]

"I usually put some butyl under all screw heads."

Putting some rubber under most anything is a great practice.

As an example a stanchion base or backing plate will be flat , almost nothing on a boat is flat .

The rubber shares the load over the entire surface.

 

[SailorGreg]

"I usually put some butyl under all screw heads."

Putting some rubber under most anything is a great practice.

As an example a stanchion base or backing plate will be flat , almost nothing on a boat is flat .

The rubber shares the load over the entire surface.

Threaded inserts were designed for exactly this application. Similar approach with oversized initial hole and epoxy. Much better grip than screws. Simpler than bolts, etc.

 

[dhays]

Dave



Good to see you back! We have missed your contributions. Always thoughtful and respectful.



I hope this means you are back for a while and will be enjoying your boat this season.

Hope to be and thanks. Life has been interesting over the past couple years. Boating has suffered as a result. If the border opens, I hope to get up into BC late in the summer for a couple weeks. As long as work, family, and pandemics don’t get in the way.

 

[dhays]

Hi dhays Guru, I am a life-long sailor on the Australian Coast. Along with over 50 years of sailing I have built many vessels. I see you have received numerous good ideas. But the following is by far the best way to fix fittings of any type to a deck. Particularly when the deck is cored. Even with foam coring this works well. I suggest you do a trial run using a bit of scrap timber; pine-board etc. Instructions: This involves casting in female threads for the chosen male screws or bolts. In this case we choose threaded bolts (or machine screws) Firstly chose the appropriate fixing bolts. Lets say 1 inch or 1 1/2 inch X 5/16" threaded bolts (Plus further length for the thickness of the chocks. choose ones with a reasonably course thread) in 316 stainless steel. The length depends on the thickness of available deck. At the same time buy the appropriate stainless washers to suite. But also buy another 316 stainless washer for each bolt. These need to have an OD or 1 inch and an ID of 5/16". Such washers will be available at specialist stainless fitting suppliers. Next center-punch where each bolt is to go. Then take a speed drill bit of 1 inch diameter (you know the flat drill bits) and drill out the holes to a suitable depth for the bolts. 1 inch or 1 1/2 inch deep. Take each of the bolts and grease their threads with any automotive grease or just Vaseline. Don't overfill the threads. You only need a smear on them - so wipe away any excess with your finger. Mix some good quality epoxy glue and 3/4 fill each hole with the glue.
Place the 1 inch diameter (1" OD) washers into the hole on top of the glue. Push the greased bolt into the washer making sure to keep the bolt vertical. You can use you fingers or small screw drivers to push the large washer further down into the glue. These washers ensure the bolt is kept center in the hole. Use a chisel or putty knife to neatly fill the hole around the remainder of the threaded bolt, to bring the top of the glue to the level of the deck. You should have sufficient length of bolt protruding from the glue to accommodate the thickness of your RIB chocks. Let the whole lot set for 24 hours. Then the bolts will unscrew quite easily. Later bolt down the RIB chocks with the bolts. Place the small stainless washers under the head of the bolts. These bolts will never come free. The surface tension between the epoxy glue plug and the deck material is enormous. For instance with the 1 1/2 inch depth of glue the surface area is about 5 1/2 square inches of surface tension. I have done this hundreds of time always with success. The threads in the glue are enormously strong.
Any questions just ask. Regards RonT

Ron,

Thanks for the detailed description. I like the idea of the washer part way down to help keep it the bolt centered.

 

[Flatswing]

For most applications screws will work fine in thickened epoxy-follow Rod's Instructions and a key hole bit on a Dremel is way easier than a bent nail-especially if you have plywood core as is common in high stress areas. Thickened epoxy is best but messy to load into the small syringes once it's peanut butter consistency. If you inject thickened epoxy with a syringe or gun, you can usually dispense with the liquid epoxy as a first step if the core is decent. If your over drilled holes are 3/8"/10mm or larger and you have a lot of them, I would recommend gunning in Thixo thickened flexible epoxy. The mixing tips fit a 3/8"/10mm hole easily, and with extra tips you can re-use the cartridge at least once. Just be sure to discard the first 3" that doesn't get fully mixed, and be aware that the epoxy cartridges hold only 1/2 to 2/3 of what you usually get out of a 10oz caulk tube. I rebedded my (40+lb each) gunwhale & transom door hinges this way (#12 screws). Pleasantly surprised.

If you use screws, you must pre drill the hardened thickened epoxy because if the screw core is too big, the epoxy core will crack and you have lost your seal (Just like using screws through gel coat, if the core is too big, the gel coat cracks) Be prepared to upsize the pilot hole more than you think, drills in 1/32 increments can be helpful here. Basically if you can't get the screw in by hand (albeit with a lot of work) the pilot hole is too small. Once you are confident of the correct size, just use a driver.

 

[HTurner]

When you use epoxy in screw holes or to fill large gaps, I have always found pastry piping bags to be the ticket.


Something like this:


https://www.amazon.com/Silicone-Reusable-Cupcake-Decorating-Professional/dp/B07JRGFKLG


A ziplock with the corner just nipped also works in a pinch.

 

[ttompkins]

When ever I mount anything I always through bolt it. If going through a cored part of decking I decide where I need to mount item, over size drill out the hole for 1/4 inch bolts I usually go 3/4 inch. Fill with West Systems 6-10 or thickened epoxy. When that is cured drill holes for 1/4 inch bolts. If wife has concerns pay attention, you may be the skipper, don't forget who is the admiral. I would suggest using stainless acorn nuts. They make for a pleasant looking finish compared to just a regular nut. You could always just hold one of these up in that area and wait for her blessing. If she is not happy with that use screws. This said I would still oversize drill holes, say 1/2 inch and screw into center of the epoxy plug.

 

[Steve DAntonio]

Step by step...

https://stevedmarineconsulting.com/wp-content/uploads/2021/01/TaskSheet189-Closeouts-05.pdf

DO NOT bed the inside portion of the fasteners, the instructions explain why.

 

[Frosty]

Step by step...

The unique thing here is that the OP does not want to through bolt. Hence the suggestions for tapping structurally thickened epoxy, etc

(Not to say through bolting isn't the way to go; just that we were responding within the OP's stated constraints.)

 

[CarlF]

I would not consider tapping into fiberglass to be strong enough for this application.

Maybe G10 but use at least 1/4". 1/2" would be better.

But I'd 2nd the suggestion of using the West hardware bonding guide. I would both bond the bolt AND epoxy the whole bottom of the davit fitting to the deck. Use West G-Flex epoxy with West 404 high density filler.

Use Rod's guide to seal the core.

If you ever need to remove it, just heat it to about 250 degrees and the epoxy will let go.