Tackling Tensioning
<< Trying to Build a Better Deadeye >>
When the hinged mast is raised on Zephelios, it will be self-supporting while undressed. When hoisting the square sail, the head loads will be transmitted to the yard and normally absorbed by the braces. As a safety backup, I will attach two running backstays to support the mast should the braces fail or be inadvertently released. To tension these stays, I plan on using a 3:1 block-and-tackle arrangement.
Block and tackle with 3:1 mechanical advantage
I used a similar system on Sqoop (for details, click HERE ) which was simple and reliable. The downside was the need to attach and feed the 1/4” lashing line (aka “lanyard”) through the thimble and D-ring each time I wanted to install each running backstay, and reverse the process to uninstall; this eventually became a tad tedious. I decided to do a bit of innovating and see if I could find a better way.
During the Age of Sail, deadeye tackles were used to tension the shrouds and stays of these primarily square-rigged ships. To stay true to this maritime legacy, and tipping my hat to Patrick O’Brian and his wonderful novels, the design goals for this tackle arrangement became:
— No moving parts (like a sheave in a block)
— 3:1 mechanical advantage
— Easy to install, tighten and adjust
— Easy to loosen and uninstall
— Simple, reliable, secure, robust and low maintenance
— Able to withstand an ultimate tensile load of 4,200 pounds, meaning the lanyards would need a breaking strength of at least 1,400 pounds.
This last design goal dictated the use of 3/16” [5mm] single braid HMPE for the upper part of each running backstay and 1/4” [6mm] double braid polyester for the lanyards.
I got some good ideas while wandering around the Pocket Yacht Palooza in Port Townsend this past summer, like this 2:1 purchase for tensioning the shrouds/stays:
I thought this was a great solution, except it uses a modern block (with a moving sheave), and I would’ve preferred something more secure than a cam cleat (more moving parts) to belay the working end. I perused the offerings available online from the various vendors, but either couldn’t find anything that met all my design criteria or the components were way beyond my budget (yes, I’m looking at you Colligo Marine).
Finally, in true PBO/DIY fashion, I decided to design my own and fabricate it myself using scrap material I already had on hand, or that I could scrounge from others. My goal was to create a modern version of a deadeye tackle.
Deadeyes were traditionally made from lignum vitae (aka “guayacan”) but I had no idea where to procure this exotic wood species (let alone how to pronounce its name!), so instead I went with readily available red oak. Finding accurate information about the tensile strength of hardwoods is surprisingly difficult, partly due to the fact that structural wood is normally used in compressive or bending moment applications and also because the construction material used is almost always a softwood, like pine or fir. I found tensile strength values of 4,000 psi to 20,000 psi (parallel-to-grain) for most types of oak, so I decided to use the low end of that range for my design; this meant the minimum cross-section of the deadeye at any point along its length would need to be at least one square inch [25mm^2].
Another building material that’s often used in marine applications is fiberglass, and it really shines in tension applications. The tensile strength of a well-fabricated professional composite made of E-glass fibers and epoxy resin can be over 200,000 psi; as an amateur, the strength of any layup I make will be much more modest. Nevertheless, you don’t need a lot of FRP to strengthen a wooden fixture under tension. I decided that this deadeye tackle would be made of oak and reinforced with fiberglass. The epoxy resin would also offer some protection from the elements.
After sketching out a number of different ideas, I settled on a version I thought would work. I first made a test assembly using some discarded redwood railing, just to ensure it would perform as envisioned. Redwood is not very strong, but it’s easy to work with and I was able to quickly knock out a prototype and convince myself that the design was both feasible to make and functional to use.
Deadeye tackle prototype
The lashing line & cleat I already had on hand and I was very fortunate to obtain some beautiful, knot-free red oak stock (1.5” x 1.5”) [38mm x 38mm] from a friend. The cutting, drilling and edging of these pieces was straightforward, but forming the radii of the deadeye holes was a bit time consuming because it involved careful shaping & smoothing with a Dremel tool, hand file and sandpaper. [NOTE: to get the recommended 8:1 ratio of sheave-to-rope diameter, 2” x 2” stock would be required]
Here are the construction drawings should you choose to make a pair. These are not set plans, but simply guides; some dimensions are intentionally omitted to allow you the flexibility to modify as desired.
After reviewing this instructional video, I made a couple soft shackles of the correct length to attach the upper deadeye to the terminating thimble of the upper part of the backstay, and the lower deadeye to the chainplate. Dyneema is really great stuff; strong, lightweight and so easy to splice! After coating the deadeyes with epoxy and assembling everything, here is the result:
