What physical characteristic might indicate a fiberglass hull dates from the pioneering phase of the early 1960s?

Answer

It seems unusually heavy or has a visibly thick, uneven gelcoat layer

Vessels from the pioneering phase often appear heavy or have thick, uneven gelcoat layers as builders grappled with early material science while trying to achieve the strength of wood.

What physical characteristic might indicate a fiberglass hull dates from the pioneering phase of the early 1960s?

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The History of Fiberglass - YouTube

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