Balsa Core Deck Repairs

[Stumble]

I am not sure about when the Challenger was designed, but you would be very hard pressed to find a current builder that is putting a mast step over a cored section anymore. There were just to many issues with crushed core, so must of them have switched to using a solid glass/G10/metal plate glassed into the mast step.

While normally I am with daddle in respecting designers wishes, in some cases we really have learned a lot since a particular boat was made. This is one of those cases.

I would almost defiantly use G10 instead of any wood. And it is preferable to standard fiberglass. But if you can't get it, then multiple layers of hand layer glass will work fine.

The metal steps work ok. But aluminum really is too soft and will distort over time, and stainless should never be sealed. This pretty much leaves either mild steel (rust), or titanium. In this application though titanium is an unjustified expense (and this is from a guy who sells it), unless you are at a very high end racing program where every ounce matters.

Because this is a pretty thick part you should degass the repair as the glass is layer down to prevent air pockets from forming (a google search on degassing thick laminate stacks should turn up more than I can type). Otherwise it isn't a particularly difficult repair if you are experienced doing glass/epoxy work.

[IslandHopper]

Originally Posted by reiner

I didn't know G10 fibreglass boards, I have to find out where to get them here in OZ. Looks like a smart way to go.
Thanks for your replies. What would we do without the Cruisers Forum.

Cheers
Reiner

Try these guys.....

Insulcut Services Pty Ltd, Product: NEMA G-10 / G-11 Fiberglass Epoxy Laminate

[Celestialsailor]

...or these guys.

McMaster-Carr

[mitiempo]

Originally Posted by daddle

Balsa core is is very incompressible. The problem was the leak not the material. I tend to respect the original designers choice and would replace with the same by removing an inner or outer skin and rebuilding appropriately.

Incompressable? Not if there is a mast sitting over it.

[roverhi]

Balsa core IS very compressible. Just got through rebuilding areas of a deck that had had the balsa core compressed by an overzealous gorilla with a wrench on the installed hardware. I assume the areas that have been compromised are not directly under the mast. If the builder put balsa core directly under the deck stepped mast, maybe Bill got a little too overzealous about light weight construction. Quality built boats with cored decks routinely put plywood or other non compressable core material where they'd assume there might compressability issues with deck mounted hardware.

Plywood is not an ideal core even though it's relatively incompressable. It is much heavier than Balsa and more prone to rot spreading over large areas from water intrusion. More care is needed in fitting any hardware to prevent water intrusion. Having seen what boat construction workers and PO's do or more accurately don't do, I'd R&R all the hardware on any new boat I bought. Just finishing up that task on my current boat.

The sandwich between inner and outer layers of glass gives stiffness to the structure because the structure works kind of like an I beam. It's not what's in the middle but the opposite forces working on the skin that create the stiffness. Having the skin bonded to the core is important when we are talking large areas not subject to high point source compression loads, In this case, if their is compression loading from the mast, that is the force that needs to be counteracted. A highly incompressable core is what you are looking for. Starboard will not stop the two halves of he core from lifting but will stop them from compressing. The downward compression of the mast will stop the skins from lifting if they are close to it. In any case, we are talking about a very limited area.

[daddle]

Balsa compressive strength is around 2000 psi. So under a typical mast shoe it might support, what, 60 tons???

That's the same as marine ply in the reference I have here.

The OP did not report compressive failure, did they?

[David_Old_Jersey]

I think Plywood is the usual support / filler under the mast - has the same problem as balsa in that once wet deteriorates beyond being useful, just that plywood does that slower!

In any event, the problem is really around the filler getting wet - cut that source out by addressing any original design "issues" (deck droop causing cracks possibly not a "problem" in design terms - especially as the designer has long since cashed his cheque!) and the filler / support material not so critical. Having said that, having gone to all the trouble and cost of replacing I would favour something that wouldn't get scared by a drop of wet.......if not solid fibreglass etc, then I would settle for a chunk of hardwood.......unless it was a cheap / small boat, in which case it would be plywood and cover in plenty of fibreglass on the basis that would probably see the life of the boat out - and certainly my ownership!

[reiner]

Just thought I let you all know the deck is repaired and a friend of mine who makes fibreglass bits made a new solid base for my mast foot to sit on. Here is a photo of what we did and its all ready to put the mast back up.
Thank you for for your thoughts on this.

cheers
Reiner


PS not sure if the photo shows up....if not I better read some help files

[Nicholson58]

Looks like a really great step & well done.

My two cents below anyway.

I would never consider burrying wood or another material that can be damaged by water in the deck especially in a place where strength is critical. In this case, I would probably use Fab Mat to make up the required bulk and then finish with a fine weave on top. (solid glass) If you are determined to go light weight, use a layer or two of woven glass followed by a fill of epoxy slurry with microballoons. Build up the top with several layers of glass. The balloon mix is totally waterproof and stronger than balsa by a lot. I use US Composites 635 low viscosity slow resin. Cost is well below West and its easier to work with. There is no amine blush so you can follow layers without prep and the low viscosity wets out very well. I don't think the G10 is justified here based on the original build in balsa mush.

[delmarrey]

The down fall of G10 is it takes metal working tools to cut the stuff. It'll burn saw blades and drill bits in short time. So who ever buys G10, beware. I use it for backing plates and epoxy it to the hull.

Great for around a windlass.

[minaret]

Originally Posted by delmarrey

The down fall of G10 is it takes metal working tools to cut the stuff. It'll burn saw blades and drill bits in short time. So who ever buys G10, beware. I use it for backing plates and epoxy it to the hull.

Great for around a windlass.

Carbide abrasives work great with G10. I run 1" plate through our tablesaw with a carbide blade on it. Also regularly use skil saw, jig saw, hole saws, etc. all with carbide abrasive blades on G10 with no problems.