we also call boat cloth as fiberglass cloth.
The types of fiberglass most commonly used are mainly E-glass (alumino-borosilicate glass with less than 1 wt% alkali oxides, mainly used for glass-reinforced plastics), but also S-glass (alumino silicate glass without CaO but with high MgO content with high tensile strength).

Fiberglass cloths are very common for marine & composite construction and repair. Light weight cloths tend to provide a smooth finish and are excellent for producing a waterproofing layer over wood or other surfaces when combined with the appropriate resin. Heavier cloths will provide greater strength and overall rigidity.
(references from www.wikipedia.org)

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