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Ask the Experts: Royalex vs. 3-Layer Polyethylene
May 23, 2011
At first glance there's not much to set these two canoe materials apart. Both tend to have a duller finish than the glossy composite hulls, both give a little when you push on them, both give a hollow kind of “thump” when you bang on them. How different are they? What makes Royalex worth the extra dollars? What kind of needs would be well served by polyethylene?Please note that we are specifically comparing 3 layer polyethylene construction to Royalex, not single layer poly. Single layer poly when used in canoes requires additional internal bracing and support to create a functionally stiff hull. This limits either the design of the hull or the lay-out of the outfitting of the canoe. Basically, single layer poly is an entirely different animal than either of these two materials.
If you were to pop off the canoe gunwales and look at the exposed top edge of the hull, it'd be hard at first glance to see any difference between Royalex and 3 layer poly. Both show a sandwich construction with a foam core between interior and exterior skins. This similarity in construction is what leads to that grey area over just exactly how they differ.
One difference is how they are produced and starts right off the bat when it comes to raw materials.
Royalex is a thermoformed material. It arrives at the canoe factory in a hard flat sheet. The actual design of the sheet varies from company to company and model to model. Working with a very finite amount of qualified materials, the designer can position various amounts of reinforcement where desired. This necessitates multiple Royalex sheets depending on intended use and the simple fact is that patch location in one length of boat doesn't work well for a longer or shorter canoe.
Working from the outside in, Royalex consists of an exterior vinyl skin overlaying a complete layer of ABS substrate. Behind that substrate will be positioned various additional patch layers of ABS substrate, usually concentrated or positioned at the stems of the canoe, along the “keel” line, and across the central belly of the canoe. The substrate layers are the armor reinforcements of Royalex, protecting the hull against impact and abrasion while adding a bit of stiffness to the hull.
Inside of the outer substrate layer(s) are the core layers which are made of an ABS foam which expands when exposed to heat in the oven. Core layers can be full end to end, side to side layers as well as selective reinforcements and provide the majority of the stiffness to the hull. Patch layers are usually positioned down the “keel” line and across the belly of the boat.
The core layers are protected on the inside of the hull by another full layer of ABS substrate and that in turn is covered with a final layer of vinyl forming the interior surface of the hull. The primary purpose of the vinyl skin is to shield the ABS substrate from exposure to UV degradation.
A sheet is then put in an oven and “cooked” until it reaches a specific temperature. The oven heat causes the center foam core layers to expand and also makes the entire sheet pliable. At that time the sheet is pulled from the oven and placed on a mold and then drawn into shape via vacuum and mechanical assist. After the hull has cooled sufficiently, it is removed from the mold and trimmed for assembly.
It should be noted that the available “ingredients” for use in Royalex are extremely limited and the biggest difference in different sheets is the amount of core (or foam) layers and patches vs. layers and patches of substrate (ABS plastic). Core is lighter and stiffer than substrate and more core and less substrate in a sheet will yield a lighter hull but also one that is not as durable.
If you were to remove the gunwale or have access to a wood trimmed royalex hull and were to look at the top edge of the hull, you will notice that the hull varies in thickness as you move down the length of the canoe. It will be thicker at the bow, then thin down as you move past the 1/3rd mark, thicken up again through the center of the hull, again thin down around the 2/3rd mark, and finally fatten up again at the opposite stem. What you are seeing is the designer putting the thickest, most durable hull where it is needed most and less hull where it is less needed. This is done to reduce the weight as much as possible while maintaining the integrity of the hull.
Three layer polyethylene is produced via a rotomolding process which is a very different process than creating Royalex. All hull materials start as a polyethylene powder and the first layer is poured into a sealed aluminum mold which is then placed in a large oven which in turns rocks and cavitates the mold as it heats.
That first “dump” of powder forms the exterior skin as the powder melts due to heat and then flows to coat the interior surface of the mold, encouraged by the rock and tilt of the mold. At a prescribed point in the process, the interior core layer of the hull is introduced. This layer is also polyethylene powder but it has a “blowing” agent added to it which causes it to foam as it melts and bonds to the exterior skin. The interior skin is then dumped into the mold and forms the solid skin you see on the inside of the canoe, sealing the foamed core between two solid skins. The mold is then removed from the oven and placed on a cooling rack that spins the hull as it cools, making sure the material is forced against the mold surface via centrifugal force as it cools and becomes rigid. The hull is then removed from the mold and the top trimmed off and made ready for assembly.
