The Ship Model Forum

There is a way to create hull lines in CAD programs (if your program supports it) that produces very accurate lines. Many blueprints have a "Table of Offsets." These offsets are XYZ coordinates for points on the hull surface (actually these "molded" offsets are usually to the inside of the hull plating, for the frames the plating attaches to).

The X coordinate is a position lengthwise (bow to stern) on the hull. The Y coordinate is the vertical dimension from a base line that may or may not be the bottom of the keel. And the Z coordinate is the distance transverse (port to starboard) to the hull center line.

Stations/frames were transverse slices through the hull. The X coordinates are given for stations or frames along the length of the hull. The distance between stations or frames varies with the vessel type, so you need to determine what this distance is. So for X distance station/frame 1 will be X distance from station/frame 0, station 2 will be 2X distant, and so on. Stations/frames are numbered between the Fore Peak and the Aft Peak (or After Peak). The Fore Peak is the point where the foremost part of the hull meets the normal load waterline. The Aft Peak is the aft most point on the hull at the load water line for modern vessels. (However, for wooden sailing ships the Aft Peak was where the center line of the rudder post crossed the load water line) Also, for American ships the station/frame numbering started at the Fore Peak (station/frame 0), but many (all) European countries put station/frame 0 at the Aft Peak.

Horizontal cross sections of the hull at vertical distances (the Y coordinate) were called waterlines. There were as many waterlines as you wanted to draw. Imagine a ship in an empty dry dock. Then as water flows in and reaches a level a foot above the bottom of the keel where the water surface meets the hull is the one foot waterline. When the water level is two feet above the keel it is the two foot waterline, etc. The light load waterline, normal load waterline and full load waterlines are special depths that are used for ship loading.

Vertical slices of the hull along the length of the ship were called butt lines. A slice on foot port/starboard from the center line was the one foot butt line, five feet from the center line was the five foot butt line, etc.

The Table of Offsets will list the Y (waterline) and Z (butt line) distances to points on the hull at each X position (station/frame). All together the Table defines the 3D shape of the hull. Many 3D CAD programs will import XYZ coordinate files and automatically generate the hull lines. This is far more accurate that trying to scan small scale line drawings.

The catch is that you have to create the plain text XYZ coordinate files in the format the program wants. Furthermore, the values in the Table of Offsets (American) are given in numbers like 12-9-3, which is 12 feet, nine and 3/8 inch, which is 12.78125 feet. So entering hundreds of these numbers into a spreadsheet is a bit tedious! But once you have the XYZ coordinate file you can regenerate the accurate hull shape in the CAD program as many times as you want.

Phil

That's an awesome bit of knowledge - thank you for sharing. Will be on the lookout for those tables of offsets.

However, I feel like we are moving further and further away from Pascal's topic and I don't want take away from his awesome work, so I started a new topic where we can discuss 3d modelling ship hulls here and we can leave this topic to discussing his amazing SS Hydrograaf model

DrPR wrote:Horizontal cross sections of the hull at vertical distances (the Y coordinate) were called waterlines. There were as many waterlines as you wanted to draw. Imagine a ship in an empty dry dock. Then as water flows in and reaches a level a foot above the bottom of the keel where the water surface meets the hull is the one foot waterline. When the water level is two feet above the keel it is the two foot waterline, etc. The light load waterline, normal load waterline and full load waterlines are special depths that are used for ship loading.

Very good summary of the ship's anatomy measuring system!

I have one important addition to make here: waterlines are always parallel to the design waterline of the ship. That is not necessarily parallel to the keel!

Until about 1900 (sailing)ships always had their rear of the keel deeper into the water than the forefoot, for getting sufficient water pressure on the rudder. With tugs and fishing boats this is still always the case, now also to have the props as deep into the water as to give good rendement. In those cases the angle between the keel and the waterline(s) can be up to 10 degrees, sometimes even more.

So the example DrPR gave is only true for a ship with this angle being 0, which is today the case for most cargo and passenger ships, most naval ships too. But you must ascertain this for every design you make.

