Wrapped foam plank

I'm wondering about techniques to eliminate the need for stringers. One aspect of foam sandwich construction has always bothered me, and that's the weakness of the foam that joins the two surface plys. I've toyed with ways to enhance strength and stiffness, and the thought that keeps returning is to lay up "planks" in an infusion layup. Each plank would be wrapped with wide fiberglass tape on a 45 deg bias, or simply wrapped in biaxial cloth. These "planks" would be pushed tightly together. The result would be two plies of bias fiberglass joining the two surfaces, at each joint. This would be infused with both surface layers on a surface closely matching the curvature of the boat side, with a fair surface corresponding to the outside of the boat. Essentially a female mold of one side of a flat sheet catamaran hull with a curved flat sheet bottom infused separately.
The rationale.... or would that be "irrational" for doing this is to achieve similar stiffness to having stringers without having to go with considerably thicker foam or more glass plies. Obviously most of the fiberglass tape wrap would end up being part of the surface layup. Essentially what you end up with is like little I beams joined to two solid surfaces, much like the aluminum decking that is used in many delivery vans. My experience with this stuff is that it is extremely strong and stiff for it's weight.

H.W.

 

You can buy foam with fiberglass imbedded in it. However, the structural foams available do a good job. As long as the weak link is strong enough, no problem exists.

 

Hi! I'm trying to figure out someting similar in the thread just below. Only with a slightly different approach.
Unfortunately it is quite complex and counterintuitive. In traditional construction, like alu skin on stringers. The skin is the "tough" surface, and the stringers add stiffnes. In a sandwich it is almost the opposite. The skins add stiffnes and the core absorbs impact forces. The Visby class warships use Divinycell HS linear PVC foam between carbon skins to handle slaming, impacts and mine blasts. The linear HS is slightly less stiff and has better energy absorption, compared to regular crosslinked PVC.

 

On the other hand, structural elements like stringers and frames, add strength. Only in that case, the boundary becomes a place where failure is likely to occure. It's usually refered to as a "hard spot". It might become more succeptible to impact, even if slightly stronger at static loads. It would almost be a sort of honeycomb. Important in honeycomb is it has hexagonal or triangular cells. Walls in three directions. Also, it has small cell sizes to mimic a homogenous material, and spreading the forces over the skins evenly. Avoiding hard spots.

 

On the other hand, by limiting panel sizes, you can gain some strength. I e tightly spaced frames. But as pointed out earlier, there is a difference to a backing frame, and an "internal" one. I am still trying to figure out if your suggested method, could be considdered fixed panels, in calculations, or freely supported. Would be thankful for pointers.

 

Here is a good startingpoint for the theory bits.

 

If adding internal stringers, i believe it would be a good idea to have the foam plank edges rounded, just like the strip planks. One side concave and the other convex. Also it would need to be a flexy resin, like epoxy. That way hard spots could be eliminated, while still adding shear, compressive, and tensile strength, and maybe to some extent, decrease "theoretical" panel size.

 

Then again, it would add complexity, and there should be a reason for it. One could be the wish for use of much cheaper, but inferior foam like XPS.

 

The whole point of foam sandwich is to eliminate stringers.
As for wrapping planks in glass it would want to be a big boat to make that viable and how would you spile them at the ends ?
What you have to do is get your head around the foam sandwich concept and trust the engineering.
In small boats where the skin thickness gets a bit fragile foam can have bashability issues you may be better off with ply and stringers or strip plank, at least from a peace of mind viewpoint.

 

Foam sandwich build pic.

No stringers per se. bulkheads and bases is all.

Sorry for the photo angle. The transom gets two knees...probably not needed.

I might put a shelf in the long 1750mm open area (hull cabin). It would behave like a stringer, but also be a nice place for stuff. The 2000mm space for the bunk has the bunk as a stiffener. Not required, but dual purpose.

The exterior is rebated at the tape seams to simplify fairing. The rebates are 2.5mm deep.

Pay attention to what redreuben said. He iterated my thoughts.

For scale; the hull you see is walkable, 32' long less 6' of bulkhead areas forward as shown. Height is about 5' 6" as shown.

 

The idea here is to convert (and lengthen slightly) a plywood design to a foam design, eliminating stringers, and spacing some bulkheads wider for more usable interior space, and not having to use really thick foam, or a lot of glass plies to compensate.
A very good point was made about impact. The internal "stringer" is a hard point that cannot flex at all without damage. Foam sandwich should allow some flex before failing at the interface. We have two competing structural needs here. One is stiffness to retain shape, and the other is flexibility in an impact situation to be resiliant and avoid failure.
**** below a bit off topic ............. context ****

This is a 30' catamaran hull with single curvature (longitudinal), and not very much of that, so it lacks the natural stiffness of a compound curved hull. Perhaps the better solution is a thicker core, and or more plies of glass. The idea of going to sandwich construction is simplicity of construction, and weight reduction. It would be infused on a simple female mold with the curvature built into the mold made from slick faced MDF. Done properly this should eliminate finishing except at the seams. I 'd like to rebate the foam a distance out from the seams to facilitate making a neat flush seam. The hulls would be "flat" (laterally) bottom, straight aft from the bows to past the center, then swept up to the transom at waterline in a gentle radius. The only curvature being longitudinal. No chines. Good load carrying, simple construction, and the ability to use the hull bottom as the sole in some areas for more head room and lower windage. In the largest central area, which is also max draft, (galley, etc) there would be bilge area for low CG stowage of heavy stuff, and an elevated sole. The hulls and bottom would be 30+ foot panels. The formers that give the table it's shape would be flipped end for end when laying up opposite sides. A single sheet plastic template would be used to trim all four sides to identical shape. The bridge deck cabin cutout would be applied to all four sides, with an abbreviated wing on the outboard sides to provide additional workspace in the galley and chartroom / workshop, and aft berths.
I'm just toying with this, in the early scale model building stage.

H.W.

 

Owly, have you read Derek Kelsell and his Kss method ?
http://www.kelsall.com/UniqueKSS/WhatIsKSS.pdf
Here’s a rough guide to substituting ply for foam.
Plywood/Glass strength Comparison | Multihull Design Blog http://multihullblog.com/2010/11/plywoodglass-strength-comparison/

 

Carolina Skiff used a 'plank' type system, although they called them 'logs'. They did it with a hand layup, but I imagine infusion would work.

US4495884A - Boat construction and method - Google Patents https://patents.google.com/patent/US4495884

Nida Core makes a product called NitaFusion STF (or STO) that might be what you want.

New sandwich pressure-tank trailer concept for powder goods | JEC Group http://www.jeccomposites.com/knowledge/international-composites-news/new-sandwich-pressure-tank-trailer-concept-powder-goods

 

you guys are posting some great material. I'm enjoying the links. One question I have about one link is the plywood to sandwich equivalencies. The numbers go like this for example:

6mm ply equals (22 oz ) 750 gsm w/12 mm core

It states that: "Assumed that triaxial amount is on both sides of a core"

Does that mean 750 each side, or 375 each side? I assume the GSM is glass only?? About 24 ounces of cloth on each face..... Probably about 10.5 pounds total per square yard total weight........... Does that add up?? Presumably the glass is considerably heavier on the outside than the inside.

About 37.3 lbs for a 4x8 sheet............ Seems heavy to me. Perhaps I calculated wrong. I was shifting between GSM and ounces and pounds. When I get home tonight, I'll spend a bit more time figuring this. The results don't add up to my expectations...... That usually means my calcs were wrong, or the inputs were wrong.

Then there is the issue of structural bulkheads where lumber and ply are both used.......

One thing is sure, I'll have to do some test samples.

H.W.

 

Read the Kelsall PDF in it's entirety ......... fascinating . The "dart cuts" to achieve he compound curves offers up the tempting alternative of making the bottom with only the central seam. The sides will curve obviously, while over half of the bottom will be a straight line, however this presents a compound curved radius where it joins the sides. The aft end being a very large radius gentle curve would present no more difficulty really than the sides, however I don't see how the exterior glass surface can take on a compound curvature once cured?? It must stretch or compress through the compound curve. This seems like a very limited potential..... Of course epoxy will soften significantly with heat.....The stiffening offered by some compound curvature on the bottom would be an asset. I would like a more or less flat bottom, and I doubt that this degree of compound curvature can be achieved. It would have to be a well rounded shape to work with the KSS system obviously.... lots of radius.

H.W.