stitching and glueing foam core

[Troutcatcher]

Does anyone here have any experience with substituting plywood with lightly fiberglassed (one side only) foam core panels in stitch-n-glue construction??

Do you think this method will work???

[PAR]

Sandwich construction and stitch and glue construction rely on quite different engineering principles and concepts. One can't be substituted for the other without re-engineering the area(s) in question for the different method. If your idea was to save money with the use of foam, substituting for the plywood, it can work, but isn't as simple as just using some foam. In stitch and glue, the plywood is the structure, with sandwich construction the 'glass layup, separated by the core (foam) is the structure. One is basically a wooden boat the other is a 'glass boat.

[yokebutt]

Yes, it should work, although we strapped the sheets onto a stations-and-battens form for shape.

Yoke.

[Raggi_Thor]

Originally Posted by Troutcatcher

Does anyone here have any experience with substituting plywood with lightly fiberglassed (one side only) foam core panels in stitch-n-glue construction??

Just remember to add glass to bothe sides after the panels are bent to place.
The foam has practically no strength with glass on one side only.

Kvedja,

[JR-Shine]

I have built one of our plans (phantom 16) from foam core using the stitch and glue method. Do not glass anything until the foam is secured to the mold. You will have to add a lot of battens to the mold/strong back to keep the foam from sagging. The lamination schedule will be quite different than whats required for plywood. The boat will cost twice as much to build and will be a little lighter - assuming your just hand laying the glass.

[PAR]

I'm under the impression that the original poster wants to change a stitch and glue plywood structure to a foam cored structure with in his words a "lightly fiberglassed (one side only)" type of modification.

I can't emphasize enough how unwise this is. As should be easily understood, foam has next to no structural strength without both sides being covered with a substantial amount of 'glass. Even if this was done, there would be considerable effort required to keep the weights of the two different methods reasonable enough to insure similar hull balance to the original plywood structure.

To directly answer the original poster's questions, NO it's not a good idea as you have it envisioned. Yes, with some re-engineering of the structure, which very few amateur builders have. Redoing an engineered composite structure (foam core) isn't something that can be just "winged" and work out with a realistic level of safety. Folks experienced with composite structures can make an estimated guess, based on other builds of similar size and configuration, but would also be aware of the requirements of the method (like 'glassing both sides for example, and why it's necessary)

Troutcatcher, if you are not familiar with composite structures, then I strongly recommend you stick with the plywood, stitch and glue method, which will produce a boat with less effort (unless you like sanding 'glass a lot) and much cheaper.

[Windvang]

Yes it works perfectly well, I build my decks, cockpits etc. that way.

Prefit the foam and glass 1 side on a flat floor. To save time later you can finish the laminated side with filler and paint, but leave the places parts that need tabbing to hull or bulkheads untouched.

Next "spotweld" the part (laminated side in) on the hull with Plexus glue, Finally shape the outside of the foam and laminate the outside. I don't use the stitch method, tape, straps and weights do the job.

[zerogara]

There are development dinghy/skiff classes where epoxy/carbon/foam sandwich built boats in female molds compete head to head with stitch and bond plywood boats and if it wasn't for minimum weight the development of the building techniques wouldn't have evolved as much.
There are pros and cons of course as with any choices, but glued plywood is a very respected boat building technique.
So, my advise! If you have an existing design of plywood & glue it would take alot of engineering to bring a different composite to lower the weight, as someone else mentioned.
Sure you can substitute plywood for something stronger and heavier, alumnum, steel, concrete, or solid fiberglass, but the weight will go up.

[Troutcatcher]

Thanks for all the useful tips and suggestions!!!!!

I have an existing stitch-n-glue design. It´s 4.2' x 67" and weights about 500 lbs. That´s a little bit too heavy for frequent beaching.

The idea is to save weight by substituting plywood with the much lighter foam core without changing the building sequence much.

By strengthening the foam core panels a little bit with light glassing on one side before cutting out the neccessary parts I imagine the resulting material should be strong enough to be stitched and glued together just as it was plywood.

By doing it this way it should be possible to bypass the male molds and battens neccessary in traditional "one-off" foam core construction.

