Composite panel design

Hello all.

My first post today as I thought I might request some input on a composite sandwich panel.
The power craft is basically stitch and glue but will be subject to heavy swells and hard use.

Okoume ply is specified for the core. My initial thoughts are Eglass on the outside of the hull under compression. Carbon fiber laminated on the inside under tension. Aramid tape may be of use on the inside seams and local high stress areas internally. This may sound rather simplistic but I wish to verify with some experienced builders before running FEA and finally building. Needless to say I want this hull light and strong.

I'm sure this has been discussed before but I am after some feedback on these materials used in the manners mentioned.

Cheers and Merry Christmas!

 

Welcome to the forum.

A lot of laminate design is dependent on the actual design specifics. Your ideas and questions seem to be not about cored laminate engineering, but about sheathings on a taped seam project.

There's no need for an FEA approach, if you have a set of professionally prepared plans. Even if you are making adjustments or a method conversion, you can apply simple formulas and guidelines instead of hours of assessment.

At this point it would be helpful to know what design you're intending on building.

Light and strong are fine goals, but usually more costly then typical approaches. Using plywood as a true core, isn't the best choice for "light and strong". Balsa, foam and honeycomb are the usual materials, in this regard. This also means you'll need an upgraded laminate schedule to get the strength and stiffness desired. This would be a full up method conversion (taped seam to cored composite), which is straight forward enough. Enter carbon, Kevlar, Spectra, etc. and your laminate engineering needs go up dramatically, if truly interested in light and strong, so does the cost.

So, what's the design and what are the things you're trying to achieve (SOR) with the modifications to the scantlings, as presented in the plans?

 

I disagree somewhat with this statement and I'll explain why. According to the engineering concept that I have, the engineering is the same for any type of fiber. Only the mechanical properties change, depending on the material used and, indeed, give rise to a lighter and more expensive structure due to the price of the material and, perhaps, the cost of labor but not to engineering.
Speaking of other materials, metal, wood, engineering is the same as for the fibers, but, actually, in the case of the fibers need to do some additional multiplications to determine the mechanical properties of the laminate. That's all, in my opinión.
I agree that it is very important to know the LOR and yes it can greatly affect the final composition of the structure.

 

The design is similar to this boat. A small power multihull, 20-30hp. The builder of this craft has used spare duflex(balsa core) material. He has since built two more almost identical craft using 4mm marine ply. Due to the thickness of the ply, the question does become one of sheething. My main goal as I see it is to increase panel stiffness between the limited number of bulkheads. While not an ideal core material,I would like to maintain the relativley thin 4mm ply due to its simple construction and sheeth to improve both strength, rigidity and durability. My main question is can the interior be sheethed in anything other than Eglass to achieve my goals? Will using other materials i.e. carbon/aramid be compatable given an Eglass exterior and thin plywood "core"? Will there be any clear advantages by internally sheething in carbon fibre? Will kevlar tape be an improvment over fibreglass for taping the internal seems? This is a very small boat so the increase cost of materials is not much of an issue.

Any thoughts or insights would be appreciated.

 

Post by Eric Sponberg

What about carbon inside the ull and glass on the outside?(essentially my question)

I did a technical paper some years ago that was published in Marine Technology of SNAME, Vol. 23, #2, April, 1986, pp. 165-174 called "Carbon Fiber Sailboat Hulls: How to Optimize the Use of an Expensive Material". It was also referenced by Larsson and Eliasson in Principles of Yacht Design. One of my references for this paper was another paper that I did before that for the Southeast Section of SNAME, Sept 1983, called "Fundamentals and Practicalities of Carbon Fiber Composites for Marine Applications." It became quite a bible for a lot of people in the marine industry.

I did physical tests and engineering analyses on 5 different laminates using unidirectional materials over foam core (4 samples) and one with balsa core (1 sample). In all the samples, the fibers were always running in the same direction, that is, I did not have any laminates with non-zero orientations.

#1: 3-ply S-2 glass, equal thickness skins both sides of 1/2" Airex core.

#2: 1 x S-2 glass + 2 x Carbon + 1 x S-2 glass, equal thickness skins both sides of 1/2" Airex core.

#3: 3-ply S-2 glass skins one side, 4-ply Carbon other side of 1/2" thick Airex core.

#4: 3-ply Carbon, equal thickness skins both sides of 1/2" Airex core.

#5: 3-ply Carbon, equal thickness skins both sides of 1/2" balsa core.

I studied:
a. computer predictions for strength and stiffness
b. test results for strength and stiffness
c. weight per unit area
d. cost per unit area
e. specific strength vs. specific stiffness (i.e. strength and stiffness divided by weight)
f. a desirability factor
g. Impact tests.

The results were:

Laminate #2 was the overall winner when considering all of the factors above. The all-carbon laminate scored very high for strength and stiffness, but at the time, a very high cost. The cost is still pretty high.

Laminate #3, where the S-2 glass is one side of the core, and carbon is the other side, was the worst laminate. It is not strong or stiff, and it wastes the mechanical advantages of the carbon, particularly considering its price.

In my practice, I always advocate for the balanced laminate. When mixing materials of different strengths and stiffnesses, it is always beneficial to make the skins mirror images of each other either side of the core. You get the most efficient use of the materials. This mirror image technique is quite fundamental to general composite design, and has been proven innumerble times in engineering analyses and tests.

