Like a Socratic question or a question used to teach Shaolin monks, this question has many layers.
First, why gansey knit? (i.e., knit with a knitting sheath and long steel needles) The gansey knitting technology allows knitting tighter than can be achieved without the leverage provided by the knitting sheath. The tighter, gansey knit fabric is more weatherproof than can be achieved with hand held needles. The fabric is remarkably thin for its warmth, which is a real advantage in the cramped quarters onboard a working fishing ship, and it is remarkably warm which in an a real advantage in the cold and windy conditions under which commercial fishing is often conducted. That is reasonable, but why cables?
Cables provide some additional ventilation between the sweater and the oil skin (water proof layer) to reduce wetness under oil skins as a result of moisture from the sailor's skin condensing on the cold inside of the oil skin. This is a good reason. It may abe reason enough. Cables provide some additional comfort when sleeping in a canvas hammock. This is a good reason. Cables provide an artistic outlet for the knitter. This is a good reason. Cable patterns help identify the sweater (and I assert, at one time the cable pattern indicated the wearer's job and fleet.) That is a good reason. However, none of these are really compelling reasons.
To really understand cable patterns, you have to go back to the reason for for ganseys; warmth with light weight. The early (13th centrury) fishermen on the North Atlantic lashed barrels to the rails of their small (70 foot) ships. Then, the fishermen stood in the barrels with straw to help keep them warm , and jigged for cod. (later they jigged for mackerel, and trawled for herring). In those days, a single cod could weigh more than 100 pounds. Bringing up a cod was like hauling a iron manhole cover up through 300 feet of water, and they would do it every 10 minutes. Except this is the North Atlantic, so there are large waves and everything is rocking. What did they do? They braced themselves against the edge of the barrel.
Put on a sweater and climb into a barrel, grab hold of a manhole cover and have 2 of your brothers rock the barrel violently as you repeatedly lift the manhole cover for a week. At the end of a week you have a big hole in your sweater where it rubbed against the edge of the barrel.
(Later generations of fishermen worked from dories and braced themselves against the gunnels of the dory.)
After they caught the fish, they cut fish. With the ship still rocking, you take a fish in one hand and a sharp knife in the other and you brace your self against the cutting table - except by now your gansey has a hole in it, and there is only a thin apron between your belly and the cold slime and wet from the cutting table. You get back to St Peter Port and you go to your knitter, and tell them that you want a sweater that will last more than a week. So they knit you one - with cables on the belly where it rubs against the barrel, and the design was so good, that in some way copied by 50 generations of knitters. Thus, fisherman's sweaters have cables or fisherman's welt on their fronts.
The third job of the fisherman was to get where the fish were, and stay where the fish were. That meant sailing up wind in all weather. The weather blew the ship off the fish, so the fisherman must constantly sail up wind. Sailing up wind is an uncomfortable business. Moreover, the harder the wind blows, the more uncomfortable it is, but also, for a commercial fisherman who must catch his catch as fast as possible, the more important it is to work to windward, to stay over the fishing grounds.
During a storm on the Grand Banks, the expected wave period is only 20 seconds. On a ship, anything that is not lashed down is going to get thrown about. Lead sinkers jump 2 feet in the air, twice a minute. Sailors get thrown about. The ships were oak and the sailors, mortal flesh. Today under those conditions, we would be wearing layers of polyester fleece (and life jackets/ float coats/immersion suits) and that would provide some protection. However, gansey fabric was thinner and provided less padding. Hence, cables all over the sweater provided some padding in an otherwise thin garment. Again, likely a concept developed by knitters on the Channel Islands, and copied by others knitting for sailors for 50 generations.
We can look at the differences between the sweaters worn by sailors and fishermen and those worn by life boat men to take another bearing on the concept. The ganseys worn by life boat men do not seem to have had cables. Life boat men did wear oil skins, so we can drop the ventilation concept. What they did not do is brace themselves against the railing or gunnels to haul fish to the surface. The lifeboat's prize was already at the surface. They rowed out, picked it up, and rowed back. Nor did the lifeboat men take the beating of sailing to weather for days or weeks on end. It is not that rowing a lifeboat is easy, just there is less to bang against. So while lifeboatmen's gamseys without cables do not prove my theory of cables as padding, they does not disprove my theory either.
Having worn ganseys with and without cables, while sailing in serious weather, I find the concept of cables as padding the most compelling reason for cables. Anybody that disagrees should have tried sailing in ganseys with and without cable patterns on them in serious weather. To have any credibility, on this topic you need to have been out fishing when the waves were bigger than the boat. You need to understand that the key to getting work done while wearing a Type 1 PFD is motivation. To have any credibility, on this topic you need to have been knitting while lead sinkers were thumping on their racks.
