weight-saving techniques

[kennyrayandersen]

little delams can be fixed if caught early too.. you just need to drill a small hole and inject some resin..


if you're smart, you can do it with your vacuum cleaner..

yep -- just like them cracks in the windshield!

 

[eamarquardt]

Gus -- I was slightly disagreeing about the thermal damage you were suggesting -- I don't think it is there. Of course -- how would you know what I was thinking -- since I forgot to write it down!!

I wasn't making any "dire predictions" but having owned GRP boats, with a cat in the back yard right now with some delamination between the glass and foam (started before we got it), and having seen surfboards damaged by the sun I am convinced that any effort to keep thin GRP/foam out of the weather to the extent "practical" is a good thing. A friend has a "junkyard" at his place. Motorcycles, boats, jetski, etc. Most of it has been rendered "junk" by the effects of the weather (and it's MILD here in Ventura County) compared to the rest of the country.

As I'm sure most who read my posts agree, I do things a bit differently than most. I do my best to keep everthing out of the weather simply because doing so makes things last longer. If I weren't married with children I think I'd have an acre or so with a big butler building on it. The exterior of Butler buildings are pretty maintenance free. In the building would be a small home facing a BIG ROLL UP DOOR, a huge open shop, a huge inside storage area/work area, with the cooking area outside the home near the door to exhaust the fumes. Never have to paint the exterior of the house, never need a new roof, all cooking fumes and vapor go outside, and everything out of the weather. On good days I'd roll up the door and enjoy the view (gotta be on a hilltop with a view) and during "inclement weather" I'd close the door and carry on with business as usual out of the weather. Heating an a/c costs would be minimized with the house out of weather. Odd, yes. Functional, yes. However, I don't think it's gonna happen as Suzy has vetoed the idea.

Cheers,

Gus

 

[eamarquardt]

little delams can be fixed if caught early too.. you just need to drill a small hole and inject some resin..


if you're smart, you can do it with your vacuum cleaner..

yep -- just like them cracks in the windshield!

How do you get a "sucker" to "inject"? What's your technique? I've done the "windshield miracle cure" several times with excellent results!

Cheers,

Gus

 

[afreegreek]

little delams can be fixed if caught early too.. you just need to drill a small hole and inject some resin..


if you're smart, you can do it with your vacuum cleaner..

yep -- just like them cracks in the windshield!

How do you get a "sucker" to "inject"? What's your technique? I've done the "windshield miracle cure" several times with excellent results!

Cheers,

Gus

basically you use the vacuum to suck out the air.. suck hard enough and the cavity will become a vacuum chamber.. you get some resin at the entrance to the cavity and release the vacuum.. the cavity will suck the resin into itself as it tries to equalize it's pressure with the outside pressure..

it's a poor explanation but it's the best I can do

 

[eamarquardt]

basically you use the vacuum to suck out the air.. suck hard enough and the cavity will become a vacuum chamber.. you get some resin at the entrance to the cavity and release the vacuum.. the cavity will suck the resin into itself as it tries to equalize it's pressure with the outside pressure..

it's a poor explanation but it's the best I can do

I can see this working with a small delam. The one on the cat is pretty big (maybe 6" in dia). I'm thinking this would be an "ideal" location for one of those small circular access hatches. Cut the circle, inject resin into the delam that's left around the perimeter, use multiple clamps to hold it together while the resin goes off, and then fill the hole with the access port. The thing that bothers me is that only one hull has a problem and if I only put a port in one hull the hulls would then be "asymmetrical". I'm big on symmetry so I'd have to put a port in the hull that's ok to keep the universe in proper balance. However, since the hulls are aysymmetrical already (but mirror images of each other) maybe I'd be ok with a port in just one hull. I digress.

Cheers,

Gus

 

[afreegreek]

basically you use the vacuum to suck out the air.. suck hard enough and the cavity will become a vacuum chamber.. you get some resin at the entrance to the cavity and release the vacuum.. the cavity will suck the resin into itself as it tries to equalize it's pressure with the outside pressure..

it's a poor explanation but it's the best I can do

I can see this working with a small delam. The one on the cat is pretty big (maybe 6" in dia). I'm thinking this would be an "ideal" location for one of those small circular access hatches. Cut the circle, inject resin into the delam that's left around the perimeter, use multiple clamps to hold it together while the resin goes off, and then fill the hole with the access port. The thing that bothers me is that only one hull has a problem and if I only put a port in one hull the hulls would then be "asymmetrical". I'm big on symmetry so I'd have to put a port in the hull that's ok to keep the universe in proper balance. However, since the hulls are aysymmetrical already (but mirror images of each other) maybe I'd be ok with a port in just one hull. I digress.

