Sheer line

[Tom Ask]

Is anyone familiar with Watson's relationship between length and sheer? Watson (Merchant Ship Naval Architecture, 1998 p. 261) uses y = 0.0166 L + 0.508 (forward) and y= 0.00833 L + 0.254 (aft). These equations don’t work out though, there must be a negative sign missing. Any ideas on Watson’s equation? I know sheer design is contentious but I am looking for a reference point for my research.

Thanks for any ideas.

[Tom Ask]

Logically, the L is at the low point amidships and L increases fore and aft from that point. However, it seems that it must read y = -.0166L + .508 or the sheer goes the wrong way. Confusing, but I have not found any other source for a "standard" sheer. Part of my research involves identifying a visual stereotype and hull surveys, including the sheer, for Malaysian fishing boats. Does anyone know of Watson's forumula or a similar one that might be applied to fishing boats (I know this matter is debated a lot, Ive seen the other threads).

Thanks!

[Eric Sponberg]

I am not familiar with Watson's book, but I do think that there is no such thing as a "standard sheer". The sheer line is probably one of the most important lines in the art of a boat design--it affects every other aspect and from all viewing angles. But I dare say nothing is standard because good looks AND practicality are in the eye of the beholder. Practicality and survivability are also dependent on sheer, at least on the height of the sheer, that is the freeboard, and sometimes that is governed by classification society rules or boat building standards.

I remember once long ago that George Hazen once described the sheer lines of his designs as being the arcs of circles, with the low point at about 60-70% back from the stem. This is a pretty good rule of thumb, and I have used it myself. Other designers do well with absolutely no curvature at all in their sheer lines. Many powerboats use reverse curve, or compound curves, i.e. curves in two directions, and they can all look really good.

Sheer lines are kind of like one of my favorite quotes: Nothin' ain't worth nothin' if is does not have that certain je ne sais quoi. It is all in the art.

Eric

[Paul Kotzebue]

Originally Posted by Tom Ask

Logically, the L is at the low point amidships and L increases fore and aft from that point. However, it seems that it must read y = -.0166L + .508 or the sheer goes the wrong way.

I'm not familiar with Watson's formula, but if sheer is positive upward a negative sign is not required. In this case "L" is constant, and the formulas give bow height and stern height above the lowest point (or perhaps amidships).

I'm assuming the formulas are not intended to give heights along the length of the boat because they are in the form of a straight line.

[gwboats]

I think you may be confused about how these formulae are applied.
The equations are normally applied to large merchant ships and are used to define the minimum standard sheer (deck at side) for application in the freeboard regulations.
To define a standard sheer, draw a line parallel to and above the DWL at a distance representing the minimum freeboard at amidships. At the forward perpendicular (FP) mark a point a distance 0.0166L+0.508 above this line. Repeat at the after perpendicular using 0.00833L+0.254 (you may have noticed this is half the FP amount). Now draw a parabolic curve through the 3 points (FP, amidships, AP) to get your standard sheer.
There are equations for 1/3L and 1/6L to make drawing simpler but Watson has not included them!
In practice most designers use greater values and modern ships, alas, have straight line sheers for ease of manufacture.
As you can see, this is not really applicable to small vessels where the designer's favourite sweep is usually applied.

Hope this helps,
Graham Westbrook
Naval Architect
www.westbrookmarine.co.uk

[Paul Kotzebue]

Graham,

Thanks for the clarification. I now remember "standard sheer" from the Load Line Technical Manual ....

[Tom Ask]

Thanks for the clarifications I have never seen an equation for sheer before the FP/AP. Sheer on ships is all gone so the equation is a vestige of a different era, alas.

[ancient kayaker]

I read once that a sheerline should not have a reverse curve when viewed at any angle. A quick bit of math reveals this means it has to be coplanar from one angle, to ensure that the entire curve reverses at the same angle. I don't think this is what the writer meant though. However, I must admit that applying this rule produces an acceptable sheerline.

[sorenfdk]

This is more relevant for sailing yachts, but still:
A nice looking sheer line can be obtained by intersecting the side of the hull and a planar plane that passes through the three points found by using Watsons formula.
Of course, you can use other points, if you like!