First, why gansey knit? (i.e., knit with a knitting sheath and long steel needles) The gansey knitting technology allows knitting tighter than can be achieved without the leverage provided by the knitting sheath. The tighter, gansey knit fabric is more weatherproof than can be achieved with hand held needles. The fabric is remarkably thin for its warmth, which is a real advantage in the cramped quarters onboard a working fishing ship, and it is remarkably warm which in an a real advantage in the cold and windy conditions under which commercial fishing is often conducted. That is reasonable, but why cables?
Cables provide some additional ventilation between the sweater and the oil skin (water proof layer) to reduce wetness under oil skins as a result of moisture from the sailor's skin condensing on the cold inside of the oil skin. This is a good reason. It may abe reason enough. Cables provide some additional comfort when sleeping in a canvas hammock. This is a good reason. Cables provide an artistic outlet for the knitter. This is a good reason. Cable patterns help identify the sweater (and I assert, at one time the cable pattern indicated the wearer's job and fleet.) That is a good reason. However, none of these are really compelling reasons.
To really understand cable patterns, you have to go back to the reason for for ganseys; warmth with light weight. The early (13th centrury) fishermen on the North Atlantic lashed barrels to the rails of their small (70 foot) ships. Then, the fishermen stood in the barrels with straw to help keep them warm , and jigged for cod. (later they jigged for mackerel, and trawled for herring). In those days, a single cod could weigh more than 100 pounds. Bringing up a cod was like hauling a iron manhole cover up through 300 feet of water, and they would do it every 10 minutes. Except this is the North Atlantic, so there are large waves and everything is rocking. What did they do? They braced themselves against the edge of the barrel.
Put on a sweater and climb into a barrel, grab hold of a manhole cover and have 2 of your brothers rock the barrel violently as you repeatedly lift the manhole cover for a week. At the end of a week you have a big hole in your sweater where it rubbed against the edge of the barrel.
(Later generations of fishermen worked from dories and braced themselves against the gunnels of the dory.)
After they caught the fish, they cut fish. With the ship still rocking, you take a fish in one hand and a sharp knife in the other and you brace your self against the cutting table - except by now your gansey has a hole in it, and there is only a thin apron between your belly and the cold slime and wet from the cutting table. You get back to St Peter Port and you go to your knitter, and tell them that you want a sweater that will last more than a week. So they knit you one - with cables on the belly where it rubs against the barrel, and the design was so good, that in some way copied by 50 generations of knitters. Thus, fisherman's sweaters have cables or fisherman's welt on their fronts.
The third job of the fisherman was to get where the fish were, and stay where the fish were. That meant sailing up wind in all weather. The weather blew the ship off the fish, so the fisherman must constantly sail up wind. Sailing up wind is an uncomfortable business. Moreover, the harder the wind blows, the more uncomfortable it is, but also, for a commercial fisherman who must catch his catch as fast as possible, the more important it is to work to windward, to stay over the fishing grounds.
During a storm on the Grand Banks, the expected wave period is only 20 seconds. On a ship, anything that is not lashed down is going to get thrown about. Lead sinkers jump 2 feet in the air, twice a minute. Sailors get thrown about. The ships were oak and the sailors, mortal flesh. Today under those conditions, we would be wearing layers of polyester fleece (and life jackets/ float coats/immersion suits) and that would provide some protection. However, gansey fabric was thinner and provided less padding. Hence, cables all over the sweater provided some padding in an otherwise thin garment. Again, likely a concept developed by knitters on the Channel Islands, and copied by others knitting for sailors for 50 generations.
We can look at the differences between the sweaters worn by sailors and fishermen and those worn by life boat men to take another bearing on the concept. The ganseys worn by life boat men do not seem to have had cables. Life boat men did wear oil skins, so we can drop the ventilation concept. What they did not do is brace themselves against the railing or gunnels to haul fish to the surface. The lifeboat's prize was already at the surface. They rowed out, picked it up, and rowed back. Nor did the lifeboat men take the beating of sailing to weather for days or weeks on end. It is not that rowing a lifeboat is easy, just there is less to bang against. So while lifeboatmen's gamseys without cables do not prove my theory of cables as padding, they does not disprove my theory either.
Having worn ganseys with and without cables, while sailing in serious weather, I find the concept of cables as padding the most compelling reason for cables. Anybody that disagrees should have tried sailing in ganseys with and without cable patterns on them in serious weather. To have any credibility, on this topic you need to have been out fishing when the waves were bigger than the boat. You need to understand that the key to getting work done while wearing a Type 1 PFD is motivation. To have any credibility, on this topic you need to have been knitting while lead sinkers were thumping on their racks.