Hi,

I am pondering a new boat build which will, in all eventuality, comprise making the hulls (it's a catamaran), foils, beams and maybe even a mast. I am pretty impressed with the Easy-preg option for an off the mould finish, ready for paint directly - I don't want the weight penalty of a gel coat if possible. The hulls would be a Nomex sandwich and I assume that this would be OK with a 215 gsm vari-preg layer (don't want to go to the 430 or would the experts recommend a resin layer? If so, where do I get this as there is nothing on the Easy Composites site that looks like it fits the bill. I'd like to work on a single vacuum / over cure rather than doing curing the layers individually.

My second train of thought is how to incorporate UD layers in the vari-preg system. The cat's beams, foils and mast will be a UD dominated laminate, so I will need to work this out! Again, is there sufficient resin within the vari-preg backing layer to co-infuse UD layers, or again will a resin layer be required?

Got to build an oven of course, but I'd like to get the construction resolved in my head at least.

Thanks for the great tutorials.

Cheers

Andy

> this would be OK with a 215 gsm vari-preg layer

You want to build a boat with only one layer of 215g as sandwich skins? This will not provide sufficient strength unless used in a small RC model. This might be barely sufficient for an aircraft traveling through air, but certainly not for a boat going through a dense medium like water if this boat is supposed to carry human beings.

For a real boat prepreg and nomex is way too expensive and will not offer a lot of advantages over more traditional building materials unless you want to compete in the America's Cup.

> I don't want the weight penalty of a gel coat if possible.

A boat needs to be able to swim in a medium with an approximate density of 1g/cm³. Gelcoat has a density of about 1.2g/cm³,  so there is no real weight/buoyancy penalty.

I don't want to be rude, 

Thanks for those positive and helpful replies. I know of plenty small (14', 4.3m) racing dinghies with a 200 gsm glass skin on PVC foam which are constrained by their rules on materials but proving fast, reliable and long lived. The boat I am thinking on is not limited by materials. I have an old 1990's vintage A Class Catamaran on my back lawn which is build in nomex and carbon & kevlar pre-preg; so please do not be so dismissive of the technology or the application or my skills (which you have no knowledge of) or belittle my thought processes.

It wasn't clear in my post but I was working on an Easy-preg on the as a first layer, with the 215gsm vari-preg as the second layer which would be the layer that the nomex would attach too. That gives a 430 gsm / 0.5mm outer skin.

I also thought that this was the Easy Composites tech support, so I was looking for guidance on whether the nomex would need a resin layer which is something that EC do not apparently sell or at least, I cannot find it on the site. Similarly, if I go down a PVC core route, this would probably need a resin layer to successfully sandwich the core. Thoughts on this would be appreciated, irrespective of any "opinion" on my project.

To quote Uffa Fox: "weight is only of use to road rollers".

Sorry if we sounded a little harsh, but please remember that not everybody here is posting in their native language.

> I know of plenty small (14', 4.3m) racing dinghies with a 200 gsm glass skin on PVC foam

This is very thin, such a sandwich is easily damaged and the first thing that everyone thinks of is "let's hope this boat never touches a rock". And of course many people will have the urge to voice this opinion.

> with the 215gsm vari-preg as the second layer ... That gives a 430 gsm / 0.5mm outer skin.

That certainly sounds better than 200g. Maybe 2 layers of easypreg could supply enough resin to bond everything together?

Let's see what the EC staff has to say.

That's the wonderful thing about free speech & open forums - everyone can express an opinion!

I know that I could always use the Gruit pre-preg system that has surfacing films, UDs and core adhesive layers, but these are all additional steps in the process. The EC system suggests that some of these steps could be eliminated, although the range of reinforcements is limited.

You could get away with a single thin layer with a closed cell foam as it won't collapse so much if damaged, it will dent and still provide some strength whereas a honeycomb would collapse and not provide any strength after impact.

Sorry if I didn't express myself well, I wasn't doubting your skills its just that I dont really see the point of using prepreg on bigger parts like boats and so on...

This is not me being critical but I have to agree with Matthieu. Out of autoclave pre pregs need carefully controlled heat cycles to perform well. It is not an easy task building a DIY oven of that size and to be able to control very accurately the temperature evenly throughout it and achieve temperatures in excess of 100 degrees C.

I did give infusion a good deal of thought - it is a process we use at work and I know well, although we inject the (polyester) resin as well as drawing it through with the vacuum. The largest parts that I am looking to make would be a maximum 6m long by 0.5m wide by 0.2m deep (i.e. half an 18' hull), so we are not talking about a huge item to lay up & it is a pretty simple laminate; so the out life of the prepreg should not be a factor.

I'd still need an oven to post cure the infused laminate, although it might be at a lower temperature, just to avoid the inevitable core block print through. The oven would still need controls, etc. so not much saving there either. I was woring on a scalable design for the oven so that I could build it in sections, say 2m at a time. I could then "practice" on some smaller parts - rudders, etc. - before working on the larger parts. This would at least give some confidence in the controllability of the oven and reduce the likelihood of scrap parts.

I could go down the route of the Gruit SE84 prepreg with the associated / compatible surfacing resin, adhesive layers, etc. This is an 80 degree C cure. Additionally, there would be no open resin mix and hence a much lower smell issue (I am looking at working in my garage in a residential area!).

Thanks for the continued contributions, all food for thought!