Mounting hardware on foam-cored boats

[Disailor]

Once the mounting hole has been drilled, use a countersink to form a conical relief. The sealant in the conical depression will act like an "o" ring to keep moisture from creeping in

[cyfishburn]

Wow. I never expected so much kind attention. Thank you for your tips. If any of you would like to vacation in Baltimore soon and maybe do some sanding while you're here, please feel free!

[roverhi]

Like to keep the skins as complete and unmolestled as possible. I use a dremel 199 bit to rout out the core around the fastener. Begin by drilling whatever fastener hole(s) required but only drill them through the top skin. Insert the dremel 199 bit at as close to a 90 degree angle as you can to minimize top layer glass damage. Rotate the bit to horizontal and rout out the core and vacuum it out. Fill the puka(s) with epoxy resin and suck out then mix the resin with silica or other structural filler and reinject. When it's gone off drill your hole(s) all the way through. The damage done by the 199 bit should be minimal and easily covered by the hardware. Lightly loaded hardware probably will do just fine through drilling and backing with fender washers. Stuff that will be carrying a significant load should have a backing plate and the anticrush resin procedure above performed.

[GordMay]

The Dremel #199 only has a cutting head diameter of 3/8" (9.5 mm), which requires a 3/8" hole in the skin, and doesn’t provide much (only 1/8") core undercutting (hollowing).
I prefer an “L-shaped” cutter (or series of increasing length), such as the Allen wrench.

[Slainte1]

Get hold of the West System manual "Gougeon Brothers on Boat Construction" They did a lot of research on resin bonding fasteners. Also have look through the EPOXY Works index. Lots of good advice

[roverhi]

Originally Posted by GordMay

The Dremel #199 only has a cutting head diameter of 3/8" (9.5 mm), which requires a 3/8" hole in the skin, and doesn’t provide much (only 1/8") core undercutting (hollowing).
I prefer an “L-shaped” cutter (or series of increasing length), such as the Allen wrench.

Inserting the 199 bit at as close to a 90 degree angle as possible only makes a 3/8" slot not a 3/8" hole. Easily hid under the hardware you're insatalling Believe it's important to keep the top and bottom surface with as small a hole and as intact as possible. If you just oversize the hole and fill with epoxy there is precious little bonding to the outer shells that could become detached with very little torquing. Undercutting the core greatly increases the bonding areas and almost guarantees the non compressible core will stay in place. Tried the bent nail and ground down Allen wrench but it didn't work well with an intact balsa core, can you say wrist threatening.

[CarlF]

Are these above or below the waterline? What sort of loads are you talking about?

Skins are surprising strong. For loads less than 100lbs above the waterline like hatch frames, I just use sheet metal screws with a good caulk or butyl

For medium load items above the waterline (small blocks, handrails, etc), I'd use one of the methods in this West guide https://www.westsystem.com/instructi...ners-hardware/ If it's a deck mounting and there's easy access to underneath, I'll usually just thrubolt with washers as backing. Crushing the core is really only an issue with high load items.

Below the waterline medium load (like seacocks) I use the Allen wrench chucked in the drill method to clean out the foam about 1/2" from the hole. When doing this, be sure to sand the inner surface after the foam is gone to get a good bond with the epoxy putty. The constant hydrostatic pressure of water a few feet under the surface on an underwater fitting will find its way through any slight crack.

For high loads (cleats, winches, stanchions, heavily loaded blocks), I cut a 2" dia or larger hole in the inner skin, and dig out the foam to the outer skin. I then fill that with epoxy putty and thru-bolt with a G10 backer.

[GordMay]

Originally Posted by roverhi

Inserting the 199 bit at as close to a 90 degree angle as possible only makes a 3/8" slot not a 3/8" hole ...

Good point.

[slug]

Originally Posted by CarlF

Are these above or below the waterline? What sort of loads are you talking about?

Skins are surprising strong. For loads less than 100lbs above the waterline like hatch frames, I just use sheet metal screws with a good caulk or butyl

For medium load items above the waterline (small blocks, handrails, etc), I'd use one of the methods in this West guide https://www.westsystem.com/instructi...ners-hardware/ If it's a deck mounting and there's easy access to underneath, I'll usually just thrubolt with washers as backing. Crushing the core is really only an issue with high load items.

Below the waterline medium load (like seacocks) I use the Allen wrench chucked in the drill method to clean out the foam about 1/2" from the hole. When doing this, be sure to sand the inner surface after the foam is gone to get a good bond with the epoxy putty. The constant hydrostatic pressure of water a few feet under the surface on an underwater fitting will find its way through any slight crack.

For high loads (cleats, winches, stanchions, heavily loaded blocks), I cut a 2" dia or larger hole in the inner skin, and dig out the foam to the outer skin. I then fill that with epoxy putty and thru-bolt with a G10 backer.

