Are Cats Made from Duflex Panel Kits Strong ?

[clockwork orange]

That last post by johny1973 reflects exactly my experience and opinion of balsa coring in boat construction,i have spent 25yrs+ repairing these things and get a little pissed whenever i read all the bs put out by the manufacturers regurgetated by people,either boatbuilders or backyard boatbuilders to justify their choice.While i will concede that if you make up a panel and do tests you get good results there are just too many variables in the life of a boat for it to be a good choice in real life. Is foam a better choice?,again based on real life experience,not just reading other peoples opinions on a computer screen i would say yes,while we get a lot less examples in our shop because they are a lot less common,i have seen wet cores for the same reasons i see wet balsa cores but i have yet to see a degraded core or delaminated skin, i have sitting on my desk a section of cabin sole which i cut out of a Lindenberg 26 which had been sitting with water over the sole in a yard for several years in Chicago,there were teak battens screwed to the sole without the holes being sealed and when i cut it out to provide access to the hull to install a transducer there was water from the foam core spraying off the jigsaw blade on the upstroke,i put the cuttout aside and over time it completly dried out and now appart from the osmosis blisters in the top skin serves as a great example of a well laminated 32 year old cored panel,balsa would have been compost. As for foam cores requiring heavier laminates,nonsense,my sons 24ft uldb which i designed and built in 1987 has a 1/2" klegecell core with skins of DB120 (400gsm) in and out as the basic laminate with of course extra local reinforcements and has endured many years of use including being backed into by a trailer while in storage and collisions on the racecourse none of which penetrated the inside skin indicating a tougher than necessary build,back then that was the lightest of the then fairly new stitched fabrics available.
Steve.

[olliric]

Well, it seems clear that cored materials in general are far from being ideal for boat building both above and below the water line. However, if one has to build a very light boat there are no really good substitutes. In a cat using solid glass of the same strength and stiffness for only the underwater part will add considerable weight (maybe around 20%) of the over all weight and that will kill the performance. To used it above it's a no go for various other reasons.
So, although the negative comments on core materials are numerous, true and well documented they do not offer alternatives and as such are of little use for someone that have to build a boat.
I want to build a cat that is strong, seaworthy, sails well and fast. I will have to take the risks associated with cored materials, I see no other alternatives if I want to achieve the design specification.
I will use only epoxy for water proofness, best bonding properties and strength and make sure that anything that penetrates the skins or put concentrated loads will be prepared to perfection. I will also have to be constantly vigilant during the use of the boat to make sure to detect surface cracks or any other damage of the the skins that may lead to water ingress. I think that if someone is competent and diligent enough it is feasible to have a 100% cored boat that can last a long time.
Being such an unforgiving material both to build with and use, it does not lend itself well to production building and casual use. I suspect that is the main reason why problems are so common. In a production situation it is difficult to dedicate extra time and skills that this material calls for.
That's one of the reasons I am thinking of building the boat myself and I will not buy a production cored cat or a custom build by someone I don't know.
I believe that cored materials are a necessery evil in boat building. I think we are stuck with it untill something better comes along.

Originally Posted by clockwork orange

That last post by johny1973 reflects exactly my experience and opinion of balsa coring in boat construction,i have spent 25yrs+ repairing these things and get a little pissed whenever i read all the bs put out by the manufacturers regurgetated by people,either boatbuilders or backyard boatbuilders to justify their choice.While i will concede that if you make up a panel and do tests you get good results there are just too many variables in the life of a boat for it to be a good choice in real life. Is foam a better choice?,again based on real life experience,not just reading other peoples opinions on a computer screen i would say yes,while we get a lot less examples in our shop because they are a lot less common,i have seen wet cores for the same reasons i see wet balsa cores but i have yet to see a degraded core or delaminated skin, i have sitting on my desk a section of cabin sole which i cut out of a Lindenberg 26 which had been sitting with water over the sole in a yard for several years in Chicago,there were teak battens screwed to the sole without the holes being sealed and when i cut it out to provide access to the hull to install a transducer there was water from the foam core spraying off the jigsaw blade on the upstroke,i put the cuttout aside and over time it completly dried out and now appart from the osmosis blisters in the top skin serves as a great example of a well laminated 32 year old cored panel,balsa would have been compost. As for foam cores requiring heavier laminates,nonsense,my sons 24ft uldb which i designed and built in 1987 has a 1/2" klegecell core with skins of DB120 (400gsm) in and out as the basic laminate with of course extra local reinforcements and has endured many years of use including being backed into by a trailer while in storage and collisions on the racecourse none of which penetrated the inside skin indicating a tougher than necessary build,back then that was the lightest of the then fairly new stitched fabrics available.
Steve.

