Core decisions on 43 DCMY

[Triskele]

Am repairing deck core under bow pulpit and forward of bulkhead on bow. Roughly a 30 sq. ft. section or one 4' x 8' sheet. Core is 5/8" thick. Since the area requires structural integrity and the ability to bear the loads of the anchor windlass, bow pulpit and cleats, based upon all that I have read plywood is probably the best replacement for the rotted balsa that has come out (at this stage I have removed the balsa and am in the process of sanding inside upper skin and prepping area to take the new core. I am working from the bottom...as in, I have removed the bottom skin. The work is also confined to a limited space inside the ship's anchor locker.) So here's my question(s)...does anyone see a concern with using a sheet of 3/8" plywood on top and a sheet of 1/4" nida-core on bottom? Is that doable? Can they be laminated together with epoxy? Also, will I maintain the structural integrity of a solid sheet of 5/8" plywood? Given the area I am working in, it will be impossible to put in one solid sheet of plywood so I will have to laminate two pieces together and overlap the seams. However, although I intend to "pretreat" each plywood sheet with epoxy to ensure it has a moisture barrier and bonds well, I was thinking that using Nida-core as one of my layers instead of plywood. It is lightweight, durable and plastic honeycomb. Thus, structurally sound and water resistant. However, not sure how well it would stand up to the test of time and degradation or whether it is appropriate for my application around the bow pulpit and windlass.

I have seen in other threads on this subject where others have recommended Divinycell, Coosa board and the like. I do not believe that these alone are appropriate for an area that would have to sustain the loads of the bow pulpit and anchor. As well, they can not be screwed into like can be done with plywood if necessary. Thus, I have ruled these alternatives out. Would possibly consider them in combination with a plywood layer, but seems that Nida-core might be a better choice depending on its bonding ability with plywood. Or perhaps it would just be better to stick with plywood all around. Thoughts?

[Glory]

I don't think I would have done that job from the bottom! I think that is much harder.
What ever you chose to use, as long as it can't be one piece I would suggest you stager the courses, like bricks. As for the load you can add a backing plate(s) to provide as much strength as you deem necessary. I don't have a pulpit, just an anchor shoot, the anchor shoot and the windlass have stainless steel backing plates a couple of feet in each direction.

[jim rosenthal]

Even with marine plywood, I don't think I'd put screws into it; I think I'd let it set up good and thoroughly and then through-bolt everything with backing plates like he mentioned. That way, you get to seal the holes that you drilled through the deck to make sure they don't leak; you can even drill them oversize, plug them with epoxy, and then redrill them to size.

At least one of our members did this deck repair from underneath. I think the results were very good, but it looked very difficult indeed while it was going on.

[BOATER BOB]

Coosa board would be better than nidacore in your case.
Bob

[sgharford]