The “rock and roll” of the oven can be carefully controlled by the computer program and is orchestrated to put more powder (and hence more thickness) down the keel line and in the stems of the canoe, and less in areas that need less strength and stiffness, such as up the sides of the canoe.
One of the differences between the two materials is that the positioning of reinforcing layers in Royalex can be done with more precision than with polyethylene. With polyethylene, you're kind of dealing with a moving target as the mold rocks and tilts and the powder flows. With Royalex, the patch layers are placed on a flat sheet of material which is then vulcanized under significant pressure which effectively locks the reinforcement precisely into place. If you look carefully at the inside of a royalex hull you can often find the patch lines of reinforcing patches revealed as straight lines running across the hull or parallel to the keel line.
One key to why Royalex canoes cost more than 3 layer polyethylene models is simply the cost of the raw materials involved. Royalex simply costs more than powder, simply due to the preliminary production phases it goes through before it reaches the canoe factory. Each sheet has to be laid up, layers cut to size and positioned and then vulcanized. This process includes more hand labor and time than does the production of polyethylene powder and there are costs in sourcing and transporting the different materials used in creating royalex. The production of royalex also involves additional processing of component materials to specified thicknesses and dimensions.
Royalex is simply a more complex material to produce and this complexity comes at a price. Royalex is also an expensive material to transport as the sheets measure as much as 18' long by 3.5' wide, not something you can roll up and pack in a carton.
Now the question becomes, does that extra expense contribute to superior performance on the water? That's the crux question, what does all this mean to me, the paddler?
For one, Royalex is lighter than 3 layer poly, on average about 20%. Mad River's Explorer averages about 72 lbs. in Royalex while the comparable Explorer 16TT tips the scales in the high 80s. The difference can be bigger, MRC's Explorer LT 16 is a Royalex canoe with a lighter sheet lay-up that hits the water at 64 lbs or so. That's almost 30% lighter than an 87 lb TT model.
And weight is the significant issue to many paddlers. Most think in terms of how hard is it to get up on my car roof and then down but one also has to think about what it might mean on the portage trail as well as on the water. The very fact that a canoe weighs on the north side of 85 lbs may be enough to discourage paddlers even attempting some portages, thereby denying them the joys of exploring the waters that lay beyond. And while the canoe weight is not so readily apparent when it's on the water, it is undeniable that the heavier canoe represents the greater mass and the more mass there is the more energy it takes to make it move and to move it in a direction you want it to go. A lighter boat responds more quickly, accelerates faster, and holds pace more easily than a heavier counterpart.
So weight is a legitimate big deal and is one area that Royalex holds a big advantage. Another is repairability, especially in the structural sense. Polyethylene is notoriously difficult to get anything to stick to; it's just the nature of the beast. Royalex on the other hand is well disposed to bond to a variety of adhesives, from contact cements to fiberglass resins. This predisposition is big in terms of making permanent structural repairs, whether it be filling dents or patching cracks or splits in the hull. Both require the use of a resin. In case of a dent, resin is used to build the area back to original shape. If hull is cracked, reliable repair involves bedding fiberglass or Kevlar cloth in a resin matrix on the interior of the hull. For both of these applications, Royalex bonds better to available resins than does polyethylene. Part of the problem is polyethylene's inherent flexibility. Initially, you may get what seems a good bond but as hull flexes and moves, this creates separation between the more rigid patch and the underlying hull material. This flexibility combined with the inability of adhesives to get a good grip on the poly makes such repair difficult to sustain.
The addition of skid plates to the ends of the canoe to reinforce the stems is something a lot of canoeists opt to install. Skids can be installed on a Royalex boat using a good epoxy resin that will bond to vinyl. To install a skid on a poly boat requires the hull to be flamed and then a special resin to be used. So, it can be done, but is a more complicated process and tests have shown that the adhesion does not match up to that achieved on Royalex and the skids are more prone to being knocked loose upon impact.
The same issue of getting an adhesive to bond to polyethylene also comes into play when you want to install custom outfitting in your canoe. A lot of outfitting requires adhesive for installation and if your adhesive can't create a good bond, where are you? Contact cement works well with Royalex (too well sometimes if applied too liberally) but poorly with poly. This makes it difficult to effectively install knee pads, D-rings, etc. in poly canoes. Knee pads are more likely to be satisfactorily installed than a D-ring. As D-rings are subject to a sudden stress or pull, they are prone to popping free of the hull at the worst possible moment, especially if used to secure thigh straps.
