Any experience with home made concrete anchors?
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Hey all,
I'm a first time kelp farmer based out of mid-coast Maine, and I'll be planting my small, single line array in about 25ft (median tide) of water this winter! Since it's a small-scale year intended for learning, I'm trying to source as much of my gear second hand or build it myself when possible. Trying to learn on a budget! In that vein, my brother built me two 100 lbs pyramid anchors out of concrete, which I fitted with stainless steel loops. I know there's about a 50% reduction in the weight of concrete once it's submerged, and I'm concerned they'll be too light to use.
Have any of you made your own anchors out of concrete? How heavy did you go? Considering weighing them down with kettle bells or making another batch with chain embedded in the concrete to weigh it down more. I'll be planting one 250ft line in a relatively sheltered part of the bay with moderate current.
Appreciate any and all insight! And if any of you are midcoast Maine based farmers, I would love to connect!
@eliza_43 Here's two previous posts around homemade anchors that may be useful: train wheel anchors, custom anchor option
I also moved your post to the Farm Design topic so it will reach more people.
Pinging @kendall_barbery & @clifford_goudey for more input as well!
@jwhis Thank you!!
@eliza_43 Concrete deadweight anchors are not a bad approach if you have the boat to handle them. You are right, they do lose 40% of their weight in the water due to their own buoyancy but the real problem is their tendency to slide along the bottom.
For a boat mooring where the load can come in any direction, just go big. For farm use where the pull is in a known and consistent direction, without digging in, they will likely fail. The 100 lb anchor you describe will barely resist 50 lbs of horizontal pull.
@clifford_goudey Got it! They are pyramid anchors in a mud bottom, so I'm hopeful they will sink which should help the sticking power. It's also only going to be one 150ft line this year. Do you think adding weights to the existing pyramids would help? Or should I make them again with more submerged rocks/metal? I still have the molds so that wouldn't be the worst thing in the world!
@jwhis Thank you!!
@clifford_goudey Got it! They are pyramid anchors in a mud bottom, so I'm hopeful they will sink which should help the sticking power. It's also only going to be one 150ft line this year. Do you think adding weights to the existing pyramids would help? Or should I make them again with more submerged rocks/metal? I still have the molds so that wouldn't be the worst thing in the world!
@eliza_43 Have you played around in the Farm Design Tool? I ran a quick calculation based on a 250 ft growline with ~3 lbs of kelp per foot (about average for Maine growers) on a site with 1.5 knots of current. The drag calculation in the model assumes the longline is positioned perpendicular to current, which may not be the case for your site, but in this scenario it suggests you need ~900+ lbs of holding power per anchor at peak growth. The model doesn't currently include a friction coefficient for your bottom type, and depending on the orientation of your longlines and current velocity, this could be oversized. But I agree with Cliff that a 100 lb concrete anchor is not sufficient for this scenario. A drag embedment anchor of that size should be more than sufficient though.