Fun to see my old post come alive regarding this topic on my 43DC. &nbsp;Note that I found the core on my 1976 43DC to be 3/4" thick and laminated 3 sheets of 1/4" marine rated mahogany plywood with West Systems (I would use whatever BoatsB recommends) . &nbsp;I'm diverging from your original post to give you my entire experience with this is about a paragraph: &nbsp;If you laminate 2 sheets of plywood together, you should wet both sides with unthickened epoxy to saturate plywood, then use a thickened epoxy to bond those 2 sides together. &nbsp;Do all at same time and yes a helper is very handy for large sections. &nbsp;Be sure to stagger seams from one layer to the next. &nbsp;I found in an experiment that just wetting both sides unthickened and putting together did not make nearly as strong a bond as adding that 3rd final thickened layer. &nbsp;If I was a betting man, I'd say you going to find the section much bigger then you thought. &nbsp;This was case on mine and honestly it would have been next to impossible to do that job from the bottom. &nbsp;If you do go from the top and decide to reuse old top skin, I still had to drill holes and pump slightly thickened epoxy thru until it popped out adjacent holes. &nbsp;This was even after layering a coat of thickened epoxy on underside of skin (and wetting the plywood side with unthickened) and placing hundreds of pounds of weights on top of skin while it dried. &nbsp;Also note if you go thru top, you will have to take headliner down and support the lower skin with plywood and 2x4's. &nbsp;Even with that I would not walk on lower skin without at least 1 layer of 1/4" plywood down from the top, so if section is truly larger than you think, do sections you can walk around first, putting 1 layer down before cutting the next section of deck out. &nbsp;The most expensive part of all this is having the decks painted, you really have to do the entire top side if you want it to look right. &nbsp;If you still have the original mast, I found the mast step leaked and caused allot of core damage over main saloon. You may as well get that done too (another headliner removal and replacement - may as well replace those lights while in there!). Take it one step further, take out all 6 of the window frames, repair any rot in the plywood around them - including adding 3M marine filler or the like to make openings smaller as Hatteras was notorious for cutting the opening too big for these windows causing leaks, and reseal the frames prior to paint as well. &nbsp;<br><br>All of above is what I did after I came to realization that the 2' x 3' section of soft deck that had been there since previous owner was actually much bigger (3/4 of the entire front deck) and the amount of work that was going to be required to do and make it look right. &nbsp;Sucked beyond belief at time and was a long, dirty, and expensive winter and spring. &nbsp;However, not a visit to boat goes by when I don't admire that all the above mentioned things are done. If I was too do it again, I would have brought boat to nearest capable boat yard within 150 miles (preferably one with a paint booth) and subbed the entire job out to them. &nbsp;As I said earlier, and the reason I 'm sure you want to go thru bottom, the big nugget is painting the top side when done. &nbsp;Another reason I went thru top, once you cut the bottom skin out you are completely committed to doing job from bottom. &nbsp;I can't see how you can do it from top without the bottom skin already in place, at least as an amateur and not a professional who does this stuff for a living.&nbsp;

[Dan Mapes]

I do not know why you would rule out Coosa. Blue water Coosa is as heavy as plywood and has layers of fiberglass within it. It is also used in transom repairs. If you read my thread you know I used it and the foredeck is rock solid now. Are you certain that area is 5/8? I believe it is more like a full inch. It could be where you measured it was compressed. Make sure you find a segment with good balsa and measure there.

[Triskele]

Fun to see my old post come alive regarding this topic on my 43DC. &nbsp;Note that I found the core on my 1976 43DC to be 3/4" thick and laminated 3 sheets of 1/4" marine rated mahogany plywood with West Systems (I would use whatever BoatsB recommends) . &nbsp;I'm diverging from your original post to give you my entire experience with this is about a paragraph: &nbsp;If you laminate 2 sheets of plywood together, you should wet both sides with unthickened epoxy to saturate plywood, then use a thickened epoxy to bond those 2 sides together. &nbsp;Do all at same time and yes a helper is very handy for large sections. &nbsp;Be sure to stagger seams from one layer to the next. &nbsp;I found in an experiment that just wetting both sides unthickened and putting together did not make nearly as strong a bond as adding that 3rd final thickened layer. &nbsp;If I was a betting man, I'd say you going to find the section much bigger then you thought. &nbsp;This was case on mine and honestly it would have been next to impossible to do that job from the bottom. &nbsp;If you do go from the top and decide to reuse old top skin, I still had to drill holes and pump slightly thickened epoxy thru until it popped out adjacent holes. &nbsp;This was even after layering a coat of thickened epoxy on underside of skin (and wetting the plywood side with unthickened) and placing hundreds of pounds of weights on top of skin while it dried. &nbsp;Also note if you go thru top, you will have to take headliner down and support the lower skin with plywood and 2x4's. &nbsp;Even with that I would not walk on lower skin without at least 1 layer of 1/4" plywood down from the top, so if section is truly larger than you think, do sections you can walk around first, putting 1 layer down before cutting the next section of deck out. &nbsp;The most expensive part of all this is having the decks painted, you really have to do the entire top side if you want it to look right. &nbsp;If you still have the original mast, I found the mast step leaked and caused allot of core damage over main saloon. You may as well get that done too (another headliner removal and replacement - may as well replace those lights while in there!). Take it one step further, take out all 6 of the window frames, repair any rot in the plywood around them - including adding 3M marine filler or the like to make openings smaller as Hatteras was notorious for cutting the opening too big for these windows causing leaks, and reseal the frames prior to paint as well. &nbsp;<br><br>All of above is what I did after I came to realization that the 2' x 3' section of soft deck that had been there since previous owner was actually much bigger (3/4 of the entire front deck) and the amount of work that was going to be required to do and make it look right. &nbsp;Sucked beyond belief at time and was a long, dirty, and expensive winter and spring. &nbsp;However, not a visit to boat goes by when I don't admire that all the above mentioned things are done. If I was too do it again, I would have brought boat to nearest capable boat yard within 150 miles (preferably one with a paint booth) and subbed the entire job out to them. &nbsp;As I said earlier, and the reason I 'm sure you want to go thru bottom, the big nugget is painting the top side when done. &nbsp;Another reason I went thru top, once you cut the bottom skin out you are completely committed to doing job from bottom. &nbsp;I can't see how you can do it from top without the bottom skin already in place, at least as an amateur and not a professional who does this stuff for a living.&nbsp;

Thanks SG for reviewing your experience with this task. Will come in handy. Makes for an excellent reference. Appreciate the specifics and detail. My project is smaller (so far) than yours so I have decided to repair from the bottom. Prefer not to have to rip off the top skin then refinish and paint. So far so good. Built a platform in the anchor locker to work from and has worked out well. No doubt it would be easier to do from the topside, but also more expensive unless painting the deck was part of the plan. In my case, it is not.

[Triskele]

I do not know why you would rule out Coosa. Blue water Coosa is as heavy as plywood and has layers of fiberglass within it. It is also used in transom repairs. If you read my thread you know I used it and the foredeck is rock solid now. Are you certain that area is 5/8? I believe it is more like a full inch. It could be where you measured it was compressed. Make sure you find a segment with good balsa and measure there.

Dan...no I am not certain the core is 5/8". After removing the balsa it measured 5/8", but it could have been a "compressed" 3/4. My boat is an '81 model year. Those were AMF boats. It would come as no surprise to think of AMF cutting corners to save on cost, but it is also possible that a thicker core was not necessary by design. Perhaps made up by glass? Anyway, the deck thickness (glass + core) is 1". I am planning to raise the question on the forum in a seperate post to see if there are any other members who know for sure. In the end, going a little thicker will not hurt things since I am working from the bottom. As for the core material, this continues to be an ongoing debate in my mind. The bottomline is...well...the bottomline. Coosa is more expensive and don't see the benefit over laminating sheets of plywood (yes more work). Also, am not convinced that Coosa is an appropriate core material when taking into consideration the loads attributed to a bow pulpit, windlass and cleats. Plywood is cheap and exceeds Coosa by multiple factors when it comes to compression tests. Coosa has its place in other areas of the ship, but not sure under the bow pulpit is one of them. Having said that, a combination of plywood and Coosa might make since. Jury is still out, but based upon the extensive research done reading threads on the topic in this and other forums, it seems that plywood wins out for structural applications (i.e load bearing).

[34Hatt]

Plywood is the best bang for the buck but Coosa is more than good enough for the job better than nida core that you mentioned and you also mention Divinycell so I didn't think you cared about $$$ but it would be the best and never have to worry about it again. With that said you can do it as you said sealing it up good and mulitple layers with some glass in-between makes for a stronger lay up. Just remember any voids or air in-between weakens it this is why it is better to work bottom up.
5/8" core is not common and my bet is it is 3/4" which is very common.
I would do two layers of 3/8" with some 1208, start with one layer, then 3/8"core, 1208, 3/8" core finished with 2-3 layers of 1208 and you will be bullet proof No screws through bolt unless its a canvas snap.

[Boatsb]

Coosa is defiantly strong and rot resistant.

I just replaced a transom on a cc that will have an Armstrong bracket and that's what I used.

I also used epoxy and 1708 biax for strength.