# Lamination schedule: foam core with epoxy



## yobata (Jul 14, 2015)

if you wanted to use epoxy instead of polyester resin to build a one-off skiff that uses devinylcell as the core, what lamination schedule would be sufficient for outside skin and inside? Could you get away with a layer of 17oz biax and a 6 oz cloth for the outside skin? 

I have never worked with foam cores but have been told that unlike wood core, the foam does not provide any structural properties. The fiberglass layers have to be increased in order to make up for the foam.

Also, here is a link to a one off skiff build (but using polyester resin instead of epoxy): https://www.microskiff.com/threads/one-off-skiffs.42755/


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## topnative2 (Feb 22, 2009)

a willy -----
wr1-kevlar layup over mat
wr3 divnicell over kevlar
wr5--biax over dyvinicell

OK my spelling sucks......The layup w/ the DC is very strong because it is "scored" and the vynilester resin in this case supports the core by filling the "scoring". IMHO I am not sure that a epoxy layup is worth it?


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## weldandglass (Sep 29, 2009)

Yobata:
The use of epoxy resin doesn't allow for appreciable reduction in skin thickness for the hull itself. 
The assumption that one lamination schedule will fit a range of applications is not a valid one. 

As vessel size and speed go up, so should the lamination schedule. If you follow basic scantling rules (I like the scantling rules laid out in Dave Gerr's "Elements of Boat Strength" you will arrive at two very different sets of scantlings for a boat that is 18ft by 70in and will be powered with a 90hp and one that is 18ft by 54in and will be powered by a 25hp. To apply the lamination schedule of the former to the latter would result in a overly heavy skiff and to apply the lamination schedule of the latter to the former would result in catastrophic failure.

Use of epoxy gives you several distinct advantages:
1. You don't have to use mat in between layers of fabric (cloth, directional, etc.)
2. The mechanical properties of the laminate will be much higher, across the board.
2. Secondary bonding is much, much stronger compared to both polyester and vinylester (this means that anything laminated to an already cured laminate within the hull such as bulkhead and stringer tabbing, deck and sole bonding, etc. will be much less prone to delamination).

Back to the reason that epoxy doesn't allow you significantly reduce skin thickness:
I've explained this elsewhere in another thread but I'll briefly go through it again. In cored structures, the out skin provides the only resistance to puncture from significant point loads (think running over a sharp stick or T-post). Wave-slamming loads are distributed across hull panels more evenly and absorbed by hull panels and structural members (stringers, bulkheads, etc.) where as point-loads are basically absorbed solely at the point of impact. You can build a skiff out of high modulus resin and fabrics (epoxy, s glass and kevlar) with incredibly thin skins (less than .1" thick). This skiff would last a lifetime under normal operating conditions but will fail rather predictably (by puncture) when exposed to significant point loads. This is the real downfall of foam-sandwich construction and the primary reason why laminates in small craft can't realize significant skin thickness reduction through the use of epoxy. The rules change a bit for larger vessels. I'm an advocate of epoxy but it won't save you significant weight. It will, however, produce a much stronger stiffer and longer lasting vessel. 

Topnative:
the use of scored foam doesn't appreciably increase the strength of the core-laminate matrix as you've suggested. Resin uptake by scored foam is one of the major downfalls of such core materials. They uptake resin which adds weight but little strength. Scored foam is used for two purposes:
1. to contour nicely into a curved mold
2. to facilitate resin infusion 

Builders can save considerable weight by not using scored foam. When a panel is stressed, the outer skin in put into compression and the inner skin is put into tension. The center of the laminate structure, in this case the core, is unstressed and just there to hold the skins in place so they can do their job. This is why higher modulus materials should always be placed as far away from the neutral axis as possible and why cored structures produce stiffer but lighter laminate structures in the first place.


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## weldandglass (Sep 29, 2009)

As an additional thought:
If you're looking at doing a home build utilizing core (like divinycell) I would strongly recommend using epoxy. Boats built with epoxy resin, directional fiberglass (or higher modulus fabrics) and PVC cores are lifetime hulls, if built properly. The epoxy won't reduce weight considerably but the above mentioned advantages make it a worth while investment.

