Train wheel anchors
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I’ve just returned from a workshop in Alaska on kelp farm gear and heard a lot of buzz about scrap train wheels for use as anchors for kelp farms. Available in limited quantities in Anchorage, they seem in attractive option up there. A couple of farmers I’m working with are using them and I’ve been thinking about their utility in seaweed farming.
Weighing around 700 pounds and being steel, they only loose 13% of their weight when submerged in seawater, resulting in 609 pounds of underwater weight. Compared to concrete, that loses 43% of its weight underwater, you’d need a 1,244-pound block of concrete to match them. So in either material, let’s look at the performance of something weighing 609 pounds underwater. As a pure dead-weight anchor, the holding power depends on its friction coefficient on the seabed, which in turn depends on the type of bottom and on how long the weight sits there. In soft mud bottoms typical of Alaska, most sources estimate this coefficient to be 0.7 and as a result, 609 pounds x 0.7 = 426 pounds of horizontal pull capacity.
However, the scope ratio of the anchor line comes into play here. A 3:1 scope ratio (line length/water depth) means that for every pound of line pull there is a third of a pound upward pull on the anchor. Doing the calculations on that situation results in a holding power of 345 pounds. That’s a pretty disappointing result given the difficulties in handling a 700-pound wheel or a 1,244-poind block of concrete. It also pales in comparison with a drag embedment anchor that typical have a holding power 20 times their weight.
There is no question that a large mass at the end of a mooring line helps to dampen jerk loads caused by waves, but I recommend caution in the use of any sort of dead-weight anchor, as the cost to rebuild a farm tangled due to sliding anchors can far our weigh the economy of what seems like a handy solution.
If you’re interested in more gory details om anchor performance, I recommend NCEL report N-1627 “Interaction of Anchors with Soil and Anchor Design” by Robert J. Taylor that can be found at: https://apps.dtic.mil/sti/pdfs/ADA116597.pdf
Cliff
@clifford_goudey how much does it cost to purchase and ship these - the physics work in my head
ben
@benedict_glover Benedict, according to folks I've spoken to in Alaska, they can be purchased from the local railroad company for $200 each. I'm guessing the cost of getting them from the facility in Anchorage to a farm site is considerably more than that initial price. But at $200 for 700 pounds of steel (28¢/lb.), it seems like a good deal, though scrap steel in general is only worth 3¢ to 5¢ per pound, depending on your location.
The problem is, and explained above, dead-weight anchors are inefficient and for their holding power they may not be worth the hassle associated with their handling when you figure they could be outperformed by a 20# Danforth.
I’ve heard it mentioned that railroad wheels are often used in oceanographic research to keep high-seas moorings in place. That is true, but often stacks of them are used and the application is very different from seaweed farming as the loads those mooring see are largely vertical and not horizontal.
This does not mean there is no role for these wheels in ocean farming. They could work nicely as a tom weight in front of a drag embedment anchor to be sure that the anchor never sees an uplifting force. They are also well suited for boat mooring where the pull can be omnidirectional and the mass of the wheel provides some immunity to jerk loads caused by waves.
@benedict_glover Benedict, according to folks I've spoken to in Alaska, they can be purchased from the local railroad company for $200 each. I'm guessing the cost of getting them from the facility in Anchorage to a farm site is considerably more than that initial price. But at $200 for 700 pounds of steel (28¢/lb.), it seems like a good deal, though scrap steel in general is only worth 3¢ to 5¢ per pound, depending on your location.
The problem is, and explained above, dead-weight anchors are inefficient and for their holding power they may not be worth the hassle associated with their handling when you figure they could be outperformed by a 20# Danforth.
I’ve heard it mentioned that railroad wheels are often used in oceanographic research to keep high-seas moorings in place. That is true, but often stacks of them are used and the application is very different from seaweed farming as the loads those mooring see are largely vertical and not horizontal.
This does not mean there is no role for these wheels in ocean farming. They could work nicely as a tom weight in front of a drag embedment anchor to be sure that the anchor never sees an uplifting force. They are also well suited for boat mooring where the pull can be omnidirectional and the mass of the wheel provides some immunity to jerk loads caused by waves.