And another remark: ususally the forward-backward axis we call X (stations/frames), the lateral left-right axis Y (butt lines) and the vertical axis Z (waterlines). But of course that may be arbitrary.

Maarten,

Good points. I was thinking only of ships with a keel angle of zero, even though I am also working on a Baltimore clipper model that has a deeper draft at the stern than at the bow!

However, in plans for those wooden ships sometimes the waterlines are drawn parallel to the load water line and sometimes they are parallel to the keel even though the vessels had greater draft at the stern. In this latter case the frame/station lines in the drawings are perpendicular to the keel, and this complicates matters when trying to use them to create framing for a model!

There is another thing I forgot to mention. The vertical distances are given from a Base Line. There was no specific location for this base line for all vessel construction.

In the case of the Cleveland class cruisers (and presumably other US Navy ships of the mid 20th century) the base line was on top of some of the keel plating. You have to add the keel plating thickness to the vertical dimensions to get the actual height above the bottom of the keel. But I have seen other drawings where the Base Line was the bottom of the keel surface, or the surface of the ways where the vessel was built.

And some small boats hulls are built upside down with the Base Line at some arbitrary position that is above the hull when it is sitting upright.

So you can't take it for granted that the Base Line is where you think it "should" be!

DrPR wrote:Maarten,

Good points. I was thinking only of ships with a keel angle of zero, even though I am also working on a Baltimore clipper model that has a deeper draft at the stern than at the bow!

However, in plans for those wooden ships sometimes the waterlines are drawn parallel to the load water line and sometimes they are parallel to the keel even though the vessels had greater draft at the stern. In this latter case the frame/station lines in the drawings are perpendicular to the keel, and this complicates matters when trying to use them to create framing for a model!

There is another thing I forgot to mention. The vertical distances are given from a Base Line. There was no specific location for this base line for all vessel construction.

In the case of the Cleveland class cruisers (and presumably other US Navy ships of the mid 20th century) the base line was on top of some of the keel plating. You have to add the keel plating thickness to the vertical dimensions to get the actual height above the bottom of the keel. But I have seen other drawings where the Base Line was the bottom of the keel surface, or the surface of the ways where the vessel was built.

And some small boats hulls are built upside down with the Base Line at some arbitrary position that is above the hull when it is sitting upright.

So you can't take it for granted that the Base Line is where you think it "should" be!

Hi DrPR,

A Baltimore clipper is even more extreme regarding the keel angle! up to 20 degrees or so.

As for the Zero of the vertical measurements: indeed these vary, and one has to study the plans of the ship in case closely. Adding to the options: some designers use the (theoretical) Load Waterline as the origin, al the measuremenst downwards are negative, upwards positive. So I go with your conclusion: don't take it for granted but study the information you've got closely before you dive into building something!

Just to give a warning: as for the longitudinal measurements you must be aware the Zero point might also be chosen arbitrarily. Most designers place this at either the Forward or the Aft Perpendicular -- the ends where the ship's stem and stern cross the waterline, but there are variations with the definition of these too. In many cases the rudder axis is used as the Aft perpendicular for instance.

One more note: should we not take this discussion off the Hydrograaf topic and start a separate discussion on ship design in the technical section?

Do we have a "technical section? "

Maybe we need a thread for blueprint reading or ship design. I started a separate thread for 3D printing to try to get folks to post information there.

I have repeated this discussion about blueprint reading, Table of Offsets, and other technical points several times in as many threads.

Phil

DrPR wrote:Do we have a "technical section? "

Maybe we need a thread for blueprint reading or ship design. I started a separate thread for 3D printing to try to get folks to post information there.

I have repeated this discussion about blueprint reading, Table of Offsets, and other technical points several times in as many threads.

Phil

There is a section 'History & Technology': viewforum.php?f=14

You need to scroll down on the Board index to find it. As you won't see it right away, I think many people will never look there though... Maybe we shoud adress this to one of the moderators? Otherwise we will repeat ourselves very often, just as you say...