Mounting hardware on foam-cored boats

[cyfishburn]

My new 33' catamaran is almost ready for hardware installation. The boat is entirely VE resin and glass on divinycell foam. There are lots of things to screw on to the boat, some of which will be heavily loaded and some of which will get almost no loading.

I'm hoping that this thread will help me manage about 200 or so fasteners, and help others who maybe are just adding a fitting or two to their boats.

I've been advised to: 1) prevent loads on the hardware from crushing the foam, and 2) seal the foam from water intrusion. Issue No. 1 I get, since foam isn't strong in compression. Issue No. 2 is a bit murky to me, since divinycell doesn't absorb water. Nevertheless...

I plan to use backing plates on any hrdwre that takes a load. I think I can make serviceable plates out of glass and resin laid up and cut to size. For other items such as ports, vents, and rub rails (except at the ends), I plan on using bolts with nuts/washers or barrel nuts.

Anything that needs a backing plate, needs a non-compressible core. If I didn't plan ahead and put such in, I'll cut out the interior glass and core, insert stiffer core (I plan on using Cousaboard), and re-glass. Ideally, I guess that I should replace rather more core than the hrdwre's footprint. I'm getting tired already!

So that's issue no. 1, or strength. For issue no.2, or sealing, there are lots of notions out there. The idea seems to be to replace the area around the bolt with solid resin. Over-drilling the hole and filling back with resin might work OK with a wood core, but I doubt the pull-out strength, since all that's left for strength is the core in shear with the resin plug. Then there's the "winkle" method, where one drills for the bolt, chucks in a right-angled blade that you make yourself, fits the blade into the hole, and grinds up the core. Suck out the core bits, fill with resin, and re-drill. I've read about this idea and tried it and had very little luck. I JUST WONDER if, for fittings with no load, why I can't just paint the holes with a q-tip and resin and leave it at that.

Ok, that's me with my project. I hope other perspectives will help inform, and I apologize in advance if I've been Captain Obvious.

Cy Fishburn

[mrm]

I think hardware palcement and loads should have been addressed at the design stage and deck structure modified accordingly. If they weren't, whatever you come up with will be a band-aid.

[Tillsbury]

Yes, anything serious like winches, jammers, standing rigging, windlass and whatever should have something big in the deck structure -- mine has thick aluminium plate substituted for the divinycell in those locations sized for the expected load, for example. What does the design specify?

[Montanan]

Originally Posted by cyfishburn

My new 33' catamaran is almost ready for hardware installation. The boat is entirely VE resin and glass on divinycell foam. There are lots of things to screw on to the boat, some of which will be heavily loaded and some of which will get almost no loading.

I'm hoping that this thread will help me manage about 200 or so fasteners, and help others who maybe are just adding a fitting or two to their boats.

I've been advised to: 1) prevent loads on the hardware from crushing the foam, and 2) seal the foam from water intrusion. Issue No. 1 I get, since foam isn't strong in compression. Issue No. 2 is a bit murky to me, since divinycell doesn't absorb water. Nevertheless...

I plan to use backing plates on any hrdwre that takes a load. I think I can make serviceable plates out of glass and resin laid up and cut to size. For other items such as ports, vents, and rub rails (except at the ends), I plan on using bolts with nuts/washers or barrel nuts.

Anything that needs a backing plate, needs a non-compressible core. If I didn't plan ahead and put such in, I'll cut out the interior glass and core, insert stiffer core (I plan on using Cousaboard), and re-glass. Ideally, I guess that I should replace rather more core than the hrdwre's footprint. I'm getting tired already!

So that's issue no. 1, or strength. For issue no.2, or sealing, there are lots of notions out there. The idea seems to be to replace the area around the bolt with solid resin. Over-drilling the hole and filling back with resin might work OK with a wood core, but I doubt the pull-out strength, since all that's left for strength is the core in shear with the resin plug. Then there's the "winkle" method, where one drills for the bolt, chucks in a right-angled blade that you make yourself, fits the blade into the hole, and grinds up the core. Suck out the core bits, fill with resin, and re-drill. I've read about this idea and tried it and had very little luck. I JUST WONDER if, for fittings with no load, why I can't just paint the holes with a q-tip and resin and leave it at that.

Ok, that's me with my project. I hope other perspectives will help inform, and I apologize in advance if I've been Captain Obvious.

Cy Fishburn

Captain Obvious, you have answered your inquiry above.

For heavy loads one should dispense with the use of core and just use non-compressible, structural fill, epoxy / glass. Truly a PITA on an afterthought of construction basis but you got to do what you got to do over.

[chrisr]

drill oversize hole
fill with epoxy n wait to cure
drill final hole in epoxy
cheers,

[slug]

Originally Posted by cyfishburn

My new 33' catamaran is almost ready for hardware installation. The boat is entirely VE resin and glass on divinycell foam. There are lots of things to screw on to the boat, some of which will be heavily loaded and some of which will get almost no loading.

I'm hoping that this thread will help me manage about 200 or so fasteners, and help others who maybe are just adding a fitting or two to their boats.

I've been advised to: 1) prevent loads on the hardware from crushing the foam, and 2) seal the foam from water intrusion. Issue No. 1 I get, since foam isn't strong in compression. Issue No. 2 is a bit murky to me, since divinycell doesn't absorb water. Nevertheless...

