Mixing Foam Brands on Same Laminate??

IMHO*, your core is (sufficiently many) many times thicker than the skins and also (sufficiently many) many times weaker than the skins, so you have a standard composite sandwich, where the core is there for shear only. No complicated one-off engineering, it will behave normally. Fatigue resistance is a function of your safety margin.

It comes down to your bond line and can you execute the bond; how big a surface, working time, do you have a extra pair of hands, a jig?

?Aerospace might use a low foaming epoxy, one that foams fractions of self, not multiples, like ones for rudder coring. Don't think "bog" can be considered an engineered material. Boeing bog tables

http://www.compositescentral.com/ These engineers might be more helpful.

 

Actually, my boat was designed for a Dutch boat building company who built the first one out of foam/glass for a couple who currently uses it for charters.

That explains why I tend to have a lot of questions. The boat plans are developed for the professional builder.

I agree with the excessive core and that logic does make a lot of sense. If panel is thicker than spec and the core is as dense (and more dense) than spec, it should be stiff enough.

The designer said I could just use the 1.22", 7lb Corecell, but he said it may not feel right under foot unless I use furniture to keep the panel stiffer. Designing the layout is hard enough without having to plan for that, so I would like to go with the full stiffness.

I am really not concerned with the strength of the panel, as it's just something you walk around on that hangs under the beams (which carry the loads). It's not about "strength", it's about longevity of the laminate, but I think your argument about the cores being over kill says they should hold up... I guess.

 

What about stringers catbuilder? Does the design have glass stringers to stiffen the bridgedeck? If not, i would consider just using the 1.22" foam only, and compensate for the reduction in stiffness by adding some uni-directional glass reinforced stringers under there so that it "feels right" under foot. This would save you considerable money in foam, and take away this dillemma your faced with...

If the bridgedeck is not a stressed member (are you sure about this?) then a panel 1.22" thick with 2000+gsm glass both sides is a pretty hefty panel already... Think carefully about taking the curvature out of it tho, it will loose alot of stiffness if you make it flat just for the sake of easier cabinetry...

 

Yep... had thought about all that too.

Stringers are out because I am against making more work for myself. It's already a grueling build. Foam/glass/epoxy, an extended 45+ft (13.75+ meter) x 25 ft (7.62 meter) boat. I can't add any extra steps to the build. It's already taking too long.

I am trying to ask the designer about taking the curve out of the bridgedeck and I know what you mean about it becoming less stiff if I do so.

EDIT: Kurt (the designer) said the curve in the bridgedeck panel isn't necessary except for drainage.

Another thought would be to just get some 1/4" 7lb foam and use that as the top layer, so the densities are the same. That is the route I'm trying to take after this discussion, to keep things simple.

However, Gurit's Core Cell unit is being bought right now and is going under a big reorganization. They are at least a month behind on orders and there is a 6 week plus wait on foam. Pretty ugly. No matter for this issue, though, since I can't do the bridgedeck until I finish my last hull and the beams, which are also waiting on delayed foam!!

 

If Kurt has given his approval for the reduction in core thickness why go to the expense, work and risk of incomplete bonding of the two cores.

 

Because it would be soft under foot. Could affect resale value and impression of boat by charter guests. Does that make sense, or an I getting too anal here again?

 

My question is two-fold: Are the 2 brands approximately the same density and are they the same material (same chemical)? If so, does brand matter?

 

Just one thing.. microballoons makes your bog more fragile so maybe something else for bonding bog..

 

Hoyt. They are not the same chemical and they are not the same density, but I am looking at getting the same brand Core Cell (SAN foam) in a 1lb density difference.

Teddy: Not sure that's right. The entire boat is made of Core Cell foam and epoxy with microballoons added. That's what people use to hold foam together if they want to do it right. Cheap/shoddy places slap a core adhesive to the foam, then laminate onto that. Core is core. If the epoxy can't pull from the foam without tearing the foam apart, you are in good shape. That's how it works... the epoxy bog pulls the foam apart.

 

That's related how much microballoons is used and how much strength is expected.. But generally it's like I said. Thou it's possible to use something else as reinforcement with the balloons to make it "even"
ps. There's a huge difference if the bog is holding two foams together to the direction of the skins or just between the end sides of two core plates in a panel.

 

No you are not too anal, but what is soft underfoot is a relative term.
Personally I would make the deck with the foam you have and take the risk of having to add a top hat stiffener later, this is not difficult to do. Usually a lighter and cheaper option too.

 

You're right about that, Andrew. I do, however have the extra foam lined up for under $600 for the entire bridgedeck. Price wise, it may be similar. Tough call, in general.

 

CB
Given stiff top and bottom glass layers the two core materials will behave like a parallel fold (structural geology sense). Attached is a picture of strain ellipses that would occur in each layer. Obviously this is much exaggerated from your question relates to. Continuous flexing would result in the internal foam walls having to bend repeatedly, likely ending in wall failure eventually.

The flexural slip that could be expected to occur between layers of different density will pretty much be eliminated by your epoxy bog, but probably at a cost of magnified strain ellipse geometry near the join of the two foams.

My guess is that your bridgedeck will not have sufficient flexure for the foams to have internal wall failure for a long time. The less dense one, if it is the thinner layer would develop such failure first, if it does occur.

If you want to go mathematical, google parallel folding and deformation and you will find equations. But your issue is regular and repeated very small deformation, so folks that look at stress failure might be better sources.

I don't think you have anything to be concerned about.