I don't always mix materials, and every design seeks to satisfy different needs. As you are aware, the science is complex, so you have to pick and choose the best combinations for any particular design.

I hope that helps.

Eric

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Very interesting!

 

In my opinion, you should put one more bulkhead at bow in each hull, approx. 20-25% of the length.
On the other hand, again I say that is my opnion, if the board is properly sized, the fiber should be used only to get the hulls watertight and to give a good finished appearance. Fiber not necessarily has to be regarded as a structural element, therefore, you put that you like. If board is not strong enough to withstand the loads to which hulls will be exposed, you should make some numbers to determine the most suitable laminate or the most appropriate way to enforce boards.

 

My question now is, is it feasible to tape the internal seams with Kevlar first before sheathing the interior with Eglass? I would apply by hand layup using the wet on wet method to increase bonding between layers and reduce epoxy.

 

Yes, the middle photo is missing a bulkhead. There are two bulkheads forward of the transom.

Unfortunately I am working the design from numerous photos and a building blog. I have tried to contact the builder for more information with no luck. I have some rough dimensions in mind but am still to finalize.

Given the relative simplicity of the hull & lack of any formal plans I thought I might run FEA to see how the structure is loaded.

 

FEA makes no sense in this boat.
I´d forget Kevlar, and rare and expensive things. Place fiberglass, as it may seem to you, and if you have doubts about the strength of the hull, add transverse bulkhead, even lightened.
The tunnel should also carry 4 or 5 treansversales (rings) reinforcements.

 

You get more stiffness by adding stringers and ribs, unless you really up the panel thickness and sheath. It is much lighter too. Relatively narrow tall stringers will give a 'lot' of stiffness. On dinghies which have an option of thick floor or taller stringers/floor battens the latter are stiffer, both by calculation and in practice. One example would be 10mm floor or 6mm floor plus 25mm battens - the latter is way stiffer.

 

Agreed Ignacio, the engineering is the same, my point was cost and conversion related, more so than developing new scantlings of similar "strength and stiffness", per the OP's request.

With such a thin core, stiffness will be an issue. The "similar" design has me concerned as well, as different engineering approaches may seem similar, but rely on different thinking in regard to the scantlings. For example, your thin plywood core is a dramatic difference, compared to the "similar" balsa core used above, which is clearly considerably thicker.

I also agree with Ignacio that you can just skip the high modulus fabrics on the internal (and external) seams. 45/45 biax of sufficient weight will do. On a modest speed craft (less then 30 knots), using 1/4" (6 mm) plywood (5 ply), you only need a single layer of 9 ounce (300 GSM) biax on each side of the plywood seam. If you anticipate higher speeds and/or higher loading I'd recommend two layers of 9 ounce on each side, with a single layer of 17 ounce as a slightly less desirable option.

The easy way to a light strong structure for a boat like this is, as Suki has mentioned, longitudinal stringers, likely with athwart ring frames/bulkheads/partitions working in concert. It's difficult to judge size, but with the milk crate as reference, this is a small boat, say 16' or so. It's not difficult to make this structure stiff and strong. I would estimate I could build this hull, using conventional taped seam techniques and quality Okoume plywood and still bring the raw hull (fully decked) in for under 200 pounds. I'd use 3/8" for the bottom panels (all 3) and two longitudinal inboard sides of the tunnel, with 1/4" for the remainder, including athwart stiffeners. Of course, this is just a set of guesses based on previous hulls of similar dimensions I've designed and built.

Simply put, don't over think this puppy. Do a weight study, with a few method and material combinations and select the appropriate one for the SOR and budget you have. I also think, once you do a cost assessment of a cored composite build and a sheathed plywood structure, you'll find the sheathed plywood very difficult to beat, at this size (16' LOD), once you count up all the variables.

 

Well PAR, I think you could build her at 65% to 75% of that 200lb, and she'd still be strong enough. Mind you I'm assuming sub 35Kn speeds. I'd guess she is a lot shorter than 16'. Even strip planked with a 9mm 3/8" core she'd be pretty stiff with internal stringers and ribs. If you went nuts and had parabolic shaped stringers and ribs in light spruce and cedar maybe you could save more. I'd proabably use tapered conventional stringers on the joints (internally) but fillets are OK too. Then glass without the tape seam externally. As you say PAR 300gsm would be plenty unless really severe service conditions.

Key point is getting enough brace for the power unit so she does not twist and flex there.

 

Thanks for all your suggestions. I definantly feel more comfortable with 6mm ply on the hull and will redesign the tunnel structure. A weight of 200lbs is close to my original design weight. I will post a model with details in the design forum shortly.

 

With unsheathed 6mm you really don't want to go more than 8 or 10 inches between stringers, and that's pushing it. If you sheath both sides with 8 oz glass you will just about double the strength and you need to do that as a minimum. Stringers are also good in that it gives you something to glue and screw the panels to and that forces the shape to be correct. Stitch and glue is really meant for thicker plywood that keeps its shape and has a lot more inherent stiffness. Really thin plywood isn't that strong or stiff and stringers are really necessary in the joints to make a the hull stiff.

 

I disagree, smaller craft, using thinner sheets, just have tighter "compartmentalization" with these lighter panels, thus compensating for the relatively flimsy free spans between stiffeners.

I don't think the scantlings for this particular boat are terribly difficult to work out and even Geer's scantling rules could do a fair job of it.