I plan to use backing plates on any hrdwre that takes a load. I think I can make serviceable plates out of glass and resin laid up and cut to size. For other items such as ports, vents, and rub rails (except at the ends), I plan on using bolts with nuts/washers or barrel nuts.

Anything that needs a backing plate, needs a non-compressible core. If I didn't plan ahead and put such in, I'll cut out the interior glass and core, insert stiffer core (I plan on using Cousaboard), and re-glass. Ideally, I guess that I should replace rather more core than the hrdwre's footprint. I'm getting tired already!

So that's issue no. 1, or strength. For issue no.2, or sealing, there are lots of notions out there. The idea seems to be to replace the area around the bolt with solid resin. Over-drilling the hole and filling back with resin might work OK with a wood core, but I doubt the pull-out strength, since all that's left for strength is the core in shear with the resin plug. Then there's the "winkle" method, where one drills for the bolt, chucks in a right-angled blade that you make yourself, fits the blade into the hole, and grinds up the core. Suck out the core bits, fill with resin, and re-drill. I've read about this idea and tried it and had very little luck. I JUST WONDER if, for fittings with no load, why I can't just paint the holes with a q-tip and resin and leave it at that.

Ok, that's me with my project. I hope other perspectives will help inform, and I apologize in advance if I've been Captain Obvious.

Cy Fishburn

You must use compression sleeves with cored decks

The poor mans compression sleeve is to oversized drill the fastener hole ... 1.5 to 2x. Oversized .... tape off the bottom of the hole ... then fill this hole. Completely with high density epoxy filler

Once cured drill thru this solid epoxy plug with the correct size drill for your application

Remember to use a backing plate ... g 10’ or similar to spread the load

[Fore and Aft]

Cyfishburn, I am not sure many deck fittings are not under load at some stage. I would be happy to do the “resin the holes” then sikaflex and a big washer on the back for things like my cockpit rope bags, cockpit lights etc. They really are light loads and should not crush the core.
Cheers

[slug]

If you look in a a specialty fasteners catalog you can find bulk compression sleeves

Typically made of plastic with a washer surface on one side ... a “ T”

They are widely used in aviation, or any application with composite panels

Very handy for small size fasteners

[cyfishburn]

Guys, thank you for your time and attention.

[Chotu]

Originally Posted by slug

You must use compression sleeves with cored decks

The poor mans compression sleeve is to oversized drill the fastener hole ... 1.5 to 2x. Oversized .... tape off the bottom of the hole ... then fill this hole. Completely with high density epoxy filler

Once cured drill thru this solid epoxy plug with the correct size drill for your application

Remember to use a backing plate ... g 10’ or similar to spread the load

What's the rich man's compression sleeve?

Edit:. You answered this already. I hadn't finished reading the thread. Please ignore

[mholtzberg]

If you look up Onboard Lifestyle on YouTube.. They just celebrated their 100th episode they have a catamaran with honeycomb stuff and look at that. I believe he would crush the honeycomb and leave the skins and fill with thickened epoxy.

[blubaju]

drilling an oversized hole? You want to keep the skin intact to distribute the load, so what we did, drill a rather small hole from one side only, no through-hole, then use old allen keys different sizes with an electric drill, take out the core, twice as large as the screwhead with large washer, fill with a dense epoxy-fiber mixture, fill, press well. When cured drill the hole you need.

I am very sure, you know this, but hoping for a miracle solution ;-)

[ahumbug1]

I saw a video of a guy that drilled a hole, put an allen tench in the chuck of a drill and he inserted this in the hole to ream out the core larger than the hole drilled. He then taped the bottom hole from underneath. Next he vacuumed the core crumbs out and then filled with epoxy. This probably would work for mounting small stuff. Not sure about wenches.

[blubaju]

Originally Posted by ahumbug1

I saw a video of a guy that drilled a hole, put an allen tench in the chuck of a drill and he inserted this in the hole to ream out the core larger than the hole drilled. He then taped the bottom hole from underneath. Next he vacuumed the core crumbs out and then filled with epoxy. This probably would work for mounting small stuff. Not sure about wenches.

That is exactly what we did, avoid drilling through else you will tape it again, avoids resin drops. works for heavy loads too, but you want staggering layers of glass/epoxy to locally enforce the wall, backing plates, do never cut them square, again make the edges WELL tapered to bring in the loads gently, still, you must support the core or it will collapse. On my cats I never used backing plates as such, always tapered layers of glass or carbon, the composite chainplates for the shrouds are up to 1 1/2 inches, you can bring in big loads, but you must know what you do.

[Paul Howard]

I mark the location of fasteners, then drill a 1/8" hole to mark. Drill an oversize hole, a small hole saw in a drill, say 3/4" to 1" through the top fiberglass and the foam, but leave the lower fiberglass layer intact. Clear out the foam to the top of the lower layer of the fiberglass. Use a small allen wrench in a drill ( I shortened the short side of the L to only about 3/8") and undercut the foam around the inside of the hole.
Tape over the underside drill hole with masking tape and fill hole with epoxy and cabosil mush to the level of the upper fiberglass and let go off. Re-drill to to correct fastener size.
This will do for light duty hardware, though for cleats and winches, windlass, etc. use a backing plate as well.
This was my procedure for every fitting I added or moved on my Airex core catamaran