Kevlar and Marine Plywood Questions

[servobot]

Will Kevlar adhere to marine plywood when wet out with epoxy?

If so, would it be redundant to first lay a substrate of glass?

Does the Kevlar add strength to the composite structure in the same manner as fiberglass, or just puncture resistance as an outer layer? (Or none of the above.)

[Ssor]

Generally one would use kevlar in a system where weight was a serious concern. Kevlar over plywood IMHO is a waste of money if weight isn't a factor.

[duluthboats]

“Will Kevlar adhere to marine plywood when wet out with epoxy?”

It’s the epoxy that adheres to the plywood, the Kevlar is suspended in the epoxy.

”If so, would it be redundant to first lay a substrate of glass?”

Yes, most Kevlar lay-up schedules I have seen have glass on the outer layer to keep the fuzzies under control.

“Does the Kevlar add strength to the composite structure in the same manner as fiberglass, or just puncture resistance as an outer layer? (Or none of the above.)”

Kevlar can add a great deal of strength if used properly, as stated above it is used mostly where weight is problem. It’s hard on tools, near impossible to sand, and wicks water like a sponge. On top of that it is outrageously expensive. So if your not building a gun boat I don’t think you need to add it to the sheathing over plywood.


Gary

[rxcomposite]

Kevlar or fiberglass over wood is indeed a waste of money. Very little gain in terms of structural strength. all it does is add weight.

Kevlar is very tough but flexible. When hit, it will flex but the wood underneath will shatter. Like a "skin and bones" construction.

Wood by itself is structural. "cladding" it with a superior layer will not enhance the mechanical properties.

The only time it works is when a low density material such as end grain balsa is used as a core material. This is "sandwich" construction.

In the old days, wood was "cladded" with copper sheet. The only gain was anti fouling and this developed today as cupric oxide based anti fouling coating.

[servobot]

Thanks for the advice guys. Sounds like anything other than some epoxy to seal the plywood adds no value, other than abrasion resistance, I suppose.

[Ssor]

One must be careful about using broad blanket statements. Yes, plywood will shatter if struck a sharp blow whether or not it is clad with glass, end grain balsa will delaminate, Airex core or corecell will absorb impacts somewhat better. pilwood clad with resin/glass will provide a better substrate for paint than plywood alone. Stitch and glue would not be viable without fiberglass/resin overlay. Allan Vaitese in his book "One off in Fiberglass" covers the concept of wood left in place and depends on the wood glass matrix to enhance the structural integrity of the boat.

[servobot]

My boat is being built of plywood over sawn frames and longitudinal stringers. All I really wanted to know is: if slamming into a big rock, reef or any other solid, immovable object, would kevlar help keep the water out, even if the underlying plywood cracked? If not, I'd rather not spend the money.

[Ssor]

Well let's see, two ton boat at twenty kts. Two thousand ton rock sittin' tight. Won't much matter if boat hits rock or if rock hits boat. The result is a proceedure called damage control. Kevlar ballistic resistant armour are multiple layers free to move and absorb the energy. Glass would probably serve just as well in a boat.

[doesitfloat]

Not worth the money. Make sure you have a collision bulkhead that makes the bow a watertight compartment.

[JEM]

Originally Posted by servobot

Thanks for the advice guys. Sounds like anything other than some epoxy to seal the plywood adds no value, other than abrasion resistance, I suppose.

That's not true. You gain strength and stiffness if applied properly.

[rxcomposite]

Ssor,

I agree with you on the excellent antiballistic performance of the aramid(Kevlar). The slippage between the fiber and the matrix is what absorbs the impact energy. This fiber slippage is sometimes enhanced by sizing the fiber with a release agent.

My statement [will flex but the wood underneath will shatter] is not a generalization but a principle of composite design.

The laminate consisting of plies behave linearly elastic until fracture. Each succeeding ply depend on each other whether they are in compression or tension depending on their position relative to the neutral axis.

Designs which features aramid fibers on the outer plies, in an attempt to make use of the superior impact properties must always be checked at the center of panel where it deflects most.

Aramid being a high performance laminate, has a very different mechanical properties than plywood. When the two dissimilar material is bonded together, it is the plywood that will crack first. The aramid, while superior on its own merit, did not enhance the properties of the plywood.

Core materials, whether end grain balsa, foam core, paper or plastic honeycomb, behave in a different way. It is in the neutral axis and is compressed by the inner and outer ply and suffers shearing at the base of the stiffeners.

While i agree with you on the general characteristics of the core material you mentioned, remember that the designer has a lot of leeway to correct whatever deficiencies the core has. The designer can vary core density, core thickness,and face thickness to achieve optimum performance/cost.

Cheers

[rxcomposite]

Originally Posted by servobot

My boat is being built of plywood over sawn frames and longitudinal stringers. All I really wanted to know is: if slamming into a big rock, reef or any other solid, immovable object, would kevlar help keep the water out, even if the underlying plywood cracked? If not, I'd rather not spend the money.

Most boatbuilders incorporate as many watertight bulkheads, collission bulkheads, and other sealed compartments. Watertight compartment will give you an extra amount of safety. Some fill up the spaces with liquid foam (expensive) while others use slabs of cheap packing foam wrapped in plastic bags. Plastic bag is needed as this type of foam dissoves in styrene.

[JEM]

Worth noting that Kevlar® 29 is the type of aramid used for Ballistic purposes. Aramid/Kevlar® used for boat building is a different type/weave of cloth.

[Ssor]

RXcomposite, I was stationed at Poro Point, San Fernando, La Union in 1961/1962. Where are you located?

[rxcomposite]

Jem,

Thanks. If you have some interest in "Ballistic Fibers" read High Performance Composite, January 2005 issue. New technologies discussed.

regards