recoring help needed

[Summercamp]

I am preparing to fix some soft spots on some decking which currently has 3/ inch plywood for the core. For my core replacement material I am questioning whether to replace the rotten core with marine plywood or use something else. My thought is to go with 3/8 inch balsa. Any thoughts as to the pros and cons?

Second question: Some of the rotten core extends to the very edge of the deck where the plywood core ends and becomes solid fiberglass and curves upward to the deck and hull joint. When cutting the top skin for removal I will leave a 1 ¾ inch flange of the original deck to aid in re-laminating the top skin. I will need to dig out the rotten core and will then have a crevice to fill. I have read that some individuals have filled this entire outer edge/ crevice with thickened epoxy and then butted the new core material against that. I guess another option would be to insert and epoxy the new core material all the way into this crevice. What is better? Other options?

Lastly……Creating hard spots… I have read about the concern of creating hard spots and am a little unclear about this. If I understand it correctly this could occur if someone were to use a core replacement material that differs greatly from the surrounding original core material. For example using aerox foam to replace a rotten core area in the middle of a balsa cored deck or perhaps using solid fiberglass. Another example could be wide epoxy joints between the old and new core material. Is this a case that by having an area that flexes more (aerox) or is stiffer (fiberglass/epoxy) than the surrounding areas there is the potential for the joint to weaken where the two materials meet? Somehow I feel like I am missing something.

Relating my last question to my first two questions….Do I need to be concerned about hard spots in the case of using 3/8 inch balsa for a repair in a plywood deck? Are the two material compatible enough? In the case of filling the crevice with thickened epoxy (question two) do I need to be concerned about hard spots where the epoxy meets the core replacement material?

I have tried to research this well but need some help.
Thanks!

[amarinesurveyor]

In your first sentence you missed the thickness (you had 3/), I'm not sure whether it is 3/8 or 3/4. As long as the thickness of the core is the same, it shouldn't matter a whole lot, what material you use, as long as the laminate thickness on top of it is the same as the surrounding areas. Is it a high stress area like under a windlass or were all of the decks core with plywood? Make sure you seal up all deck penetrations where the water may have entered.
Brian

[mitiempo]

If the deck is cored with plywood and you are only replacing some core I would stick with plywood. For a total recore I would switch to balsa.

At the edges where the core tapers I would shape the plywood for a close fit and the thickened epoxy will fill in the gaps. If it is not shaped to fit you will use more epoxy and plywood is less expensive than epoxy.

I wouldn't worry about hard spots. Many manufacturers replace core with solid glass for hardware attachment. As long as the skin is strong enough it is not an issue.

Here's a link to a recoring project on Glissando, Tim Lackey's own boat. A lot of good information from an expert at this - he restores boats for a living and does a great job.
Pearson Triton #381 Glissando | Deck Recore Project

[Summercamp]

It is 3/8 plywood.

The entire deck is 3/8 plywood and with the exception of the stanchion the soft area is not under a high stress area such as a windless. My plan is to use fiberglass where the stanchion mounts. Your question does bring up something I am wondering about. The soft area is in the same neighborhood of where the bowsprit mounts. I will know more once I cut but it appears the area where the bowsprit mounts is solid. At least I am hoping because there is a glassed in raised pad (4" x 6") where the bowsprit mounts and this will complicate things. If the area below the raised bowsprit pad turns out to be soft would you use plywood in this high stress area or fiberglass? There is a nice heafty aluminum backing plate. If the raised glassed in pad for the bowsprit mount also turns out to be soft (hard to say what was glassed under there...solid block of wood?) and has to be removed I have considered not trying to reglass in a new raised pad but having an exterior pad for the bowsprit to mount to. That said I am open to ideas.

[mitiempo]

Plywood has enough compressive strength so should be ok. But where each fastener passes through the deck you should overdrill and fill with thickened epoxy before drilling the proper size hole. Here's another good link.
Sealing Deck Penetrations to Prevent Core Rot Photo Gallery by Compass Marine at pbase.com

What kind of boat is it?

[tommays]

There are no small wet spots in plywood you may be able to live with wet in some areas BUT


What boat you are repairing is a factor as someone who has owned a J24 were the inner and out skins merge into a very solid single piece at the hull deck joint and above deck repair is no problem



On my plywood core Cal 29 all the good advise in the world will not make up for the fact that the inner skin on the Cal is just sealed to the core and while it is important in keeping the core sandwich stiff


It will come out with the plywood leaving a big hole in the boat

[Summercamp]

There are no small wet spots in plywood you may be able to live with wet in some areas BUT what boat you are repairing is a factor as someone who has owned a J24 were the inner and out skins merge into a very solid single piece at the hull deck joint and above deck repair is no problem

It is a Pacific Seacraft 25 and the inner and outer skin do merge.

[mitiempo]

Here's a pic and a diagram that might help everyone get a better idea of how it is built. The diagram is of a Pacific Seacreaft Orion but I would think the 25 is very similar. The pic is of the bowsprit on a 25.

[Hudsonian]

I have found balsa core very easy to work with. The grid of blocks on a scrim conforms easily with compound curves. It cuts easily with a utility knife. The epoxy you apply between the blocks creates a system of closed cells, thereby, preventing transmission of water intrusion that you experience with plywood. Of course, through deck penetrations should be potted.

[tommays]

With the thick outer skin/ thin inner if the non-skid is in good shape i would try and repair from below as messy as it is it would be ashamed to destroy good looking non-skid