Nida core layup

[Jimbo2004]

Making a bimini hard top using three bow 1" frame that came with the canvas.

about 8x9.5 long

Was going to use 1/" Okume but now thinking to use

5/8 Nida it really isn't going to support weight just replace canvas

How much of a layup do I really need?

thinking 10 oz E on top/6 oz E on the underside. weight 20-22lbs

Or double bias(12oz) stitched at 45degrees along with one layer of 3/4oz mat. on top with 6 oz E on the underside weight 22-25lbs?


Not sure of the weight gain per sq ft on either layup, I thought it is 1:1

Can't find the weight of core I think It's about 5lbs per 4x7

[MN Dave]

Typo? "Was going to use 1/" Okume" Maybe "Was going to use 1/2" Okoume"

Weight: Nidacore 5 lb/cu.ft. So 3/4" x32 sq.ft. = 24" = 2 cu.ft. 10 lb. Add 12-15 lb for 36 oz cloth or half as much for 10 plus 6 oz. 1/2" okoume 42 lb.

I would use a little more glass and apply it to both sides. Say 2 layers of 6oz glass both sides. I think 6 and 10oz will dent too easily, but for a biminy, It could be enough.

Just a FYI, it might not matter. Fiber orientation: Fiberglass, plain weave is much stronger (5-7 times) and stiffer in the fiber direction than at an angle to the fibers. On a structure like this, I think (not same as know) that you might want the fibers running parallel to and perpendicular to the framework, not at 45°. Unless you do want them on the bias. I found some examples of similar layups. One of them with no additional detail said that a layer of 1810 (18oz) cloth was OK for a deck.
If you want to read about fiber orientation: Link
This figure from the link has strength values for carbon fiber. Divide by 10 for glass, but the shape of the curves won't change.

This link gives and example of a similar layup, 1/2" okoume vs 3/4" Nidacore with 1708 glass and mat. It is heavier. 1708 is a 17oz bias cloth with mat, so the glass total is ~35 oz versus 16 (10&6) or 24 (2x6 both sides). the ratio of resin weight would be similar.

https://downeastboatforum.com/thread...-plywood.1595/

Both core materials measured 18" x 48". 3/4" thick NidaCore vs. 1/2" marine plywood.
Test parameters: Both core materials measured 18" x 48". 3/4" thick NidaCore vs. 1/2" marine plywood.

The 3/4" NidaCore was covered on each side with 1-layer of 1708 and then 1-layer of .75 oz mat. The mat was used to cover the 1708 to create a smoother finish.

The 1/2" marine plywood (no voids, good stuff) was covered on each side with 1.5 oz mat. The mat adds very little if any strength, but it is what I use to seal plywood in bulkheads.

The weight difference between the 2 finished materials equated to 15 lbs per 4'x8' sheet. So if you were to use say 30 sheets of 4'x8' 1/2" plywood to build your boat, you would save 450 total pounds by using the NidaCore instead of the plywood. Those sound like good weight savings, but it gets better.

The NidaCore was way more rigid and stiff than the plywood (which surprised me a little bit). For plywood, marine plywood with no voids is pretty strong/stiff material. I've always been satisfied with it's strength. I'm just estimating here, but I would guess that you would have to jump up to at least 5/8" marine plywood to equal the rigidity/strenth of the 3/4" NidaCore. 5/8" plywood is about 11 pounds heavier per 4'x8' sheet than 1/2" plywood, so now the weight savings for a 4'x8' sheet of NidaCore is about 26 lbs lighter than a 4'x8' sheet of 5/8" plywood. 5/8" marine plywood is sturdy material in my opinion, but the NidaCore blows it away in weight savings.

For every yard of 50" 1708 biaxle it takes about a half gallon of resin.

[Jimbo2004]

Thanks for the feedback

Actually was going to use 1/4" Okoume, but thought it would come out just looking flimsy, not say it would be, but would need to add edge to it to give it balance.

Also costs more as a core

The 12 or 17 biaxle with mat will get a heavy with that much resin

Instead of two layers of 6oz why not one layer of 10 or 12 oz E glass?

