Mould making alternatives - can you lay carbon fibre hull directly onto plug

Hi guys, looking for advice on boat building via mould. Firstly an intro, we are a university team building a 'water-bicycle'. Basically a monohull of 5.5m length, beam of 85cm and depth of 40 cm. Propeller is mounted on vertical shaft 2m from aft powered by cycling. We will be working with carbon fibre, using two outer layers, a layer of foam/honeycomb and an inner layer or two (some final decisions still have to be made here) but i need to get the moulding decisions under wraps as soon as possible.

Everything i come across seems to be the standard plug to mould to boat process. However we are only building once off and this is quite a costly method, does anyone have alternative ideas?

What i was thinking is laying the carbon fibre around the plug, this method would already save us having to make a mould. Does anyone know the pro's and cons of this?

Cons i might imagine:
-Having to add internal structure afterwards, two bulkheads as well as the gearing/transmission system, seats and deck. Would this affect the rigidity of the bonds?
-Having to smooth the outer surface, light sanding would be my idea, would that affect the strength of the hull?

Any help and ideas would be much appreciated!

 

Felix,

the pros are obvious, you can save yourself the effort and expense to make the female mould.

Regarding the cons you've brought up:

- An internal structure being glued in later is pretty much standard, even with a female mould. Make sure you thoroughly clean and roughen the areas where your internal structure touches the hull shell to produce the best bond possible (I assume you will reinforce the joints with fillets and some tabbing). A very good idea is to use peel ply as the first layer to go on the plug.

- If you use carbon fibre it's best to have a glass fibre layer on the outside of the hull so that you have something to sand into with the carbon being left intact. Also don't expect to smooth the hull with sanding (i.e. removing material) only, but go for some lightweight filler - otherwise you risk sanding into the carbon, thus of course weakening the structure. Obviously this adds a bit of extra weight, which is another downside of using a plug.

In general I think you are on the right track with your plug. As long as weight isn't that crucial, the extra trouble with a female mould is usually not justified for a one-off project. To be able to pop out one smooth hull after the other is a different matter.

Just my € 0.02...

 

Depending on the technology and expertise you have access to,there is another way.You can create a female mould directly.Perhaps most easily done by CNC machining in sections and then going for a respectable,but maybe not perfect,finish.

 

Just had a walk to the harbour to catch some fresh air, thought about this and that, and suddenly remembered something that might be interesting for you, Felix.

Admittedly it's model boat building here, but you may be able to scale this up for your project: *Click here!* (It's in German unfortunately, but since I read you're from the Netherlands it's likely you will understand the text well - the photos are quite self-explanatory, though.)

These guys use latex sheet to smooth and compress their laminate on a male mould and the result is stunning. I also like the way they transfer the wet carbon fabric without distorting the weave - very clever in my opinion!

 

How thick is the core you'll be using? A jig set-up might be possible and form the foam over this, laminate the outside, flip over and laminate the internals. Would need to ensure the hull is braced sufficiently before flipping over. Also, glassing rebates would be required for overlaps of fabric.

If you do go with the male mould route, you will need to consider how you plan to bond the core to the inside skin. If the hull shape is all developable surfaces then hand-laminating should be fine, otherwise you might need to consider vacuum bagging

 

John and Ian Lindahl of Fennville, Michigan, USA have built several A Class catamarans with carbon fiber and epoxy on either side of a foam core. They start with a set of female station molds. The core is created using strips of foam, vary similar to building strip plank canoe. Then the foam cores are cover with carbon fiber inside and out. Bulkheads are then glued in place. I lightweight fairing compound is used for final smoothing.

Photos of the build sequence for two boats are at:
LR5 BUILD https://lindahlcompositedesign.weebly.com/lr5-build.html
LR 6 https://lindahlcompositedesign.weebly.com/lr-6.html

 

Hello Felix,
If you'll build in foam core, an easy way is building in vertical foam strips in a female mould. Farrier trimarans are built this way (a lot of them also being built in the Netherlands so you could have a look how it's done).
In short : you build a female mould from stations and fir battens lengthwise. Then you bend the foam in (vertical strips, about 20 cm wide) using a litle heat. Seal the gaps between the strips with thickened epoxy. Laminate the inside. Glue in the bulkheads. Release the half laminated piece and reverse the mould frames. make the other half of the hull. Then glue the two halves together, and fair the foam outside (schuren). That is easy and pleasant work because the foam is very light to sand/fair. And last laminate the outside.

