Forward crossarm / Headstay attachment

The forward cross beam area got a bit of a critical look the first time I ever saw the boat. It seemed like the lower edge of the trampoline could attach to the underwing to clean up the aerodynamics and I also wanted to make the structural part of the headstay attachment point really strong.

I'm doing many things to this boat (while it is in the shop and stripped of gelcoat) that may or may not be necessary, but I'm doing what seems right to me with the view that this boat may have some hard sailing in it's future.

This photo shows the original trampoline and the stock aluminum strap bobstay.

As can be seen, I went a bit nuts in this area from a structural point of view, but I figure that if this had not been a production boat, a lot of these areas would have been tied together in a similar fashion to what I have done.
In this boat, the headstay loads are taken in an unusual way for a catamaran.  it is more like a monohull with a bowsprit, where the headstay loads are transferred to the bobstay and the bowsprit is in compression.
This photo shows that the "bobstay" is now a carbon tube and the "bowsprit" part has been tied into the cabin front and hulls with carbon tapes.
The front crossarm is now tied to the hulls with carbon (was mechanically fastened before), and yes, there is now a carbon chainplate for the headstay.

Notice anything different between the two photos? The gelcoat chippers have been busy.

I cut away the flange that joins the upper and lower halves of the boat at the cabin front. I did this with a sawzall and what's left of that flange can be seen on the right.
I wouldn't do this again most likely, but the idea was to reduce obstacles for the trampoline (and air and water) to pass by in this area where lots of air and water will be passing by.
Biaxial carbon tapes on both the inside and outside will more than make up for cutting away the flange, but the work involved was significant.

 A relatively simple job was carbon taping the forward crossarm to the hulls.
The gelcoat seems to be well bonded to the crossarm, but I removed enough for the taping with chisels, a scraper, and a sander.


I applied filled epoxy to the upper & lower edges of the flange (straight parts) before filleting around the beam ends to make it possible for carbon to wrap around the sharp fwd & aft edges.

 

After a thorough sanding, two layers of light woven carbon were applied to the upper and lower edges of the crossarm ends.


These were cut to a length that did not extend past the fwd and aft ends of the beam to make it possible for the next step.

A large patch of heavier twill weave carbon was added to the forward and aft ends as shown. These pieces were able to conform around the complex shapes before applying peel-ply to all but the really complex curved areas.
These areas will take some carefully applied filler and some careful sanding before painting.












The carbon chainplate and carbon-tube bobstay were a bit over the top, but I have never related to metal all that well and like to get rid of metal parts whenever possible.

I have made lots of carbon chainplates, but this one is most unusual, being attached to the front of a crossarm and then tied to a carbon tube "bobstay".

I won't try to explain everything I did here, hoping that you won't fall asleep.  The pictures will provide the general idea.

I toyed with many ideas, including suspending a stainless thimble in just the right place and winding carbon strands over it and down onto the carbon tube, but the eventual method was to make a chainplate that tied around the forward edge of the crossarm and in a second step, tie the chainplate to the carbon tube.

These two photos show what the chainplate was formed around.
The aluminum shaft was used as a mandril to make a light fiberglass tube.
The fiberglass tube has a wedge of epoxy with low-density fillers bonded to it as shown. That step was easy, I just laid the tube on a flat waxed surface and applied epoxy with a wide putty knife and block sanded later.

The laminates for the chainplate were cut from 9 oz (300 gsm) unidirectional (uni) carbon and 5.7 oz (200 gsm) woven carbon cut on the bias so that the fibers ran at 45 degrees to the uni pieces.









THE NEXT SIX PHOTOS SHOW ONE "WET LAYUP.

The laminates were fully wet out with epoxy on a piece of plastic taped to the bench.

The unidirectional layers were wet out before applying and wetting the woven pieces to them.
A putty knife was used to lift a corner of these pieces off the table for application.

The longer piece in this photo was not used because we ended up with more thickness than expected.









The resin-rich layers of uni (with bias -cut woven layers underneath) were applied to the tube-with-the-wedge-thingie one by one and pressed firmly onto it by hand from top to bottom, layer by layer.
The thingie had thickened epoxy applied to it first.

Thickened epoxy was applied to the front of the beam. A bit too much down low...

Note the hot-melt-glued together and clamped-on rod-holders....

The wet laminate-on-the-thingie were brought to their new home and applied as shown.

Heavy roller pressure were used to both pull the laminate downward and to force it firmly onto the beam.



This was allowed to cure and was sanded and shaped a bit with a random-orbit sander.

                                                       

THE NEXT 7 PHOTOS SHOW ANOTHER "WET" LAYUP, THIS ONE VACUUM BAGGED.

The laminate above could have easily had some extra long layers that would tie the chainplate to the carbon tube (bobstay), but no, I wanted to vacuum bag those layers in a separate step to compact the strips of uni onto the tube.

The photos make this chainplate look massive and it is a bit large for the loads involved, but it does get trimmed, notched, and drilled.

These strips of uni were mostly applied on a slight diagonal so that they would wrap around the tube with lots of contact to the tube and the strips were narrow so that they could pass the beam/tube intersection without wrinkles or puckers.




Most of the layers came up over the top of the chainplate and onto the crossarm. They were left "bridging" as shown and then pushed into the corner when vacuum bagging to help tighten the laminate and keep wrinkles from forming at the top of the chainplate.

The uni's were covered with bias-cut woven carbon.
I had ground the flange away to laminate the chainplate, so carbon was applied there as well to tie the beam back together.

Vacuum bagging laminates requires many layers of stuff that all go in the trash afterwards (consumables they call them) and since there are many layers, there will be many photos.

The entire wet area is covered with peel ply and then perforated film.
Most of these pieces were cut before hand, so applying them was easy.

The peel ply sticks into the wet epoxy, but all the perforated film parts are lightly sprayed with spray adhesive while laid out on a scrap of cardboard and then applied as shown.
Overlaps and wrinkles are placed in areas where bridging could occur in both the peel ply and the perforated film to allow those non-stretchy
materials to push into the low spots

and inside corners.




Layers of breather (fuzzy blanket material) are taped in place....











Vacuum bagging an area like this requires a bit of planning and a bag that is cut much bigger than seems necessary.

 This piece of bag film had just the tips of it's corners taped to the bench, stretching the bag tight before applying the mastic to the edges all the way around.
The mastic should be as close to the edge as possible to avoid the edges of the bag folding over into the mastic.


This bag was applied by draping the bag over the area with half of it hanging off the front of the beam. The backing was peeled off of the mastic just where needed to attach the bag to the top of the "bowsprit" before folding the hanging part of the bag back to wrap around the tube, joining the bag on the way up to the underside of the bowsprit, then splitting off to meet up with the top half of the bag.

The breather can be seen absorbing epoxy through the holes in the perforated film and the vacuum hose can be seen on the upper right of the photo.
Patches of breather were placed over the end of the hose.

The lower end of the carbon tube was also "bagged". Long, narrow strips of uni run from the tube well out onto the underwing. These strips were run a bit off-axis, so that the strips crossed the tip of the tube on center, but the upper end of the strips wrap halfway around the tube and the lower ends of the strips fan out onto the underwing.

One of the risks of vacuum bagging laminate is having wrinkles develop and we got some here. I filled over these areas with thickened epoxy before sanding.












It looks a bit more aerodynamic after a bit of trimming with a hacksaw and sanding blocks.

Grinding the flange away was necessary, but I could have just removed the flange just where the chainplate went.
After the paint goes on I want to forget that I was a hack, so the hollows either side of the chainplate must be filled.

Since it's so hard to shape areas like this by sanding, I did it as shown and it didn't take long.

The skinny clamped-on stick had a strip of plastic tape stuck to the aft edge and was clamped with it's lower edge parallel to the flanges.

The blue tape was placed carefully and stuck down well before lightly thickened epoxy was poured from the top to fill the void.

After popping off the stick, the top edge had to be filed down the the thickness of the flange. A small fillet was applied to the top edge to make the final radius from flange to beam.







The holes are for another time, though I can tell you that the lower one has something to do with the bowsprit attachment.

The new trampoline was patterned before all the above was even a twinkle in my eye...
Compare this photo with the one at the top of this post.