J24 wet core replacement material

jamesmalcolm

Member
65
9
USA
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.

 

Sail4beer

Super Anarchist
9,510
3,077
Toms River,NJ
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.  

image.jpg

4C8D6458-ADDE-4567-B6BC-D18DA2F52C00.jpeg

5F604D82-83B3-4B23-AA2E-C9C23FDFA632.jpeg

 

12 metre

Super Anarchist
3,745
612
English Bay
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.

 

Sail4beer

Super Anarchist
9,510
3,077
Toms River,NJ
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. 

 

jamesmalcolm

Member
65
9
USA
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.

 

gkny

Member
335
25
Balsa core is pretty possible so close as possible sounds like balsa if you want to be one design

 

Sail4beer

Super Anarchist
9,510
3,077
Toms River,NJ
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

 
Last edited by a moderator:

Gouvernail

Lottsa people don’t know I’m famous
36,980
4,765
Austin Texas
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. 

 

Will1073

Anarchist
688
103
Balsa. Foam won’t sound the same, and would make the boat illegal for one design racing — so worthless. 

 

Jubblies

Member
169
78
Toledo, OH
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.

 

Jubblies

Member
169
78
Toledo, OH
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.

 
Last edited by a moderator:

Zonker

Super Anarchist
8,871
4,781
Canada
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. 

 

jamesmalcolm

Member
65
9
USA
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.)

 

jamesmalcolm

Member
65
9
USA
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.

 
Last edited by a moderator:

Zonker

Super Anarchist
8,871
4,781
Canada
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.

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.jpg
     
20115BitCutAwayOfPottedDeckHole-235x350.jpg


 

jamesmalcolm

Member
65
9
USA
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.jpg
     
20115BitCutAwayOfPottedDeckHole-235x350.jpg
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?

 

SloopJonB

Super Anarchist
65,079
10,593
Great Wet North
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.

 
Last edited by a moderator:
Top