The rest of the process would involve glassing the hull inside-out and supposedly adding extra fiberglass as needed to compensate for the lack of the relatively heavy plywood.

P.S. In order to obtain further weight savings and to virtually eliminate airbubbles (not to mention the smelly and hazardous fumes) the more technologically advanced builders might even want to apply vacuum infusion techniques at the later stages of the process?!!?

What´s your opinion???? Do you have any experience doing it this way???

Thanks in advance,

JS

[yokebutt]

Trouter,

Never mind infusing it, not worth the trouble. What you can do is vacuum-bag the inside skin on a flat table before cutting the panels to shape, and then bag on the outside skin when the whole thing is assembled.

Sealing it up before bagging the outside skin is tricky, consider attaching the skinned-on-one-side-panels to the wooden frame with double-back tape, and then sealing all the joints carefully with putty for an airtight envelope. With that in mind, have the inside skin extend below the foam at the sheer a bit, so you can seal the bag directly to it.

Yoke.

[zerogara]

Troutcatcher: It´s 4.2' x 67" and weights about 500 lbs

Can you repeat those figures please?

[PAR]

Yes, you can add some stiffness to the foam by a light sheath of 'glass before assembly and sort of free form the hull shape, less the molds and battens, but shape control will be difficult, at least more so then tacking, taping, stitching or whatever to a mold in a more conventional fashion.

Then there's the shape issue. The boat is heavy, by your description. This isn't that adjustable, maybe 10 to 15%, but not by a lot. So, at 15% you'll save 75 pounds, not a significant amount in the big picture.

I think you're looking to save much more weight then this, hence the foam core conversion, right?

Back to the shape thing. The boat, as designed, should hit it's target displacement, assuming the intended structure, LWL, beam and draft are within the design parameters (you built it to the plans) Bingo a boat that weights 500 pounds, will have a performance envelope of such and such, with a level of safety and comfort that could be expected of the type. You with me so far? Lets say you want the same LWL, beam and draft, but 250 pounds instead, what do you think will be required of the shape (mostly beam and draft) of this boat to hit the new displacement target weight? All the calculations covering stability, performance and safety, go out the window without redoing the shape of this boat to reflect the new, much lighter displacement target.

In other words, aside from the complete redesign of the structure, from one of plywood to a cored build, you still have a 500 pound displacement boat, even if it only weights 250 pounds when you're done with the changes. If it's a flat bottomed boat it will pound the fillings out of your mouth in comparison to the original design. If it's a "V" or round bilge boat, it's stability will be "tender" to say the least, likely requiring ballast, which puts you back in "the boat is to heavy to beach easily" column again.

If you need a canoe, don't try to convert an outboard skiff into one and expect good results, it just doesn't work well, it's been tried.

Look, you seem to be a nice guy, with a genuine interest in getting a boat built. You've done some investigating on one off 'glass methods and techniques and are interested in a skiff, if my memory serves me (it doesn't always) There are several methods and techniques that have been developed for the differing types of 'glass and composite structures used in boats. Most of these methods have stood the tests of time and mother nature's occasional ill will. Select one, of the literally thousands of one off 'glass or composite skiffs (or whatever floats your desire) and have a go at it. Stick to the plans, because you don't seem to have the skills or experience necessary to make as radical the likes of changes and conversions you've requested. I build them, fix 'um, restore them and design them and it's not as easy as it would appear, particularly with composite structures. Every composite I've ever run across has been a highly engineered structure.

So, in engineering these new panels to replace the plywood, would you think a 400,000 PSI is a good enough flexural modulus rate on a epoxy matrix of a 12" x 12" square cloth laminate, say a 40:60 'glass/resin ratio a 1/16" thick? Or would 700,000 be a better suggestion? How about something easier like how much would that 12 x12 x 1/16 inch panel weight?

[Windvang]

Originally Posted by zerogara

Troutcatcher: It´s 4.2' x 67" and weights about 500 lbs

Can you repeat those figures please?

4.2 m. is more likely?

[Troutcatcher]

Your are right....the LOA is 14 ft = 4.2m, sorry!!