Tortured composite panels?

Foam isn't all that flexible, and once a skin is attached to one side it will be less so.

It will depend on how thick the foam is. The thinner the better.

Scoring the foam will help, but will take time and add weight and cost.

 

The issue here is that the skin is itself not going to be amenable to compound curves to anything beyond minimal, you will be able to bend the panel to tighter developable shapes than a panel with a skin on both sides, but the addition of even a single skin, loses the ability to conform to compound curves, which non-skinned foams will do to varying degrees according to the material and its thickness. The old Airex foam, which was one of the early PVC foams, was relatively pliant and would take up compound curves better than the stiffer PVC's. But if a thin GRP panel ( no core) won't take up the shape you desire to torture it into, then neither will a sandwich panel with that thin GRP skin attached.

 

Maybe I'm misunderstanding what KSS method (Kelsall Swiftsure Sandwich) does but that seems to be exactly what Robert is describing:

http://www.kelsall.com/TechnicalArticles/WhatIsKSS.pdf

From what I understand basically you are pre-tensioning the fiberglass which can be elastically pulled like 4.8% elongation at break. I assume this would be better done with epoxy not PE or VE because epoxy is also more flexible so the skin can stay intact (although if you do it with PE or VE that is still a bit "green" and not fully hardened it might not matter.

You cut relief into the foam so that it creates a kind of fulcrum for the compound curvature to stretch into a compound curvature. Then laminate the inside, filling up those darts too I assume.

Rob Denney has tried that method but then switched to just using developable surfaces and designing for "intelligent infusion". Which just infuses on flat tables or very simple to build moulds with large fillets and with all the dimensions and things pre-planned to just glue together without much work. I think this allows more precise planning of positioning of interior bulkheads, hatches, shelves and doors etc. KSS hulls would never be exactly the same and a bit harder to design in software for.
I haven't used it, but if the moulds are simple enough to build then it should be faster than KSS since you infuse both sides in one go. But you only need moulds for the hulls anyways.

But it should also be no problem to combine both. Like you could make just the bow with compound curvature and build with KSS and the rest with developable surfaces.

I also recently asked if a foam core panel with laminate on both sides could be bend into a gentle curve like his proas, and he says yes they would bend enough for his 80' boat. That is like 0.5meter deviation bend over 6 meters. I imagine though that structurally, that is not the ideal.

PS: So technical difference could be that afaik in tortured or bend plywould / wood you compress the wood. You don't stretch the fibers, you compress the inside of the wood. With KSS you probably do stretch the fiberglass fibers and also compress the core a bit. But that is speculation on my part.

 

well, you need to ask another question first

what advantage do you expect from tortured panels?

The answer is to speed the process. That is the reason.

The slow part of boat building is fairing. If you can reduce fairing by laying the hull without lots of glass variation; you've won the goal.

 

Ignore all this discussion about conics etc.
Look at the Gougeon book on building wood structures.
One chapter is about tortured ply hulls.
You can't just bend ply or laminate around any old frame structure, most people who do that end up with broken ply (or laminate).
The shapes you can get are very limited and are generally found by trial and error. There is some info about a model building process which reduces the risk, but I don't know how you translate ply to your laminate.
If the laminate will survive the torture then a foam core will almost certainly remain intact. Thinner foam will be less risky as said above.
There is a risk that the thin inner laminate (I'm making an assumption here) will buckle and crack where a piece of ply (with greater thickness) will be successfully tortured into shape.
You may also need to add reinforcement locally on the inside for special features after the hull is completed.
Usually you need to remove the foam and add a hard material where ever you need penetrations or fasteners thru the hull.
Any interior bulkheads need to be fitted after the hull is completed.
Experiment and experiment again, unless you are a lot better engineer than I am.

As Fallguy said, fairing is tedious and you will be doing the entire outside laminate.

Let us know how your experiments go.

All that assumes you are trying to get a 3d final shape and not flat sheets bent in simple curvature ( which has been done and proven lots of times).

 

It isn't the foam core that will resist the torture, it is the single skin, and it won't matter how thin, it still won't happily take up a compound curve, and what have you achieved by it anyway ?

 

The process, in my opinion, makes sense for smaller craft, but not larger.

 

KSS method seems to have worked for them from 8' to 100' boats.

I figure larger boats = less torture

 

Like I said, the only achievement I can see is speed of completion.

Wet one side of light fabric on all panels.

Assemble the boat, perhaps the third day.

Day four install bulkheads.

Flip the boat. Finish it in say a week.

Flip it back over, a few weeks and something like a dinghy is done.

 

Yeah, but.

The larger boats have bigger panels and require full bagging, like what I did on the Skoota. There is no speed gained by one siding....unless you can laminate a whole hull as a result

 

Yeah I agree (although I still don't have experience).

I don't think there is a critical advantage to compound curved hulls for multihulls except that it might sell better. Because people want it and maybe think it is faster or looks better.

KSS advantage is that you can build one offs without a compound curved mould and without a strip planking. You don't need to fair but you still have to hand laminate the inside. I don't think you can vacuum bag or infuse the inside with KSS.

Overall developable surfaces in simple molds (Skoota / "intelligent" infusion) should be faster and more precise. Or easier to design and build precisely (and probably uses less resin). And hydrodynamics should be just as good.

But for some areas combining this could be useful. Like for the bow, if you have slightly flared out sides but want a plum / axe bow. Then you need a very slight compound / convex curvature. Or for creating a round canoe stern to have a smooth waterline. So for those areas only you could have that part single sided and torture it into place. I think...

 

You guys really need to look at the Gougeon book closely.
You don't get to choose just any shape.
Round canoe stern - not on a monohull. Too much curveature.
Oh well, too many people won't believe even when its been done.
And won't believe the people who did it.
That means I'm assuming single skin foam will work like ply.
Of course I suggested single skin strip planked done on a flat table for a sailing kayak, and that worked. Not the same as the OP suggested but got similar can't be done responses.

Dejay, why do you say compound curvature has no "critical" advantage. A little backup to that statement would be interesting.

Mr. Efficiency - have you ever seen a Tornado catamaran? It was initially designed and built in plywood, and it took the compounding without a problem. Of course the design was done right, in the 6o's. I've never heard if it took multiple tries when it was first done. Anyone know?

 

Well this might be ignorant but I thought for multihulls anything you need a hull to do can be achieved with developable surfaces. Maybe not 100% as good but no critical reason to need them.
But what do I know? Maybe with rocker this is more important?

 

Yet back in the 1930's to 60's it was far from unknown for capable boatbuilders to build a complete dinghy and launch it in less than two weeks.They would have had the advantage of patterns for some of the parts and a good deal of skill.I see very little application for the process being described here as plywood is a good deal more elastic than composites and if you try to distort composite panels you are inviting micro-cracks to form.I doubt they would lead to a long lived boat and it the total context of the cost of a boat,the hull is not that significant a part of the whole amount of time and effort,particularly when you get above dinghy sizes.

 

oh, i am certain you can build a dinghy in two days. Woods did it.

But perhaps not an ultralight dinghy.