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.html3 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/18404 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_425 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.pdfwessexresins.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-deckMore
7 These videos look of use
8 You might try and save one of the existing glass laminates - say the top one - and bond the plug in to it directly with epoxy glue, and just put the taper on the inside ply deck and plug then the laminates. This will keep the deck curve fair but not get the benefit of the deck side taper.
9 The laminates in the taper troughs will be sized progressively to sit in the trough. Assume the hole cutting out the rotten ply will be rectangular with generous corner radii.
10 I can do a mudmap schematic through the plug to deck join if you want. Don't want to make a meal of it but a picture is worth a thousand words.
11 The trough valley at the centre can ideally be faired round with the epoxy glue to make a rounded surface for the glass to sit on, not a broad vee surface.
12 The finish laminates 150-200 past the outer taper perimeter can be lightweight epoxy filler faired and smoothed so as to not look too bad a bulge.
13 Section 5.3 of the WEST System covers it well. An 8:1 taper is recommended. Using the taper both sides is probably overkill - they just use an 8:1 taper on one side.
