Polyethylene

[Kevin B (RPS Coach)]

Does anyone know the pros/cons of the different types of polyethylene on the market for plastic kayaks? For example super linear poly versus a triple layering system?

[Bill Gwynn]

My understanding of plastics in kayaks is...there a basically two types of plastic used (3 if you count the new Airalite stuff) They are High Density Linear Poly, and Cross Linked Poly.

Most high quality poly boats use Linear Poly, and the Cross linked poly is generally for less expensive rec kayaks. Cross linked poly is nearly impossible to repair.

Linear Poly boats typically are manufactured either as rotomolded single-layer, rotomolded multi-layer, and blowmolded. I believe that Prijon uses blowmolding, and most others use rotomolding.

Both P&H and now Valley are using triple layer linear poly, and these are considered to be some of the best made poly boats on the market. So, I would say triple layer poly is the best.

[Kevin B (RPS Coach)]

Thanks Bill. Your response verified much that I had read. Your comment about the Valley boats was particularly helpful.

[Dee Hall]

The triple layer consists of two layers of dense polyethylene surrounding a foam core of polyethylene. It's thicker than the other boats and more rigid without being significantly heavier.

-Dee

[Brian Nystrom]

Although it's not the primary consideration for a kayak, IIRC linear poly' can be recycled, but cross-linked cannot.

Bill's right about P&H and Valley. Prijon is also well-regarded and the Wilderness Systems Tempests seem to be made of good material. Perception and Dagger are sub-par and I haven't been particularly impressed with Necky's plastic, either. Old Town uses multi-layer construction, but their plastic is very soft and deforms rather easily. They probably don't make anything you'd be interested in, anyway.

[Kevin B (RPS Coach)]

Brian,

It's interesting that you brought up Perception. I'm trying to convince someone of the merits of the Valley plastic boats over the "equivalent" Perception models. I happen to agree that the Perception plastic is sub par since I started with an Eclipse, but I'd be interested to know why you think so as well. Each company seems to use different lingo for their plastic. Both Dee and Bill's comment regarding the three layer system used by Valley cleared up a lot. As for Perception, they say they use Super Linear Polyethylene (straight plastic not the Airalite stuff) and I'm curious as to why it's so super.

[chpaton]

If you are asking this question because you or someone you know is considering a NEW poly 'yak, I advise doing some current research. The chemistry of the polyethylene used by a significant few kayak makers is evolving rapidly. Think lighter, harder, stronger.

As far as manufacturers with "good" formulas previously in use, I'd add Current Designs (I'm only personally familiar with the Storm) to the list of those mentioned by other posters.

[Brian Nystrom]

From what I've seen Perception boats are very thin and as you say, single layer. They frequently mold stiffening grooves in their hulls to stiffen the flimsy material, then give them a misleading name such as "tracking grooves". On some models, they have to resort to bolting metal tubing to the keels to keep them from oil-canning. If a hull cannot maintain adequate stiffness without these "band aids", it's a bad sign. A friend of mine is a Perception/Dagger dealer and constantly bemoans their substandard construction. Since Dagger became part of the same company, he says they've done the same thing to their boats, made them thin and flimsy. Basically, they're "McKayaks".

The bottom line is that there are better boats for not a lot more money. A used Valley, P&H or Prijon would be a better boat than a new Perception.

[Dee Hall]

My understanding is that Wilderness Systems bought Watermark (Perception, Dagger, and others), so it will be interesting to see what happens to the polyethylene on these boats. Also, there is the competition from the new thermoformed, harder plastics from some of these manufacturers. For now, it's a little more expensive, but the end result is a more rigid, attractive boat that can be glued and repaired more easily.

-Dee

[Kevin B (RPS Coach)]

>Also, there is the competition from the new thermoformed,

>harder plastics from some of these manufacturers. For now,

>it's a little more expensive, but the end result is a more

>rigid, attractive boat that can be glued and repaired more

>easily.

Dee,

Are you referring to what Perception calls Airalite?

[Brian Nystrom]

You're may not be old enough to remember Mattel's "Vacuform" toy molding machine, but that's the way thermoformed boats are made. A sheet of plastic (most are derivatives of polycarbonate) is held in a frame and heated until it's pliable. It's then pulled down over a mold and the air is evacuated, pulling the plastic tight to the mold. After a few minutes of cooling, the newly formed hull or deck is popped off the mold. As with fiberglass boats, the molded pieces are then trimmed and then joined along the sheer seam to make a complete kayak. It's a pretty efficient process, as the molds are about the same size as those for fiberglass boats, the sheet plastic heats easily and quickly and the molded panels can be popped off the molds pretty quickly.

Polyethlylene boats are rotomolded. That entails pouring a predetermined amount of polyethylene powder or beads into a two-piece mold that's sealed. The mold is heated and rotated to distribute the plastic once it melts. I understand that the rotation speeds and patterns can be quite complex in order to achieve the desired result. Once the mold has cooled sufficiently, it's opened and the boat is popped out in one piece. When Walden was auctioning off their inventory and equipment, I had a chance to look at some of the boat molds. They're large and heavy and the ovens to heat them were pretty massive. It required gantries and such to move the molds around, ovens built into the floor to heat them and pretty large mechanisms to rotate the molds as they were heated. Apparently, the cooling process is also much longer than for thermomolding, which slows the production time.

Although thermomolded boats currently command a premium over the cost of similar rotomolded boats, I wouldn't be surprised to see that disappear in the future, at least for sea kayak sized boats.

[donperry]

Vacuum forming tends to thin the sheet material where it is stretched over high points in the form. What do they do to reinforce the high wear areas along the keel in the bow & stern?

[Brian Nystrom]

[EEL]

>

>Vacuum forming tends to thin the sheet material where it is

>stretched over high points in the form. What do they do to

>reinforce the high wear areas along the keel in the bow &

>stern?

FWIW, I have seen it claimed that the sheets can have variable thicknesses and by using molds properly they are able to have high wear areas formed with greater thickness.

Ed Lawson