The is was the sword of the seas the cutlass. A cutlass was a short bladed (compared to other swords) singled edged sword. The blade was usually slightly curved and only sharpened on the outer blade. In appearance the Cutlass resembles a sabre, only the blade is slightly heavier and shorter. The reason the cutlass had a shorter, heavier blade is because of what it was called upon to do. Besides having to run through your foe, the cutlass was also called upon to cut through heavy marlin lines, break down heavy oaken doors, and so on. A regular sword may not have always been up to the task.
Furthermore, the slightly shorter blade was not seen as a disadvantage when fighting aboard a ship, because there was rarely enough room to swing a cat let along a longer sword. The shorter blade allowed the blade to be sturdier without adding weight to the overall sword.
When fighting, the general rule was to hack at one's opponent such as was done in sabre duels. Thrusting or stabbing, was done with rapiers or swords. Thrusting took longer, and if you were not careful your blade could get stuck. Hacking meant that you could just as easily immobilize your opponent by chopping off his hand rather that stabbing him in the gut. This also explained the design of the blade. Curving the blade made it more easy to control while hacking, and the added thickness insured that it would cleave through bone and muscle. Of course curving the blade made the cutlass slightly less effective as a thrusting weapon.
Types of cutlass
Typical 16th thru 18th Century (1500-1700s) Cutlass. Note the shape of the blade. This early cutlass has an almost straight blade with the one shapened dged. The blade is also thicker near the point, making it more finctional as a hacking weapon.
Another example of a cutlass, this one with an open basket and more curving blade. In this case the top of the blade is sharpened near the point. The blade is more curved and also shorter than the example shown above
Slightly decorative version of an 17th thru mid-19th Century (1600-1800s) Cutlass. Note a full basket on the handguard. This solid handle would cracksheads as well as protect the hand.
Late 18th, 19th Century (1776-1860) French Officers Cavalry Sabre. Note the similarities to a cutlass.