Foam core hull compression damage

[bunnins3]

Hi all,

I am looking maybe purchasing a Corbin 39. It has been sitting in a cyclone pit in tropical climate for 2 years and tyres were supporting it. When hauled up and surveying, we found compression dents about 15-20mm deep at the worst point from where the tyres were sitting on the hull on both sides, in between the bulk heads/supports. Approx. 20cm long. No visible cracks under the anti fouling.

Photo of core sample is not mine. Just for reference from another Corbin.

Anyone had similar experience? Any experience repairing a airex foam core? What would be the best way of doing it?

Please see attached photos for reference.

Thank you

[s/v Jedi]

Have you checked on the inside? Hard to believe the core has compressed to half its thickness…

[bunnins3]

Unfortunately very difficult to assess the inside as it’s below the tanks, which everything in the pilot house is built on top off.

[Tetepare]

I’d not “repair” any dent. Seen lots of boats set improperly on cradles and stands, and for the most part the hull flexes back. Discount accordingly.

[bunnins3]

Originally Posted by Tetepare

I’d not “repair” any dent. Seen lots of boats set improperly on cradles and stands, and for the most part the hull flexes back. Discount accordingly.

Thank you.
Should we be worried about possible delamination of the core?

[psk125]

Originally Posted by bunnins3

Thank you.
Should we be worried about possible delamination of the core?

Yes. Delamination is always a worry. Sound for it with your hammer. Looks like it is very likely deformed, but not necessarily delaminated.

[DeValency]

See if you can insert a video flexible borescope under the tank to see the area inside. You can get a reasonable one for less than $100. I’m using it all the time to inspect my boat in difficult to reach areas, so it is a well worth investment. Also check really well moisture around and in the impact area. A mallet and a sensitive ear can actually be the most useful tool in evaluating a possible internal delamination or cracks. You could also hire a surveyor that has a good Flir thermal camera.

If everything proven solid, this is more of a cosmetic inexpensive repair, which I would do myself but even if you hire a pro, it shouldn’t be a deal breaker.

[boatpoker]

I've surveyed two Corbin 39's that sat in wooden cradles for many years. Both hulls were badly deformed and one had four areas of about 3'sq each of core separation. Airex gets soft in heat and that is what I believe caused the issues.

I don't know what happened to the boat with core separations but the deformed hull was put in the water and lived on while the owner finished her out. I saw her about five years later and the hull was still deformed.

The deformations I saw were considerably worse than your photos.

Other than those I've only surveyed 3 other Corbin 39's. Two hulls were good and one had about a 5'sq area of core separation about 2' outboard of the keel, amidships on the starboard side.

From BoatDesign.net ...
SAN and L PVC Foams: These foams are based on the thermoplastic Styrene Acrylo Nitrile (ex. Corecell) and a non crosslinked or "linear" PVC (ex. Airex R63), respectively. Typical reasons for using these materials are: high toughness, good impact resistance/energy absorption, good fatigue resistance, and a closed cell structure. Some drawbacks to using these foams are: comparatively lower strength/stiffness, high temperature problems, and is susceptible to styrene attack (the styrene in the resin may seep through the foam, leaving the resin uncured and the foam degraded). These foams are typically used in areas where high impact is prevalent, such as hull bottoms and sides.

More about foams here ... https://www.boatdesign.net/articles/...ties/index.htm

[OneBoatman]

Even a non-cored hull can acquire similar compression "marks" if put up in for extended periods and particularly where steps that could prevent local load concentrations are ignored. Fiberglass composites do not exhibit memory so it is expected the depressions in the hull will remain after the load is removed. Corbins were sold in various states of completion and some as finish yourself kits, but I believe it is correct to say that that the hulls were all molded at their factory. The hulls have an Airex core sandwich which extends well below the waterline before transitioning to solid glass approaching and supporting the keel. Airex is a closed cell foam which essentially means it does not absorb water. However this what I would do if liked the boat and I felt pretty comfortable with the price and the rest of the condition of the boat. I put it as four steps: 1) Get moisture readings in the local area of the depression, the technician should also be able to identify and define the transition to solid glass boundary as knowledge for the sake of knowledge is always good in addition to the confirma of construction in the area of the depression. 2) If it is solid in area go to step 3 & 4. If not wait for the second day of high pressure, a nice warm day and then satisfy your curiousity by drilling an array of 4-6 holes about an 1/8" diameter only through the outer layer and vacuum the cuttings out of each hole, then apply heat to the area locally for about 20 minutes. Take an early lunch after 2 hours or so check inside using a Q-tip to see if moisture is present in the core. My guess is probably not. 3) Using a rotary grinder or a belt sander with 50 grit belt remove all bottom paint and scatch deep enough to a get a bite on the outer layer in the area you plan to bring back to profile. Wipe the area clean first with laqcuer thinner and after it evaporates give it a final scrub with clean cheese cloth dampened with 70% isopropyl alcohol. Let dry and then apply fine line tape to establish the repair boundary. Bring it back to profile in two to three mixes of slow cure epoxy stiffened with microballons. After final sanding the restored area of the hull will be one tape thickness or about .004" above profile. Do not sand the bottom below profile outside of where you apply new resin. Your bottom paint can applied applied directly to the repaired hull area because you used epoxy resin. If you use polyester resin it would be prudent to apply several barrier coats before re-applying bottom paint.

[boatpoker]

Originally Posted by OneBoatman

a nice warm day and then satisfy your curiousity by drilling an array of 4-6 holes about an 1/8" diameter only through the outer layer and vacuum the cuttings out of each hole, then apply heat to the area locally for about 20 minutes. Take an early lunch after 2 hours or so check inside using a Q-tip to see if moisture is present in the core.

