January 2013

Frames and thermoforming

January 2013:  We took a week off following the New Year celebrations, and therefore have some catching up to do. Despite two weeks of frost we’ve still managed to make some progress…

Battens and Frames

As you might already have read in the ‘Building Method’ section, after positioning the frames, the longitudinal wooden battens then have to be fitted. These battens will enable us to form, shape and fasten the foam panels. Because we are using a female jig, we had to take into account the 20mm thickness of the battens by adding 20mm to the frame profiles.

The distance between the frames varies between 50cm and 100cm. The battens we used are 420cm long, the long length gave us the advantage of being able to bridge four frames at a time, thus giving us a nice, smooth curve. There are also fewer joints to make between the battens-ends.

The first step was to accurately fix battens left and right of the Center line and along the complete length of the keel. The Center line previously been marked out on the floor before position the frames. These two battens together form the keel, the backbone of the hull.

Battens on keel as a reference  Battens on keel

Schionning designed the G-Force 1500C with the home builder in mind. The hull above the water line is fairly straight compared to that below the waterline. This means the foam will be easier to shape and require less support. Battens above the water line could therefore be placed further apart. The hull below the water line is heavily curved, so the panels require more support, we therefore had to mount the battens much closer together.  Joining the battens is easy and doesn’t take long. It is however, important to make sure that they follow the profile of the hull; this will prevent bulges in the finished hull.

Reference batten on the waterline  Battens installed on the outside of the mold

Connecting battens at the back  Still a smooth surface for the Corecell panels

The bow requires the most attention

The bow was very tricky to lay-up because of the extreme compound curves. The hull bends round not only lengthwise, but also in height. Our 20mm battens are simply not flexible enough to follow these curves. This problem was solved by laminating two 9mm thick multiplex slats and bonding them to form a single batten. The missing 2mm was filled with a thin wooden strip.

Strong curves in two ways  two thin battens are more flexible to create the right curve

The outside of the hull will be foam stripped first. To make access easier, any internal battens will be fitted at a later date.

In mid/January we reached a milestone:
the two hulls were ready for installing the foam.

Building frame two hulls  51 ft catamaran

Working with Corecell, a nice challenge.

This is the first time that we’ve worked with foam, we did however do a lot of research beforehand. We decided to use as wide a panel as possible; this will reduce the number of seams in the hull and also reduce the chance of air leakage (see the section on ‘Building Method’).

Our ‘CoreCell’ panels were originally 122cm wide by 244cm long. Using a circular saw, we cut each panel into three narrower strips each 40.5cm wide and finished the strips off by chamfering the edges to 45° with an electric Router. Chamfering the edges makes filling the seams a lot easier.

Sawing Corecell with circulair saw is easy  Milling a bevel   Bevel can be filled with micro balloons later

Forming the Corecell foam panelsrengen

CoreCell foam is easier to form when warm. The optimum temperature for forming is about 60°C. Henny  (www.fram.nl) suggested we use a hot air paint stripper as a heat source. He said it would work up to a width of about 40cm, and that it would take a little practice, probably 6 panels or so, to get a ‘feel for it’.

The first panel went well, but was very tricky. The problem was getting the right amount of heat into the panel and keeping it there without causing local overheating. Foam cools off quite quickly and the panel loses its flexibility. Paul decided to see if he could find a way of improving the technique.

When Henny built his boat he used 16mm CoreCell panels, ours are 20mm. His panels needed less heat. One possible solution was to reduce the panel width to 30cm, but that meant having more seams. After a bit of head-scratching Paul decided to use two hot air guns mounted in a simple wooden frame. He was then able to introduce more heat and cover a greater area in one pass. After four or five panels using the new, improved double hot air gun we had it down to a tee.

Corecell thermoformen with fohn

Foam that has been formed should be kept in place while it cools. We temporarily held the panels in place using wooden slats that were themselves either screwed or clamped. The foam panel was then fastened to the battens from the outside using self-tapping wood screws.

It was nice to be able to work for so long without using a drop of epoxy. This is definitely one of the advantages of this method of construction.

temporary fiaxting foam panels with battens and clambs  Fixate foam panel at the back with screws  Easiest to start at the middle (less curves)

We started laying-up by placing the first panel roughly in the middle of the hull where it’s less curved. The next panel was placed to the left of that panel and the second to the right, slowly working our way out towards the bow and stern.

We developed an efficient routine. Paul first formed and clamped a panel, then moved on to the next; at the same time Jolanda ran a line of tape over the battens down what would become the seam for Pauls’ next panel. She then moved on to fixing Paul’s previous panel from the outside, always staying one panel behind Paul. The forming and clamping, and fastening and taping activities took about the same time, so we were able to work perfectly in sync.

The tape forms a non-stick barrier between the seams and the wooden battens, protecting the battens from any leaking resin.

We found ourselves having to modify the panels slightly as we got closer to the more curved bow and stern. This slight re-shaping along one edge was necessary to keep the panels vertical and maintain a good fit. At the bow and stern the curves became so extreme that we were unable to form the 40cm panels, we had no option but to use much narrower, shorter strips.

Make it fit at the curves  Smaller panels in the bow

Once we got used to working with foam we were able to work quite fast. Laying-up the foam panels took all-in-all about 90 hours.

You can see a slideshow showing step for step how we did it in the “How do you do it” section.

nice view

Timelaps thermoforming de foam panelen in de romp

Cold, cold and even colder.

We were hoping for a mild winter this year, but temperatures have been hovering around freezing for a long time now. Our plan was to have the starboard hull ready for filling the seams now, but it’s been too cold. The resin/micro-balloons mixture used to for the beading can only be used above 10°C and to do the epoxy infusion we need temperatures of at least 18°C. Final curing of the hulls will require heating them to 50°C for approximately 24 hours. This seemed like a good time to invest in thermal insulation for the workshop. Insulating the workshop would be a project on its own. We finally managed to install insulation panels over the 10mx20m work floor without using additional vertical supports.  Any supports that we added would get in the way of the hulls.

With the insulation in place we can now concentrate with the task of building a boat.

Isolating the shed