Centreboard lay-up

This is a photo of the rudder layup after the peel ply had been removed and the surface was given a light sand ready for the foaming epoxy. Notice the shape of the uni layers, i did this to try and remove excess material were it isn't needed for example the front and rear sections of the foil. As the middle section (thickest) takes the majority of the bending load, and the narrow areas contribute very little to strength and stiffness.


This is the hole drilled into the top of the rudder mould so that excess foam could be allowed to escape from the mould. There is a straw in the hole to prevent the foam sticking inside the hole.


This is the amount of resin we used this time, as you can see from the image it looks like possibly a little to much (this stuff expands to 4 times its volume). We brushed all surfaces as suggested by SP Systems.


This is the excess foam pouring out of the mould, turns out after our failed test pour we got a little carried away. At least there shouldn't be any gaps in this one.


This is the centreboard layup that i used, similar to the rudder in profile and shape, except 1 extra layer for good measure. The layup is as follows:

1x cloth 200gsm cloth at 45 deg
1x full uni 300gsm
6x tapered and progressively smaller layers of uni 300gsm
1x small patch of 200gsm cloth at hull exit.

We applied to the mould i felt that this was way too much carbon, time will tell i guess you never know the extra stiffness might be worth it.

Applying the clear epoxy as a gel coat to the mould beforehand. This was done with a spray gun on the rudder moulds but we could not get it to spray the epoxy well and this gave us a very poor surface. The image above shows us using the gun as cup to hold the resin to brush on after we gave up on the spray gun.


The above images shows the centreboard under vacuum, with the bleed out clearly visible.

Plans for the weekend are the join the centreboard and remove them from the moulds.

Rudder Lay-up

The above photo is showing how i cut the carbon cloth for the laminate. Instead of cutting the cloth dry to a rough fit or a close fit that frays every where and becomes messy. I laminated a large piece of cloth (measured beforehand) onto a plastic sheet (vacuum film in this case) once the cloth is wet out a second layer of plastic is smoothed out over the top. This gives a "Poor mans Pre-preg" where you can handle the cloth without it distorting or covering everything in resin. Then using a template and marker pen, the shape is traced and cut with scissors to give a neat non-fraying edge.

The backing plastic is then peeled off one side and placed into the mould carefully. With the backing plastic still on the carbon is stable and you can pull it up as many times as needed for a good fit.
Once it is placed in the plastic is peeled off and the rest of the laminate is applied like normal. I cut the carbon cloth on the +- 45 deg so give the board a greater stiffness from twisting. I also read recently that placing fibres on the 45 deg over Uni's helps to support them in compression so that they resist buckling better. This should help to give a stronger skin in compression. the rest of my rudder lay-up consisted of:

1x full cloth 200 gsm
1x full 300 gsm uni
4x tapered and progressively smaller layers of 300gsm uni.
1x cloth 200gsm on 45 patch at box exit.

The tapered layers resulted in the majority of the material on the max cord of the foil and around the box exit, (some tapered above this to the head of the board as well).


Both mould halves under vacuum. The bagging process was fairly standard, peel ply, perforated release film, bleeder, breather (shade cloth) then the bag. If you look closely at the above image you can see the bleed-out through the perforated release film.

Expanding foam test pour

Clean garage, ready for work.



This photo is of the first test pour of the expanding foam into the mould to make sure that the process would work without producing large air pockets.

Using a rough solidworks mould it calculated the volume of the rudder to be 2.35 L. The amount poured into the mould was calculated as enough to fill this space.

As can be seen from the photo below, this pour did not turn out so well. Firstly the largest problem was that obviously there was not enough foaming resin in the mould. After checking the volume of the mould with water it turned out to be about 4L not the 2.3L that was calculated. This resulted in a large number of voids and unfilled cavities as well as not filling the mould the whole way.

After talking with SP it was suggested that the resin should be brushed on all mould surfaces before joining the halves together, as well as use the correct volume of resin.

Expanding foam for the foils arrived today, hopefully going to run a few sample / test pieces on saturday aswell as a test blank to make sure the mould will fill without any air bubbles or atleast if air bubbles form we can see what to expect. I'll update with photos after the weekend.
If anyone is interested here is the data sheet for the AMPREG F230 expanding epoxy foam:
http://www.gurit.com/core/core_picker/download.asp?documenttable=libraryfiles&id=1546

The JIG has arrived

The jig was completed at the CNC cutter this morning and I was able to pick it up shortly after. The initial rough quote was $500 for materials and cutting time. After i simplified the jig design to reduce the cutting time a little. The final cost this morning was $374, this covered approx 1 hr of cutting time (@ $220 an hour) and 4 sheets of 18mm MDF. The images below are just a quick mock assembly of some of the jig to check if everything was okay, which it appears to be. I haven't double check any measurements yet but i will do in the near future.

I'm still waiting on the expanding epoxy foam, which should be here before the weekend so that i can make the foils and return the moulds so there is room for the jig to be assembled. Also now that i have the jig we need to drill holes around the edges so that we can attach the foam sheets. We are also going to paint the jig with old paint to help protect it from moisture and hopefully allow another hull or two to be built off it provided the design is okay.