Water penetration into the core is a issue as important as compression

Priming the exposed core surface with epoxy , before injecting thickened epoxy is a critical detail

Attention to detail and correct engineering is needed

[FAST FRED]

Originally Posted by 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

I use this same technique but add a few steps I bedd a 1/2 inch thick piece of laminate in epoxy and lay it on the outer hull. This raises up the mounting holes , so when any sealing goop finally fails it only fails with over 1/2 inch of water on the mount , giving plenty of notice.


I also bedd the metal backing plate below decks as GRP is never level and smooth , so the backing plate gets to load the deck better.I use a piece of aluminum foil under the backing plate as a barrier , when its final assembly,I use a 1/16 inch of rubber between the backing plate and the cast smooth area to help spread the load or overload. Folks will push on stanchions even when advised not to.

[thinwater]

Originally Posted by cyfishburn

... Issue No. 2 is a bit murky to me, since divinycell doesn't absorb water. Nevertheless...

You are in Maryland. When water gets between the skins and core and freezes, it jacks the skins apart, causes delamination, and pulverizes the core.


A compression sleeve (many good descriptions--we all have our favorite way) only resists the bolt pretension. It has very little to do with the ultimate strength of the installation. (I've tested these to failure).


Solid glass is nice and high density cores like Coosa Bluewater 26 are nice. However, if the backing plate is big enough the core will not be compressed.



If you are still doing glass work, you can either bond pre-lamainted FRP to the hull or layup 4-6 layers of 1708 to the hull. This will be stronger and lighter than conventional backing plates, particularly in shear.


As for the washers, by extra thick fender washers. They are about 8 times as stiff and actually function and mini-backing plates, unlike regular fender washers, which just bend into cones when challenged. Using 2 fender washers is not a substitute.

[slug]

Originally Posted by thinwater

You are in Maryland. When water gets between the skins and core and freezes, it jacks the skins apart, causes delamination, and pulverizes the core.


A compression sleeve (many good descriptions--we all have our favorite way) only resists the bolt pretension. It has very little to do with the ultimate strength of the installation. (I've tested these to failure).


Solid glass is nice and high density cores like Coosa Bluewater 26 are nice. However, if the backing plate is big enough the core will not be compressed.



If you are still doing glass work, you can either bond pre-lamainted FRP to the hull or layup 4-6 layers of 1708 to the hull. This will be stronger and lighter than conventional backing plates, particularly in shear.


As for the washers, by extra thick fender washers. They are about 8 times as stiff and actually function and mini-backing plates, unlike regular fender washers, which just bend into cones when challenged. Using 2 fender washers is not a substitute.

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

[Chotu]

I used plywood backing plates BECAUSE they bend a little, plus huge fender washers on my cleats. I'm surprised no one has mentioned this.

The whole boat is corecell except where deck hardware is being mounted. I chose a balsa core in deck locations to prevent crushing the core. Balsa has huge compression strength.

I drilled my normal size hardware holes, maintaining skin integrity for tension loads, ripped out extra balsa core with an Allen wrench on a drill, and filled the hole up with thickened epoxy.

Then I re-drilled my normal size hardware holes for the cleats.

Below deck, I used a generous size 1/2" okoume marine ply X2 and massive fender washers.

Butyl tape keeps the drips out.

This setup is nice because the plywood bends a little bit as you put pressure on it by tightening up the hardware. That curve very nicely distributes the load all over the slightly curved surface it makes contact with.

A pair of these cleats (and no other attachment) held my 50' cat through a direct hit by the eye of hurricane Ivan. I moved it to anchor for the hurricane to keep it safe.

[slug]

The viscosity , density of butyl rubber makes it less than ideal
You torque down fasteners , compress the butyl , then next month the hardware settles , the fasteners loose their torque setting
the hardware is now under torqued
I know of no professional boatbuilder who uses butyl tape for general purpose bedding

[Chotu]

Originally Posted by slug

The viscosity , density of butyl rubber makes it less than ideal
You torque down fasteners , compress the butyl , then next month the hardware settles , the fasteners loose their torque setting
the hardware is now under torqued
I know of no professional boatbuilder who uses butyl tape for general purpose bedding

5 years full time use, as in the cleats are tugged on hundreds of times a day by the lines that have been attached to them for 5 years. Even through 2 hurricanes, one a direct hit of the eye.

No leaks.

It's working just fine because there is no movement of the cleat and probably that I also used plenty thick butyl and squeezed it out on initial torque down trimming excess away. I torqued these down hard.

Note: just did some math. The cleats are pulled 6 times a minute conservatively. That's 8600 tugs a day. Or... 15.7 million pulls on the cleats so far without a drop of leaking. I'm putting that in the "it works" column.