they were designed by sir thomas slade, based on the prototype 28-gun frigate (launched in 1748), "with such alterations as may tend to the better stowing of men and carrying for guns". these alterations involved raising the headroom between decks. they were originally ordered as 24-gun ships with 160 men, but re-rated while under construction to 28 guns with the addition of 3-pounder guns on the quarterdeck and with their complement being raised to 180 men.