screw mounting in cored deck

[dhays]

Thanks Mike,

Yeah, I was hoping to get up your way last year but COVID nixed that. So far it doesn’t look good for this year either. I’m taking a couple weeks in August and if we can’t get to BC, I’ll putz around the South Sound again this year.

I keep playing with different ideas based on all the good ideas that folks are giving me. I have plenty of time as the new RIB won’t be available until August (hopefully) so I can experiment in the mean time.

 

[dhays]

I did a couple more tests today. I used the West Systems 404 thickener. I also used some 1/4-20 x 1.75” set screws that I set in the epoxy to cast the threads. This made it easy to use to the mounting bracket to properly get the screws centered and aligned. It worked well other than….

I leveled the surface of the epoxy even with the surface of the wood. However as the epoxy set, it sunk down below the surface by maybe 3/16”.

Any idea how/why the epoxy contracted like that?

 

[Comodave]

You probably didn’t get the hole filled completely and the thickened epoxy settled into the hole some. There is a bit of shrinkage with epoxy but nowhere near that much.

 

[angus99]

I did a couple more tests today. I used the West Systems 404 thickener. I also used some 1/4-20 x 1.75” set screws that I set in the epoxy to cast the threads. This made it easy to use to the mounting bracket to properly get the screws centered and aligned. It worked well other than….

I leveled the surface of the epoxy even with the surface of the wood. However as the epoxy set, it sunk down below the surface by maybe 3/16”.

Any idea how/why the epoxy contracted like that?

Did you work any bubbles out? Rod Collins (Mainsail) recommends poking around the fresh epoxy inside the cavity with a piece of weed whacker string to get any trapped air out.

 

[dhays]

You probably didn’t get the hole filled completely and the thickened epoxy settled into the hole some. There is a bit of shrinkage with epoxy but nowhere near that much.

Did you work any bubbles out? Rod Collins (Mainsail) recommends poking around the fresh epoxy inside the cavity with a piece of weed whacker string to get any trapped air out.

Yeah, I did poke, prod, twist and swirl a toothpick around in the cavity before inserting the wetted screw into place. It could be that I wasn’t diligent enough doing that. I bet that both of the above could be the reason. Thanks

 

[Northern Spy]

I did a couple more tests today. I used the West Systems 404 thickener. I also used some 1/4-20 x 1.75” set screws that I set in the epoxy to cast the threads. This made it easy to use to the mounting bracket to properly get the screws centered and aligned. It worked well other than….

I leveled the surface of the epoxy even with the surface of the wood. However as the epoxy set, it sunk down below the surface by maybe 3/16”.

Any idea how/why the epoxy contracted like that?

Could be a void or gap in the core that the epoxy slowly filled while setting. I've had that issue before.

 

[dhays]

Could be a void or gap in the core that the epoxy slowly filled while setting. I've had that issue before.

Yup. However in this case it was just a test in a piece of 2 x 12, so there shouldn’t be voids. My guess is that I need to be more careful about making sure the thickened epoxy fill all the space without bubbles.

 

[AKDoug]

Probably too late for my two cents, but rather than drill and fill, I simply used 4200 to attach 1" G10 drilled and tapped for mounting bolts. The G10 is only loaded in shear for my application and 4200 is more than strong enough. My cabin roof is balsa cored and I didn't want to put any holes through it.

It's been there for eight years now and appears to be as sturdy as the day I mounted it. Additionally the G10 spreads the load over a little more of the roof so I feel it's a better method (for my application). If I want to remove it, I can saw it off with spectra fishing line and the roof is still intact.

 

[stubones99]

Dave,
Do you have pre-fab bunks for the tender or are you going to make them?

You could take a pair of 2x6 boards and wrap them in fiberglass, and then tab them to the deck, with no deck fasteners and no deck intrusions. Just epoxy the encapsulated boards parallel to give you a chock for your tender, and if someone else didn't like them, cut them loose with a grinder.

Your eye bolts could also be screwed to a starboard block with the nut countersunk and then epoxy the block to the deck.

If you choose to use the method of replacing the coring with epoxy, warm the area and that will speed curing of the epoxy and also make the epoxy a little less thick, so it will auto-level, filling the void in coring. You want good adhesion with the coring since that is your water tight barrier. You might squirt in some un-thickened epoxy first, then push the thickened epoxy into the hole, so the thin epoxy would be a gap filler that thickened epoxy won't go in.

 

[Frosty]

I leveled the surface of the epoxy even with the surface of the wood. However as the epoxy set, it sunk down below the surface by maybe 3/16”.

Any idea how/why the epoxy contracted like that?

I usually let the thickened epoxy make a small "muffin top." Then even after any settling it's still slightly proud of the surface. When it's in the green stage (hard cheese), I then slice it off perfectly flush with a sharp chisel.

That said, if I understand you correctly you have a screw in the center of the area so maybe you can't chisel flush (?).

A couple of other tips are to not whip a bunch of air into the mix as you are stirring it together, and to, if possible, fill from the bottom of the hole (in your case you probably can't literally do that, but you can put the tip of a syringe or pastry piping bag down at the bottom perhaps when you start out).

Also, and again this may not work for your situation, I will sometimes drill a small relief hole in a blind hole situation. Say if there is a place off to the side or above that will end up being covered up with a backing plate or piece of hardware. Then the air can come out that hole and once epoxy mix follows then the air is out.

