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J24 wet core replacement material


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I have yet another J24 with wet core. I have read the West systems guides and plenty of threads on this forum and others and they have all been very helpful. The only thing I'm not sure about is what to replace the wet core with. My boat was built in 1988 and was cored with balsa. Should I replace it with balsa or something else? I would like to use something that won't absorb water in the future. My boat is a little heavier than class minimums so something lighter than the original core would help get the boat down to weight. Any input would be great.

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3 hours ago, Sail4beer said:

Corelite. PVC board is what I am using to replace 3/8” wet balsa core on an old Ensign using polyester resin and not epoxy.  

Except Corelite is 28 lb/ft2.  That's almost 3x heavier balsa and over 4x more than a lightweight foam.

If the OP thinks his boat is overweight now - yikes.

Quick and dirty estimate here:  J/24 has roughly 400 sq ft of FRP surface area.  Assuming 1/2 inch core, that's about 17 cubic feet.  Balsa core would be about 160 lbs,  Corelite would be about 475 lb.  So over 300 lbs more than balsa.  Probably weigh in as the heaviest J/24 on record.

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42 minutes ago, 12 metre said:

Except Corelite is 28 lb/ft2.  That's almost 3x heavier balsa and over 4x more than a lightweight foam.

If the OP thinks his boat is overweight now - yikes.

Quick and dirty estimate here:  J/24 has roughly 400 sq ft of FRP surface area.  Assuming 1/2 inch core, that's about 17 cubic feet.  Balsa core would be about 160 lbs,  Corelite would be about 475 lb.  So over 300 lbs more than balsa.  Probably weigh in as the heaviest J/24 on record.

 

l’m doing an old cruising Ensign.  About 15 sq ft. My distributor also had nothing else in stock in 3/8”. Im sorry I forgot to mention that. Lightweight foam is the best. 

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1 hour ago, 12 metre said:

Except Corelite is 28 lb/ft2.  That's almost 3x heavier balsa and over 4x more than a lightweight foam.

If the OP thinks his boat is overweight now - yikes.

Quick and dirty estimate here:  J/24 has roughly 400 sq ft of FRP surface area.  Assuming 1/2 inch core, that's about 17 cubic feet.  Balsa core would be about 160 lbs,  Corelite would be about 475 lb.  So over 300 lbs more than balsa.  Probably weigh in as the heaviest J/24 on record.

It's about 50 lbs over weight. Class rules say the replacement material must be as close as possible to the original but I don't know how much the original weighed and I want something that won't absorb water in the future. Divinycell looks like a great option because I can choose the density. Thanks all.

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2 hours ago, Diamond Jim said:

Core replacement is no-fun work.  How much do you want to invest in a J/24?

Thank you.

 

The job I’m working on has new mahogany bulkheads and the original coamings and benches in Awlwood. Way expensive $6,500 marina job on a $500 boat. The family of 3 weighs about 900 pounds, so the core material wasn’t too important to me:D

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A multi time J-24 champion was banned from racing for a while because his boat was inspected and it was discovered his lazarettes were recited with foam 

 

if you plan to race the boat, check with a friendly trusted Certified Official Recognized currently active J-24 class Association measurer before you alter ANYTHING about your boat’s construction. 

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Balsa, Balsa, Balsa!

1. It's cheaper than the alternative, at least in the United States.

2. It's not really any heavier than the other foam solutions.

3. I feel like this is the most important one...Very few if any foam or plastic alternatives will have the same quality of adhesion as what you get from the resins bonding into the end grain of wood whether it be plywood or balsa. Many foam core boats are notorious for delamination.

So...balsa will be your best blend between cost, weight, strength, stiffness, and adhesion.

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On 5/26/2020 at 9:40 PM, jamesmalcolm said:

It's about 50 lbs over weight. Class rules say the replacement material must be as close as possible to the original but I don't know how much the original weighed and I want something that won't absorb water in the future. Divinycell looks like a great option because I can choose the density. Thanks all.

Regardless of the core you choose, water will be absorbed and delamination will occur not from the core material, but due to improper mounting of hardware. Every screw in the deck should be over drilled, backfilled with thickened epoxy and the re-drilled. Then each hole should be counter sunk so that whatever sealant material you choose compresses into the hole and around the screw and not just spreading around the base of the hardware.

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1 hour ago, Jubblies said:

Balsa, Balsa, Balsa!

1. It's cheaper than the alternative, at least in the United States.

2. It's not really any heavier than the other foam solutions.

3. I feel like this is the most important one...Very few if any foam or plastic alternatives will have the same quality of adhesion as what you get from the resins bonding into the end grain of wood whether it be plywood or balsa. Many foam core boats are notorious for delamination.
 

Foam, foam, foam - unless you want to keep it one design.