I prefer larger holes (first 2 photos in link below) so I can actually see the core condition. Rather than a Q-tip I tape a piece of Saran Wrap over the hole and wait for a couple of hours. If there is moisture it will condense and be easily seen on the inside of the Saran.

A word of caution using moisture meters on cored hulls ....
Moisture Meter Mythology

[Captain Bill]

The foam used in these cores are great for dealing with dynamic loads as they can stretch significantly and return to their original shape. Their performance under static loads however presents some issues, especially when subjected to large point loads. I did some work using klegecell foam in the 80s which is PVC foam. We had some marine architects build us some large foam cored fiberglass tanks which were rectangular in shape. When first filled they looked great, but over time these tanks stretched and deformed. After a second attempt and another failure I decided to study the failures myself. It ends up that under high static stress these foams "cold flow" and stretch and flow away from stress points. These boats were not designed to sit on support points on land for any length of time. They were designed to float in the water where there are essentially no point loads. I believe that the deformations you are seeing are probably caused by the foam core's reaction to the point loads. I doubt that the foam is fractured or damaged in a conventional sense, but more likely "cold Flowed" under the point loads. I don't know of any practical way to return it to its original shape short of cutting it out and rebuilding it. I would probably grind the area down to bare glass and add some more layers of glass to fair it to shape if it bothers you too much.

[Celestialsailor]

Bunnins3...believe it or not there are actual boats out there without major defects like core collapses and bent rudder shafts. That said...I was going to look at a Corbin 39 in Emeryville, Ca. Once the seller told me the hull was Airex cored, I passed. He was pissed at me and told me the core was for insulation only. I just wanted a solid core hull.

[bunnins3]

Originally Posted by boatpoker

I've surveyed two Corbin 39's that sat in wooden cradles for many years. Both hulls were badly deformed and one had four areas of about 3'sq each of core separation. Airex gets soft in heat and that is what I believe caused the issues.

I don't know what happened to the boat with core separations but the deformed hull was put in the water and lived on while the owner finished her out. I saw her about five years later and the hull was still deformed.

The deformations I saw were considerably worse than your photos.

Other than those I've only surveyed 3 other Corbin 39's. Two hulls were good and one had about a 5'sq area of core separation about 2' outboard of the keel, amidships on the starboard side.

From BoatDesign.net ...
SAN and L PVC Foams: These foams are based on the thermoplastic Styrene Acrylo Nitrile (ex. Corecell) and a non crosslinked or "linear" PVC (ex. Airex R63), respectively. Typical reasons for using these materials are: high toughness, good impact resistance/energy absorption, good fatigue resistance, and a closed cell structure. Some drawbacks to using these foams are: comparatively lower strength/stiffness, high temperature problems, and is susceptible to styrene attack (the styrene in the resin may seep through the foam, leaving the resin uncured and the foam degraded). These foams are typically used in areas where high impact is prevalent, such as hull bottoms and sides.

More about foams here ... https://www.boatdesign.net/articles/...ties/index.htm

Thanks for the information and the links! Since you’ve surveyed that many corbins you’re a good man to know. How’d you determine the one Corbin had core separation? Through percussion testing or other ways?
Would you be worried about the structural integrity of the one we’ve shown in the pictures or putting it as more cosmetic? Thanks.

[bunnins3]

Originally Posted by Captain Bill

The foam used in these cores are great for dealing with dynamic loads as they can stretch significantly and return to their original shape. Their performance under static loads however presents some issues, especially when subjected to large point loads. I did some work using klegecell foam in the 80s which is PVC foam. We had some marine architects build us some large foam cored fiberglass tanks which were rectangular in shape. When first filled they looked great, but over time these tanks stretched and deformed. After a second attempt and another failure I decided to study the failures myself. It ends up that under high static stress these foams "cold flow" and stretch and flow away from stress points. These boats were not designed to sit on support points on land for any length of time. They were designed to float in the water where there are essentially no point loads. I believe that the deformations you are seeing are probably caused by the foam core's reaction to the point loads. I doubt that the foam is fractured or damaged in a conventional sense, but more likely "cold Flowed" under the point loads. I don't know of any practical way to return it to its original shape short of cutting it out and rebuilding it. I would probably grind the area down to bare glass and add some more layers of glass to fair it to shape if it bothers you too much.

I’d read bits and pieces explaining this but you’ve explained it much better. Thanks for the detailed reply!

[OneBoatman]

I agree with Captain Bill, it is true that that under high point loads the core will try to flow away from the stress and the skins act more brittle by comparison. The good news is you don't need to be an engineer or materials expert to address "cradle waves" because they are not a specific sign of anything amiss. Most of everything else that has been offered here is great to know but the takeaway should be that if you like the boat, the deformation you see is more than likely not a result of moisture penetration and not a dealbreaker per se. Most all boat hulls will locally deform at the stess points if left on stands or in a cradle for a long time, and certain constructions are more sensitive than others. It is also generally true that assuming uniform construction, the deformed areas are likely not now nor were they any weaker than the rest of the hull when the vessel was put to rest. What to do. Assure yourself there is not also local weakness present or moisture in the core for any other reason is prudence. I like Boatpoker's suggestion of using Saran Wrap. Doing this extra bit of investigation will either result in a deal or could push you to keep looking. Building the area back to profile is done for aesthetic reasons not just because sailors like their bottoms smooth but because you need permission to drill holes in a boat that isn't yours yet.