Even with all that though, I don't think I'd have consistent luck if trying to fill a hole to "flat" and then not have it change at all in flatness (hence the muffin-top and chisel slice).

 

[dhays]

Dave,
Do you have pre-fab bunks for the tender or are you going to make them?

You could take a pair of 2x6 boards and wrap them in fiberglass, and then tab them to the deck, with no deck fasteners and no deck intrusions. Just epoxy the encapsulated boards parallel to give you a chock for your tender, and if someone else didn't like them, cut them loose with a grinder.

Your eye bolts could also be screwed to a starboard block with the nut countersunk and then epoxy the block to the deck.

If you choose to use the method of replacing the coring with epoxy, warm the area and that will speed curing of the epoxy and also make the epoxy a little less thick, so it will auto-level, filling the void in coring. You want good adhesion with the coring since that is your water tight barrier. You might squirt in some un-thickened epoxy first, then push the thickened epoxy into the hole, so the thin epoxy would be a gap filler that thickened epoxy won't go in.

One of the goals was to be able to have as clear a deck as possible when the dinghy wasn’t being stored up there. I initially was going to go with the removable Weaver chocks. However, the plate that is left on the deck would still be nasty to hit with a bare foot.

I ended up going with Versachocks. I think the rails will work well but I can also create my own supports using the Versachock system. The rails they have are just Starboard that is screwed into place. I like the design of the base as it won’t be like stepping on a Lego with bare feet.

 

[RonT]

Dave, I think you had too much filler in the glue. Everyone has a fixation on over thickening the epoxy. Let me quote from the West people: "While certain fillers will increase the density of epoxy, this does not correlate to higher physical properties or increased adhesion strength. Epoxy is strongest without filler added to it." If the glue is too viscous the epoxy will move marginally into the end grain over a short period of time after you have leveled the surface off; giving the later impression of shrinkage. WEST SYSTEM and other Epoxies have a very low percentage of shrinkage. <.01%. The epoxy glue for casting needs to be quite runny. You don't need to poke, twist and prod for bubbles. As long as you have mixed it carefully, you pour it in and let it set. Look back at the last photo of the sample I did. It is level with the surface. Also note there is about 2-mm of soakage into the timber. Below the surface right down to the bottom of the casting the timber is end-grain and the soak depth is even greater. The amount of penetration depends mostly on how the wood fibers are oriented. Face grain will not absorb nearly as much epoxy as end grain. But since Epoxy has very high strength and excellent adhesion, it strengthens the wood and makes it waterproof. By having the glue reasonably runny that absorption take place quickly and the surface will remain level. (I have later cut down sample castings to check the radiation depth of the absorption. Usually about 1/4" in ply.) Looking at the Versachocks system I believe you could use my 3/4" washer method and the complete casting surface would be covered by the Versachocks bases. I always use the masking tape for the reasons I explained. Furthermore when I trowel off cast glue the thickness of the tape pretty well compensates for that minute shrinkage (0.01%) Regards Ron

 

[cks]

Definitely go with the filled epoxy. A following recommendation was giving to me by an experienced ship wright.
If your drilling a 1/2 hole, take a nail and bend a 3/8” right angle into the nail. Mount into the chuck of a small drill. Insert this nail with the bend into the hole and essentially route out the underside of the core just below the fiberglass deck so that epoxy just doesn’t fill the hole but also the void underneath the fiberglass deck. This adds significant strength.

I’ve utilized this method a couple of times with excellent results.

 

[Weraz]

Drilling is the best option always

 

[Xlantic]

How about this, in SS:

The large diameter of the insert spreads the load. A machine screw allows easy seemly & disassembly without weakening attachment point. You can also use epoxy on the insert for greater strength and better sealing of the wood.

 

[Scooby5959]

Seems like a whole bunch of stress to me. Why don’t you just relax and go for a nice cruise in that beautiful boat you have. The way you all were analyzing things you think you were sending a man to Mars

 

[Frosty]

Seems like a whole bunch of stress to me. Why don’t you just relax and go for a nice cruise in that beautiful boat you have. The way you all were analyzing things you think you were sending a man to Mars

Why a judgemental tone?

There are lots of different types of people and situations.

--- Maybe he cannot cruise right now for other reasons (recovering from surgery or, taking care of a relative, etc.). So this is a way he can interact with his boat.

--- Maybe he just likes to experiment and learn things? (I know I do.)

--- Or maybe he's doing both concurrently so already is taking out his beautiful boat.

 

[dhays]

Seems like a whole bunch of stress to me. Why don’t you just relax and go for a nice cruise in that beautiful boat you have. The way you all were analyzing things you think you were sending a man to Mars

Several reasons….

1. I’m afraid of my wife. If I screw it up, I will have to deal with the consequences.
2. I’d like to do the best possible job that I can.
3. I like to learn about stuff and TF has a wealth of very smart and experienced folks to learn from.
4. I still work for a living so don’t have the opportunity to get out on the boat.
5. The dinghy won’t be available for at least another month so I have time to figure it out.

 

[SoWhat]

Seems like a whole bunch of stress to me. Why don’t you just relax and go for a nice cruise in that beautiful boat you have. The way you all were analyzing things you think you were sending a man to Mars

Why? Because over analyzing trivial details is relaxing. It's a boat. Do whatever you want to it. No stress,. No drama. (and I only sent things to the moon. Worked on LEM, Polaris subs and missiles, Shuttle, GPS satellites. Enjoy it just as much now as I did then)