In the Talmud (Tractate Menahot 94b) there is a famous debate between Rebi Hanina and Rebi Yohanan about the shape of the Showbread –
“R. Hanina says like a ‘Teiva Perutza’ (open box) R. Yohanan says like a ‘Sfina Rokedet (dancing ship)”.
The commentator Rashi (ibid.) interprets the Sfina Rokedet (dancing ship) shape as follows –
“קווי"ש (an abbreviation of a word in ancient French) ..and it does not have a base, but is wide at the top and at the bottom narrows down to a finger כזה (such): and its two heads are sharp and rise upwards and do not touch the water and it is thus called ‘dancing’ as it dances and travels quickly”.
Every time when Rashi says “כזה (such):” he attaches a drawing to illustrate his commentary.
In the Vilna Shas edition of the Talmud, published around 1880, which serves as the basis of many modern editions of the Talmud including Steinzaltz, Schottenstein, ArtScroll etc., the drawing appears as follows-
like the English letter V. This seems to correlate with Rashi’s written explanation “is wide at the top and at the bottom narrows down to a finger …. and its two heads are sharp and rise upwards”.
An interesting point to note is that the opinions of R. Hanina and R. Yohanan are both referring to different types of vessels that float on water.
The term “Teiva” (box) appears in the Torah and refers to a vessel similar in shape to Teivat Noah (Noah’s Ark) or Teivat Moshe Rabeinu (the basket that Moses floated down the Nile in). This is what is referred to in maritime terminology as a “punt”, a vessel with a flat bottomed/base. Teiva Perutza (open box) means it was not closed above, but open, as follows 
The term Sfina Rokedet (dancing ship) does not appear elsewhere in scripture and requires further clarification what type of vessel it is referring to.
As part of our research into this matter we approached a number of maritime archeologists from different universities around the world and we asked them to send us pictures of boats/ships, large or small, that existed around the time of, or prior to this Talmudic debate (100-200CE). From the multitude of responses we received (you may view them at the Showbread Institute), we tried to find one that had a V shape.
It is not certain whether Rashi was referring to a side view of the ship, or a frontal cross sectional view, so we looked for either. From all the sketches and photographs we could not find a single vessel that had a V shape, neither from a side view nor a cross sectional view.
We again wrote to these professors and asked them (we did not want to do this at the outset and possibly bias their response) if there was no vessel they could think of with a V shape. Their unanimous response was negative. Not only was there no ship/boat with that shape, but there could not be, because a V shape has never been used in ship building due to its unstable structure.
This poses a question on the depiction of the V shape in the Vilna Shas Talmud.
In Rashi’s commentary above, he begins with an abbreviation of a word in ancient French קווי"ש. According to Moshe Katan’s lexicon of Rashi’s non-Hebrew words, the word in question is “cojet”, an ancient kind of boat/ship. The maritime archeologists we approached think that “cojet” is referring to a ship called a cog, which looks like this –
This is not a V shape, neither from the side nor from the front, but rather a U shape.
At this point we hit an impasse, as the archeology seems to conflict with Rashi’s commentary. When I approached him with this dilemma, Prof. Zohar Amar from Bar-Ilan University offered a suggestion – perhaps the drawing that appears in the Vilna Shas Talmud is not Rashi’s original drawing.
We began to trace back in time from the Vilna Shas edition to see what the drawings in prior editions looked like. It appears that the Vilna Shas did not “invent” this shape, but rather copied it from previous versions of the Talmud, like –
Berlin 1748 –
and Frankfurt on Main 1708 –
We searched further back than 1708. With the help of Dr. Ezra Shvat from the manuscript department of the Israel National Library we uncovered a hand written manuscript from 1563, part of the Firkovitch collection, in the National Library of Russia, St. Petersburg which is Rashi’s commentary on Tractate Menahot. In this manuscript the drawing appears as follows –
and bears a striking resemblance to the "cog" ship, particularly the front end and the read end (what Rashi was probably referring to as "heads").
The only known manuscript in existence prior to the 1563 Firkovitch manuscript, is from the 13th century and is in the Vatican Archives in Rome, but does not include the page with the above mentioned debate, the manuscript only goes up to page 93, one page before the debate. It appears that the remainder of the manuscript was either lost or damaged.
What is clear though is that as you go further back in time, the shape of Rashi’s drawing changes and more assumes the shape of a ship, similar to Rashi’s “cojet”, a cog. This ship has a U shape, not a V shape.
Rashi’s written explanation above “is wide at the top and at the bottom narrows down to a finger …. and its two heads are sharp and rise upwards” applies equally to a U as to a V.
From Rashi’s subsequent commentary (ibid.) where he states “They rounded it דעגיל להו מיעגל” and “circle צירקל"א”, it appears that the base of the Showbread was curved like a U and not pointed like a V and I believe this 
is what Rashi actually meant.