1. For a few sheets, cost is not that big a difference. Cheaper is why boat builders use it.

2. Yes balsa is heavier. Lightest stuff available is about 110-120 kg/m3.  Typical foam used in this size boat is 80 kg/m3 but lighter boats can use 60 kg/m3. It sucks up more resin too. Lots of balsa weighs more than spec too. (well so does foam but less variance)

3. Lots and lots of boats have been built with foam cores. There is nothing magic about core adhesion when comparing balsa and foams. Use vacuum and clamping pressures will be more than strong enough. Use epoxy in small areas and weights.

Delamination due to structural loads in regular sailing conditions is rare. What is far more common is "never bonds" where the builder used bonding putty and just laid the core into wet putty to bond the core and the core didn't properly stick. They don't build Gunboats with Corecell because balsa is "better" or it sticks better. They build it with Corecell because it's the best material for the job.

4. Many J boats are notorious for wet balsa rotten core. When foam gets wet it's not a death sentence. 

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On 5/28/2020 at 1:53 PM, Jubblies said:

Regardless of the core you choose, water will be absorbed and delamination will occur not from the core material, but due to improper mounting of hardware. Every screw in the deck should be over drilled, backfilled with thickened epoxy and the re-drilled. Then each hole should be counter sunk so that whatever sealant material you choose compresses into the hole and around the screw and not just spreading around the base of the hardware.

You're right. I had a friend take a moisture meter to the deck and the core is wet around and downhill of hardware that was removed and improperly sealed by the previous owner. What is counter sinking? If I've already backfilled and redrilled the epoxy do I still need to seal the hardware because water won't get through the epoxy anyways? (Of course I will reseal regardless but I'm just wondering what if.)

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On 5/28/2020 at 3:54 PM, Zonker said:

Foam, foam, foam - unless you want to keep it one design.

1. For a few sheets, cost is not that big a difference. Cheaper is why boat builders use it.

2. Yes balsa is heavier. Lightest stuff available is about 110-120 kg/m3.  Typical foam used in this size boat is 80 kg/m3 but lighter boats can use 60 kg/m3. It sucks up more resin too. Lots of balsa weighs more than spec too. (well so does foam but less variance)

3. Lots and lots of boats have been built with foam cores. There is nothing magic about core adhesion when comparing balsa and foams. Use vacuum and clamping pressures will be more than strong enough. Use epoxy in small areas and weights.

Delamination due to structural loads in regular sailing conditions is rare. What is far more common is "never bonds" where the builder used bonding putty and just laid the core into wet putty to bond the core and the core didn't properly stick. They don't build Gunboats with Corecell because balsa is "better" or it sticks better. They build it with Corecell because it's the best material for the job.

4. Many J boats are notorious for wet balsa rotten core. When foam gets wet it's not a death sentence. 

I found a foam that is the same density as balsa and if the class measurer in my fleet doesn't object I will use it over balsa. The only potential issue is shaping it to the areas of the deck that aren't flat, but the wet sections should be small enough and the curve of the deck gradual enough that it doesn't matter.

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Most PVC cores and Corecell can be heated slightly and held in position until they cool and will take the curve with minimal spring back. Watch the fumes if you overheat Corecell though. You can also get single cut and double cut (slit) foams (for single curvature and compound curvature areas. Or you can slit them yourselves with a razor knife if the area is pretty small. Slits at about 3/4" apart.

10 minutes ago, jamesmalcolm said:

What is counter sinking? If I've already backfilled and redrilled the epoxy do I still need to seal the hardware because water won't get through the epoxy anyways?

It will change your life. Well maybe not as much as peel-ply but still...

Anyway after your epoxy potted hole has cured and you have redrilled the epoxy, use a countersink bit to provide a small countersink recess in the top. The sealant then has a bigger area and seals around the fastener / hardware interface just like an O-ring. It really prevents leaks. And of course you still need sealant.  

Excellent pictures of it (though I just use an oversize bit and drill through the top laminate instead of just gouging out the core :)

 19654BitCutAwayOfPottedDeckHole-233x350.     20115BitCutAwayOfPottedDeckHole-235x350.

 

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7 minutes ago, Zonker said:

Most PVC cores and Corecell can be heated slightly and held in position until they cool and will take the curve with minimal spring back. Watch the fumes if you overheat Corecell though. You can also get single cut and double cut (slit) foams (for single curvature and compound curvature areas. Or you can slit them yourselves with a razor knife if the area is pretty small. Slits at about 3/4" apart.

It will change your life. Well maybe not as much as peel-ply but still...

Anyway after your epoxy potted hole has cured and you have redrilled the epoxy, use a countersink bit to provide a small countersink recess in the top. The sealant then has a bigger area and seals around the fastener / hardware interface just like an O-ring. It really prevents leaks. And of course you still need sealant.  

Excellent pictures of it (though I just use an oversize bit and drill through the top laminate instead of just gouging out the core :)

 19654BitCutAwayOfPottedDeckHole-233x350.     20115BitCutAwayOfPottedDeckHole-235x350.

