@PaddleDog52 said:
Kevlar makes a stronger kayak or boat. Use many combinations of it in offshore race boats along with carbon fibre. Kevlar is more impact resistant.
Here is a better resource for comparing the properties of aramid, fiberglass, and carbon fiber fabrics:
http://www.christinedemerchant.com/carbon-kevlar-glass-comparison.html
Aramid can make a stronger boat when used properly but does not necessarily make a stronger hull. Less fabric is often used in aramid hulls because many people who buy aramid hulls prize light weight. If equal weights of aramid fabric and fiberglass fabric and equal volumes of identical resin are used to construct two hulls of identical strength and dimensions, the all aramid hull will probably be stronger since aramid does have an overall higher strength to weight ratio. And racing hulls are not necessarily designed for maximal strength. They are designed to be light and stiff.
But aramid is best used in conjunction with other fabrics for paddlecraft hulls. Aramid has some disagreeable properties that go part and parcel with its virtues. A big one is lack of compression strength. The compressive strength of aramid fibers is only about 1/10th of its ultimate tensile strength and is considerably less than that of fiberglass and much less than that of carbon fiber. The significance of this is that aramid is best used for interior layers. When the convex external surface of a boat hull is impacted, the fibers of the external layer are subjected to compressive forces. Same thing with a sideways tangential blow that applies a shear stress to the external fibers. There is also the tendency of aramid to become fuzzy with extensive abrasion. All good reasons not to use aramid fabric on the hull exterior. Either fiberglass (especially S fiberglass) or carbon fiber are better choices there.
Another rather big downside of aramid is rather mediocre resin bonding to aramid fibers. Both fiberglass and carbon fiber bond much more strongly and durably to resin. The result of this is that the aramid fibers may remain intact, but with a strong enough blow will disassociate from the resin matrix. It is very common to see cracks or punctures in hulls in which the external non-aramid fibers broke and the interior aramid fibers remained intact, but broke away from the resin matrix.
A third liability of aramid is its hydrophilic tendency. Any aramid fibers that do begin to delaminate from the matrix tend to absorb water which can further weaken the aramid to resin bond strength.
Carbon fiber has excellent tensile strength and compression strength, a strength to weigh ratio a bit better than aramid and almost twice that of fiberglass, excellent resin bonding properties, and a high Young’s modulus, meaning high stiffness. But like the brittle tree trunk that breaks instead of bending, it has a disturbing tendency when it fails to fail catastrophically with little prior evidence of strain or deformation. I have seen boats with carbon fiber external layers, and paddles with a wood core blade sheathed in carbon fibe,r simply break through along the weft or warp of the weave leaving a straight and very sharp edge. For this reason, carbon fiber is not always a good choice in hulls that are expected to sustain repeated impacts, such as whitewater hulls, unless again absolute light weight and stiffness is the priority.
And as for the stiffness of a hull, yes material choices are important but the primary determinate of the stiffness of any hull is the thickness, regardless of material. This is why foam cored and ribbed hulls can be made so light but still retain adequate stiffness. The foam ribs and core themselves contribute very little strength, just thickness.