Cheers,

Gus

6" is nothing... here, this is the basic principle.. it's similar but not exactly how you would infuse a delam but it will give you the idea..

 

[NathanL]

How set on a traditional teardrop profile? You could do a tortured plywood stressed skin "teardrop" out of 1/4" ply and be done with it inside and then put your insulation on the inside....but putting a second layer inside would be next to impossible so you would have to live with either insulation or something you could bond to it in a non flat/tortured layout.

 

[kennyrayandersen]

Gus -- I was slightly disagreeing about the thermal damage you were suggesting -- I don't think it is there. Of course -- how would you know what I was thinking -- since I forgot to write it down!!

I wasn't making any "dire predictions" but having owned GRP boats, with a cat in the back yard right now with some delamination between the glass and foam (started before we got it), and having seen surfboards damaged by the sun I am convinced that any effort to keep thin GRP/foam out of the weather to the extent "practical" is a good thing. A friend has a "junkyard" at his place. Motorcycles, boats, jetski, etc. Most of it has been rendered "junk" by the effects of the weather (and it's MILD here in Ventura County) compared to the rest of the country.

As I'm sure most who read my posts agree, I do things a bit differently than most. I do my best to keep everthing out of the weather simply because doing so makes things last longer. If I weren't married with children I think I'd have an acre or so with a big butler building on it. The exterior of Butler buildings are pretty maintenance free. In the building would be a small home facing a BIG ROLL UP DOOR, a huge open shop, a huge inside storage area/work area, with the cooking area outside the home near the door to exhaust the fumes. Never have to paint the exterior of the house, never need a new roof, all cooking fumes and vapor go outside, and everything out of the weather. On good days I'd roll up the door and enjoy the view (gotta be on a hilltop with a view) and during "inclement weather" I'd close the door and carry on with business as usual out of the weather. Heating an a/c costs would be minimized with the house out of weather. Odd, yes. Functional, yes. However, I don't think it's gonna happen as Suzy has vetoed the idea.

Cheers,

Gus

No disagreement on the UV damage -- it's a killer. But UV damage and thermal damage aren't the same thing. Thermal damage actually is a little non-specific. When I think of thermal loading it is due to the difference in the coefficient of thermal expantion of two material joined together witch I was thinking probably wouldn't happen in this case. Of course, things can get damaged due to elevated temperature as well, but it likely that the glass transition temperature (temperature that the resin goes to junk) is higher than one one gets just by setting it outdoors. I'm thinking that the gelcoat, or whatever UV protection that someone had on there breaks down and the the epoxy or polyester resin goes to crud in no time. WAY better, as you suggest to keep it indoors.

And with regard to your vision of a house/barn/shop... nice, but women, generally don't have the same 'vision' that men do...

 

[kennyrayandersen]

How set on a traditional teardrop profile? You could do a tortured plywood stressed skin "teardrop" out of 1/4" ply and be done with it inside and then put your insulation on the inside....but putting a second layer inside would be next to impossible so you would have to live with either insulation or something you could bond to it in a non flat/tortured layout.

I think if you wanted to do a curved sandwich, you'd lay the foam on one side of the playwood (or fiberglass skin), slit the foam on the other side parallel to the axis that you are rapping the curve around and then bend it with the plywood to the inside, (or fiberglass plies if you are going that way) which will cause a litle gapping in the core to the outside -- no big deal if you are using plywood on the outside (since it will bridge the gap). If coming back over the top with fiberglass, I think it would be best to fill the gaps with microballons, to get a smooth surface before adding the fiberglass and resin (holes would still need to be poked in the outside). Either way, its not so hard.

I think you are right -- you can't build it from the outside in -- it's got to be from the inside out!

 

[glassice]

say what I thing this has got over thunk about kiss is the best way

 

[eamarquardt]

No disagreement on the UV damage -- it's a killer. But UV damage and thermal damage aren't the same thing. Thermal damage actually is a little non-specific. When I think of thermal loading it is due to the difference in the coefficient of thermal expantion of two material joined together witch I was thinking probably wouldn't happen in this case. Of course, things can get damaged due to elevated temperature as well, but it likely that the glass transition temperature (temperature that the resin goes to junk) is higher than one one gets just by setting it outdoors. I'm thinking that the gelcoat, or whatever UV protection that someone had on there breaks down and the the epoxy or polyester resin goes to crud in no time. I think we are quibbling, bickering, or bun fighting in a minor way. Whatever you want to call it hot sun damage, thermal, or UV, the sun damages GRP over time. This statement is not a "dire prediction of catastrophic failure" but just something to be considered in the overall plan. "Protection" from the effects of the sun can be thicker layups, paint, indoor storage, etc. WAY better, as you suggest to keep it indoors. Agreed.

