by Tad Roberts, Naval Architect, Vancouver BC
Having looked at the ends of the hull in the past two newsletters, let's now consider the sheer line which ties those ends together. The sheer is the uppermost edge of the hull, often it's the intersection of hull topsides and deck, but not always as in the case of bulwarks. As it's so prominent, the sheer will play a major role in establishing the character of any vessel, a deep sweeping curve says something very different from a long straight flat line. Both are common in the Classic Yacht Association fleet, both are handsome in their own way, and both indicate different intentions on the part of their creators.
As we've discussed in past articles, key aesthetic features always also have important structural functions and will bear on how a boat might be used. For instance, a raised deck forward can vastly improve the liveability of a smaller boat, while also improving seakeeping and strength.
The sheer can take numerous shapes following four basic forms as seen in profile.
The classic sheer (see below) is a full-length sweeping concave curve with its high point at the stem head, a low point about 2/3rds aft, rising again to termination at the stern. Variations of this sheer have the low point further forward and in Asia the highest point might be at the stern.
While long unbroken gradual curving lines are generally considered more graceful than short, chopped up ones, there can be some downsides to consider. One common shortcoming is that freeboard at the low point can be too low, limiting interior volume, allowing water to board more easily, and cutting off peak stability prematurely. Alternatively, a long smooth curve properly executed with moderate freeboard can be breathtakingly beautiful.
The proper way to draw this concave curve is as a spiral in profile, relaxed at the stem head and gradually tightening the bend moving aft. This can be done with a stiff batten and three batten weights ("ducks"). One weight is placed at the stem, another at the stern, and the third well beyond the stern pushing the batten up to form the spiral. See the photo below.
A variation of the concave sheer is the straight sheer. Those sheer lines which appear straight in profile are usually very slightly concave, this is to correct for perspective. All sheer lines are threedimensional and as the ends are further from your eye, they recede towards the horizon. A slight lift to the ends corrects for this and the shape appears flat. The flat sheer is often used on higher speed boats such as runabouts, and on daysailers and one onedesign racing sailboats. The attached photo shows a runabout of my design with sheer line that appears flat or straight from some angles but is in fact very slightly concave.
The third form of continuous sheer is the reverse or hogged sheer line. This shape first appeared in the 1930's and it's use probably peaked in the 1950's powerboat design. Originally the reverse curve was utilized for pragmatic reasons, mainly to gain more internal volume amidships where it made a difference in the interior. Incidentally it also added to stability with a higher deck edge, and reduced windage forward. Of course, it quickly became an edgy styling feature, and we still see hints of it in current design. The photo of my speedboat design Rooster shows a reverse sheer.
The last type of sheer we'll consider is the broken sheer. This style may have one break or many, and I usually consider two breaks to be the maximum but sometimes we see 3-4. Most often the broken sheer is to create a raised deck forward, or sometimes aft, and classic English motoryachts often had two breaks which formed a raised deck amidships. I've used the broken sheer in a lot of my motoryacht designs to gain internal space while simplifying construction. The raised deck also increases the strength of the foredeck by eliminating a trunk cabin. The downside to a raised deck is that it can appear ungainly on smaller and narrower boats. Good examples of excellent incorporation of the raised deck and broken sheer are seen in the hundreds of small power cruisers designed by Edwin Monk Sr. in the previous century. The last photo is my Ironbark design with sheer line broken amidships.
Next time we'll start looking at underwater hull forms.
