Does anyone know what properties spectra brings to a canoe hull? Is it extra resilience to impact? Any known negatives to the material when used with kevlar?

Similar to Kevlar. Strong in tension,
but inferior to carbon and S-glass in compression. That so few manufacturers and private builders are using Spectra says something. If the builder uses compatible resin, there is no reason not to use it with Kevlar, but the question remains, what does it add?

I heard
that it needs to be between cloth as gel coats don’t stick to it,something like that.

Well, since Spectra fuzzes like Kevlar,
you would not want to use it right under gelcoat or skincoat anyway. The outer layer would better be carbon or S-glass. Novacraft, one of the few builders using Spectra, is now offering boats where the outer layer is a crossweave of carbon and colored Kevlar. That would not be my choice, but it looks slick and avoids gelcoat.

One of the big issues in composite boats is how well the resin sticks to the fibers. Probably the best stiction occurs using polyester cloth and vinylester resin, because prior to setting, the resin literally melts the outer layer of the polyester, and then bonds to it. All glass fibers have special coatings on them, and must be used within a few years of production, or the coating will not produce a firm bond to the fibers.

Kevlar is like a modified Nylon. I have heard that Spectra is like a modified polyethelene, or a modified polypropelene. Wish I knew. If it is a modified polyethelene, one would be concerned as to whether ANY resin would form a good bond to it. It is not enough for a resin to interpenetrate the fibers of the fabric. It has to form a good bond with the surface of each fiber.

Googled 20 pages on "Spectra fabric"
and found many applications to sails, backpack fabrics, bear-resistant food bags, police body armor, but almost NOTHING on resin laminates. Tells you something. Following is a quote from a sailing site, talking mostly about sailcloth.

"Can woven fabrics be laminated?

“Certainly. While one of the advantages of laminated fabric is that the load-bearing yarns can be applied as a scrim, i.e., loose grid of fibers, you can still laminate a woven substrate to a film. These fabrics are very good if you are looking for rugged durability like a Spectra for an Around Alone race or for a large offshore cruiser where you need a high yarn count to handle the loads. It’s fairly difficult to engineer these fabrics if Spectra is going to be he principal yarn because Spectra is a slippery fiber that does not adhere very well, and it’s better if there are gaps between the fibers, as in a scrim, so that the adhesive can bond between the fibers. On the other hand, some fabric engineers have developed adhesives that will bond to Spectra without delaminating.”

Evidently very few engineers have figured out how to get epoxy or vinylester to bond well to plain Spectra.

Reported advantages of Spectra, which IS a high molecular weight polyethelene, are high modulus, high UV resistance, and no water absorption. One key disadvantage is that, under a persisting strong load, Spectra fibers will gradually elongate, permanently. Kevlar is much less inclined to stretch like that, and Kevlar bonds nicely with conventional resins. These sources say Kevlar is relatively vulnerable to UV damage, and also, if Kevlar fibers are “worked” a lot, they lose a good deal of strength. For this reason, making long-lived Kevlar sails has been difficult. Spectra sails stretch, but they can be put into composite weaves which do not stretch.

A well-designed boat laminate should not be limited by its weakest link. It should be MORE than the sum of its parts. Has anyone achieved this using Spectra? I don’t know.

More info
Spectra (aka Dyneema) has three undesirable characteristics.

  1. It doesn’t bond well.

  2. It’s prone to “creep”, permanent stretching under load.

  3. It’s very heat sensitive. As the temp goes up, the creep rate increases dramatically. It’s polyethylene, so it melts at a relatively low temp.

    The Kevlar issue of “working” is the fabric abrading itself. Vectran does this, too, but moreso.

    I’ve seen greatly improved performance with materials that are blends of Kevlar or Vectran and Spectra. Their properties seem to cancel out each other’s weaknesses.