Deadeye tensioning system
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Hey everyone,
Yesterday @lindsay_olsen told me about the deadeye tensioning system, and I was just reading about it on this hub. This was our first year of farming, and we didn't use a spreader bar/bridle. Do we need to use a spreader bar for the deadeye to work correctly in tensioning all lines? Also, if we do add spreader bars, would it need to be stainless and a specific size?
Thank you!Skye at Noble Ocean Farms
@skye_steritz -Hi Skye, you don't need a spreader bar to use a dead-eye tensioner, but it might be overkill if you are just using it to tension a single line. We use dead-eye tensioners on spreader bars and our larger arrays because we need the mechanical advantage to get the desired tension. If we have a 5 line spreader bar and want all lines to be under 100lbs of tension we need 500lbs (100lbs x 5) on the bridle/anchor side.
There are a bunch of different ways to tension a single line system but it really depends on your configuration. Happy to provide some tips once I know more about your farm setup.
Spreader bars don't need to be stainless. I have seen spreader bars made out of all types of material: aluminum, steel, PVC, and wood. The size also varies, depending on the strength of the material.
@david_bailey - Thanks, Dave! I'll let Skye weigh in on the specifics of her farm design. (Skye, maybe it'd be helpful to upload the sketch you showed me?) From our conversation I understand that she's using a multi-line grid-like system with corner anchors that aren't easy to adjust the positioning and tension of. Her farm experienced some line sag throughout the season and she's looking for ways to adjust the tension of the entire array without having to reset her anchors. I thought @alf_pryor17 and @clifford_goudey might also have some additional thoughts on whether a deadeye system could be integrated at the corner of a grid-like design.
@david_bailey - Thanks, Dave! I'll let Skye weigh in on the specifics of her farm design. (Skye, maybe it'd be helpful to upload the sketch you showed me?) From our conversation I understand that she's using a multi-line grid-like system with corner anchors that aren't easy to adjust the positioning and tension of. Her farm experienced some line sag throughout the season and she's looking for ways to adjust the tension of the entire array without having to reset her anchors. I thought @alf_pryor17 and @clifford_goudey might also have some additional thoughts on whether a deadeye system could be integrated at the corner of a grid-like design.
Skye, it sounds like the tensioning system Lindsay mentioned might be just the ticket. Anchors take time to settle in and achieve their intended holding power. Moving then to adjust rig tension re-starts that process. So we are left with the need to shorten lines somehow and with larger systems with pretension for controlling growline sag, such adjustment can be difficult even at low tide or at the down-current end of things.
Deadeye tensioners are an underwater block and tackle with no moving parts. You can insert whatever adjustability you need based on the amount of line you include. We've found 10' of adjustment is enough. This is what they look like molded in HDPE.
Let me know if you're interested.
Cliff
Thank you all so much for your inputs, @david_bailey @clifford_goudey and @lindsay_olsen.
Yes, we had 16 total grow lines in a grid-like system. Lines are roughly 200 feet each. They attach to the perimeter and the mid-line with "tag lines" connected with a seine ring and snaps. Here is a rough schematic.
It does seem like we need an alternative way to tension besides moving the anchors. I am interested in getting some deadeyes in place this fall. It sounds like we might need 16 of these dead eyes? Is that correct? Could you advise on the placement please?
Thanks again!
Skye
Skye,
OK, I see what you're trying to achieve.
The main problem with your drawing is the assumption that your two end lines are straight. They are not and that's why they are more properly called catenary lines. Like the cables on a suspension bridge, in order to provide uniform tension across your eight growlines, you have to build in the right amount of inward sag. You can still have uniform growline lengths but you have to make up for things by having variable length tag lines (we call them extensions). When this is done right, things work pretty well and you can enjoy close growline spacing because everything moves in unison.
We generally put two anchors, 90˚ apart, at each corner to provide some redundancy. That also allows us to stack these catenary modules both width-wise and length-wise. However, a single anchor at some outward angle can work.
In your case, a tensioning rig should go at the very top of the anchor line where you can best reach it for adjustments.
I'll reach out to you via email with some more specific advice.
Cliff
Hi @clifford_goudey
Just wanted to follow up from our previous discussion. We have a better drawing of our array now, which I am attaching here. Our array is somewhat (not completely) perpendicular to the flow of the current.
Questions I'm still wondering about:
Thank you!