And with regard to your vision of a house/barn/shop... nice, but women, generally don't have the same 'vision' that men do... As my "psychotherapist", a woman (but the HMFIC is a man, not that it matters), agrees: Women are not only plumbed differently from men, they are wired differently. I just love saying "my psychotherapist" as it implies I'm more than a half bubble off plumb but I've been released from two insane asylums which proves I'm at most only a 1/4 bubble off plumb or they wouldn't have released me. I do, however, admit (one of my few admissions) I told em what they wanted to hear to get out of the first place as the showers were cold and the food lousy! After 17 ECTs at the hands of a quack, it was determined that my problems were a reaction to a prescribed pain medication. I can only look back and laugh as no one cares when I cry!

say what I thing this has got over thunk about kiss is the best way.

My thoughts:

1) I agree: KISS!
2) Proper Prior Planning, Prevents Piss Poor Performance (PPPPPPP) or put differently, I just hate admiring my great paint job from the corner I've just painted myself into.
3) Overkill in moderation!

Cheers,

Gus

 

[Cappy2081]

Although my experience is 25 yrs ago, some underlying thoughts are still true.

The delamination of the panels (in whatever vehicle) are generally caused by 3 things:
1. poor layup/ adheshive mixture.

2. too large a panel, with no recurring connection between inside and outside lams.

3. No/ failed adhesion between core and skins.

After many many many destructions repairs I found many examples of NO connection between the inside and outside lams, regardless of plywood, foam, balcore, airex, honeycomb paper. If there was too large an area of a panel, there MUST be frequent stitching between the inside and outside skins to maintain a panels integrity. However that is done is up to the laminator. But Some of the ways done are: small core panels, with tapered sides, saw cuts through frequently to allow eposy/ester resin to connect between inner and outer skins, and holes with fabric brought through, and incorporated into each side skin.

I cant say enough about the correct mixture ratio. I have worked on fiberglass boats with the interior (in places) that was litterally a gooey mess of uncured puolyester resin that someone had obviously goofed up on in the factory. (This was a Hobie cat) Once the deck was mushed onto the hull it was outta sight, outta mind. Until the hull started to flex bow and distort.

The failed laminations are GOING to happen regardless of how you make it. The trick is making the panel size small enough or with cuts through the core panel so there is frequent attachments all over the panels. then what you have is a series of small localized delams, which don't effect the structural strength of the panel as a unit.

The cost of the particular core material is directly related to its marketability to the use. That may sound dumb, but balcore (for instance since its the oldest, most well known) has tensile strength, is adherable (both by epoxy and ester resin) progressing through the other known core material, until you get to the blue/orange board being discussed frequently on this forum. These dow poly iso, or styrene boards have -0- holding power, NO structural significance, and must only be looked upon as the means to provide separation between skins.

It is NOT the foam core that is providing any structural strength. it is the "channel" of two skins, separated by space. We used to use cardboard tubes, to lay on the hull to glass over, to make a channel to stiffen hulls (and also provide wiring/pipe runs down the hull) so even an open space between layers of glass forms a strong panel!

 

[GeoDrop]

I drilled some lightening holes in my hatch frame. Look how much I saved!

 

[kennyrayandersen]

Although my experience is 25 yrs ago, some underlying thoughts are still true.
The delamination of the panels (in whatever vehicle) are generally caused by 3 things:
1. poor layup/ adheshive mixture.
2. too large a panel, with no recurring connection between inside and outside lams.
3. No/ failed adhesion between core and skins.
After many many many destructions repairs I found many examples of NO connection between the inside and outside lams, regardless of plywood, foam, balcore, airex, honeycomb paper. If there was too large an area of a panel, there MUST be frequent stitching between the inside and outside skins to maintain a panels integrity. However that is done is up to the laminator. But Some of the ways done are: small core panels, with tapered sides, saw cuts through frequently to allow eposy/ester resin to connect between inner and outer skins, and holes with fabric brought through, and incorporated into each side skin.

