Glassing over puzzle joints

Hi,

Question from someone locally who is building a C17. 

With the puzzle joints, do they need any additional strengthening other than the glass/epoxy inside & out (as would be done with scarphs)?

I can't imagine the stresses in bending the ply into a C17 curve would force the joint apart, and the in/out layers of glass would be enough to hold it?

The instructions weren't clear.

Thanks,
Darren


8 replies:

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RE: Glassing over puzzle joints

A single layer of glass tape on the inside will give you all the strength you need.  The outside will be further strengthened by the glass you apply over the entire boat after it's assembled.

RE: Glassing over puzzle joints

And there may be another layer if you glass the cockpit inside.

RE: Glassing over puzzle joints

When I took a class to build a Chesapeake 17LT last spring, we did not tape the inside of the puzzle joints. They were glued with celofil and glassed on the outside only. This was, I'm sure, to save class time. I've been meaning to drag that puppy inside and put glass tape over the inside of the joints just to say that I have done it.

But the joints have held up fine as they are and seem plenty strong. If I was taking my time though, I'd tape over the inside of the puzzle joint. I'm pretty sure that instruction is in the manual? Unless you are concerned about building extremely low weight, why not?

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Ogata (eric)

RE: Glassing over puzzle joints

Hi,

Thanks all for the responses.  As I suspected, the normal boat glassing will do the trick.

Thanks,
Darren

RE: Glassing over puzzle joints

Eric,

That's a pre-stressed joint. Since it bends outwards, the outside is where you really need the extra tensile strength that a layer of glass gives you. Inside, it's a compressive force and epoxy/celofil has a reasonably good compressive strength.

The only time you really need tensile strength on the inside is if you hit something really hard, hard enough to push the joint inwards. So it probably wasn't just a shortcut.

Finally, a warning to readers - no matter how plausible my analysis may sound, if the designer says to tape both sides, do what the designer says.

Laszlo 

 

RE: Glassing over puzzle joints

Sounds reasonable to me Lazlo. The joints seem plenty strong as is.

Also I took a look at my manual for the Chesapeakes dated 12/2002, version 2.2 and seems to only show construction of panels using scarf joints. Presumably the puzzle joints were added to the kits later. There is no instruction (that I can find now) about taping the inside of the joints (scarfed) so just the glass on the outside of the hull and deck supporting those.

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Ogata (eric)

 

RE: Glassing over puzzle joints

I have a recent WD 12 manual and it does say to add a strip of glass tape over the puzzle joint on the inside & trim it flush with the panel edge prior to stitching the hull together.

 The pictures are from a Shearwater, so I am guessing it applies to most assemblies using puzzle joints.

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