On this blog (my brothers's boat building blog) you can see how it's done, and also how the "poor mans vacuum bag" (the laminating with plastic sheet to get better laminate quality mentioned a few posts back) works.
F22 trimaran building by Menno http://f22bymenno.blogspot.nl/

Regards, Hans

 

Hi instead of using carbon fiber that is pricey, I've been using basalt and found it to be an absolutely amazingly strong material. It's fairly new to the market although it's been used for a CPL years in the construction market. You will be amazed at it's strength and it does not mold or oxidize and weather hardly affects it. It's volcanic ash.

 

Well, not Ash. Rock. High in Silica and Low in Iron. eg What is Basalt Fiber - Basalt Fiber Tech - Main Section http://www.basaltft.com/hist.htm

 

Sorry for the late reply. You guys have certainly been a great help! I do need to pick your brains once more if i may.

What density of foam would we require to CNC cut the female mould directly??

We have access to a CNC machine as well as some building space in a shipyard thats kind enough to let us use any other equipment necessary.

 

The foam density typically isnt a question for the forum. For a start, is the foam part of the structure? If so, your engineers will specify it.

 

Wet feet, maybe you could shed some light on this, i just imagine the foam would need to be dense enough to be able to pop out a smooth surface. Or maybe to coat it in epoxy, but then again a lower density would just absorb this.

Would you have any more tips?

 

Generally polystyrene foam(styrofoam) is used in CNC machining of female moulds. Depending on the size is can be done in blocks and glued together. This does require additional coatings on top of the foam to ensure a smooth surface. It all depends on the level of finish you desire and the lay-up method you wish to use?

 

The type of foam depends on the surfacing filler,which in turn depends on the budget for the project.The polystyrene recommended by our Australian comrade is indeed in quite common use.The downside is that you need coat an oversize cavity with epoxy paste which is subsequently machined to the desired shape and then painted and polished.Epoxy in Europe is rather expensive and a polystyrene mould is very limited in terms of the number of mouldings you are likely to extract before pieces begin to flake off.You could use a poyurethane foam and skim it with polyester filler and then paint for a less expensive job and in saying as much on a forum,I expect a barrage of criticism-but it can work for a few components.If you need dozens,you just need to make a heavy layup at say number five or six and save it for a conventional plug and mould-making process.I wouldn't recommend foam of less than 35Kg/cu.metre and even this is a bit light.
Depending on the intricacy of the shape and the depth,not to mention the risk of the router motor or collet fouling the job you could cut the mould from slabs of 50mm MDF and then apply Durabuild or similar.A bit of effort at the CAD modelling stage can allow you to break the job into slabs form which the sections can be made rather than turning the pile of MDF into dust.If the slabs are made 10mm over size it can save a lot of time and material.If you can glue two layers of 50mm together with joints staggered and then machine the surface and ideally a few dowel holes for aligning the stack of machined slabs,it isn't too hard to achieve an accurate outcome.If my description of the process isn't clear enough a search for "bread and butter hulls" for model yachts may yield helpful diagrams.

 

Thanks for the input guys! We will be finalising decisions by the end of the week hopefully and starting the build in about a months time. At which point i will try to upload as much info on the process and methods as possible.

At the moment it looks like we will be going with a wet layup and vacuum bag method, where i have been told that even a lesser quality foam can give a good finish once coated nicely. Cutting the foam into blocks, stacking them up and then milling will definitely be used to save costs. We might still be go with prepreg carbon, in which case we would mill a plug and then make a mould from carbon to ensure that hull and mould expand at the same rate when going in the oven. The two options will be analysed costs wise soon so i will be able to share those results.