I’m with you on 1 and 3, but 2 is not a reason for failure if the panel skins are sized correctly. Now, it may be true that once a delamination starts, that the strain level in the panel is such that the delamination grows, but size of the panel itself is not a failure reason. The scarfing of the core as you suggest, or the pocking of holes at 50 mm intervals as suggested by the core manufacturer (and discussed previously) should act to prevent the core from failing in a tension-type failure mode between the skin and the core. In the case of honeycomb core, the resin drips down the cell walls and prevents this type of failure better than most other core products, but is it quite expensive. Cheaper would be to score or poke holes in the foam core to allow resin to get a grip for out-of-plane loading.

The failed laminations are GOING to happen regardless of how you make it. The trick is making the panel size small enough or with cuts through the core panel so there is frequent attachments all over the panels. then what you have is a series of small localized delams, which don't effect the structural strength of the panel as a unit.

Again, if the panel is sized properly for the loads, it shouldn’t delaminate (not that your suggestions of scoring the panel are good ones BTW). Delaminations generally (for thin facesheets) are caused due to facesheet wrinkling, which will exhibit itself as a delamination between the core and facesheet, but the separation may not be due to merely an adhesive failure. If the facesheet has bits of foam stuck to it (which I have seen on man test specimens) then it’s not an adhesive failure but due to a facesheet overload (for the given thickness) the core actually fails. The facesheet wrinkling is influenced by the core stiffness (denser cores are more stiff), laminate stiffness, and laminate thickness. So, it’s not always easy to determine the exact cause, but the skin thickness and layup (stiffness) overall has a bigger impact. Delaminations can also occur do to impact loading, for instance (especially on a boat that may smack rocks, docks, or other boats). I have the equations, if anyone is interested, but I don’t think they are so useful for the average user.

The cost of the particular core material is directly related to its marketability to the use. That may sound dumb, but balcore (for instance since its the oldest, most well known) has tensile strength, is adherable (both by epoxy and ester resin) progressing through the other known core material, until you get to the blue/orange board being discussed frequently on this forum. These dow poly iso, or styrene boards have -0- holding power, NO structural significance, and must only be looked upon as the means to provide separation between skins.

It is NOT the foam core that is providing any structural strength. it is the "channel" of two skins, separated by space. We used to use cardboard tubes, to lay on the hull to glass over, to make a channel to stiffen hulls (and also provide wiring/pipe runs down the hull) so even an open space between layers of glass forms a strong panel!

I have to completely disagree with your statement on the home depot foam since I know of several planes that are flying that use it as well as another friend that is building one now that has already successfully gone through full-scale testing of the wing.

It’s more proper to say that the strength is not high, but it is known and it can be design with those allowable in mind. Without the core in fact, the skins are quite weak – especially in the transverse shear direction. It can’t be used willy-nilly without thought for the expected failure modes, but it can be made to work (there have already been a couple of tears built that way, so it actually not a particularly new approach). What is helpful to understand is that it probably isn’t adequate for high local load introduction (which has already been discussed in detail). In that case, plywood plugs (or epoxy pucks, as we call them)) should be buried in the core to transfer that concentrated load.

I’m not trying to say you are not bringing up good points, and in fact, the scarfing or poking of the foam core is critical to getting good out of plane adhesion, so your points about that are well taken. However, the 2 lb/ft cu. core does have structural properties and when properly designed can be relatively robust. Whether it would work for your particular application is something I couldn’t answer

 

[Cappy2081]

It is interesting to note the different ideas here.

The foam you mention being used in a wing is a pretty good example. but the foam has -0- structural strength. it is the forming of the complete panel, with the adjoining attachment of the edge of the skins that makes the strength. Any of the mentioned foams have little or no strength, it is when they are joined as a unit they have strength. This is unless you have the $$ to buy some carbon foam. Not me!

To make the point, you could (if you had enough money and time) make a camper wall (or the entire camper) out of JUST fiberglass and resin. But it would be HEAVY. The flex of the walls and floor necessitate the need to have a stiff construction. Thus the fiberglass would have to be about 3/16 or 1/4" thick all over. It would still need stiffeners, all over to make the walls and roof not flex. To fill you in, in early fiberglass boat building, they built them the same thickness as the wooden boats they were copying. some hulls were 1" or more thick. It was soon found out that that was WAY overkill. Now we are just trying to make as light a wall as possible, using the cheapest stuff around. (At least I am!)

Or you could make a camper out of just Foam. No matter how much liguid nails you use, the foam box will fail. Yes, it will work, and function until...... a bump, a thump, or a gust of wind and poof its all gone.

