Mounting hardware on foam-cored boats

[thinwater]

Originally Posted by slug

Sheer strength is a function of fastener diameter
When you use a compression sleeve, epoxy. plug in core construction you greatly increase fastener diameter and as a result sheer strength of the installation

Strength in tension is a different issue

We are getting into subtleties here, but good point... mostly.



This depends on the density of the core and how well the plug is bonded. In testing, the plug is not that well bonded to the skins, the skins flex, and the plug comes loose from the skins rather easily, often before the skins begin to fail. If the core is soft, it adds little. Obviously, this depends on the size of the plug and how well the core is cleaned out. But remember, you are not bonding to the skin, you are bonding to skin with bits of core still attached.



However, a backing plate that is bonded makes a big difference.

[Chotu]

While we're at it, does anyone see a problem using polyester in the holes drilled into a core?

As in, drill and fill with polyester instead of epoxy?

[Fore and Aft]

Chotu for me personally I don’t think polyester has the adhesive qualities of epoxy.
Nothing wrong with plywood backing plates, I also like the bottom of an ice cream container filled with glass mat and maybe 18mm high of polyester resin. Once cured, pop it out of the container and you have a nice solid backing plate.
Cheers

[Kylahi]

My O’Day 40 has a cored hull. She has had a few hull penetrations installed and a few replaced while here in La Paz, Mexico. I did most of them myself by the allen key method. Worked out fine.

After reading your post, I belief that you are well prepared to tackle the job. I am a semi-retired consulting structural engineer from Canada. Go for it.
By the way, if I was 10 years younger, I too would be designing and building a 35 to 37-foot cat. I would have a cantilevered mast with but one stay (backstay to shape the main). Yes, A larger diameter mast would be required. I would core the hull with PVC foam as you have done. Two small Yanmars, some electronics, some vittles, raise the main and head south. Good luck, sweet winds and calm seas.

[col50]

Originally Posted by slug

Sheer strength is a function of fastener diameter
When you use a compression sleeve, epoxy. plug in core construction you greatly increase fastener diameter and as a result sheer strength of the installation

Strength in tension is a different issue

Do you mean sheer strength or shear strength?

[slug]

Originally Posted by col50

Do you mean sheer strength or shear strength?

Are you insecure ?

[col50]

Originally Posted by Fore and Aft

Chotu for me personally I don’t think polyester has the adhesive qualities of epoxy.
Nothing wrong with plywood backing plates, I also like the bottom of an ice cream container filled with glass mat and maybe 18mm high of polyester resin. Once cured, pop it out of the container and you have a nice solid backing plate.
Cheers

Polyester resin by itself is to briddle and should not be used as a backup plate without fiber reinforcement.

[betwys1]

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

This puzzle reminded me of making a Rogallo hang-glider, in the long-ago. It was necessary to fasten aluminum tubes together with slender long bolts. The curt advice was to use wood plugs to avoid crushing the structural tubing, or drill oversize for a slender aluminum tube through the main tube, which could be swedged in place with a sawn-off wood screw.
In this case, a compression tube in aluminum seems inappropriate, but something more appropriate to gluing in place with epoxy, and well placed to resist the compression of a through bolt would be carbon fiber tube - say six millimeter, like these:
https://www.amazon.com/Carbon-Fiber-...95409971&psc=1
qty 4, 6mmod,4mm id and 20 cm long (8 inches) for $16
I expect you could find the ids that would suit your through bolts? Possibly backing plates of carbon fiber would also work for you?

Brian W

[Uncle Bob]

Originally Posted by Chotu

While we're at it, does anyone see a problem using polyester in the holes drilled into a core?

As in, drill and fill with polyester instead of epoxy?

Understanding your problem with epoxy I would suggest that polyester resin with lots of glass fibres in the mix forced into the cavity would be a close second.

[Icarus]

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

Depends on what you intend to mount.
Lets say you are using 8MM screws, drill 6mm through then cut out the top layer of fiberglass and foam only with a 20mm or so dia holesaw, fill with an epoxy filler, sand smooth and mount the fitting usind a backing plate.
Apart from gaining compression strenght the fastener wont leak.
Or drill a hole and gauge the foam out with a sharpened allen key fitted in a drilling machine, works as well

Good luck

[Uncle Bob]

Originally Posted by Icarus

Depends on what you intend to mount.
Lets say you are using 8MM screws, drill 6mm through then cut out the top layer of fiberglass and foam only with a 20mm or so dia holesaw, fill with an epoxy filler, sand smooth and mount the fitting usind a backing plate.
Apart from gaining compression strenght the fastener wont leak.
Or drill a hole and gauge the foam out with a sharpened allen key fitted in a drilling machine, works as well

Good luck

Given the guy has allergy problems with epoxy I would guess this is a non starter. Any other suggestions?