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Old 04-26-2004   #1
Join Date: Nov 2003
Posts: 75
Injection or rotamold kayaks

i have been researching this topic on which is better, injection molded or rotomolded kayaks. Its seems to me that injection molded kayaks are better in terms of safety, durability, and speed (specifiaclly in creekers). Roto-molded kayaks seems to have an advantage in their ability to have more dynamic styles and colors yet are lacking in durability and safety. any thoughts?

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Old 04-26-2004   #2
Join Date: Oct 2003
Posts: 388
Yea, I think you've got it right.

Making the injection mold is expensive, so you can't constantly change the design. You need to produce many number of boats to pay for the mold.

I think Prijon and Eskimo are the only ones who use the injection mold.

The injection molded kayaks are stiff enough that they supposedly don't need center pilars.

Since creek boat designs seem to have stabilized and strength is important for a creek boats, you'll see a lot of Prijon creekers. But, play boat design still seem to be improving. It seems like Prijon is a little behind here.

I don't understand why injection moded kayaks would be faster.

I've got one injection molded kayak and one rotomolded kayak, and I'm not sure there is a big difference. I don't mind the center pillar. Perhaps the Prijon will last 15 years instead of 10, but I don't care.

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Old 04-27-2004   #3
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I wanted to mention that I think the injection molded creek boats are the way to go. The plastic is a whole lot stronger so they don't need a center pilar to maintain hull integrity. The center pilar may not be a big deal when you're sitting in the boat, but it could be a huge deal if you get pinned. You should be able to exit your creeker without your hands - a lot of people cant do that and not having a center pilar can help. Also, as far as speed, I don't know the mechanics either, but I do know that the Salto and Embudo are two of the best hole punching creekers on the market. If you're looking to buy a new creeker, definitely check out a Salto and take it out for a demo. I should have one soon that you can borrow if you're interested.

P.S. try standing on a rotomolded kayak and on an injection molded kayak - and you only weight 100-250 lbs compared to the 4000 lbs of pressure the water can easily exert on a pinned boat.
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Old 04-27-2004   #4
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Eric, you're an idiot! I didn't even see that you started this post. My advice still stands.
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Old 04-27-2004   #5
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Denver, Colorado
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$.02 from a mechanical engineering geek who's worked with plastics a bit...

The Prijon and Eskimo boats are blowmolded, not injection molded. OK, so it's a two step process starting with an injection molded "plug" about the size of a nalgene bottle (probably only the mass of plastic varies for the whole stable of boats) that gets placed in a mold and "blown up" to the shape of the hull.

The molds are way more expensive, so they can't go changing hull shapes on a whim like you can with rotomolded boats. That's the downside, which as someone mentioned, makes it tougher for these companies to keep up with the ever-changing playboat market.

The upside is that you can use higher density polyethylene (same molecule, longer chain) than is possible with rotomolding. The plastic is undeniably stronger and tougher, so the durability argument is sound.

I'm not so sure about the safety argument. The CFS had a pillar that was short enough that it wouldn't impede exit, and could provide a step for a vertical pin situation. I'm also not convinced that a pillared boat is more likely to fold in the vertical plane around your legs, which is how entrapment seems most likely. Then again, you can't run a pillar thru the cockpit. Either way, no pillar in back is nice for storage.

Any speed differences have to do with hull and deck shape, not molding process, IMHO.
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Old 04-28-2004   #6
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Roy is absolutely correct (from another Mech Eng who happens to make plastic bottles for a living).

I'd like to add a few thing though. With today's shorter creekers, this old folding/entrapment issue is less of a problem than it was in the days of kayaking with yachts (larger surface area = higher force, and longer levers produce more torque). It could just be a sales/marketing issue at this point and less of an actual safety concern in the majority of situations. But I don't have the money to do the tests with a sufficient statistical sample to give a solid answer to which is safer. This would also depends on how you like to get pinned.

As for folding, a rotomolded kayak would be more likely to fold in the cockpit area than a blown boat OF THE SAME DESIGN. This is due to the longer polymer chains that make it stiffer as Roy pointed out. This chain length is refered to as the Intrinsic Viscosity (IV for short). I've heard people say "this plastic sucks" (from a person bashing LL's plastic earlier) and others say that all Roto boats are made of the same plastic (JK's will soon be an exception). I don't know if they all buy the same resin or not, but I can say that different methods of processing the same resin can drastically affect the IV. So someone who says this plastic sucks may be right.

Now I'm rambling... back to the first sentence in the previous paragraph. I said "of the same design." Introducing ribs, folds, curves, etc., will dramatically increase the strength of the boat. A flat piece of paper can't support any weight, fold it and you can hold a roll of quarters. Take a look at a Safeway water bottle next time you are shopping and you'll see what I mean, any less plastic and it would be a bag instead of a bottle.

Next, I am absolutely convinced though that a pillared boat will take a higher load on the front and back deck than an unpillared blown boat. This is based off of personal experience of having to uncave the back deck of an Embudo after taking it off of a rack and from paddleing a Gradient and CFS for years. I could park my car on the Gradient but just try jumping on an Embudo, front or back, and you get serious deflection.

I've also stepped out of my Gradient in a vertical pin situation and the ABS plastic center pillar made it clean and easy. I prefer it. But I guess it is like I said earier, it all depends on how you like to get pinned.

All pinning and entrapment scenarios are going to be different and you can come up with infinite situations where one is better than the other but it all boils down to this- If you are boating stuff that makes you concerned about this issue, boat with people you trust, get swiftwater training, scout, and check you ego.
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Old 04-28-2004   #7
pnw, Washington
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You guys kick ass. Never ceases to amaze me the knowledge that is possessed by some of the people on this board and of course the ignorance of others. Excellent posts guys!

"Depends on how you like to get pinned", crack me up. For my money I like to get pinned between some chick half my age(and yes she would still be legal) and the keg.
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Old 04-28-2004   #8
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How is the plastic that Jackson Kayak uses different?
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Old 04-28-2004   #9
Join Date: Mar 2004
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Most kayaks rotomolded today use Linear Plastic. When linear resin is heated then cooled in the shape of the kayak the molecules "bond" together by interlinking strands of the molecules. Like if you throw a bunch of coat hangers together and try to pull them apart. You might hear of "Super Linear" (Dagger calls it Excel or used to anyway). These moleculse are engineered to have longer more consistant sized strands on the molecules thusly makeing the final product stronger.

Crosslink actually has a chemical reaction when heated and cooled. The molecules make a chemical bond and are one.

There are pro's and con's in the manufacturing of both types of plastics.

That is part of the reason you can repair linear and not crosslink.
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Old 04-30-2004   #10
Join Date: Apr 2004
Posts: 39
The moment of calapse is the most important!!!!!!!!!!

One thing that no one has said, and is the most important, is what is safer at the moment of calapse!!!!!!!!! If you have a beam between your legs and the boat is going to fold that beam has to go one way or the other. Your leg will be trapped. I have had the bad luck to be around 2 deaths in the water and both were trapped by there beams!!!!!! So I started to paddle a Slato and now an Embudo that I truly love by the way, but I realized that I can corkscrew my way out of the boat and I could not do that with a roto molded boat. Fact is that blow molded plastics are far stronger. Prijon behind in the playboat design???????? Have you paddled a Release???? OMG that thing is the poo take a smell!!!!!!!!!!!
Be safe out there Adios......

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