moldless foam core High Performance hull

I am foolishly planning to build a fireball, a fast 16ft 175lb trapeze sailboat. The traditional approach is 1/4in and 1/8in plywood "stitch and glue". I seek advice on modern construction techniques. It has been suggested that 1/2in corecell (or divinlcell or balsa or nomex or etc) panels could be used instead of plywood. The core material would require glassing inside and out to create a sandwich. I have a bunch of questions.

Can I do this with just a jig - without a mold?
Do I need a lot of frames in the jig or would 3 or 5 frames suffice? how stiff/floppy/hard to work with are corecell panels?
Can I (non expert) succeed with hand layed s-glass or aramid and epoxy or will it end up way overweight without vacume bagging?
How many layers of what type and weight fabric with what resin - s-glass outside, hybrid carbon/kevlar inside, epoxy on everything?
How thick do the layers of glass need to be? 1mm? 3mm?

The genesis of my thoughts is the fact that old wood strip 505s weighed 280lbs, new cored prepreg 505s weigh over 100lbs less and are much much stiff and far more durable. But I am not talking about prepreg in a female mold, vacume bagged in an oven, I am talking about hand layed glass on a stitch and glued foam core on a jig. Plywood performs well, it is inexpensive, and any fool (me) can obtain good results, but I really would like a lighter stiffer high tech hull. What are my chances for success?
TIA

 

You can make this hull from strips of foam or wood. It will need a form every 50 cm because the hulls seem to have compounded curves. It will need another set of forms to make the inside wall of the cockpit. Think about making these two walls first, then making your hull on female forms (18 mm 3/4 partical board). Finish out the inside of the hull and bond in the walls before you remove it from the forms.

Perhaps look at light weight wood that is easily available to you. Eastern White Cedar for example. You can get the thickness from McNaughtons sheated strip scantlings ... also the carbon fiber thickness.

Hand layup will work. use epoxy

Once you have the surface area of the hull and parts you can calculate the weight of the boat. Make up some test samples to get the weight per area.

Where do you plan to get the offsets for the hull?

 

Since the fireball is a DIY one design class I would get the plans from the the Internation Fireball Class Association. The boat has hard chines and there are no compound curves. It is usually built with plywood panels that are stitched and glued together. My big idea is to stitch and glue the hull from corefoam panels then glass them.
The question is - how thick(and how heavy) a coat of hand laid glass would be required to build a strong hull?

 

Well, there is at least one compound curved surface on a Fireball - the deck!

Don't try to save too much weight on laminate thickness as you'll only end up having to put more lead in for correctors. And the class rules specify a minimum laminate thickness (although the Pro builders count their gelcoat, bog, etc in order to get up to that minimum required thickness... if they even do meet it).

 

core foam instead of plywood

I think a mold is necessary for your hull if you want to use core foam. You can build your mold from plywood as a female mold or male mold. In first case you will build a plywood hull having the frames on outer side to stiff the structure and you will put the core foam inside of mold, in this case your hull exterior will be smooth and will need no sanding, fairing or polishing just the inner of hull will need your attention. In a male mold you will put the core foam on the outside of your mold , you have here an example of male mold:http://bateau2.com/content/view/53/28/. In this case the exterior of your hull will need more attention then in sanding and polishing to obtain a smooth surface. Divinycell is good for you project, same is Airex too, just take care to use a core foam with minimum 80 kg/cubic meter density, this a minimum density for hulls required by ABS rules.
You need to use E-glass chopped mat and bi-axial roven. First layer near core foam must be a chopped mat layer, next will be bi-axial roven , so on ,alternate them.
You can use polyesteric resin in a hand lay up process or epoxy resin.
Polyester resin is cheaper, epoxy is expensiver but has much more mechanical strength and durability. Both are suitable for core foam and E-glass.
You need to calculate the thickness of inner and outer layer of fiberglass, the weight of E-glass, so I recommend you a book " The Elements of Boat Strength" by Dave Gerr where you will find out how tthe scantling rules to calculate your hull thickness in a sandwich composite using core foam.
The success is yours, don't give up.
I hope I was helpful.

Signum Delta

 

Class minimum is 1.5 mm for 2 skins = 0.75 mm/skin = 600g/m2 glass or 350 g/m2 Aramid. Use stitched BIAX for max. results, directions +/-45 and 0/90 to centerline. With a 1/2" core that is abouth as strong as the plywood version but much stiffer.

Only use epoxy and build a mould from the cheapest ply available.



Arthur

 

600g glass + 600g epoxy = 1.2kg
1.2kg x 2 = 2.4kg.
12mm divinycell ar 80kg/m3 = 1kg
2.4kg + 1kg = 3.4kg.

6mm Okume is less than 3kg/m2, approx 2.8kg.

 

It is hard to beat wood for weight in boats under 30'