Slop

John Marchant’s Dictionary (2d ed 1760) defines jacket as a short outward garment such as a sailor might wear. Thus, what you would call a gansey or Guernsey would be called a shirt if worn as an under garment and a jacket if worn as an outer garment, and a frock if rather loose.  And if long, it would be a gown. Consider John Boyd’s A manual for naval cadets, where only in one reference are they called “knitted worsted jackets” and other times simply “jackets". Note well the use of “jackets” by Liardet, for the garment the crew is wearing to sleep in on cold nights. (Liardet was a sea captain who wrote a large number of texts on seamanship and military/ merchant ship management.) Likewise, knit wear may be called a “frock”, per Mary Wright

If sold by the ship (slop chest), they would all be known as “slop”. See for example; The Royal Navy, 1790-1970 by Wilkinson-Latham.

Additional information is in the Fourth Report of the Commissioners for Revising and Digesting the Civil Affairs of His Majesty’s Navy (1806). These reports include full details and history of the operation of the slop chests. In The laws, ordinances and institutions of the Admiralty, (1746, Vo.l 2 pg 301) we see both broad cloth coats and “Kersey gowns” are to be sold at 19 shillings. each. (~twice the price of a "striped suit" of clothes, or 3 time the price of a "blue suit") In The accounts of the Lord High treasurer of Scotland: 1473-1498, we see that “Kersey” is a finely knit fabric. From both contexts, Kersey was expensive.

Edited to say this is clearly wrong.  Kersey was a woven fabric.  Thus we still have the question of, " how did the top men stay warm." Twill fabric, even with a nap does not solve the problem.

Thus, we have knit upper body garments in the British navy slop chests in 1746, based on official government reports available in the national archives.  And the garments brought aboard by volunteers and merchant seamen are repeatedly referenced in the basic texts of the time.  The existence of such garments given the authority and the number of such references is beyond doubt.

If we must look to Admiralty documents, 

The Nautical Magazine: A Journal of Papers on Subjects Connected ..., Volume 2 (1833), at that time there are worsted, white knitted jackets. and in Admiralty Orders, &c. &c.

(Circular, No. 83.) Admiralty, 23d October, 1832, we have blue knitted jackets.

And note the that the slop chests did not sell “oil skins”. We know that some war ships had infrequent  “make and mend” days.(see for example Liardet on warship management. Other sources say some war ships went months between M&M days.  Sailors on warships were not given time to knit. This is no surprise as the Captain made a profit on all slops sold to his crew.) Thus if a man was pressed, and came aboard, he could buy a knit gown, and various kinds of cloth, but it might be months before the sailor could fashion a set of oil skins. Thus, we know that the “gown” had to be very warm. It had to be weather proof.

It is worth noting the difference between the Navy's selling slops to seamen and the way the British Army and Marines were clothed. In Reports from Committees of the House of Commons: Repr. by Order ..., Volume 13 (1803) pg 622 et seq., we see that clothing and equipment was issued to these forces, and if they remained in the service for 8 months, the soldier owned the clothing/equipment.  As we look at navy accounts pg 166 et seq we see line items for purchase of slops (to be sold to seamen) and clothing for prisoners of war,but no item for any other kind of clothing for seamen.  Thus, there was no funding for oil skins to be issued to seamen.  The sailor either brought an oil skin aboard, or made an oil skin on a "make and mend day" or went without.  On the other hand the  "striped suits" were lined with duck would have been reasonably wind proof, but not comfortable against the skin when wet, and very slow to dry. 

Academic and scientific works do not cite common knowledge such as is available in basic texts and encyclopedias. The above information is in the basic seamanship texts and encyclopedias of the time.    It was in the government reports on the topic from the time. It can be found in multiple other sources. Anyone with a basic knowledge of British or European shipping or fishing or naval operations circa 1750 should have be aware of this information.

The only question was, “How warm or weather proof, are such fabrics?” The answer is VERY! The proof is in the knitting and testing of such fabrics. That is what I have been saying, ‘the proof is in the knitting!”  The only peer review would be of the knitting, and that would require a knitter with expertise in knitting with long needles and a knitting sheath.