The plywood core is completely rotten on the coachroof on my Swanson36. I'm replacing it with the 20mm PVC closed cell foam.
My understanding (but I have no experience in boatbuilding) is that the vast majority of the load is handled by keel step, chain plates and standing rigging. There is no compression load there. Only "sideway" loads..
question. do I need to use different core material around the mast footprint? Some 20-30cm around the mast. Should I use marine ply (sealed) or solid fiberglass (epoxy and woven-rowen) layup? Or am I good with PVC foam (from Trojan fiberglass)
1 To allow a focused and effective response, photos of the deck and deckhead (ie inside) showing the coachroof area about the mast would be very helpful obviously. You may not be able to add photos until you have 10 or more posts - I don't know what the exact number is in the rules. Photos in this advert - eg number 12 in the series - show the area well. Assume your boat it similar or essentially equivalent? ie mast directly behind the main bulkhead with a significant doubler on it all around the bulkhead perimeter at the hull, topsides, deck, and cabin/coachroof junctions, plus a decent lateral stiffener beam high up under the coachroof.
derwentboats.com.au/boat/327753/swanson-36-capable-and-comfortable-cruising-yacht/
2 In general terms you have the loads on the coachroof hole area (mast partners and collar see below) valid but there are fore and aft loads from numerous sources was well as uplift loads from halyard and control line turning blocks bolted through the collar about the partners.
navalmarinearchive.com/research/docs/mast_partners.html
3 Typically tierods are used inside from the mast step up to the collar to stop the deck flexing upwards and pumping - see below. Racing yachts usually have the lower end of the tierods hooked into the mast down near the lower end. If your arrangement is like in the above mentioned photo 12 these should not be needed.
www.jordanyachts.com/1840
4 For your question imho I would not use a different core material. Broadly, the end result of the repair panel should be equivalent steady state and fatigue strength and stiffness to the original panel. Also impact strength. The in-plane loadings on the panel are tension/compression and shear, and the out of plane loads are bending, shear and torsion. I would use marine ply of the same thickness as the original plywood used in the sandwich panel, and an epoxy resin and biaxial glass laminate each side. Depending on the thickness of the glass used originally each side, you could use ~3 laminates of a suitable weight cloth to make up the thickness each side. Then there would be another ~2 laminates on top of the whole lot inside and out, extending say 150-200mm out past the outer edge of the taper.
boatcraft.com.au/Shop/index.php?main_page=index&cPath=40_42
5 The repair must have a suitable taper on the original deck ply each side at the cut-out perimeter, and the ply plug set in each side. The process should be clearly laid out at one of these links. A taper of 5-6:1 could be ok - this taper should be in a schematic in the links. The taper and associated glass laid on extended outside the taper provides suitable structural and stiffness continuity so as to not create a weak spot around the perimeter.
www.westsystem.com/app/uploads/2022/10/0617-Wooden-Boat-Manual.pdf
wessexresins.co.uk/west-system/find-the-right-product/
6 The plug will probably need tapering in thickness to account for the lateral coachroof curve, and any fore and aft curve. The inside joint to the bulkhead and doublers (both sides if they are fitted both sides) needs to be suitably epoxy glued.
Trust this is of use. Check the general statements as above with schematics from the links and other sources? eg
www.yachtingmagazine.com/yachts/boat-deck-soft-spot-fix/
forum.woodenboat.com/forum/building-repair/106527-repairing-plywood-deck
More
7 These videos look of use
In my case the "plywood core" was the mix of veneer, plywood and fllor boards (!). All the layers were loosely glued together with huge cavities between them. Generally it was a 3D labirynth of water channels with stagnant water inside....
Additionally, throughout the life of the boat there were multiple holes drilled and then plugged with silicone. So the wole thing leaked inside in heavy rain.
That is why it is not a "soft spot" repair. It is a complete rebuild of the roof. The outer shell was a thin fiberglass, with a lot of fairing compaund on it and a sheet of a dyneel. Needless to say, it was delaminted from the "core".. moreover, there were quite a few of nails. That is why I decided to leave only the bottom skin (around 4mm thick fiberglass). It gave me some curvature.
I've opened the roof, and thrashed everything till I saw the bottom skin. Then, I put 2 layer of biax "for a good measure" and started building on top of that. first 20mm closed cell PVC on thickened epoxy and then 3 layers of the 600gsm woven roven. The resulting sandwich is lighter, stiffer and stronger than the stuff I've started with.
That's how it looked when I opened it. I was ready to pull out the hands of person who did that. These floorboards weren't even glued to each other.
Below is the intermediate result. White stuff is the thickened epoxy (you can see it through the woven rowen). It fills the gaps between PVC sheets too, so it worms sort of a beam. There will be a lot of fairing when I'm done with structural part.
Now, pay attention to the white section around the mast. I haven't touched it yet. Will be lifting up the collar and replacing that section as well. On my boat I have no running rigging that is attached/connected at the mast base. Main halyard terminates on cleat near the kicker (knee high from the roof) and riffing lines / outhaul terminate on boom. I wanted to re-route everythung to the cockpit.. but I think I won't be doing that this time as I'm a fair weather sailor now.
The next photo is the close-up of how "collar" looks like. It is screwed into the roof and mast goes through it. There are a few wooden wedges between them and a lot of old silicone-like substance. I'm not sure what's the material the collar is made of. My bet is that it is thin aluminium. Surprisingly, this thing has only minor leaks. Will be installing mast booth after I finish with the roof.
Inside the boat it looks like this. The mast is in front of the main bulkhead (approx 5cm). There is a beam that goes alongside with the bulkhed. The roof rests on them. I have no questions to the beam and bulkhead condition. You can see multiple silicone-filled holes on the ceiling. And mould (especially forward starboard corner) .. because the whole thing was wet.
That's how it looks from underneath the roof:
So, my question remains the same :-) . What should I use as a core for 20-30 cm around the mast (underneath the "collar"). I'm inclined just to use the marine grade plywood.
