KEVLAR … a suggested Guide for its USE, ….. and, it’s Problems & Promise.
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First of all, lets clear up what is KEVLAR*, and is it the same as DYNEEMA* ?. (These terms* are used as well known Trade Marks of companies manufacturing the fibres noted below. There are now other similar brands to which the following also applies)
While they are similar in many ways and often competitive, they are NOT the same. Here is it in about 100 words.
What is Dyneema?
DYNEEMA is a high-performance polyethylene fiber known for its incredibly high strength-to-weight ratio. It is created through a patented spinning process that aligns the fibers in the same direction, resulting in its superior tensile strength and durability. One of DYNEEMA's characteristics is its remarkable lightness. With a SG of less than 1, it floats, which, combined with its extraordinary strength, makes it a favored material in many industries. This includes the marine field (primarily as a rope) as DYNEEMA has excellent resistance to moisture, UV light, and chemicals.. While there is some initial ‘creep’ until the material ‘settles in’, DYNEEMA ropes have extremely low stretch. Yet despite low stretch, Dyneema lines are flexible as the individual fibers are very fine. Cost of manufacturing is presently (2024) higher than for KEVLAR.
What is Kevlar?
KEVLAR is an aramid fiber, a type of synthetic fiber with performance attributes similar to DYNEEMA, as it’s also strong and light. However, KEVLAR has less resistance to UV but more resistance to heat resistance, making it the go-to material for high-temperature uses. KEVLAR fibers are extremely durable and cut-resistant, so became famous for bullet-proof vests as well as for surfaces requiring resistance against sharp objects and rough surfaces. KEVLAR has an admirable ‘stretch before failure’ value, that lays between the nearly non-elastic Carbon Fiber (that can make it brittle) and Fiberglass, but still has significantly less elasticity than Fiberglass that in some applications, can stretch too much. KEVLAR is however, relatively poor in direct (linear fiber) compression, which makes it totally unsuitable for items like a mast or even for beams. Due to its surface ‘furring up’ when abraided or sanded, its best to apply a fiberglass overlay over Kevlar that will be exposed to abrasion.
Strength and Durability
DYNEEMA is known for its highly impressive strength-to-weight ratio, being up to 15 times stronger than steel on a comparable weight basis,. However, KEVLAR doesn't fall far behind. Known for its use in bulletproof vests, KEVLAR offers high cut resistance. It's worth noting, though, that while both materials are strong, they handle wear and tear differently. DYNEEMA may have the edge in terms of maintaining its strength over time, even after repeated use. KEVLAR offers a little more stretch and its UV aging is not an issue if the material is covered.
But having cleared that away, this ARTICLE is about KEVLAR …. a cloth fiber potentially for boat construction.
KEVLAR Issues
Personally, I think this fiber has earned a bad rap ... so we need to look and understand why.. Four main issues in my experience.
It quickly fuzzes up when exposed to abrasion
It’s hard to saturate with resin as it tends to float
It wicks water, especially when the fiber ends are exposed
It’s relatively weak in direct linear compression
While it’s also hard to cut, there are ways to solve that. Either with special scissors or by temporarily stiffening the cloth with a tape which then makes cutting, even with a pair of strong but regular scissors, very doable. See here, with just an added Masking Tape
But before we turn away for those disturbing issues, let us also look at ‘the sunny side’ of this 'woven gold' fabric.
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KEVLAR Perks
KEVLAR is significantly lighter than either fiberglass or carbon fiber
It is typically less costly than carbon fiber ... quite often discounted.
It is far more resistant to abrasion and penetration than either FG or Carbon
It has double the strength of fiberglass with half the elasticity
Yet has double the elasticity of carbon fiber (often overly brittle) with over 60% of its strength and
57% more strength than E-glass that weighs 33% more.
The attached table confirms the above values .. (though I acknowledge that each test on composite materials will give slightly different results due to the unrepeatability of exact material composites). Percentages in the Material column, refer to Fiber relative to Total composite
So IF it were not for the earlier mentioned issues, it could actually be a really great fiber for boat building.
Of course, some already know this, as when you use KEVLAR for say a canoe or rowing skiff, you can avoid most of the noted issues while enjoying the benefits. But in its early days, it was used for many larger boats especially for large foam-cored composite ones that needed something strong, light yet not too elastic. But it was not long before we heard and saw stories of literally litres of water being drained out of soggy laminates, generally caused by construction that did little to prevent this from happening So for these larger boats, it’s propensity for absorbing water became such a major issue that it lost favor for common use.