Skye
Skye,
Your sketch is beginning to make sense to me. The precise lengths of the individual growlines can be calculated but a lot depends on the shape of the catenary line. Because I usually use a pair of orthogonally positioned anchors of the same size, I like to have the ends of the catenary approach those anchor lines at 45˚. In such a case, the center growlines are shorter than the outer ones by half the catenary width. In your case 200’ outer ones would call for lines in the middle to be 162’ long.
Of course, by using a single anchor at each corner, you can decide ahead of time what that angle should be. On the other hand, a shallow catenary means most of the system loads are from the tension needed to supply the outward force. IMHO, 45˚ is a good compromise.
Also, instead of shortening those center growlines and wasting seed string, consider making up your catenary lines with permanent growline extensions of specified length and have a straight row of C-links or other connector across that intersection with full-length growlines.
Here is a table of the extra length needed for the growline extensions as you move out from the middle with 16.67’ spacing between growlines.
Growline # Growline extension (ft)
1 0.46
2 4.17
3 11.57
4 22.69
5 37.50
Also, note that the separation between attachment points along the 172’-long catenary line increases as you get to the end. I’ll send all the dimensions to your email. This is what the catenary will look like.
Yes, a deadeye tensioner at the intersection of the catenary and the each anchor line should do the trick. Here is a photo on one being used on the ARPA-E farm in Puerto Rico.
Cliff
Thank you all so much for your inputs, @david_bailey @clifford_goudey and @lindsay_olsen.
Yes, we had 16 total grow lines in a grid-like system. Lines are roughly 200 feet each. They attach to the perimeter and the mid-line with "tag lines" connected with a seine ring and snaps. Here is a rough schematic.
It does seem like we need an alternative way to tension besides moving the anchors. I am interested in getting some deadeyes in place this fall. It sounds like we might need 16 of these dead eyes? Is that correct? Could you advise on the placement please?
Thanks again!
Skye
Skye,
OK, I see what you're trying to achieve.
The main problem with your drawing is the assumption that your two end lines are straight. They are not and that's why they are more properly called catenary lines. Like the cables on a suspension bridge, in order to provide uniform tension across your eight growlines, you have to build in the right amount of inward sag. You can still have uniform growline lengths but you have to make up for things by having variable length tag lines (we call them extensions). When this is done right, things work pretty well and you can enjoy close growline spacing because everything moves in unison.
We generally put two anchors, 90˚ apart, at each corner to provide some redundancy. That also allows us to stack these catenary modules both width-wise and length-wise. However, a single anchor at some outward angle can work.
In your case, a tensioning rig should go at the very top of the anchor line where you can best reach it for adjustments.
I'll reach out to you via email with some more specific advice.
Cliff
Hi @clifford_goudey
Just wanted to follow up from our previous discussion. We have a better drawing of our array now, which I am attaching here. Our array is somewhat (not completely) perpendicular to the flow of the current.
Questions I'm still wondering about:
Thank you!
Skye
Skye,
Your sketch is beginning to make sense to me. The precise lengths of the individual growlines can be calculated but a lot depends on the shape of the catenary line. Because I usually use a pair of orthogonally positioned anchors of the same size, I like to have the ends of the catenary approach those anchor lines at 45˚. In such a case, the center growlines are shorter than the outer ones by half the catenary width. In your case 200’ outer ones would call for lines in the middle to be 162’ long.
Of course, by using a single anchor at each corner, you can decide ahead of time what that angle should be. On the other hand, a shallow catenary means most of the system loads are from the tension needed to supply the outward force. IMHO, 45˚ is a good compromise.
Also, instead of shortening those center growlines and wasting seed string, consider making up your catenary lines with permanent growline extensions of specified length and have a straight row of C-links or other connector across that intersection with full-length growlines.
Here is a table of the extra length needed for the growline extensions as you move out from the middle with 16.67’ spacing between growlines.
Growline # Growline extension (ft)
1 0.46
2 4.17
3 11.57
4 22.69
5 37.50
Also, note that the separation between attachment points along the 172’-long catenary line increases as you get to the end. I’ll send all the dimensions to your email. This is what the catenary will look like.
Yes, a deadeye tensioner at the intersection of the catenary and the each anchor line should do the trick. Here is a photo on one being used on the ARPA-E farm in Puerto Rico.
Cliff
@Michael_teasdale this conversation in the comments below may be of interest to you. @Cliff_goudey developed the modified deadeye tensioner to improve tensioning capability on ocean farms without having to make anchor adjustments. See below.