Thickness of a sandwich composite panel, and a few other related questions?

[Peter Griebel]

I might have some 600g panels around here; if you know the bulkhead spacings midship largest spans, I can give you some idea of what 600 does under load

my bulkheads are made with 600g

Fallguy, that seems like way to much trouble to put you trough, but thanks

 

[Peter Griebel]

Using the boat drawings/particulars posted, I can estimate that your largest panel size is 0.9 x 0.6 m. ISO calculates 7.7 kN/m2 of bottom pressure for a sheltered water operation. Round of to 8.

Using core sizing spreadsheet I posted here in the forum many times ago. With 9 mm Okoume plywood, the core shear required is 0.67, Okoume has 2. That leaves plenty of room with a safety factor of 2.

Encapsulating the plywood with 300 g/m2 of WR, derived skin stress is 72 (N/mm2), allowable is 76. Close but still within the safety factor of 2.

With a recommended 15 mm H80 core, I need only 0.134 N/mm2 of core shear stress. H80 has 1.15. Too much.

Let us try H60, 15mm with 600 g/m2 of skin lamination. Derived skin stress is 20 (N/mm2), allowable is 76. The skin is barely stressed. Problem is the skin thickness is only 0.7 mm thick. The minimum allowed is 1.5 mm.

With a 1.5 mm skin thickness of 600 gm WR + Biax, Skin stress went down to 11.3 N/mm2. Still strong but compliant.

Further evaluation shows you can go down to H45 10 mm thick, 8 kN/m2 pressure and still pass the criteria. This has been validated by a software.

There are many approaches.

Thank you very much for running the calculations . This is quite interesting, and even though I wouldn't be able to calculate this myself, I understand the reasoning behind. This minimum skin thickness requirement makes the foam core a second player. Maybe H60 @ 10 mm would be the safe way to go.

I will go ahead and buy the plans for the boat. I wish to change the rig into a free standing un-stayed rig if possible within a reasonable budget. I have asked the NA if he will help with this, if he will, it will probably take some time to make the necessary changes to the plans.

 

[fallguy]

1.5mm/25,4mm/inch is 0.060 inches

two skins of 600gram glass exceed this by about 0.010" based on my experience

@rxcomposite

What do you arrive at for skins of 600g biax and a 400g wr or 400g uni for thickness? And would this work with 15mm h60? It seems like a great option to me, because despite it being two different glass types; for hand laminating; he would lay the wr option second and be able to reduce his resin content for the final skin as the wr wets out so well. He may actually be able to wet out below 100% resin to glass. The uni will not behave as well. Less resin is a better laminate Peter.

I am quite certain it is at 1.5mm by hand.

Also, he should understand rebates and reliefs here for his tape seams. In a hard chine hull; those relieved seams will save a LOT of unwanted expense ito time to fair and fairing compounds and weight and cost.

Also, this boat is a prime candidate for female stations. Why? Because he can lay up the finished panels into the hull and place the bulkheads inside and have a rigid structure before flip like I did. And the work can be done walking insode through the transom/stern and that is the final piece which is so handy to work from bow back. Then the boat is rigid and can be flipped for the outside seam work. It id a dreamy way to do a foam build. The male stations are not as good here. I'd be curious your comment. It is a matter of rigidity. Otherwise, I think he needs to leave the male stations in to flip or risk losing shape.

Peter, this is a 2 year project for an amateur in foam.

 

[fallguy]

Fallguy, that seems like way to much trouble to put you trough, but thanks

I won't do it until you hash out the skins a bit more with the na. Let's just stick to the assumption they are too light at a single akin of 6oog each side for now. I have good intuition on this as all my bulkhead and cabin are built with these skins. They are terribly flimsy I'm afraid. The problem not mentioned here is too light a skin can fail fast or slow. An impact to 600g easily dents and on the bottom; the hull is subject to hydraulic delamination which happens fast. Long term, if the hull is flexing a lot; the same; stresses will create tiny delaminations and water pressure on them makes the skins come apart from the hull. And it could be a serious nightmare. Also, I'd never walk on 600g panels. I have 800g decks with 12mm on my boat; the decks are really narrow and flex quite a bit.

Personally, I'd see if you can drop to a 12mm core before considering the h45, and I'd avoid dropping to 10mm to allow for rebates.

For the same skins, a 15mm core is 1.5 times stiffer than 12mm. This boat with hard chines is already gonna be pretty stiff, but you may want to be strategic varying skin thickness. No idea your cockpit spans, but 12mm core is awfully thin and should have minimum spans of about 20". Places you walk with wider spans you may want 18mm core even. All my really wide spans over 20" use 800gsm both sides or wider spans use a 25mm core and 1700gsm bottom and 850gsm triax top. Those spans are 50"x50" unsupported to give you an idea. It is really nice to not have the sole or deck move under you a lot.