At this point, you're probably wondering why even consider a polyethylene canoe? They're heavier, hard to fix and a challenge to outfit. What do they have going for them?
For one, price. As the raw material for the hull is considerably less expensive, the finished boat is also less pricy. If you're on a limited budget, or if you're not really sure how involved you'll become in paddling, the 3 layer poly canoe makes a lot of sense. No pun intended but it's a good way to get your feet wet in the sport at a modest cost.
And 3 layer poly boats are tough, plenty tough. It's rare that we hear of one failing. They shrug off repeated impacts, barely missing a stride. This makes them great boats in which to learn how to paddle as they are extremely tolerant of the learning curve. It's very unlikely that any of your mistakes will be terminal and more likely that it will be your pride that's more damaged than your canoe. Polyethylene after all, is the material of choice for whitewater kayaks, boats that are designed for, and use in extremely demanding and unfriendly waters. You can count on a 3 layer poly canoe to stand up to just about anything you want to throw its' way as well as things you don't intend to encounter.
Polyethylene is also as close as you can come to a care-free maintenance material. You don't have to provide protected storage, the boat doesn't have to live indoors, you don't even need to rinse it off before you put it up for winter. Probably the most critical thing you need to do when it comes to storing your canoe is to lock it up so it can't be stolen.
Royalex canoes are quite carefree but do better with a little TLC. If you live in cold climates, it's advisable to take a few simple precautions to minimize what are called cold cracks in the hull. Cold cracks result when the plastic hull contracts due to cold faster than the gunwale attached to it and the result can be stress fractures at the fasteners. This isn't something that happens frequently at all but it does require some preventative maintenance that a poly canoe does not. So in terms of care-free maintenance, the poly canoe gets the nod.
As expected, each material has strengths and weaknesses compared to the other. How do you sort these out and line them up in terms of what will serve you best?
Perhaps you should take a good look at your aspirations as a canoeist. Are you anticipating getting heavily involved in paddling, that it becomes an activity of choice and a primary recreational outlet? Do you feel ambitious about your paddling, envisioning running class III whitewater or heading off into the wilderness, near or far? Is paddling an end unto itself, are you feeling the urge just to head out to wet a paddle?
If the answer is yes to most of the above, then you'll probably be better served by a Royalex canoe. Yes, the initial outlay is a bit steeper but think of the rewards in terms of what you want to accomplish. A lighter boat that is still very durable, one that can be custom outfitted and structurally repaired when needed. In a sense the Royalex canoe can do anything that you'd ask of a poly canoe and more. Think of it in terms of a choice you'd make if a similar boat of each material lay before you on the beach and it was up to you which one you'd grab to head out on a ¼ mile portage or to get down a Class III run. Given the option, most folks would opt for Royalex.
So who would choose polyethylene? Again, if your discretionary budget is a major factor in the buying decision, you're getting a heck of a buy with poly. Virtually identical hull shape to a Royalex canoe for considerably less money and you're getting a boat you don't need to worry about, that readily absorbs abuse and neglect. If you look at canoeing as more of an occasional exercise, something done now and then and you're not particularly ambitious in your visions of your paddling experience, the poly boat will more than meet your needs. Yes, it's heavier but if you are only putting up with that on occasion, it's not such a big factor. If your paddling horizons are smaller and closer to home, poly could be the ticket.
Either material makes a great platform for hunting, fishing or birding (given the appropriate hull shape). The foam core deadens sound and also provides insulation against cold water. Compare that capability to using a booming aluminum canoe to hunt stealthy, wary prey.
Either material makes for a great family canoe (again assuming appropriate hull design). Both are very forgiving of error whether it come from inattention to where you're going while you referee a sibling squabble or if you let the young ones take the boat out on their own for some exploring.
Either material makes an impressive canoe, one that would have been eagerly embraced by the native cultures to which the canoe was a survival tool. Instead of fragile bark or wood, they'd be thrilled to have a canoe that wouldn't deteriorate and could take on adverse conditions with impunity. So, instead of anguishing over this decision, think of just how fortunate you are to face this dilemma. Give it some thought and realize that it's actually awful hard to go wrong.