Comment on my options for adding "Hard Points" to Divinicell Hard Top

I need to add some hard points to the hard top I'm building that will support my VHF antenna mount, GPS mast, etc. The 1/2" H80 Dcell top has been vacuum bagged using epoxy resin and 10oz cloth on both sides so far.
I plan to rout out the core and top layer of glass (with an 1/2" undercut and bottom lamination remains) and pour in either:
a) Thickened epoxy (cabosil)
b) Epoxy mixed with 1/4" chopped mat
c) one or two layers of wetted out 1708 embedded with "b" above.

The plan is to fill the oversized areas with one of the mixtures above and then after curing apply 1708 on top. The real question, finally... is which of the materials above would be better for a drill and tap operation to hold 1/4-20 stainless screws.
I do plan to test at least a and b but wanted to hear from the guys here. BTW - I know that I could have used some G10 but that stuff is really expensive!

Just to clarify... this is NOT for through-bolting. The bolts will not penetrate the bottom of the laminated core. This is to anchor and spread loads from something like an 8' VHF antenna mount base that is about 3"x4".

 

Wood plug

 

That would be an option, and I have nothing against wood, but it's a no wood boat so trying to stay true to that theme.

 

Themes don't really matter much.
Holding the screws do.
You would be better off with thru bolts so that both skins would directly provide strength to the bending induced by the antenna.

Should have put in the insert before so you don't have a joint/ weakness in the outer skin.

 

I would not disturb what you have done, you can epoxy on to the top of the canopy a suitable plate to screw into, if you don't want thru bolting. Thru bolting with backing plates would be easier, if you don't like holes, how is your antenna cabling going to be run ?

 

The usual practice is to rout out the bottom, bevel the edge and add several layers of fiberglass epoxy. The better practice is to insert a thick solid laminate then glass it over, filling the voids and sharp corner with bog. The minimum diameter is twice that of the fender washer you are going to use, plus the taper which should be minimum 6:1.

Sandwich laminate do not stand up to compression load and shear, hence the need for hard points and load spreading.

Seal all mounting fender washer pads and thru holes with silicon or Sikaflex.

 

west 404 or 407 depending on how dense you want it.

 

either route out the bottom and tie the bottom laminate to the top or rout out the top and take top down to bottom laminate then place whatever material you like in the depression and glass over it again. that leaves both top and bottom layer competely in tact and prevents any migration of water into the laminates

 

Exactly what I did - routed out from the top, left bottom lamination intact then added 7 layers of 1708 and chopped mat also. Then I laminated over that after it cured.

 

I have built a few hard dodgers with balsa core and usually pre plan what is going to be mounted and install inserts in place of the core, coosa board if the hardware is going to be through bolted and G10 which I drill and tap for machine screws if it is going to be blind fastened for light loads such as solar panel mounting. This is done prior to applying the second skin but no real reason not to install the inserts after the fact like you are doing. Coosa is lighter and gives more than adequate compressive strength for through bolting and the G10 is similar to lying in glass layers but is much more dense than you would achieve hand laying so takes a thread well and is easy to make on the drill press with a hole saw.

Steve.

 

I would have laminated G10 or Coosa in place at the beginning when I vacuum bagged the top but I didn't have the exact locations I needed for the mini-tower supports at the time. Adding them later is more work for sure because it requires careful routing of material so you don't go through the bottom lamination. It also requires some grinding before the next lamination goes on top of these areas.
One piece of advice I can offer if you do this after the fact as I did... either use some very slow resin or fill those cutouts with resin and 1708 in two passes. I did the first cutout that is 1/2" deep and 6" in diameter with one fill. It didn't melt the existing foam or glass but it did get VERY hot and some bubbles formed at the top. I did all the remaining cutouts in two passes which generated a lot less heat.