Tuesday, February 1, 2011

Bulkhead repair

After the sail at lane cove sailing club I noticed that there was an uneven surface that had appeared around the port side stay mount. After further investigation it turns out that strands of unidirectional fabric buckled and were poking up through the filler and paint layers. Also the plain weave fabric closer the stay mount had also buckled.

This damage was more evident on the port than the starboard side, yet since both sides were pretty much symmetrical i first thought it may have just been voids from manufacture since this area was not originally vacuum bagged.

Upon further investigation the unidirectional fabric had buckled where it did not have a covering layer of fibreglass or carbon cloth. Yet in areas the carbon fabric had also buckled in a few places. This suggested that we had underestimated the loads on the bulkhead in compression along that top surface/corner.

Yet speaking to other class builders they had mentioned that they usually just put a simple layer of carbon cloth over the corner. My laminate was two layers of glass cloth and essentially two tapes 50mm wide of uni carbon running along the corner. With carbon cloth closer to the side stay mount where the load was expected to be higher.

After further thought i concluded that if the failure was not from voids and from the boat experiencing higher loads than expected. Then it would need to be strengthened significantly since the boat had only sailed in moderate conditions. This higher load could potentially be from shock loading, as a stiffer structure will generally experience higher peak loads during shock loading when compared to a "softer" one. So at least on the positive side it my suggest that the boat is quite stiff :)

To bulk up the laminate I used 3 layers of 300 gsm uni-directional carbon 50mm wide running from the chain plate towards the centre of the boat, and tapered each layer to avoid a large laminate drop or stress concentration. Then uni strips from the ratchet blocks and possible side stay positions the meet up with the main laminate.

Then to improve the compressive strength of the unidirectional laminate a covering layer of 200gsm carbon fabric was applied at the -+ 45 deg to the uni-directional laminate. This improves the strength by allowing compressive stresses to transfer from fibre to the fibre next to each other via shear stresses through the 45 deg laminate, or to simply support and stabilise the unidirectional fibres .

The laminate was applied using the "Poor mans Pre-preg" technique and then vacuum bagged to the hull to improve consolidation and reduce void content.

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