Boal is a name used to distinguish both a style of madeira, and a number of indigenous Portuguese white skinned grape varieties used to make the style.
Boal is sweeter than those madeiras made from the other noble grape varieties Sercial and Verdelho, with residual sugar in the range of 2.5 to 3 degrees Baumé, giving a rich and raisiny style of fortified wine with an acidic edge for lift and length. However, wines rendered from Boal are not as sweet as the lusher Malmsey.
Interestingly, the acerbic nature of Sercial and to a lesser extent Verdelho, sees the must separated from the skins before fermentation. Malmsey and Boal on the other hand, are largely fermented on the skins giving them a grippier texture, colour and lifted aromas.
Like its noble brethren, Boal too was threatened by the ravages of phylloxera in the late 1800's. Now, however, it is the most widely planted of the bunch due to its relatively sturdy nature; thriving on Madeira's warmer southern mountainous sites.
The name Boal is often Anglicized to Bual. Nevertheless, most of the Boal grown on the island of Madeira is the clone, Boal Cachudo. Portugal's Instituto da Vinha e do Vinho (IVV) then lists four other subvarieties of Boal: Boal Barreiro, Boal Branco, Boal Espinho, and Boal Ratinho, all with a litany of synonymous progenies under different names, throughout Portugal and Spain. Boal is known as Gual in the Canary Islands.