So this is about how to meld the two together. Someone else mentioned that delamination is caused by exposure to the sun. That is true to an extent, but this is only important when you are trying to KEEP the core attached to the skin. In an ultra lightweight construction the core adhesion is relatively UNimportant. This is what is being lost in these conversations. This switching between these two construction types seems to be getting people confused.

[/img]

Sorry about the cheesy paint thing!

 

[kennyrayandersen]

It is interesting to note the different ideas here.

The foam you mention being used in a wing is a pretty good example. but the foam has -0- structural strength. it is the forming of the complete panel, with the adjoining attachment of the edge of the skins that makes the strength. Any of the mentioned foams have little or no strength, it is when they are joined as a unit they have strength. This is unless you have the $$ to buy some carbon foam. Not me!

To make the point, you could (if you had enough money and time) make a camper wall (or the entire camper) out of JUST fiberglass and resin. But it would be HEAVY. The flex of the walls and floor necessitate the need to have a stiff construction. Thus the fiberglass would have to be about 3/16 or 1/4" thick all over. It would still need stiffeners, all over to make the walls and roof not flex. To fill you in, in early fiberglass boat building, they built them the same thickness as the wooden boats they were copying. some hulls were 1" or more thick. It was soon found out that that was WAY overkill. Now we are just trying to make as light a wall as possible, using the cheapest stuff around. (At least I am!)

Or you could make a camper out of just Foam. No matter how much liguid nails you use, the foam box will fail. Yes, it will work, and function until...... a bump, a thump, or a gust of wind and poof its all gone.

So this is about how to meld the two together. Someone else mentioned that delamination is caused by exposure to the sun. That is true to an extent, but this is only important when you are trying to KEEP the core attached to the skin. In an ultra lightweight construction the core adhesion is relatively UNimportant. This is what is being lost in these conversations. This switching between these two construction types seems to be getting people confused.

[/img]

Sorry about the cheesy paint thing!

The foam that you say has no structural strength has, conservatively speaking, about 30 psi worth of shear strength. Were it not so, the 2 face sheets could NOT work together. It is solely because it has this shear stiffness [and strength] that the sandwich works. You absolutely do NOT need to allow the resin to go all the way through the panels. Just for your information, I've been analyzing composites structures for about 25 years and can speak with some authority on how they function.

I don't disagree about the adhesive strength, since they all are generally stronger than the base material that they are bonding (core). For lightweight sandwich construction, the failure is almost always either core shear or facesheet wrinkling. The skin can separate for several reasons and since your exposure seems to be primarily boats, I suggest that they can easily be secondary causes for core delaminations than usual loading (impact, poor detail design, or lack of understanding of the total loading that the craft will see).

Designing a teardrop that is only loaded by air, and one that has accounted for local load introduction is not that big of a deal. There are several manufacturers who have already made many examples and have done so without long-term failure. So, sorry if I'm not seeing your gloom and doom scenario coming to pass (though it is possible if done incorrectly).

The REAL key is understanding 1) the loads, and 2) the strength of the material that you are working with and 3) how to design and engineer with the target material system. Lack of understanding of the underlying engineering principals, or an incomplete load spectra will easily result in a premature failure.

 

[Cappy2081]

I am not gloom and doom as much as trying to learn from other past experiences. There were several builds on TTT&T that documented delamination, foam board construction and similar methods. I, like others, am simply trying to wade through the info to garner the best info possible to not WASTE $$ in the construction of my trailer.

I have a wife who is only half aboard for this project. If the boss sees a waste of expenditure I will be permanently shut down! The bank will be closed. So I am trying to do it right the first time.

 

[kennyrayandersen]

I am not gloom and doom as much as trying to learn from other past experiences. There were several builds on TTT&T that documented delamination, foam board construction and similar methods. I, like others, am simply trying to wade through the info to garner the best info possible to not WASTE $$ in the construction of my trailer.

I have a wife who is only half aboard for this project. If the boss sees a waste of expenditure I will be permanently shut down! The bank will be closed. So I am trying to do it right the first time.

If you want the lowest risk while maintaining most of the benefits of the sandwich construction you have a couple of options.

The first way is to skip the structural fiberglass composites altogether and use 1/8 plywood sheet bonded to the home depot foam (the foam being framed out with 1X2 or 1X1 fir/pine. The thickness of the 1/8 plywood is enough that you shouldn't have ANY skin failure modes to cause delamination of the skin and core. You could then use a really thin ply of fabric only for sealing the plywood followed with a UV resistant paint or alternately you could cover in aluminum if that's the look you are going for.

