Friday, July 17, 2009

Carbon Boom Rigging - Part 2

Gooseneck fittings, aft mainsheet lead, outhaul sheaves:

The boom came the proper length which meant there were no scrap pieces to use for molding backing plates or to use for reinforcing. I made a mold out of foam to help mold the gooseneck fittings. This was made by hot gluing sheets of foam insulation together to make a block and then sanding it using the outside of the boom and sandpaper.

The outhaul fittings provided from Windrush would have put the top of the boom below the boom band and required moving the gooseneck. I also did not like the idea of installing a goosneck fitting with fasteners. I first made a small riser pad using epoxy and 1/4" chopped glass and some colloidal silica. I also used some black pigment for cosmetic purposes.

The idea I came up with was to use the supplied part and incorporate it into a bonded gooseneck fitting as Larry Tuttle and others have done. I liked the idea of using the stainless fitting so that the mating surfaces would be a metal/metal interface and would not wear over time.

The mold was lined with packing tape and sprayed with McLube. I wetted out 3 layers of 6 oz carbon and layed them up in the mold. Next, I set in the pre-fabbed risers along with more epoxy/chopped glass/filler/pigment mixture. I set the stainless fittings on a fid(sprayed with Mclube) that perfectly matched the ID. This way I could align the fittings on the center line of the mold even when I burred them in the epoxy mixture. At this point I was not optimistic as the whole deal looked super ghetto-fied when it was all laid up.

The parts were un-molded the following day after 24 hours cure time.

Next I trimmed the parts with a hack saw.

I then began sanding the parts. This was more of an undertaking than it had to be due to my sloppy molding job. That said the parts tuned out far better than expected after they were sanded and fit to the inside of the tubes.

Gooseneck fitting inside the newly fabricated part. Note that this stainless liner will not "wollow out" as a composite receptacle will over time.

Fittings shown inside the boom sections. Note this is not super high end fit/finish, but they should be bomb-proof which is what I care about.

Fittings were then bonded in using the same mixture used to fabricate the parts. Tonight I will laminate a few layers of carbon cloth over the top of the fittings and onto the tube. I am very happy with how these turned out.

This fitting was also provided by Windrush. These will go on the aft end of the boom on the bottom to lead the aft mainsheet block. The block will be lashed around the top of the boom and then through this fitting to keep it from angling forward. Note that I am NOT tying the block directly through this fitting. It will be subject to shear and very minimal peel loading.

The areas were masked off and the bonding surfaces were then sanded.

Parts were bonded on with epoxy/silica/glass mix. Not that these parts were a hollow skin. the entire cavity was filled with the bonding mixture for strength and I will have to re-drill the hole after cure.

The first holes I put in the sections were the outhaul sheaves. Note that before I did anything I very carefully marked the top and bottom along the center line of the boom with masking tape. The booms have a thicker wall at the top and the bottom so it is important to get those types of details from the manufacturer before you cut anything. Also note the hole has no sharp corners which have a higher stress intensification factor than curved corners.

Outhaul sheaves fit. These will be through bolted with machine screws/washed/nylon lock nuts. Cutting holes like this is far easier than in an aluminum section. I used a Dremel and a cutoff wheel to make a box and then sanded it to size with a small fine-grit abrasive wheel.


Stay tuned, more to come...

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