Why?
More generally, I'm curious as to the relative cold crack potential of Royalex canoes with wood vs. vinyl vs. aluminum vs. vinyl w/aluminum insert gunwales. I've never thought about this issue.
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Can Royalex safely freeze?
- Thread starter MyKneesHurt
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Why?
More generally, I'm curious as to the relative cold crack potential of Royalex canoes with wood vs. vinyl vs. aluminum vs. vinyl w/aluminum insert gunwales. I've never thought about this issue.
Cold cracking is generally considered to be the result of different rates of contraction between Royalex (ABS plastic), wood, and metal during very cold temperatures. A vinyl rail generally has a contraction rate that is most similar to a Royalex hull so is least likely to produce a cold crack. Wooden and aluminum rails (or vinyl rails with aluminum inserts) have contraction rates that are significantly different from a Royalex hull and are most likely to create cracks in very cold conditions. Your mileage may vary...
Benson
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That sounds reasonable.
Maybe I don't fully understand why different cold contraction rates would cause Royalex cracks. Of course, I've heard about it for at least 30 years and have stated so myself, but now I'm not sure I understand the physics of it. What contracts more, the wood or the Royalex?
Moreover, I sort of have the feeling that differential expansion rates in heat would be more likely to cause cracking than different contraction rates in cold. But I've never heard about Royalex heat cracks, so I guess that intuition may be empirically wrong.
Pondering further, I wonder if the cracks happen as the ambient temperature goes from warm to freezing, or do the cracks happen when the temperature reverses and goes from freezing to warm.
I just put away my royalex mad river explorer for the season. I recently moved to Saranac Lake, NY, and I now store my canoes in an attached, but unheated, garage. At my old house, this canoe was stored in my basement and never saw temps below 50 degrees F. Now, I expect it will see temps down to around 0.
To prep it, I completely removed the 8 screws in the outwales at bow and stern, as this is where I previously repaired cold cracks in the hull. This allows the ends of the gunnels and decks to “float” freely. I then loosened all the other screws in the inwales 4 or 5 turns.
Does this seem acceptable or should I completely remove every screw?
To prep it, I completely removed the 8 screws in the outwales at bow and stern, as this is where I previously repaired cold cracks in the hull. This allows the ends of the gunnels and decks to “float” freely. I then loosened all the other screws in the inwales 4 or 5 turns.
Does this seem acceptable or should I completely remove every screw?
Pondering further, is it a rapid change in ambient temp that causes the issue, or the overall low temp, or both (a rapid change below a certain temp)?
I'm not surprised high heat isn't an issue because the royalex should be more pliable the warmer it gets, up to melting temp.
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There's a material property called coefficient of thermal expansion that tells how much a material expands or contracts with each degree of temperature change. It's very low for wood and much higher for Royalex. So I think the failure mechanism is that the wood holds still and the Royalex shrinks (in every direction) so the gunwale screws may start to tear through it and since the Royalex itself gets less pliable (more brittle) at low temps any small tear could grow quickly. In this case it's not the rate of change of temp (thermal shock). That's my guess and I'm sticking to it for now.
TLDR: With a temperature drop from 70°F to 20°, a 16' Royalex hull wants to shrink over 3/8" but a wood gunwale only wants to shrink less than 1/32". This mismatch causes a lot of stress between the two.
As @gumpus said it's the difference in the coefficients of thermal expansion, not the rate of change. Different sources give different numbers for these coefficients but they don't disagree too much. One source gives these:
ABS plastics 0.0000410
Vinyl2 0.000045
PVC1(rigid) 0.000035
ALum 0.0000117
Wood, Oak 0.0000027
These values are all in inches per inch per degree Fahrenheit. As you can see, ABS and vinyl (the two materials in the Royalex sandwich) are pretty similar, aluminum is less, and wood is a lot less. What this means is that if you have a 16' (192") long canoe and you reduce the temperature from 70° to 20°, it will shrink 0.0000410*192*50 or 0.39". A piece of wood (presumably ash isn't that different from oak) the same length will only shrink 0.026". So the Royalex hull has grown by over 3/8" but the screws in the gunwales have only moved less than 1/32", a mismatch of 0.36 inches, nearly 3/8". Something's gotta give!
Now if you assume (not a good assumption, but this is just a thought experiment) that the hull is bigger than the wood gunwale so it's gonna do what the hull wants to do and the wood is along for the ride, we can calculate how much tension it takes to stretch the gunwale 0.36". To do that we look the modulus of elasticity, which for ash is 1363000- 1740000 psi, let's use the average of 1551500. That is in psi per inch per inch, and for a 1" square gunwale the stress in psi is the tension in pounds. The actual load to stretch the 16' gunwale 0.36" is then nearly 3000 lbs! And that's just one strip; there are two each side of the hull.
Now the Royalex hull is actually going to be more stretchy (it has a lower modulus of elasticity) so it'll probably stretch (or rather, be prevented from stretching) more than the wood but the shape is complex enough that I'm not going to try to calculate it but either way, it's a lot of force so as I said, something's gotta give.
If you have aluminum gunwales the mismatch will be 0.28", less than wood but still significant.
As @gumpus said it's the difference in the coefficients of thermal expansion, not the rate of change. Different sources give different numbers for these coefficients but they don't disagree too much. One source gives these:
ABS plastics 0.0000410
Vinyl2 0.000045
PVC1(rigid) 0.000035
ALum 0.0000117
Wood, Oak 0.0000027
These values are all in inches per inch per degree Fahrenheit. As you can see, ABS and vinyl (the two materials in the Royalex sandwich) are pretty similar, aluminum is less, and wood is a lot less. What this means is that if you have a 16' (192") long canoe and you reduce the temperature from 70° to 20°, it will shrink 0.0000410*192*50 or 0.39". A piece of wood (presumably ash isn't that different from oak) the same length will only shrink 0.026". So the Royalex hull has grown by over 3/8" but the screws in the gunwales have only moved less than 1/32", a mismatch of 0.36 inches, nearly 3/8". Something's gotta give!
Now if you assume (not a good assumption, but this is just a thought experiment) that the hull is bigger than the wood gunwale so it's gonna do what the hull wants to do and the wood is along for the ride, we can calculate how much tension it takes to stretch the gunwale 0.36". To do that we look the modulus of elasticity, which for ash is 1363000- 1740000 psi, let's use the average of 1551500. That is in psi per inch per inch, and for a 1" square gunwale the stress in psi is the tension in pounds. The actual load to stretch the 16' gunwale 0.36" is then nearly 3000 lbs! And that's just one strip; there are two each side of the hull.
Now the Royalex hull is actually going to be more stretchy (it has a lower modulus of elasticity) so it'll probably stretch (or rather, be prevented from stretching) more than the wood but the shape is complex enough that I'm not going to try to calculate it but either way, it's a lot of force so as I said, something's gotta give.
If you have aluminum gunwales the mismatch will be 0.28", less than wood but still significant.
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