Case Study
So let’s look at one of these large boat cases as a learning tool. And let’s not fool around here … let’s go really big .. to a beautiful classic boat over 100ft long designed by the famous William Tripp and built back in 1992 .. over 3 decades back when KEVLAR was 'looking fabulous'..
Courtesy of a composite hull repair article published in Professional Boatbuilding* (see below),
Here is a section through her hull as she was originally built. Particularly note two major issues. First the Kevlar used was VERY thick … about 36oz (1100gsm) in one cloth. This made it almost impossible to wet out all the fibers, so some stayed dry, serving as perfect wicks for the moisture that seeped in over the next 15 years. Helping this ‘inner core water supply’ were the kerfs (foam slots) that were common ar that time to help form the thick foam into place. As this was ‘pre-infusion days’, those kerfs could only be filled from the outside so although the surface would have looked good before final sheathing, much of the kerf was still just void, to soon be filled with water seeping up the dry Kevlar fibers.
Secondly, ‘the barrier coat’ such as it was, over the Kevlar, was apparently only one relatively thin layer of matt and fairing putty. That would never have passed inspection with todays knowledge of how attractive dry fibers are to wicking moisture.
Also, what you cannot see here, is that holes were later drilled in the hull for thro hull fittings. While I have no detail of this, it’s very likely that nothing more than compound would have been used to make such fittings watertight through the hull .. but with that, water would soon seep in past the outer flange and then wick it’s way into cut ends of the Kevlar, and over a relatively short few years would soak out the outer Kevlar layer, especially since that outer KEVLAR layer was too thick for full wetting out by the resin during the construction.
So ALL issues hazardous to KEVLAR were innocently thrown at this one boat construction, namely:
Inadequate wetting out of the Kevlar from using too thick a cloth.
Likely inadequate sealing of the ends of the Kevlar at the thro-hull fittings, and
Inadequate barrier coats between the outer Kevlar layer and the sea, enabling water to seep into the inadequately impregnated KEVLAR from all sides.
Needless to say, the whole outer layer exterior of the foam core itself had to be removed and then vacuum tubing set up in relatively small panels over the bottom area to suck out all moisture from the foam core itself. Interestingly, tests showed that the fairly dense H100 Divinycell PVC foam core was not too harmed by the water and only lost about 10% of its physical performance from the experience.
Although the repair is beyond the need of this article [which is ‘what to watch out for with Kevlar’], the repair was quite fascinating and ingenious, so for anyone interested, this can be studied in the excellent PBB Article (Professional Boatbuilder #...)
But this example, very effectively teaches us just what we need to take care of, if we want to use KEVLAR in our hulls to take advantage of the positive features (Weight & Strength to Cost) that this material can offer us.
In summary, I would say: “Use relatively thin layers of KEVLAR (not over 12oz each) and take care to thoroughly wet them out before sealing with any vacuum. For any area underwater, close KEVLAR off with at least one layer of fiberglass and apply at least two barrier coats of epoxy to prevent osmosis … water coming in through the surface. ….. more of both for larger vessels over 9m.
For the flat bottom of the W17 or W19, one 5oz layer of KEVLAR is recommended to add resistance to the bottom when sitting on rocks at low tide or on stony beaches. As noted above, this layer must be covered with a layer of 6oz boat cloth to fully surround the KEVLAR with a long term water barrier and if any of this surface covering is damaged, it’s important to dry-out the area and re-cover as soon as possible.
1” wide KEVLAR tape can also be added over the lower sharp keel of the W17/W19 amas but then encase under the outer sheathing of 6oz fiberglass boatcloth, with the same warning as above, to jump on to any needed surface repair quickly. Such a tape on my personal W17 Magic, is still very intact after 10 seasons of bouncing occasionally off rocks, so it was probably worth the effort to use it.
Also make certain the ends of all KEVLAR layers are well sealed …. preferably with NO holes through the skin. If there HAS to be holes, then stop the Kevlar at least 20mm short of the hole and seal that area off watertight with epoxy and coiled fiberglass fibers, or something equally effective. 
I can see KEVLAR effectively being used under the flat inner bottom of say a large trimaran hull, like my W32 concept that has hulls like a large W17. Such a boat has an inner bottom with 100mm of foam and with each plywood layer sheathed with Kevlar, properly wetted out and encased, would make a really tough bottom highly resistant to rocks and accidental groundings.
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