Sven Yrvindvideo
The bumpkin backstay
7 March 2026 · 4 min · 2K views
What this video covers
Sven Yrvind is building a small cruiser and working out how to route the bumpkin halyard — the line that runs from the masthead down to the bumpkin — using the same pulley-wheel principle applied to a masthead halyard.▶ 0:07 Because the mast runs straight up rather than at an angle, he adapts the geometry accordingly.▶ 0:51 The current design uses four wheels, but he is considering reducing to two to simplify the system.▶ 0:48 The central engineering question he poses to his subscribers: is a 25 mm axle strong enough?▶ 1:45 The boat weighs approximately 1,000 kg, the load center sits 35 mm out from the axle centerline, and the load may distribute unevenly between wheels when the hull hits a wave.▶ 2:12 He notes that duplex chain — roughly twelve times stronger than 316 stainless standing rigging — is part of the setup.▶ 2:47 He has only a pipe on hand that day, which he judges too weak, and plans to test a proper 25 mm axle on Monday.▶ 3:26 He asks anyone with the relevant engineering knowledge to help calculate whether that axle diameter is adequate.▶ 4:14
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