[clockwork orange]

Olliric, you have the right attitude to own a cored boat,even a balsa one,unfortunatly most boat owners dont have a clue that by far the majority of production boats have balsa cored decks and by the time that they get tired of water leaking from a piece of hardware onto their bunk and decide to do something about it its been leaking into their core for years. Even though i personally dont care for balsa as a core when there is always the choice of foam there is nothing inherently wrong with it,i have never seen a rotten balsa core where it was the fault of the core, it has always been the fault of the builder or an owner,mostly the builder,all of the lessons were learned in the 1960s,but in the interests of saving a buck the builders cut corners all over the place.That said,the only reason to chose balsa over foam is price,period. any structural advantages that builders will tout are only to justify their choice of a cheaper material.
Steve.

[olliric]

Yes, unfortunately there are no care free materials that would make a light cat.
So, you seem to indicate that balsa is the worst of core materials. Yet it is the one with greater mechanical properties. Here is the spec of duflex panels with the same skins and core thickness.
Balsa: 5.1 Kg/Sqm, 3.4 mm deflection, 35 Kpa pressure to failure
Foam: 3.6 Kg/Sqm, 4.2 mm deflection, 30 Kpa pressure to failure
The bonding of skin to core is likely lower with foam in addition 20% more flexing will increase chances of delamination. Given that foam is lighter it makes sense to increase skin thickness to compensate for the lower strength. Doing so will also reduce the risk of water penetration.
Here are some opinions on the issue.
Structural Issues : Core Materials
With duflex panels I have the option to chose the core type. I am still debating what's better for the underwater part of the hulls. I am considering Airex duflex below waterline and balsa above.

Originally Posted by clockwork orange

Olliric, you have the right attitude to own a cored boat,even a balsa one,unfortunatly most boat owners dont have a clue that by far the majority of production boats have balsa cored decks and by the time that they get tired of water leaking from a piece of hardware onto their bunk and decide to do something about it its been leaking into their core for years. Even though i personally dont care for balsa as a core when there is always the choice of foam there is nothing inherently wrong with it,i have never seen a rotten balsa core where it was the fault of the core, it has always been the fault of the builder or an owner,mostly the builder,all of the lessons were learned in the 1960s,but in the interests of saving a buck the builders cut corners all over the place.That said,the only reason to chose balsa over foam is price,period. any structural advantages that builders will tout are only to justify their choice of a cheaper material.
Steve.

[clockwork orange]

Olliric, as i said in my last post,builders will tout superior mechanical properties to justify their choice of balsa when what really drives the choice is cost pure and simple. You dont need to have the strongest, whatever that means,you need to have strong enough and foam cored boats are plenty strong enough.I wonder if anyone has done a comparison test of a wet balsa cored panel and a wet foam cored panel of the same core thickness and skins, when the balsa turns to compost there is nothing joining the skins and at that point the properties are not looking too good. We did a lot of work on a Choate 40 a long time ago that had a klegecell core, that thing had water entering the core all over the place and travelling throughout the hull by way of the scoring in the core,it was very frustrating as water would weep out of the many penetrations of the inner skin such as wiring saddles etc, in short it was a poorly built boat yet it suffered no delamination, had it been balsa it would have been junk. If i were building a cored hull for myself again i would not even entertain using balsa even though i have the skill and experience to do it properly,there is simply no reason to do so except to save a buck of course.
I wonder how many of the raceboats bashing around the southern ocean are balsa cored,my guess is none,i would guess most are honeycomb or foam.
Steve.

[Robertcateran]

Originally Posted by olliric

Yes, unfortunately there are no care free materials that would make a light cat.
So, you seem to indicate that balsa is the worst of core materials. Yet it is the one with greater mechanical properties. Here is the spec of duflex panels with the same skins and core thickness.
Balsa: 5.1 Kg/Sqm, 3.4 mm deflection, 35 Kpa pressure to failure
Foam: 3.6 Kg/Sqm, 4.2 mm deflection, 30 Kpa pressure to failure
The bonding of skin to core is likely lower with foam in addition 20% more flexing will increase chances of delamination. Given that foam is lighter it makes sense to increase skin thickness to compensate for the lower strength. Doing so will also reduce the risk of water penetration.
Here are some opinions on the issue.
Structural Issues : Core Materials
With duflex panels I have the option to chose the core type. I am still debating what's better for the underwater part of the hulls. I am considering Airex duflex below waterline and balsa above.

A certain amount of deflection before failure reduces the chance of delamination and allows grater energy absorption . There is less chance of the inner skin being ruptured in the event of a collision. I am going with polyprop honeycomb, more for the ability to absorb impact, rather than the cheaper price.

[Harryg]

I think the posts on this topic relating to balsa cores are misleading. I have not come across any cases, though they may exist, of water entering the core of an epoxy composite at a molecular level and destroying the core material. So, assuming the epoxy barrier is not breached any core material should last a long time. However, as explained in other posts, negligence by the builder or owner can result in the epoxy barrier being breached causing significant damage to the core. Interestingly, the damage to timber cores described above result from poorly installed deck and hull fittings. Using the correct installation techniques this type of damage should be eliminated. Another interesting point is the recommended method of installing hardware to a foam cored epoxy composite. That is, to cut out an oversize section of the foam and replace it with balsa, plywood or high density foam. High density foam may crush if bolts are over-tightened causing leaks and any type of timber is capable of rotting if not properly protected. So what you have is a timber core around the areas that cause most of the problems – so why not have a timber core throughout and take advantage of its superior mechanical properties. Also, there is just as much damage caused by incorrectly installed fittings above and below the waterline so why bother using different cores above and below the waterline. Having built a foam cored boat I have to say the weight saving over timber is minimal given the additional laminate specified by the designer to support the foam core. Exotic fabrics, foam core and vacuum bagging is another story and should be used if weight saving is a priority. My latest project is strip-planked DuraKore. Harry

[Therapy]

Originally Posted by Harryg

I think the posts on this topic relating to balsa cores are misleading. I have not come across any cases, though they may exist, of water entering the core of an epoxy composite at a molecular level and destroying the core material. So, assuming the epoxy barrier is not breached any core material should last a long time. However, as explained in other posts, negligence by the builder or owner can result in the epoxy barrier being breached causing significant damage to the core. Interestingly, the damage to timber cores described above result from poorly installed deck and hull fittings. Using the correct installation techniques this type of damage should be eliminated. Another interesting point is the recommended method of installing hardware to a foam cored epoxy composite. That is, to cut out an oversize section of the foam and replace it with balsa, plywood or high density foam. High density foam may crush if bolts are over-tightened causing leaks and any type of timber is capable of rotting if not properly protected. So what you have is a timber core around the areas that cause most of the problems – so why not have a timber core throughout and take advantage of its superior mechanical properties. Also, there is just as much damage caused by incorrectly installed fittings above and below the waterline so why bother using different cores above and below the waterline. Having built a foam cored boat I have to say the weight saving over timber is minimal given the additional laminate specified by the designer to support the foam core. Exotic fabrics, foam core and vacuum bagging is another story and should be used if weight saving is a priority. My latest project is strip-planked DuraKore. Harry

I kept thinking some of those things as this thread continued.

Hmmmm........

[olliric]

Originally Posted by Harryg

Also, there is just as much damage caused by incorrectly installed fittings above and below the waterline so why bother using different cores above and below the waterline. Having built a foam cored boat I have to say the weight saving over timber is minimal given the additional laminate specified by the designer to support the foam core. Exotic fabrics, foam core and vacuum bagging is another story and should be used if weight saving is a priority. My latest project is strip-planked DuraKore. Harry

The reasoning behind having foam core underwater has to do with the much slower progress of water penetration damage in foam core sandwich. Let's say you hit a broken pallet or any other floatsome with a nail or anything else sharp sticking out. There is a good chance you may get the outer skin pierced without knowing it. You may notice something only months or years later. With balsa core you are likely to end up with greater damage as water will spread faster and cause greater decay of the core.