The lamination schedule you've suggested would absolutely work, up to a certain size and speed of vessel and with the proper bulkhead and stringer configuration. If you're going down this route, I would first and foremost make the laminate stack more balanced. If you were to use 17 oz. 45/45 bias directional fabric and 6 oz 0/90 bias cloth, you would have unbalanced stiffness between the 45/45 axis and the 0/90 axis. A 12 oz 45/45 biaxial layer paired with a 12 oz 0/90 layer would give you much better balance between the axes. 

If you're looking to produce your own design and your own lamination schedule, I would highly recommend doing one of the following:
1. Get familiar with a good set of scantling rules and follow them
2. Make a study of similar sized and powered vessels particularly with respect to core thickness, lamination thickness (of all structural members), stringer and bulkhead configuration, etc.
3. Befriend a naval architect and have him engineer the structure for you.

The lamination schedule is just one part of the engineered structure. In a properly designed foam-sandwhich structure, the bulkheads, stringers, cap, hull etc. would all have a different lamination schedule and may utilize different core thicknesses and types of fabrics. As the number of bulkheads and stringers increase and spacing of framing decreases, laminate thickness and core thickness can be reduced. As such, if you're just blindly applying a lamination schedule you're probably either building something overly heavy or under building the hull in key areas.


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## topnative2 (Feb 22, 2009)

Ah man! You are busting my bubble......

What would be "appreciable" stronger for the weight v. the scored foam or epoxy? Is the scored foam weak? V. what?
(The pics are of a setup for vacuum infusion. )

Kudos for your post....enlightening


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## yobata (Jul 14, 2015)

Scantling rules... is there a formula? I've found a few references, but there must be some calculation that would give you the layup schedule for inside and outside skin thickness on (lets say) 1/2" H80 divinycell foam. I guess I wanted to know the difference (if any) in the schedule for epoxy instead of polyester resins

For reference, the skiff I have in mind:







http://chrismorejohn.com/2017/05/04/conchfish-16-plans-for-sale/


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## weldandglass (Sep 29, 2009)

Yobata:
if it's one of Morejohn's designs then I would imagine he would be happy to provide the lamination schedule for the vessel. 

But since you're asking the question, scantling rules are a set of formulas that pertain to a particular method of construction, typically. You input variables like hull length, width, depth, intended speed, etc. For scantling rules covering foam-sanwhich construction, there's typically individual equations for core and laminate thickness of the hull, deck, bulkheads and stringers, number and spacing of bulkheads and stringers, etc. The scantling rules go as detailed as to specify width and thickness of tabbing, distance of fabric overlap a the keel etc. 

Because you're asking the question, you really should pick up "Elements of Boat Strength" Give it a read and build an excel spreadsheet of his scantling rules and you'll come to a much better understanding of laminate structures. ABS also publishes free scantling rules but they are much more complex. If you get stuck feel free to send me a private message and I can offer assistance. 

Top Native:
Scored foam isn't "weak". I was just suggesting that the resin uptake in the scores doesn't add appreciable strength to the laminated structure, for reasons previously mentioned. The ways to add "strength" (stiffness is what I assume we're talking about) a laminated foam panel is to 1. increase the core thickness or 2. increase the laminate thickness and 3. increase the modulus of the laminate/resin matrix.

There are several different brands of foam suitable for core structures in a skiff. Divinycell and Corecell seem to be the most easily obtained. In most of the brands, you can find plain core, perforated core, single scored core, double scored core, double scored scrim-backed core, etc.
Each has it's place and purpose. My point was that the resin uptake by the scored core adds significant weight while not increasing the stiffness of the resulting foam-laminate panel. if you were to measure deflection of two laminated foam core panels, one with scored and one with plain core, I don't think you would see any difference. I doubt any other mechanical properties of the laminated panels would be significantly different other than the scored core panel would be heavier due to resin uptake. Builders use scored core because it contours nicely to a curved mold surface and facilitates infusion, thereby reducing production time and cost.


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## Forcefed (Aug 5, 2016)

Awesome reads here weldandglass, thank you for dropping some knowledge!!


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