'main mast' is correct and usable in written English.
You can use it to refer to the tallest or most important mast on a boat or ship. For example, "The sailors repaired the main mast of the schooner."
As we talked, he pointed out the splintered section of the main mast, which had recently been struck by lightning.
And the 115-foot main mast, jutting from the mostly submerged Bounty.
He was at his battle station in the main mast when the ship was hit, his son said.
An image of the discovery shows the wooden vessel has remained largely intact, though the main mast has been sheared off.
The main mast is nearly as tall as a 10-story building, but the cramped spaces of the 'tween-deck seemed like an awfully small place to live with a dozen other souls for months at a time.
It has a white steel hull, a flush deck with a pilothouse forward of the main mast, a wide bridge and an exhaust funnel between the main and mizzen masts.
It has square-rigged fore and main masts and a fore-and-aft rigged mizzenmast with topsail.
The spotting tops at the head of the fore and main masts controlled the fire of the Invincibles' main guns.
At low tide, the Gothenburg ground and twisted and broke her back between the fore and main masts.
Defence and Orion attacked the fifth French ship, Peuple Souverain, from either side and the ship rapidly lost the fore and main masts.
The ship had been completely dismasted except the stubs of the fore and main masts and the bowsprit, and was leaking badly from dozens of shot holes.