[MN Dave]

First question: "How much of a layup do I really need?"
I don't know. I can estimate the weight, but the strength needed to resist a strong wind gust is not something I would be able to estimate. I worry about driving with 1/4" plywood on a roof rack unless I have something to support the leading edge.

I was giving some figures on the weight based on the erroneous guess that you were thinking of 1/2" plywood. I tried to give enough information to recalculate in case I missed my guess. Having reread my post, I can see where it could have been explained a lot better. I took the weight figures from the web site that I linked. They seem to use a lot of resin for the weight of cloth, but there is a fair amount of mat involved. Nidacore has a mat bonded to the surface to help with adhering fiberglass. and the cloth that they used had a mat layer too.

Second question: "Instead of two layers of 6oz why not one layer of 10 or 12 oz E glass?"
The strength pretty much goes with the weight, so the only reason might be that the heavier weave takes a bit more resin to fill. I think this question might be based on my failure to explain things well. When I said 10 plus 6 oz I was referring to your "thinking 10 oz E on top/6 oz E on the underside" Like I said, I didn't explain it well.

Lots of numbers for many cloth layups: http://pdf.directindustry.com/pdf/he...85-348279.html The strength does not always correlate well with weight for reasons that I have never seen explained.

So you are looking at 1/4" okoume vs Nidacore plus glass. 1/4" okoume weighs 21 lb per 4x8 sheet. A 4x8 sheet of 3/4" Nidacore weighs about 10 lb and 6 oz glass on both sides would come to around a total of 12-13 lb, not accounting for the mat which might add as much as another pound. Using 12oz plus mat on top and 6oz on the bottom I think would run lighter than you estimated, maybe as low as 15 lb. You can't beat okoume for simplicity, but you can save some weight if you work hard enough.

[JimConlin]

Nidacore is itself light, but I think you need to use glass mat or a biax-mat combination like 1708 to achieve good bond with the honeycomb. This gets heavy fast. Most mat is not compatible with epoxy, as its binders need the styrene of polyester or vinylester resin. That brings on other issues for a home builder.

I'd suggest using a structural foam like CoreCell or Divinycell and maybe 10 oz. glass cloth epoxied to each side. Total weight about 0.5 lbs./ft^2 .

3/8" okoume plus 6 oz. glass sheathing would be less costly, but about twice the weight.

[MN Dave]

Nidacore is itself light, but I think you need to use glass mat or a biax-mat combination like 1708 to achieve good bond with the honeycomb. This gets heavy fast. Most mat is not compatible with epoxy, as its binders need the styrene of polyester or vinylester resin. That brings on other issues for a home builder.

I'd suggest using a structural foam like CoreCell or Divinycell and maybe 10 oz. glass cloth epoxied to each side. Total weight about 0.5 lbs./ft^2 .

3/8" okoume plus 6 oz. glass sheathing would be less costly, but about twice the weight.

Nidacore already has a mat bonding layer on the surface.

Features• The honeycomb cell walls are fused into the non-woven polyester scrim. That coupled with the scrim’s affinity for virtually any resin or adhesive system provides exceptional bond and peel strength.

[JimConlin]

Nidacore already has a mat bonding layer on the surface.

Then why use 1708?

[MN Dave]

Then why use 1708?

That was not a recommendation. That was just the style cloth used in the example where someone was looking at the trade off between 1/2" plywood and a Nidacore deck. https://downeastboatforum.com/thread...-plywood.1595/ The OP left out the thickness of the plywood, and I inferred from the 3/4" Nidacore that he was looking for something equivalent to to 1/2" plywood. He was thinking 1/4" and I was thinking 1/2". You just aren't going to find an example where someone substituted 3/4" Nidacore for a sheet of canvass.

I started with what was available did a poor job of trying to describe a lighter alternative. I figured if I got the ball rolling, the OP would correct me on the plywood thickness if needed and someone would improve on my initial botched recommendation.

Substituting a brittle rigid structure for a thin, tough, flexible one gets into wind loading and dent resistance issues that can be complicated.