A bobstay is a part of the rigging of a sailing boat or ship. Its purpose is to counteract the upward tension on the bowsprit from the jibs and forestay. A bobstay may run directly from the stem to the bowsprit, or it may run to a dolphin striker, a spar projecting downward, which is then held to the bowsprit or jibboom by a martingale stay.
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The dictionary definition of bobstay at Wiktionary