Or you could frame the foam out in 2X1 fire/pine with fiberglass skins on either side. In order to avoid any skin wrinkling issues Glassice uses 4 ply skins (I'm assuming something like 6 oz fabric). It would be nice if you could orient some of the plies at 45 degrees, but at 4 plies it probably won't be a problem. Personally, I'm willing to take more risk, but I can understand others not wanting to.

Either way, the cost to make a few test specimens is low and you can satisfy yourself that your sandwich configuration is strong and robust enough for your project.

One other thing that enters into it is the size of your project. I'm looking at a very small (tow behind a Smart car, or one of the new FIAT 500s coming out... or maybe even towing something slowly behind my original 1963 FIAT 500, which only has about 20 HP so it has to be VERY light). Also, it will have very minimal gear (overnight only) like a foam mattress, a portable camp stove and a small propane can. The total target weight loaded will be about 250 Lb (empty around 200 Lb. That is VERY light, so the loads will be very small (even if I miss the target a bit).

If your project is 10-12 ft long tear and loaded up, you will need a much more robust sandwich. That's why I tried to point out some technical aspects of your blanket comments. Rarely are blanket statements always correct. What I can do, and get away with, designing a 200-250 Lb tear, may not be even close to being acceptable with a 750 Lb+ teardrop.

Again, I think you have some good points about doing something to help the skins stay attached. I'm thinking, per the core manufacturers recommendations, that poking the holes every 50 mm (similar to what Glassice recommends only a bit closer) will be sufficient to resist the peal type failure you are referring to. Also, as I mentioned previously, making the skins thicker helps the wrinkling allowable go up considerably.

There isn't that much transverse shear in the wall of a teardrop anyway. Most of the load is reacted vertically in the wall (assuming the corner details are well done), so the foam doesn't really have to be that strong. A boat is MUCH different as the forced from the water are orders of magnitude more and the loading is normal to the surface. Airloads on a tear are very small by comparison.

 

[Classic Finn]

I just happened to notice this thread and also with great interest.

We are using a sandwich building technique on our tears. Light and durable. Also here in Finland the sandwich method has been used in boats of all types as well as caravans. Many caravans in europe use this method and have been for years.

We are working together with our friend who owns this company that makes sandwich elements according to customers specs and drawings.
http://www.kerroslevy.net/index.html This website is only in Finnish however there you can see a few examples of what purposes they are used for.

The outer core can be aluminum, fiberglass, ply or whatever a person prefers. The Inner can be laminate or vinyl or whatever you decide.

Here,s one of the so called elements.

 

[Larry C-TNT]

If you want the lowest risk while maintaining most of the benefits of the sandwich construction you have a couple of options.

The first way is to skip the structural fiberglass composites altogether and use 1/8 plywood sheet bonded to the home depot foam (the foam being framed out with 1X2 or 1X1 fir/pine. The thickness of the 1/8 plywood is enough that you shouldn't have ANY skin failure modes to cause delamination of the skin and core. You could then use a really thin ply of fabric only for sealing the plywood followed with a UV resistant paint or alternately you could cover in aluminum if that's the look you are going for.

Or you could frame the foam out in 2X1 fire/pine with fiberglass skins on either side. In order to avoid any skin wrinkling issues Glassice uses 4 ply skins (I'm assuming something like 6 oz fabric). It would be nice if you could orient some of the plies at 45 degrees, but at 4 plies it probably won't be a problem. Personally, I'm willing to take more risk, but I can understand others not wanting to.

Either way, the cost to make a few test specimens is low and you can satisfy yourself that your sandwich configuration is strong and robust enough for your project.

Kenny, Glad to see your back, I was wondering where your were.
It seems to me your your first method might be the lightest. Four layers of 6 oz. glass on both sides of the foam would require a lot of heavy epoxy to wet out and fill. Bagging would reduce the epoxy, but not everybody can do that. Also the perforations in the foam would add more epoxy weight.

1/8" plywood is only 10# per sheet. A thin tight weave cloth wouldn't add much weight. This is the method I am using, but I am using 1/8" Cedar strips with light glass on both sides of the strips instead of using 1/8" plywood. I hope my skins will be strong enough. What do you think?

I know your goal is 250#, mine is 500# (600# on the high side) A little heavier sandwich wall won't be as big an issue for me as it will for you. However, I am counting ounces every step of the way.



Larry C