Creating a compound curve mould with vaccum or pressure alone.

[Skint For Life]

Hi guys,

No experience working with vaccum infusion etc. I'm just interested in it. I had an idea for creating compound curved hull bottoms, topsides, nose cones etc. To work in conjunction with single flat panel hull sides. I've searched youtube and the forums and I can't find anything like what I'm thinking of so I thought I'd post it up for your thoughts on it.

It is my observation that vaccum infusing one piece complete hull sides is a great idea, but what about when it comes to all the compound curved parts? strip plank? strip foam?

I thought about natural ways that compound curves are created, an inflated balloon is a compound curve, and pretty much the basis of the idea. Once the desired compound curve is designed, a frame is made that will most likely produce the desired compound curve, an inflatable membrane is placed between the frames, the membrane is inflated till the desired shape is attained, the membrane is then left inflated at that pressure while a layup is done on it, either the finished product is created or a mould created for vaccum infusion. Using the same idea but a vaccum rather than postive pressure will create the opposite mould.

Just rough pictures to show the idea, the first picture shows two full hull length infused flat panels, the second shows the inflatable membrane placed on the flat panels and partially inflated, the third shows the membrane inflated more. The reason I used a line drawing is so that I could show the uniform curves, to give a realistic impression of shapes that could be created. Would it work? you tell me

[rwatson]

yes - it will work, and has worked for many projects.

The problem is how accurately you can inflate things, to get the hull design you want.

[gonzo]

It will most likely require more fairing than other methods, but it will work. If you use a core, it will help a lot to produce a more fair surface.

[Skint For Life]

rwatson. Thanks for that. Could you please link me to projects or pictures using this method? What projects do you know of that have used this method?

gonzo. Thanks for this info Can you please explain your comments? Why would it not be just as fair as an inflated balloon is? Why would it be better with a core? BTW I'm imagining the finished surface is the one contacting the inflated surface, if that helps to clear things up.

I was hoping that the method would produce very fair compound curved panels, just as fair as flat panel layups, the idea being that all the parts of the boat get taped together and there is very little fairing required other than the joint areas.

Thanks guys

[JRD]

The balloon is fair because its round in section, there are no flat edges to deal with. Even if you lay it flat in profile, the changes in breadth are smoothly radiused.

The ultimate shape will depend on the elasticity of the membrane material, the span and how you restrain the edges. Also the length to breadth ratio will affect how the part deforms due to differring amounts of tension in each direction at equal pressures. As RWatson mentions at end of his post, its hard to control what shape you end up with.

If you wanted to influence the shape of the keel line for instance you would have to alter the tension or the edge clamping along the length, to let it move more where you want maximum rocker. However would that give you more rocker or more compound curvature?

At best it would be like using tortured plywood to form shapes, repeatable but with alot of trial and error to get what you want, and most likely some shapes you want would just not be achievable.

Why dont you try it out an model scale, all it would take is a plywood waterline profile (your chine), inner tyre tube and a bike pump. I dont think you will easily get the result you are after, but it would be entertaining to try if you have a couple of spare hours.

[rwatson]

I have used 'tensioned fabric' formers myself, with mixed results.

I don't have pictures now, but I had to 'fill in' a pod on the side of a Proa, and the edge of the pod was a square, that had to intersect with a double-curved hull. ( primitive sketch attached)

It would have been hard work to make a mould to handle the compound curves, so the easiest way was to stretch nylon between all the edges, and pulled it really tight. Then I simply put several layers of fibreglass on top of the fabric.

This 'filled in' the 'hole', with a nice curvaceous 'deck'

I tried the same with the skeleton of a kayak, light fabric pulled tight over a frame. I laid fibreglass on it.

In both cases, the curing Polyester resin distorted the surface of the fabric, and cured 'bent', which mattered little for the Proa deck, but looked really bad for the Kayak.

If in both cases had used some inflation to 'stiffen' the fabric, it would have resisted the distortion from the curing resin, but then, it would have also altered the shape. If I had inflated the kayak, it would have stopped the curing distortion, but created a 'fat boy' effect where the fabric bulged from the air pressure.

The proa deck could have been enhanced by laying plastic bags with sand in them on top of the fibreglass, to force more bend in the pod.

So, laying glass on tensioned fabric works fine, but the fine control over the finished shape is the problem, as JRD explained so well.