How do S-glass, E-glass, Kevlar (aramid), and carbon composite fabrics compare in compression strength, tensile strength, stiffness, and weight—and where in a canoe hull should you put each fabric?
I've posted the following information, graphs and video buried in the middle of two build threads here, but thought the topic deserved its own thread for future searchers and researchers.
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Just considering strength, which is not the only factor in hull building and patching, you want a material with high compressive strength as the outside layer of a canoe hull, and a material with high tensile strength on the inside.
The green bars in this first chart show that S-glass followed closely by carbon have the highest relative compressive strength/weight, so that is why those two materials are frequently used as the outside layers of a canoe hull. Carbon is lighter and stiffer than S-glass, as can be seen from the red bars in the third chart below, so it is often chosen as the exterior layer to lighten and stiffen the canoe overall, but at a much higher dollar cost than glass. Kevlar is very low in compressive strength, so it is rarely put on the outside of the hull. The exception is the rare all-Kevlar hull, which might be made simply to prioritize total canoe weight as much as possible because Kevlar is lighter than glass or carbon.
For the inside hull layer where you want tensile strength, this second chart shows the relative ultimate tensile strengths. S-glass is the strongest here, too, followed in a close almost-tie between Kevlar and E-glass.
So, to build the strongest canoe, the first two charts above would suggest S-glass on both outside and inside of the hull. And, indeed, such canoes are made. But they are heavy because fiberglass is much heavier than carbon, which is slightly heavier than Kevlar.
The blue bars on this third chart take fabric weight into account and show the relative tensile strength/weight of the fabrics. Kevlar is very commonly used as the inside layer of a composite canoe hull because its tensile strength is high and its weight is the lowest, resulting in the best tensile strength to weight ratio by a significant margin.
Kevlar would also be a very good choice for an inside patch on a hull because of its tensile strength and low weight. However, there are other considerations besides tensile strength and fabric weight for an inside patch. On a wood strip canoe, for example, one likely would want the internal patch to be as invisible and transparent as possible so that the wood will show through. Only glass is transparent, and no one would likely want a yellow Kevlar patch on wood.
In addition, Kevlar is harder to cut, easily frays, is hard to sand, and is generally reputed to be harder to work with than fiberglass. That's why even on a canoe with a Kevlar interior an inside repair may be made with fiberglass instead of Kevlar, unless the repairer is quite experienced working with Kevlar. Even then, transparent fiberglass may be preferable for aesthetic reasons if the the new Kevlar patch can't be closely color matched to the older, sun-darkened Kevlar hull interior. This aesthetic factor for a patch may not apply to a banged-up, dedicated whitewater canoe with a Kevlar interior.
The three charts above were taken as screen shots from the following video, which compares the different composite fabrics in even greater technical detail:
I've posted the following information, graphs and video buried in the middle of two build threads here, but thought the topic deserved its own thread for future searchers and researchers.
********************************
Just considering strength, which is not the only factor in hull building and patching, you want a material with high compressive strength as the outside layer of a canoe hull, and a material with high tensile strength on the inside.
The green bars in this first chart show that S-glass followed closely by carbon have the highest relative compressive strength/weight, so that is why those two materials are frequently used as the outside layers of a canoe hull. Carbon is lighter and stiffer than S-glass, as can be seen from the red bars in the third chart below, so it is often chosen as the exterior layer to lighten and stiffen the canoe overall, but at a much higher dollar cost than glass. Kevlar is very low in compressive strength, so it is rarely put on the outside of the hull. The exception is the rare all-Kevlar hull, which might be made simply to prioritize total canoe weight as much as possible because Kevlar is lighter than glass or carbon.
For the inside hull layer where you want tensile strength, this second chart shows the relative ultimate tensile strengths. S-glass is the strongest here, too, followed in a close almost-tie between Kevlar and E-glass.
So, to build the strongest canoe, the first two charts above would suggest S-glass on both outside and inside of the hull. And, indeed, such canoes are made. But they are heavy because fiberglass is much heavier than carbon, which is slightly heavier than Kevlar.
The blue bars on this third chart take fabric weight into account and show the relative tensile strength/weight of the fabrics. Kevlar is very commonly used as the inside layer of a composite canoe hull because its tensile strength is high and its weight is the lowest, resulting in the best tensile strength to weight ratio by a significant margin.
Kevlar would also be a very good choice for an inside patch on a hull because of its tensile strength and low weight. However, there are other considerations besides tensile strength and fabric weight for an inside patch. On a wood strip canoe, for example, one likely would want the internal patch to be as invisible and transparent as possible so that the wood will show through. Only glass is transparent, and no one would likely want a yellow Kevlar patch on wood.
In addition, Kevlar is harder to cut, easily frays, is hard to sand, and is generally reputed to be harder to work with than fiberglass. That's why even on a canoe with a Kevlar interior an inside repair may be made with fiberglass instead of Kevlar, unless the repairer is quite experienced working with Kevlar. Even then, transparent fiberglass may be preferable for aesthetic reasons if the the new Kevlar patch can't be closely color matched to the older, sun-darkened Kevlar hull interior. This aesthetic factor for a patch may not apply to a banged-up, dedicated whitewater canoe with a Kevlar interior.
The three charts above were taken as screen shots from the following video, which compares the different composite fabrics in even greater technical detail:
