Sunday, November 6, 2016

Sailing 18ft Skiffs

Plans for getting the cherub back on the water have been delayed for a little while as I've picked up a ride on the "ILVE 18ft skiff team" for the 2016/17 season. 

Watch this space :)



Rudder Mould

Carving out new rudder moulds on my CNC router.  Section profile is NACA 64012, mould material is MDF, epoxy coated and re-machined.



Machining the foam core with western red cedar timber stringer.



Having a go at some female spreader arm moulds.




A spreader bracket mould.  This bracket will allow the spreader arms to be adjustable.



Wednesday, April 6, 2016

Putting the CNC Router to work.

Finally got a chance to put the CNC router to work.  The first image is a trial run of a goose-neck fitting mould to be made from MDF.
Below is the MDF mould after roughing out with a end mill.

After finishing passes with a 8mm ball mill.

Single coat of epoxy resin prior to sanding.  Note: A single heavy voat of resin was sufficient.  The surface sanding back to a very nice mould surface.  I was certainly impressed.


The component after vacuum bagging.

I used the router to cut a slot for a standard stainless steel Reilly fitting.  This was test fitting the depth of the slot.

Using a trace function to cut the profile of the part in the mould.  I used small spacers of 3mm Perspex to keep the tool away from the mould surface.

Trimmed component



Test fit on the mast :)

The cavity between the mast and fitting will be filled with Q-cells to improve the compressive strength (Vang loads). 




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

Spreaders



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.





Monday, August 31, 2015

Step 1: Getting the trailer in good shape.

So the old trailer was never going to make for a trip to Brisbane with the cherub on top at the end of the year.  The rust holes were getting bigger and patch jobs weren't cutting it anymore.

My old man first built the trailer nearly 30 years ago, which it has served our family well.  So i also wanted to make something that would last as long.  I looked at some of the cheap trailers on ebay, which were mostly just painted steel.  These cant be expected to last much more than 10 years left outside in the weather think my one was.  The galvanized versions of these, still had the thin tie down rails or low sides or short draw bars.  So i decided to build my own.


Building it myself allowed me to make a nice long draw bar which allows the cherub to fit forward over the draw bar, reducing the amount of overhang out the back.  I also drive a 4WD with bard doors at the back so i need as much room between the bowsprite and the door as possible.  (plus long draw bars make a trailer easier to reverse)

I made the frame 50x50x3mm steel box section, with 2mm steel chequerplate sides and floor.  Everything was TIG welded as its the only welder I own.


I used a drill press to put 15mm holes through all the ends of each section to allow for the trailer to be hot dipped galvanized.  I learnt a handy trick afterwards which was to cut a "V" out of the end of each bit of box section with a angle grinder, instead of drilling holes as it is far easier and quicker.  It also allows for a bigger opening to allow liquid zinc to flow in and out.


I used the same axle from the old trailer which I cleaned up, reinforced some old welds and repainted.


The tie rails were 20x20x2mm so that ratchet strap hooks would still fit around them, while the rest of the frame work was 25x25x2mm.  The whole trailer took me about 6 months to weld together, which was a lot longer than i expected it would take.


I got the trailer galvanized through a local trailer builder.  All up the trailer weighed 280 kg after galvansing and cost me $700 for the galvanizing and about $200 in labour at the trailer place to strip the axles off, then re-fit them when it returned.  So all up i spent about $450 on steel and $900 on galvanizing.

Another handy trick i had seen was to use an old hub as a spare wheel mount.  Even better was to weld on a stub axle and use a complete working hub with bearings to mount the spare wheel.  You then have a spare hub and bearings that goes with the trailer as well.  There have been a few times on trips with various trailers that we have lost a wheel, which has chewed out the studs when it came off, always handy to have some spare studs.



 Shortly after i started on a lid for the trailer.  I wanted to be able to get to the stuff underneath while the boat was ontop.  So i decided to hinge it from the side and use gas struts to hold it up.  The LED tail lights were $20 each from whitworths and was pretty happy with the quality.


In the bottom of the image below you can see the hinges that i have used have a clevis pin as the hinge.  This means they can be easily removed without tools to allow the trailer to be used for other things.  Although having a flat surface on the top is super handy for transporting sheets of material or anything large.  The plastic strip is just a trimmed down trailer slid from whitworths for about $10 and makes it a lot easier to slide the cherub on its aluminium dolly up onto the trailer by yourself.


Wednesday, August 12, 2015


So its been a few years,  I've finished my engineering degree.  I've worked in the industry for a few years, tried my hand at a bit of 4wding, power boat builds, casting metals at home and TIG welding a new box trailer.  I've grown a little older and hopefully I've grown a little wiser, but finally the itch is back.

I've lifted the covers on the old girl and shes all still there just as I remember her.  I've had a good look over the condition of everything and identified a bit of the list that needs to be worked on.

The plan is to have a serious go at the 2015/16 Cherub National Titles in Brisbane over the new year.  I've been out of the class for a few years (in fact sailing altogether).  So there are going to be a few challenges along the way.

So stay tuned and watch this space, i'll be posting updates on my strategy, component design, boat repair, rig setup and analysis (The class has changed the rules in regards to sails, so that will be an interesting development to look at) and hopefully manufacturing some new foils.

Cheers

Tuesday, February 15, 2011

Self-tacker design

To work out the radius of the self-tacker track i started with working out what i wanted from it as follows:

  1. The leech tension to remain the same on all positions of the track: This was so that i could adjust the car position without having to adjust the jib sheet as well, so that simple tweaks could be made.
  2. The foot remains that same tension on all positions of the track: This was to keep the foot tension on the jib an independent variable from the outward position of the car. I adjust the foot tension by the position of the jib sheet on the clew board.
Some people like to set the boat up so that the leech tension decreases as the car is eased or have the foot tension increase as the car goes out. I'm personally not a fan of this as you are effecting two parameters with one variable and i feel you could end up in a position that you need to constantly tweak two controls just to setup the one parameter you want to vary.

If order for these criteria to be meet the jib car will need to follow the jib clew as it moves exactly. To do this it means the distance from the head to the jib car must remain the same as it travels. Secondly the distance from the tack to the car must also remain the same. This gives us the simple problem where we have a triangle with constant length sides that pivots about the forestay.

Therefore it can be found quite easily. Either by the geometry of the jib and then find the line length that runs through the clew and is perpendicular to the forestay. Or once the boat is rigged up you could just use a tape measure and builders square to find the distance perpendicular to the forestay.

The only other things you need to consider is to account for the fact you self tacker track will sit something like 30mm above the deck at the centreline and to allow for pulley's etc.

Also if the track has a slightly smaller radius than needed the sheet tension will increase slightly as it passes the centreline during a tack. If the track is on a slighlty larger radius then the sheet will loosen slightly during the tack. I personally favour the track to be on a slightly larger radius as i don't want the sheet to tighten up and jam during a tack.

From memory my current track is a 1580mm radius