A Dolphin striker is a small vertical or near vertical spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

 spanning between the bowsprit

Bowsprit

The bowsprit of a sailing vessel is a pole extending forward from the vessel's prow. It provides an anchor point for the forestay, allowing the fore-mast to be stepped farther forward on the hull.-Origin:...

 and martingale

Martingale (rigging)

A martingale is a fore-and-aft stay lying directly beneath the bowsprit strengthening it and, if extended from the sprit a jibboom, against upward force created by the head stays....

 thereby redirecting the direction of the tension in the forward end of the martingale slightly more vertically. This vertical component is necessary to more effectively oppose the forestay's mostly upward tension on the forward end of the bowsprit than would be the case in the absence of the dolphin striker.

On a catamaran such as the Tornado

Tornado (sailboat)

The Tornado is an two person multihull class recognised as an International Class by the International Sailing Federation. It was used for the Olympic Catamaran discpline for over 30 years.-Background:...

 and the Cobra where the mast is stepped on a beam between the hulls the dolphin striker provides support for the beam in order to support the mast load.