Preliminary FEA on centreboard

Here is a very preliminary finite element anaylsis for the centreboard for the new cherub hull.

Basically this was performed in COSMOS works, which is the solver for Solidwork 2008. The board is treated as a solid board made a aluminium. The head of the board is fixed, e.g. restrained in all directions. Then the top surface is loaded with a pressure acting to bend the board downwards. The purpose of this analysis was to try and determine what is the best shape of the individual layers of carbon cloth so that i'm getting the best use out of it as possible for the weight.


This image is a plot of the Von Misses stress, which is a method of determining the overall stress in an object. the red areas are under high stress, while the blue is under lower stress. From this i can select a certain stress level and see what its shape is over the surface.

The image above shows the outline of a certain stress level. This suggests that if i make the individual layers close to this shape i should be getting the most out of the material for its weight.

Hopefully this arvo i will be able to use the newer version of this programme at uni to actually make i model that has carbon skins and be able to not only determine the shape, but also how many layers are needed and how much the board might flex under load.

Whats happened so far:

1. I'm currently still waiting on the AMPREG F230 foaming epoxy resin that i order from SP systems so that i can do some tests with it. Then hopefully use it to make a centreboard and rudder. I was told it was going to be in the warehouse on the 6th of April, but they are still mucking around and haven't got it sorted. So best case at this stage is that it arrives on monday.

2. The jig is scheduled to be finished at the CNC cutters by Monday 19th of April. So at this stage i won't be able to put the jig together until i'm done with the centreboard and rudder moulds as they are using up all the table space. So best case at this stage is that the jig is assembled and ready to go in two weeks time.

3. I did some quick calcs today with regard to volume of foam and area of glass to be used in the hull shell. As a blank shell that is glassed without a centrecase or reinforcements etc i'm looking at the following:

Target weight: 11.7 kg
Epoxy used: 3.0 kg
Cloth used: 15.2 m^2

Also had a quick look at the fore deck:

Target weight: 2.4 kg
Epoxy used: 0.5 kg
Cloth used: 2.6 m^2

These calculations were done by taking the surface area of each section (extracting from solidworks) then based on a 50% Fibre Volume Fraction, which is reasonably achievable by careful hand-layup i'm told.

These calculations are all part of trying to put together an accurate list of materials needed so that we don't purchase too much at the beginning.

What i've got planned in the near future:

1. The 2009 version of Solidoworks (a 3d modeller) is now available on the computers at uni, which has the ability to analyise simple composite structures. With the use of this i intend to model a centreboard or rudder and perform a simple Finite Element Anaylsis (FEA) on it to see how i should stagger the lay-up within the board, so that i can remove exess material in areas that are under-stressed. I will post the images and results from this on the blog once it is completed.

2. Complete a materials list and have materials for the boat hull delivered before May hopefully.


Also if anyone has weighed their bare cherub shell from the mathews brothers when it was delivered i would be interested to find out how much it weighed. Thanks

These are a few photos of the garage in a moderately clean state for us before we begin cleaning / construction, just as a simple before and after reference.

Also went to FGI this arvo to obtain a rough quote for materials. With a rough estimate using the "NSW Cherub Schedule" spreadsheet provided by Rolf as a guide to the amount of foam needed, the following was estimated.

8 x sheets of 10mm, 80 kg/m^3 Klegecell foam = $1150
46m 200 gm^2 E-glass = $315
4m 200 gm^2 Carbon = $180
10 kg Epoxy resin = $220
Plus consumables

Rough quote is $2000 for hull materials, using mainly E-glass.

CNC machining of jig (including MDF) is rough quoted at $500

2-pack paint for hull and tools for construction approx $500

At this stage a rough cost for the complete hull is $3000 upon which a rig, sails and fittings are needed.

Design of the jig

(top) - All componets for jig
(middle) - Aft view of jig
(bottom) - Front view of jig

The jig is designed on having a vertical frame at a one-foot spacing from the bow to the midpoint with one and a half foot spacing from the midpoint to the stern to reduce the amount of materials used in the jig.

My intentions with this jig design were to employ the use of CNC routing to create very accurate profiles and positioning mechanisms so that it would drastically reduce the amount of time spent on constructing and aligning the jig. The panels will slot into place on vertical boards that are attached to a table. The table will be constructed out of 140 x 45 mm pine for the edges to create a very stiff and sturdy table (as each full sheet of 18mm MDF weighs approx 30kgs). This is important because of how the jig is setup, without the use of battens to fair in the sections it must be very stiff to provide the accurate hull shape.

Another design consideration with the jig was transport and storage. With this design the flat jig MDF panels can be laid flat in a small box trailer and the table tied on top or stored flat somewhere saving space, also the table will act as a work bench once the hull is flipped over and a sheet of form ply is placed on the table top.

The design was given to the CNC cutting workshop just before Easter and are expected to be completed in mid to late April. At this point hopefully centreboard and rudder will be completed and the focus can be placed on the jig and hull shell.