A competent knitter can knit ANY style sweater, on gansey needles. Garments for sailors and fishermen can be knit quickly, and so tight as to be weatherproof. (Gansey needles can also be used to produce the finest lace. One millimeter gansey needles are much easier to work with than 1 mm circs. I have used 0.5 mm gansey needles, but I do not have any 0.5 mm circs to compare them too.)
The essential characteristic of a sailor’s or fisherman’s sweater is the warmth it has from being tightly knit. More loosely knit sweaters are not as warm. I find that a difference in stitch gauge of only 7% can result in a sweater having catastrophically less warmth. Sailors and fishermen wearing sweaters that are not as warm tend to get hypothermic and perish. If several of the sailors on a ship get hypothermic and perish, then the ship will be lost and all on board will perish. Thus, loosely knit sweaters were inherent not suitable for wear at sea on sailing vessels. (No problem for modern vessels with engines and heaters.)
Up to Victorian times, gansey needles and a knitting sheath were how all seaman’s sweaters, mittens, hoods, watch caps, socks, & comforters were produced. Gansey needles with a knitting sheath were used from the Arab world to Greece to Spain to Brittany, Normandy, Channel Islands, England, Scotland, Ireland, Flanders, and through out the Hansa. The style and decorative stitches did not matter as much as the inherent warmth resulting form the leverage of the gansey needles packing the yarns together. Tightly packed yarn prevented air from moving through the fabric and carrying the heat away from the body. Air will flow freely and carry heat through any gap in the fabric larger than ~40 microns. (twice the thickness of a single fiber of merino)
Such goods can also be knit on circular needles. However, knitting such a fabric on circular needles will, take much longer and will stress the wrists resulting in Carpel Tunnel Syndrome (CTS). Perhaps not on the first tight fisherman’s sweater that one knits, but such knitting will take a toll on one’s wrists. The replica sweater that Mary Wright (Cornish Guernseys & Knit-frocks) first knit circa 1976 resulted in wrist surgery. In 2005, there was gansey sweater knit along on circular needles. Five of the 6 finishers required wrist surgery within a year. Therefore, with circular needles the tendency is to knit more loosely. Knitting 10% looser, takes much of the stress off the wrists, but produces a much less warm fabric.
In contrast, in 2008, I finished a Filey Guernsey with 750 cable crosses in 9 days, with no wrist problems. When I teach, I pour a bottle of water on a “gansey”, talk for a while, pickup the sweater and pour the water into a sink or bucket. The table or floor where I poured the water on the sweater is still dry. Those sweaters are weatherproof. I have half a dozen sweaters that I can use for this demo - and most have been worn a great deal. They are very comfortable for sailing. The styles and yarns used vary significantly, but they are all weatherproof.
If you want to hand knit sweaters for a boat load, or ship full of seamen, you will have use gansey needles or you will either knit too loosely and all will be lost, or you will ruin your wrists before all the sweaters are knit, and those seamen without a warm sweater will die.
“Gansey” is not the style of the garment or the decorative stitches. “Gansy” is a technique of fabric production, that can be used for all types of fabrics that can be hand knit. Gansey knitting has different physics than knitting with hand-held needles. Some kinds of objects such as gloves that can be gansey knit are better swaved. And, under some conditions, some kinds of objects can be more conveniently knit with hand held needles. However, the last time I thought I had a good example of such an object, some careful swatching showed that gansey knitting was better.