The 'normally presented' BM for HBMS, would give a slightly shorter gun-deck length for 1502, on further investigation this should calculate at 175'4" for the recorded 43'6" beam. (President for reference is 175', 157'10" (keel) and 44'4" beam... 1576 BM )
The 181'6" could be interpreted as hull length OA or some other 'wandering' stat poorly defined.

This is quite distinctive, and different from e.g. a 74 gun (Canada 170', 138'1", 46'9" - 1605 BM), 64 gun ship (Anson - 159'6", 130'1.5", 44' 5.25"), a 50 gun ship (146', 119'9", 40'6" - BM 1044 77/94)