Building a BIG hard top with foam

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ddesilva
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Building a BIG hard top with foam

Post by ddesilva »

Gday,

with materials from e-boat, I'm going to be building a hard top circa 15' wide, 14/5' long for larger vessel. This will be my first project utilizing foam.

While I intend to double up the thickness on the edges for strength and aesthetics - I'm looking for some input on core thickness.

Bearing in mind that I will have stringers and 'beams' if you will running lat and long for added strength:

Can I use 3/4 - or do I need to go to 1"?

With 3/4 it would mean the edges would become 1.5" thick - and a half round will be added on the edge for aesthetics.

Thanks for any input!

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peter-curacao
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Re: Building a BIG hard top with foam

Post by peter-curacao »


ddesilva
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Re: Building a BIG hard top with foam

Post by ddesilva »

What core thickness did you use there Pete?
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Tungsten
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Re: Building a BIG hard top with foam

Post by Tungsten »

Thickness of the foam is what makes it stiff,so if your plan includes beams and the such then the top can be thinner.1" foam is incredibly expensive its also 7 times stiffer then 1/2"

Fuzz
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Re: Building a BIG hard top with foam

Post by Fuzz »

Just wondering here. One inch Nidacore is much cheaper than foam but then you have to deal with the edges. Could you make the outside edge with foam and then only need to buy one sheet?

fallguy1000
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Re: Building a BIG hard top with foam

Post by fallguy1000 »

I have a couple of points, but not definite answers.

A hardtop is typically full of screws and fasteners, railings, rod holders, antennaes, the list is really never ending. A honeycome core would be a huge pita for such. In fact, for this reason, I would definitely prefer high density core.

Of course, high density core is VERY expensive, but you do get higher strength in almost all directions. I would start off by comparing say a 3/4" corecell and a 1/2" corecell with the same in high density. Make a spreadsheet with cost and strength comparisons.

The other thing that is really important is matching. Matching?

Matching the CTE of the core and the glue and the laminate is vital to something this massive succeeding. Otherwise, when the sun starts to cook it; it WILL delam or fail at the bonds. The CTE is the coefficient of thermal expansion. It is real and it will ruin this project if done wrong. Black paint might not help either (just for fun). By the way, this is the same reason doing the edge thing with different cores is a bad idea. It will be very hard to match the CTE of NIDACORE to something else.

As a matter of guessing, I would guess that I would try to build such a beast with 3/4" high density corecell, which is about $30 a square foot. And that hardtop would cost you $6,750 in core alone. So, you are probably going to end up building it with something less expensive. An alternative would be to use some high density core where you expect fasteners and some regular core for big spans where nothing exits.

Another option for you would be to build it skin on frame. More work to build, but less cost of materials. Depends on who is picking up the tab. You basically build a mold and laminate the glass and remove it and then glue it onto the frame and enclose the airspace between. Done right, this is strong as nails for the cost of extra glass and epoxy, but essentially nearly zero cost on the core, or very little.

Just make sure you match the CTEs of the glue, glass, and core.
My boat build is here -------->

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fallguy1000
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Re: Building a BIG hard top with foam

Post by fallguy1000 »

oh, and btw, I can't tell you how to build the skin on frame for such a monster; you need an engineer to help with that, but a guess is 2"x3" boards on edge with bridging in between would get pretty strong, then instead of an edge detail for looks, you could reduce the edges to a thinner size with some curvature and finish the edges to look thinner than 3", and even put a gutter around it if you wanted, etc.
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Evan_Gatehouse
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Re: Building a BIG hard top with foam

Post by Evan_Gatehouse »

Cabin top on my 40' catamaran:

15' wide x about 6' fore/aft
one transverse very deep 4" top hat style beam with carbon flange - 15' long. This was key to keep spans short

Thus panel size was effectively 15' wide x 3' span between bulkhead/deep beam. The shortest span matters for deflection.
evan cabin top.jpg
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Core: 3/4" 80 kg/m3 corecell

basic laminate: single layer 666 gm/m3 triaxial each side - but a custom weave of +/- 45 of 200 gm each and one layer of 90 deg 266 gm. This way I had most of the fibers running across the short span direction for max stiffness. 666 gm/m3 = about 20 oz fabric.

Very stiff structure. Almost zero deflection with me on it. Often supported many kids running around on top of it.
designer: FB11/GV10,11,13/ HMD18/
SK17,MM21/MT24

cape_fisherman
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Re: Building a BIG hard top with foam

Post by cape_fisherman »

Fuzz wrote: Sun Oct 22, 2017 1:40 pm Just wondering here. One inch Nidacore is much cheaper than foam but then you have to deal with the edges. Could you make the outside edge with foam and then only need to buy one sheet?
Yes...you can add foam along the entire outer edge. That will give you a smooth edge without having to massage the Nida-Core into shape.

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Re: Building a BIG hard top with foam

Post by cape_fisherman »

As for attaching things...the guys around here usually bond aluminum plates under their honeycomb cores for that purpose. Whether it be the tops or cockpits.

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