A dolphin striker (an older term for a martingale boom or simply a martingale; sometimes called a striker) is a small vertical or near vertical ancillary spar spanning between the bowsprit and martingale thereby redirecting the tension in the forward end of the martingale slightly more vertically. This vertical component is necessary to more effectively oppose the forestays' mostly upward tension on the forward end of the bowsprit than would be the case in the absence of the dolphin striker.
Around 1813 some large sailing vessels experimented with double strikers: these had two downward-pointing spars forming an inverted V in the middle of the bowsprit. However, the practice was short-lived as it did not seem to provide any additional benefit. The dolphin striker's length was typically half the length of the spritsail yard or, when spreaders were used, the same length as one spreader.
A short spar perpendicular the bowsprit, used with martingales for holding down the jib-boom. The position is such that a dolphin leaping at the bow of a vessel could possibly be struck by this spar.