Monday, September 21, 2015

Getting Stuck into It

It was a busy weekend.  First priority was stripping down the carbon mast so i could design and laminate on new carbon fittings that wouldn't corrode.  (And that I could trust)

I bought a new compressor from ALDI about a months ago, which supposedly had a good flow rate of air.  After using it a few times it only allows you to use a die grinder for about 50% of the time.  You then need to let the compressor catch up.  So I figured why not combine my old mans compressor and the ALDI one in parallel and get air flow for 100% of the time?  Worked pretty well in the end.  In hindsight I probably should have just bought a good compressor.

The ALDI right angle die grinder did really well and the 3" sand discs tore the carbon away with ease.  I held a garden hose in one hand, grinder in the other (air tools allowed me to do all the sanding wet and keep the dust down without getting electrocuted).  I had used these air tools and die grinders previously when I worked at McConaghy Boats for a short period of time and I had forgotten how effective they are. 

Once i had carefully stripped off all the carbon that had been laminated over the fittings.  I was able to give the metal brackets a sharp blow with a cold chisel and a hammer and the edge popped up.  From there they could just be peeled back by hand.

While I was covering my overalls in dust and had all the respirators and safety gear out, I figured it was wise to repair the rudder at the same time.  In hindsight I should have done this outside as well what a mess.  For this used "scotch brite" pads on the 3" air grinder.  This worked a treat, wasn't too harsh on the carbon but stripped the filler and paint easily.

I think if I need to do much more sanding work I would set up a bit of a "lean-to" or shelter/tarp outside against the garage wall and either put some felt or sand down on the floor.  I could then grind away with disposable overalls and a respirator till my heart is content.  At the end just walk in and hose the place down and collect all the dust in the sand or felt.

Using a carbide grit blade on the jig saw to quickly cut the damaged section of the blade out at right angles to the trailing edge.  Then bogged it up with Q-cells, aerosil and epoxy, left it to cure over night.

These blades usually have about 3-4 layers of 300gsm unidirectional carbon and a 200gsm plain weave cloth at this point.  So I used 4x layers of uni carbon and 1x 410gsm layer of carbon double bias cloth.  Each uni carbon layer was 25mm wider than the last.  The laminate was then vacuum bagged to consolidate it.

The plan for this rudder blade is to turn it into a fixed rudder with gudgeon plates carboned to the blade.  I will use an old windsurfer mast tip as a tiller.  This should keep me out of trouble for a bit while I design and build a new dagger rudder and rudder box.  I suspect my old rudder box used to un-evenly load the boards and cause them to break starting at the leading edge.  (Obviously the human attached to the end of the stick had something to do with it as well.)

Tuesday, September 8, 2015


Upon inspecting the mast for the first time in a year, I lifted the rig by the spreader only to find the bracket to start separate from the rig. It was clear that this was going to need to be repaired so I dug a little further.

These spreaders where attached to the rig in 2010 using a Plexus brand methacrylate adhesive.  The stainless steel bracket was prepare with the correct etch primer supplied by Plexus.  The carbon section was abraded and cleaned with acetone.

I think the done fall was that these spreaders had come from a 50-55mm OD mast.  We bent the bracket in as best we could, but some large gaps still needed filling near the mast track.  The adhesive used was supposed to support gap filling properties of up to 8mm.  Large gaps do present problems though.  The stiffness of the adhesive is different in areas of different thicknesses, which can mean that load might not be shared evenly by the adhesive. 

 Generally adhesives sensitive to the glue line thickness, where the strength peaks for a given adhesive thickness (I have seen 0.3mm thick thrown around a lot for lower viscosity adhesives).

Side note: An interesting technique for ensuring a 0.3mm bond thickness can be to mix in a small quantity of 0.3mm OD glass spheres/balls into the adhesive.  This allows the parts to be clamped together without squeezing out too much glue.  I have also seen the appropriate thickness fishing line used as well to space surfaces/parts. 

Additionally, corrosion and corrosion cracking can be a common failure mode in adhesives particularly used in a marine environment.  

In this case it seems the bond in the substrate or carbon laminate was the weakest link when the adhesive line was thin.  Towards the aft edge of the fitting it looks like the thicker layer seperated from the steel and also had a lot of trapped air pockets, which would have held sea water promoting corrosion.

The way ahead.

My plan is to design and build a complete new set of spreaders in carbon fibre based on the original CST composites adjustable spreaders.  My plan is the build the bracket off the mast using a 3D printed mould and vacuum bagging techniques.  The spreaders will then be bonded to the mast with epoxy resin and glue powder/fibre over a larger area. 

Unfortunately the hound fitting is bonded on the same way.  It was wrapped in carbon as a regatta repair when the corner of the bracket was seen peeling off.  So the hound fitting will be getting ground off and also re-designed / manufactured.

While i'm at it I will be assessing if my original spreader and hound position choice was a good one.  The image below shows the original positon on the right and a more conventional position on the left.