Titanic's Waterline

[Kelp]

I have just read an article about how Titanic's waterline  curved upward at a gradual slope towards the bow and stern.  I've always thought waterlines were flat,  so what would be the reason for having a slope in the waterline?

 

Thanks,

Don

[popeye2sea]

Were they talking about the water line or the sheer? 

Regards,

 

Henry

[Roger Pellett]

There are several definitions of the term waterline. Two relate to this question.  The first is the line defined by the locus of points between wind and water, ie the line at which the vessel floats.  Gravity makes this line straight but not necessarily parallel to the keel.  Changes in loading; cargo, fuel, stores, etc can change both the level  at which she floats (her draft) and the angle at which she floats (her trim).

 

The term as it relates to paint schemes really is more applicable to vessels where draft and trim do not change; think yachts.  Classic yachts and I believe most modern warships are usually painted with a straight waterline taken from the Naval Architect’s design drawings.

 

The “waterline” on commercial vessels is just the border between the owner’s chosen topside color and the bottom paint because draft and trim depends on the cargo they are carrying and how it’s loaded.  Here on the Great Lakes in the past it was common to run the topside paint all the way down to the top of the bilge strake.  As this strake curved up at both the bow and stern, the border between it and the bottom paint followed this curve.  The draft of these vessels could vary widely wether loaded with iron ore coal, or ballast water.

 

It is quite possible that Titanic’s “waterline” curved because it was following a particular plating strake

 

Roger

[Kelp]

Hi Henry,

The article was describing the waterline, where the oxide red and black colors would have met. As Roger says in his message, it may have had to do with the loading of the cargo.  I’ll do a little more research and let you know what I find out.  Thanks for your reply.

 

Don

[Kelp]

Roger,

Thanks for your response regarding the waterline.  As you mention, maybe it had to do with the cargo and loading.  I’ll check into this topic a little more.

 

Thank you,

Don

[wefalck]

There were two (traditional) and one psychological reasons, why the separation between the top- and the bottom-colour or the bootstrapping where made to curve slightly upward.

 

- the viewing psychology reason is that when you have a curved line (the sheer-line) and a straight line have above each other the straight line appears to be curving slightly downwards (hogging). So to counteract this the separation line was given a slight curve upward towards the ends.

 

- a curved line looks more dynamic than a straight line and gives the impression of speed

 

- older (wooden) ships tended to 'hog', i.e. because the ship ends have less buoyancy then the middle, prolonged exposure to heavy seas leads to a slight sagging of the ends. So painting the 'water-line' curved upwards will mask this effect visually.

[Kelp]

Hello Wefalck,

 

I have just looked at a model of the Titanic, and yes, what you have said is very true in Titanic’s case.  The waterline on the model appears to sag on each end.  Having the slight curvature would eliminate this illusion.

 

I had always thought waterlines were perfectly horizontal, and am now seeing situations were that isn’t necessarily the case, where cargo loading and simple aesthetics come into play to want to give the waterline a curve.
 

Thanks for your reply.

 

Don

[Bob Cleek]

Eberhard's answer is the best of the bunch offered above. 

 

There are waterlines and there are boot stripes. Naval and commercial ships generally only have a waterline, the theoretical line of demarcation between the water and air which is generally struck a bit above the actual line of demarcation when the vessel is fully loaded to provide antifouling protection in the "splash zone" above the actual waterline. These waterlines are generally flat.

 

The boot stripe is a stripe of a contrasting dark color painted between the bottom paint and the topsides paint on yachts and highly-finished commercial vessels. The purpose of a boot stripe (sometimes called a "boot top,") is to disguise the waterborne dirt and oil that would otherwise stain a vessel's white (or other colored) topsides when it splashed and collected at the waterline. In part for reasons Eberhard explained, and also 1) because the sides of most vessels curve inwards towards the centerline at their ends, which place the ends line farther away from the viewer viewing from the beam, which causes the line to appear narrower, from the viewer, and 2) because the sides of a vessel above the waterline may not be perpendicular to the waterline, but rather curve inward, as is common at the stern, thereby "twisiting" a boot stripe, making it appear thinner that it was to a viewer viewing from the side, a boot stripe that was equally wide over its entire length would appear to "droop" and would not be pleasing to the eye, particularly in a vessel with a contrasting sweeping sheer. Also as mentioned, a slight graceful curve upward in the upper edge of the boot stripe provides and illusion of grace and speed, as well. Therefore, boot stripes generally were horizontal at their bottom edges and gradually curved at their top edges, more so at the bow and less so, if at all, at the stern. It takes a considerable "eye" for a fair curve to lay out a proper boot stripe using a suitable batten and a poorly done job can cause considerable deteriment to the appearance of an otherwise good looking yacht.

 

 Below is an apparently colorized photograph of Titanic immediately prior to launch. One can draw their own conclusions as to whether or not her waterline was "curved upward" or not. If a good job was done striking curves which compensated for the optical illusions, you shouldn't be able to tell from looking.  I expect if the surface of the hull were projected to a flat plane, a slight upward curve towards the bow would be apparent. Note that the waterline is not parallel to the plating schedule. 

 

[wefalck]

A small correction: in those pre-grey naval days certain navies had a white stripe about half a metre above the CWL. 

[Dr PR]

Some military vessels do/did have a boot topping. US Navy vessels have/had boot topping - a wide black band between the bottom anti-fowling paint and the grey above.

 

The boot topping width depended upon the difference between light loading and maximum loading. Basically, both the minimum and full load waterlines were within the width of the boot topping. On ships like oilers where the minimum load and maximum load varied greatly the boot topping could be very wide - see the attached photo.

 

Edited May 27, 2023 by Dr PR