 

[Rumars]

Before you commit to foam consider a probably cheaper option, make your own ply. This might seem crazy, but cold molding is a normal construction method, and for this size boat it's perfectly feasible to make your own panels, either full size or in commercial ply dimensions.
I'll explain the process briefly.
First you need a local grown wood of similar weight and strenght to okoume, for example yom hom (Toona ciliata, Australian red cedar), champa (Michelia champaca), hinoki (Chamaecyparis obtusa, Japanese cypress), mai kok farang/waan (Spondias cytherea, Kedondong), mandong (Cordia dichotoma). Then you have someone saw and sand it into 3 and 4.5mm veneers with planed parallel edges. If the wood you buy is not klin dried, sticker it under a black plastic plane and put a fan at one end (solar kiln), then after a week or two it's ready to machine. The adhesive needs to work at room temperature, so either epoxy or a waterproof PU (D4 rating). The press consists of a flat concrete floor, two pieces of thick construction ply, plastic sheets your adhesive does not glue to, sandbags or water filled 200l barrels. Alternatively you can use vacuum, for that you need film ply or melamine faced chipboard, plastic sheet, butyl tape and two fridge compressors. If you want to make full size panels directly instead of scarfing them later, you need space for three ply sheets end to end. The process is simple, place plastic on ply, align the first layer of veneer so there are no gaps, secure to the ply with staples at the edge. Lay out and number the second layer, apply the glue, put the veneers in place, staple the edges making sure there are no gaps. Over it another plastic sheet, then the ply sheet with weights on top, or vacuum bag. When cured remove and cut away the edges with the staples. The bottom, deck and bulkheads get three layers of 3mm at 0/90°, the sides two layers of 4.5mm at +-45°, or the designer can provide you with alternative cold molding scantlings.

 

[Peter Griebel]

Also, he should understand rebates and reliefs here for his tape seams. In a hard chine hull; those relieved seams will save a LOT of unwanted expense ito time to fair and fairing compounds and weight and cost.

It took me some time to understand, let's just blame my English skills. I found a post on the forum from last year that explain the concept, it makes a lot of sense.

Regarding the female stations, if building directly on the bulkheads like Niels Helleman do in the picture I posted yesterday, I would also get a rigid structure before flipping the hull. The other benefit with building on the bulkheads would be that I could use plastic brad nails to fasten the hull panels to the bulkheads, and then just leave the nails in the structure. If building on either male or female stations, I would have to cut the the hut loose from the stations. I feel confident that you will let me know if my thinking is flawed.

 

[fallguy]

Building on bulkhead with foam is very, very sketchy and I advise against.

why?

Well, laminated foam does not behave so well and you will probably end up with uneven sides as the hull will sag between wide stations.

To build a foam boat properly requires a gap between the bulkhead and the side panels for bonding and to prevent hard points or high points. I am not certain how you can rest side panels on bulkheads and avoid building a 'lumpy' boat. Perhaps you could use shims?

But spacings beyond about 900mm or maybe a meter and the foam will start to flatten.

a female jig is more work to setup, but a much better way to build this design if using foam

 

[Peter Griebel]

Before you commit to foam consider a probably cheaper option, make your own ply. This might seem crazy, but cold molding is a normal construction method, and for this size boat it's perfectly feasible to make your own panels, either full size or in commercial ply dimensions.
I'll explain the process briefly.
First you need a local grown wood of similar weight and strenght to okoume, for example yom hom (Toona ciliata, Australian red cedar), champa (Michelia champaca), hinoki (Chamaecyparis obtusa, Japanese cypress), mai kok farang/waan (Spondias cytherea, Kedondong), mandong (Cordia dichotoma). Then you have someone saw and sand it into 3 and 4.5mm veneers with planed parallel edges. If the wood you buy is not klin dried, sticker it under a black plastic plane and put a fan at one end (solar kiln), then after a week or two it's ready to machine. The adhesive needs to work at room temperature, so either epoxy or a waterproof PU (D4 rating). The press consists of a flat concrete floor, two pieces of thick construction ply, plastic sheets your adhesive does not glue to, sandbags or water filled 200l barrels. Alternatively you can use vacuum, for that you need film ply or melamine faced chipboard, plastic sheet, butyl tape and two fridge compressors. If you want to make full size panels directly instead of scarfing them later, you need space for three ply sheets end to end. The process is simple, place plastic on ply, align the first layer of veneer so there are no gaps, secure to the ply with staples at the edge. Lay out and number the second layer, apply the glue, put the veneers in place, staple the edges making sure there are no gaps. Over it another plastic sheet, then the ply sheet with weights on top, or vacuum bag. When cured remove and cut away the edges with the staples. The bottom, deck and bulkheads get three layers of 3mm at 0/90°, the sides two layers of 4.5mm at +-45°, or the designer can provide you with alternative cold molding scantlings.

You are right, it does sound absolutely crazy, but maybe my head is full of foam. It sound very interesting, and I can see that there are some Thai names in your suggestions for wood. I'm not sure if it is possible to make it cheaper though, the price for the foam will be around USD 1.000,-, but maybe the biggest savings is in the amount of fiberglass and resin that are saved by using plywood.

One question, I thought having more layers of veneers was necessary when using plywood for boat building. Wouldn't 5 layers be better?

Not sure what will demand most work, making my own plywood or laying the extra amount of glass in a foam build. Maybe someone else are more cable at making that judgment.

 

[Peter Griebel]

Building on bulkhead with foam is very, very sketchy and I advise against.

why?

Well, laminated foam does not behave so well and you will probably end up with uneven sides as the hull will sag between wide stations.

To build a foam boat properly requires a gap between the bulkhead and the side panels for bonding and to prevent hard points or high points. I am not certain how you can rest side panels on bulkheads and avoid building a 'lumpy' boat. Perhaps you could use shims?

But spacings beyond about 900mm or maybe a meter and the foam will start to flatten.

a female jig is more work to setup, but a much better way to build this design if using foam

Thanks, that settles that then

 

[rxcomposite]

two skins of 600gram glass exceed this by about 0.010" based on my experience

I use the standard formula that uses specofic gravity of resin and fiber and should be easy to find. If you don't have it, I will post it.

For a standard resin with a specific gravity of 1.2, 820 gr/m2 at 50% glass content is 1 mm. 600 gr will yield 0.73 mm. It changes a little with epoxy if you are chasing numbers to 3 decimals.

 

[rxcomposite]

Perhaps it would be wise to dig up Catbuilder build in this forum. It is well documented. He built a cat with wood frames and minimal battens. Screwed the foam from inside, laminated the outside. Used squeege to remove excess resin. After laminating the outer skin, he removed the part from the jig and laminated the inside. Fitted the bulkheads then the stiffeners.

Yours should be easy to build. It is all flat panels.

 

[rxcomposite]

This minimum skin thickness requirement makes the foam core a second player. Maybe H60 @ 10 mm would be the safe way to go.

The foam core takes the shear stress. The thicker the foam, the less shear required.

The skin takes up the stress (tensile/compressive) and is calculated from the distance to the neutral axis. The greater the distance (thicker foam), the lesser the stress. But the skin has a limit. Impact strength or concentrated load. Say impact from floating debris in the sea (no puncture rule) or walking along the deck, placing heavy material, ect. 1 mm skin would suffice for side panels of superstructure unless you have mastered the art of walking vertically.

 

[fallguy]

Here are pics from my build. The jig cost 1300 to be cnc made at about 34' of jig on 900mm spacing. I had to make two, (cat), so had to cut the jig, but this gives you the idea. I only had one place up in the axebow that was 'off' from the mould a bit. The panels are screwed to the ribbands with a wood washer and screw and then the seams are bonded. The advantage to this method is there is no crawling under. The bulkheads can be build without rubbing and creating a bad hull form. And the hull is rigid when removed from the forms. The challenge is getting the designer to use his software to develop the female jig patterns. Up front, some beveling of the forms may be needed so the female jig doesn't create a bump in. I used a 22 degree router bit. You may need more angle than that. Use battens to check and when placing the foam at the bow; you'll see if any issues with fairness.

so nice to walk in and build the boat and it holds shape so well by screwing into ribbands in the jig; the only part for me that sucked was the bow was super narrow. And hard on the ankles; probably would build a nosecone next time, bit nonissue for you.

bulkhead five is removed on the top, so tabbing is short, but you can see how nice it is to tab; the backside is ladder in an out though

Bulkhead 3 in and four ready to trim to fit. They must be short for puttying. I used hotglue to temp hold them in place.

bulkhead two and the deck for the bed is installed

Jig from outside. The fuel tank is sitting there waiting for the base to be formed which was also installed before popping the boat out. We just worked out way to the back.

Tabbing, you can see I did not use peelply on the right and dripped bad. The left used peelply and no drips. Amateur hour.

dryfitting, note wood washers and clamps

lower panels and some canoe glass to spec, clamps holdi g shape

bottom panels are walkable with good enough glass schedule, but a good pic of the jig

final panel is transom

deck shoe could be done now or later after bottom work; we did it post bottom work

 

[Peter Griebel]

Thanks Fallguy for all the pictures of your project and explanations about how the female jig works. Did you finished the ship, or is it a work in progress?

 

[fallguy]

Thanks Fallguy for all the pictures of your project and explanations about how the female jig works. Did you finished the ship, or is it a work in progress?

I work on it fairly full time and am in the 4th year. It is a 10m powercat. I had a hip replaced in the middle of the project, couldn't walk much after about a year of work. It is cold here from October to April, we can't paint outside or work with plastics. But I have a slip rented for the summer and expect to launch sometime this spring. I spent about 6 months developing the worksite; insulating and heating systems; so 4 years build is really ending about August. Even this tent took over a week to build because we had to lift it for hardtop clearance a year later. Do not be dismayed by my numbers; this catamaran is a lot more complex than the lovely sailboat you want to build. You might finish in 18 months; so many variables.

The upholster is coming to rough fit cushions Sunday. I spent a week just making the stands even. My other hip is not too good either, but not a candidate for replacement, so I can be lazy sometimes if I have a lot of pain. I would say in the winter; I am working about 6 hours and summer about 10 a day. I lost three weeks of build time to Covid two times as well. The cold weather affects so much, for example, I have two side nets and a trampoline and I can't tie the sidenets now because when warmer weather comes everything will expand and my 100 knots will all be loose. Engine wiring has stopped because at 10F, everything plastic likes to crack and gets super stiff.