[44'cruisingcat]

Or not. In Duflex, the balsa is roughly 100mm squares, divided by solid epoxy glue lines. So it's quite likely the water and the damage would be confined to a small area.

With foam cored Duflex this would not be the case, so there is the possibility the water could spread further.

[Harryg]

Impact damage to foam cores may also causes delamination over a larger area due to flexing without necessarily penetrating the skin. If the skin is penetrated the water may spread further. Experiments have shown that the spread of water in a balsa core is also extremely slow across the grain. So, a diligent owner would surely notice and repair any damage before it becomes a problem. Harry

[olliric]

Originally Posted by 44'cruisingcat

Or not. In Duflex, the balsa is roughly 100mm squares, divided by solid epoxy glue lines. So it's quite likely the water and the damage would be confined to a small area.

With foam cored Duflex this would not be the case, so there is the possibility the water could spread further.

Although the core is divided in squares there is no guarantee that the water will not travel past the block that is water logged. Quite the contrary.
The epoxy boundary between the blocs is almost non existent (just a few tenths of mm or less). It will certainly fail in a dynamic situation. I have pieces of Duflex panels and I scratched the core trying to isolate the epoxy boundary between the squares to no avail. It is so thin that I could not even get a glimpse of it. In addition balsa density is so low that can be broken off using your fingernail. Being so low in density is also extremely light but water will travel across no problem despite being laid accross the grain. The spread will be guaranteed by the hydrostatic pressure below the water line.
By looking and playing with a piece of Duflex panel it is easy to come to the conclusion that if water gets through the skins it will quickly cause substantial problems. I have no doubts about it, yet I'll take the chance of using it.
I still think that below the water line foam core is probably a better choice. Airex C70.75 foam as used in the Duflex panels is a syntetic closed cell foam and therefore more resistant to moisture than a fiber based organic material like balsa. Although I have not done tests on them nor do I have a sample of a foam cored Duflex panel to play with, I think that it is quite intuitive to see which core will cause the slowest water diffusion or which will last longer once waterlogged.
Regarding the lower strength of foam core panels for the hulls shoes, I don't think it is an issue which should call for thickening of skins. The bottoms of the hulls are supported by a web of floors that drastically limit the free span of panels and create the watertight buoyancy compartments. It is an extremely strong structure.

[olliric]

Originally Posted by Harryg

Impact damage to foam cores may also causes delamination over a larger area due to flexing without necessarily penetrating the skin. If the skin is penetrated the water may spread further. Experiments have shown that the spread of water in a balsa core is also extremely slow across the grain. So, a diligent owner would surely notice and repair any damage before it becomes a problem. Harry

I agree that given the same strain, foam core is more likely to cause delamination. However, if the panel is well supported (see my previous post) it should not make much difference.

[44'cruisingcat]

Originally Posted by olliric

Although the core is divided in squares there is no guarantee that the water will not travel past the block that is water logged. Quite the contrary.

Quite the contrary? You mean there IS a guarantee that the water will travel past solid epoxy barriers?

I had a Duflex scrap in a bucket of water for a few months. Not very scientific, but the water hadn't moved much more than 10mm into the balsa by the time I gave up checking it. Where there was a glue line near the edge, the water was stopped completely. As for hydrostatic pressure forcing the water through, the deepest point of my hulls below the water is 450mm, so there isn't much anyway.

[olliric]

Originally Posted by 44'cruisingcat

Quite the contrary? You mean there IS a guarantee that the water will travel past solid epoxy barriers?

I had a Duflex scrap in a bucket of water for a few months. Not very scientific, but the water hadn't moved much more than 10mm into the balsa by the time I gave up checking it. Where there was a glue line near the edge, the water was stopped completely. As for hydrostatic pressure forcing the water through, the deepest point of my hulls below the water is 450mm, so there isn't much anyway.

In my opinion water will travel through the balsa core. At which rate and how long will it take it is not easy to say. In a dynamic condition with varying pressure, flexing etc I very much doubt that the ultra thin epoxy layer between the blocks will stop the water progress. My point is that foam is more water resistant than balsa, I think that is also the opinion of ATL in their brochures. That's why I would prefer having foam core under the waterline.
If water can eventually get through solid polyester GRP, it will cruise through balsa. It is a very common problem in older boats even in just the deck core and it is very well documented.