Hi,
Eventhough I am a Naval Archtect by training, that was still an informative video for me, as some terms/concepts (specifically the A, B and C Divisions) seem to have changed between WWII and now. Specifically I am not more familiar with ships being divided into Fire Zones, less tan or equal to 135ft in length, potentially leading to more than 3 main divisions.
Other than that the only things that stuck out to me was that, although the author mentions the fore and aft perpendiculars, he never specifically explains how they are defined (perhaps since it can be a little vague).
In general though, usually when first laying out a ship the Fore Perpendicular is "typically" set as where the stem of the ship intersects the "design waterline". Unfortunately though, as a design progresses, sometimes the actual eaterline that the ship ends up sitting at is not the same as this "assumed" "design waterline". Typically though you don't redefine a new "design waterline" since over the life of the ship (or even between sisterships) there will be issues that constantly cuase the actual operating waterline to vary, so in the end the "design waterline often typically just stays as your best 1st initial guess at something close to a warerline at where the ship will typically be operating, and the FP (or forward Perpendicular) remains as defined where the stem crosses that Design Waterline (or DWL).
However, occassionally on some ships it is possible that a decision is made to redesign the bow or something, leading to the FP not actually even being on the ship. If I am recalling correctly one Naval Auxiliary that I worked on (the AFS class of ships) actually had its FP defined a few feet forward of where the DWL and Stem intersected because of reasons like this.
For the AP, or After Perpendicular, things can get a bit more confusing though, as on commercial ships it is often defined as coincident with the main rudder stock. On many/most(?) naval ships though it is often defined as where the design waterline (DWL) intersects the stern (if I am recalling correctly).
In addition, during ship design, although the ship will eventually get divided up into Frame lines like shown in the video, before this happens it is often still necessary to know what the shape of the ship looks like, so the ship will often be divided into regularly Stations, that can make it easier to compare the shapes of different ships, etc.
In the US a ship is typically divided into 20 stations between the FP and AP, frequently with some half stations at the ends where the ship's hull may have more shape or curvature. Typicvally in the US, for large ships, Station 0 is the FP and Station 20 is the FP, and anything forward the FP has a negative number and anything aft the AP is numbered 20.X (or something similar.
In te US since these large ships are typically divided into these 20 stations you may hear reference to the ship's Frame Spacing, which really has no relation to it internal structure, but rather is just the distance beween the FP and AP divided by 20.
For smaller ships and boats in the US, as well as many ships of any size over seas though, the ship is typically only divided into 10 Stations, instead of 20, and often the umbering starts at the FP and moves forward, instead of the FP and moing aft.
Anyway, thanks for posting the video, I found it very informative.
Pat