 

Thanks for posting the great images. Is it worth resealing all of the deck hardware like this or just what is on the areas of core I'm replacing?

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All of it. :D

The ones that haven't been done that way will leak and soak the core - count on it.

It's one of those "Pay me now or pay me later" things.

FWIW I have never had a countersunk fastener hole leak - ever - in 6 boats over 25 years.

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You insert a bent nail (shaped like a L) or small allen key into your drill and use that to gouge out the core without touching the top skin.

IMO it's overkill if you use decent backing plates. The epoxy plug transfers the shear loads into the skins just fine. And if your laminate is so delicate that a slightly bigger hole weakens it so that the tensile / bending loads are an issue, you're probably sailing an AC boat...

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21 hours ago, SloopJonB said:

FWIW I have never had a countersunk fastener hole leak - ever - in 6 boats over 25 years.

I had a 1986 Pearson where it looked like all of the fittings were original (I know the owners of the previous 15 years didn’t rebed anything). Everything was counterbored and no leaks even with standard polysulfide type sealant.  I rebedded almost the entire boat and never found wet core. 

I wish all manufacturers would do this. I understand not epoxying in the balsa, but adding the counterbore only takes a couple of seconds. 

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5 hours ago, Zonker said:

You insert a bent nail (shaped like a L) or small allen key into your drill and use that to gouge out the core without touching the top skin.

There are good dremel bits that do a more elegant job. I like the multi-flute 1/4” or 5/16” ones. They made a clean cut and remove just the right amount of core. 

Edit: I found the bit numbers.  115 (5/16") is my go-to.  196 (7/32") works on smaller 1/4" holes (probably most deck penetrations on a J/24).

Dremel 115 High Speed Cutter - Power Rotary Tool Accessories ...

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17 hours ago, Zonker said:

You insert a bent nail (shaped like a L) or small allen key into your drill and use that to gouge out the core without touching the top skin.

IMO it's overkill if you use decent backing plates. The epoxy plug transfers the shear loads into the skins just fine. And if your laminate is so delicate that a slightly bigger hole weakens it so that the tensile / bending loads are an issue, you're probably sailing an AC boat...

I believe it is better to remove the core and leave the skin if you can do it. If the top skin is drilled to the same size as the core, the epoxy fill is bonded to the skin only at the skin's edge - a small area, two different materials with different stiffnesses, etc. If the core is undercut, the epoxy annulus bonds to the area undercut. I agree that the transfer of shear is OK in either case, but under flexing or thermal cycling more area, bonded to the surface rather than the edge has got to be better. Tools like Alex mentioned make this easy, just make sure when you reef out the core that you get it cleanly to the skin, otherwise you are bonding to remnant bits of core stuck to the skin, rather than the skin. 

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I agree it's better to remove the core and leave the top skin intact. I've not always been in places where I can get bits like the Dremel one pictured.

But I'm not sure it actually matters, having been doing the oversize hole for years with some highly loaded hardware.

If your boat has quite thin skins, I'd be more likely to suggest keeping them intact.

It would be a good research study for a grad student!

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20 hours ago, Zonker said:

It would be a good research study for a grad student!

Yeah, I'd like to see that. If the skins are as thick as in your pictures, it probably makes little difference. 

An additional tip: the West Six10 is an excellent product for this as it will not run out of the hole, even if it is overhead. 

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six10 is such great stuff for small projects.  I avoided it for a long time because it's expensive per ounce, but it's so easy to work with and also make up tiny batches of.

I recommend ignoring the mixer tip and just dispense into a container and mix yourself.   You can clean up the tip and push the stopper back in and use it again.  I've gotten years of small projects out of a tube this way.  The mixer tip wastes a lot of epoxy!

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The tip wastes material, and I don't use it to mix a small batch. But for larger jobs, saying filling a bunch of deck holes or a long line bond, the placement convenience of the mixing tube is well worth it. When backfilling deck holes, the mixing tip allows sticking it down to the bottom of the hole and filling up from the bottom. That eliminates most of the air pockets vs. trying to shove it in from the top with a stick. Also for en-route repairs, the mixing tip eliminates about 90% of the mess of cans with pumps, cups, thickeners, sticky mixing sticks, etc., all of which conspire against me to contaminate everything from the decks and counters to my underwear. 

The stuff is expensive, but for some things it is the bees knees. It will not run unless it gets hot, is sticky enough to be self fixturing in a lot of cases, yet still wet enough to saturate 6 oz cloth (with a little effort). And it won't run out of the cloth. If working left handed, backhand, and blind around a corner on a vertical surface this is great. It is slow to harden giving you lots of working time unless really hot. In fact it takes a couple of days to really harden at 50 deg., but you have a hour or more working time even at 80. 

One complaint I have is that the thickeners have a shelf life - after about a year they get lumpy and are harder to mix and smooth (and won't go through the mixing tip) and a little while later it becomes unusable. Still hardens though. 

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