NEMAH: The Birth of a Skiff - Part I
   
 
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NEMAH: The Birth of a Skiff
Part I: Concept to Lines

by Tracy O'Brien
http://www.tracyobrien.com/

Part II - Part III - Part IV

In this, the first of a four-part series of articles on the design and construction of the sharpie skiff, Nemah, we will look at the "whys" that affect boat designs in general, and then go through the actual process of developing lines, offsets, and panel shapes for a 15-foot sailing sharpie.

The first consideration in the design process is purpose: What are the intended uses of the boat? We have to ask this question in the plural, as boats are often expected to be suitable for a variety of tasks. Implicit in this question is a host of other questions, such as intended load, anticipated local conditions, speed requirements, physical abilities of the users, intended method of transportation, and storage environment. In this regard, the designer usually finds himself faced with the unpleasant task of explaining to a would-be client that what he wishes to accomplish is either physically impossible or unaffordable!

Once we are able to state our intended uses in order of importance, we can begin to assemble a list of hull shapes that are well suited to our
primary purpose, and yet will lend themselves to the secondary uses listed as well. In practice, the range of shapes seriously considered by individual boaters will be limited by the notion of what to them constitutes a "boat", both structurally and asthetically, as well as a list of details they feel strongly about, one way or the other. These would include materials (wood, aluminum, etc.), and may include an almost endless list of other considerations; such as bottom shape, outboard location, leeboard/centerboard, rig type, and so forth.

In choosing the hull shape and rig for Nemah, my major considerations were: 1. Sailing ability. 2. Ease under oars. 3.The ability to accept a small outboard motor. My interest in hard-chine hulls in general and skiffs in particular could be regarded as my "notion of what constitutes a boat," in
light of the intended purposes of this design.

Sailing hulls require at least moderate amounts of bottom width to keep on their feet, while ease under oars requires that bottom width be held to a minimum. By fitting the hull with a fairly wide transom that is out of the water when under oars, we can provide increased waterline length and lee displacement under sail, while still ensuring a good wet shape when rowed. By fitting the transom at between 12 and 15 degrees, we can establish a proper thrust angle for an outboard. (The wide transom also allows a loose-footed sail to be properly trimmed close-hauled, which we will go into in more detail in the second part of this series.) My desire to maintain rowing ability affects my choice of sailing rigs, since if the heeling effect of the rig can be minimized, then the bottom width need only be moderately wider than would be optimum for ease under oars. With a sprit rig, the heeling effect will be significantly less than a jib-headed rig of equal area, with the added convenience of spars that can fit inside the hull.

After deciding upon a general hull configuration (in this case a sharpie skiff), we need to consider intended load, because it plays a major roll in the determination of the actual size of the boat. I arrived at the overall length of Nemah by determining how much boat I could build using side panels cut from a 16-foot sheet of plywood, given a predetermined hull width of about 4'4". Nemah's load capacity of 2 to 3 adults or 2 adults and a couple of kids is based on beam and hull depth as well as length. (A 6-inch reduction in beam would reduce this hull's safe capacity by at least one-third; in addition, her handling with the same sail area would go from "lively" to "treacherous,")


figures 1&2 (click to enlarge)

Lines

After I've cleared the mail off my drawing board, washed my triangles, taped down a sheet of vellum, and made myself a cup of tea, I've nothing left to do except put pen to paper.

 


figure 3 (click to enlarge)

For a design such as Nemah, l do what is known as a "cylindrical development", which will yield a hull whose panels are sections of a cylinder. This type of hull is often called "straight sided", but the term is confusing, because these hulls can have flare, tumblehome, deadrise, and so forth, as well as boldly developed curves. The only thing "straight" about straight sided designs is that their section lines are parallel to one another when viewed end-on (see Figure 3).

I generally develop my designs from three lines in two views: the sheer line in plan view (a top view of the hull), the sheer line in profile (the hull viewed from the side), and the chine line in profile. Because I derive all additional lines mathmatically, I normally do not draw an "end" view of the hull. Because an end view is needed to do this type of development without a calculator, I have included one in this article.

The first line drawn is the sheer line in plan view (see Figure 1). In Nemah's hull, the widest point is just aft of mid-length, and has a full- scale half-breadth of 25.8". At the transom, the half-breadth is 17.64". (Once down on paper, this line is definitive. Stations are layed out and half-breadths are taken off with an engineer's 1/50" scale to the nearest
1/100th, then upscaled by calculator and recorded on the table of offsets to an accuracy of two decimal places.)

Usually the sheer line in profile is layed down by "eye" and its heights upscaled and recorded. On Nemah, I opted to "straight plank" the sheer; that is, make the top of the side panel dead straight, and so derive its curve in profile from hull breadth and side flare (see Figure 2), This was a common practice among old-time dory builders and usually results in a nice sheer line. Station heights are easily derived either by calculator or from the end view drawing once you establish the angle of side flare.

The chine line in profile is the most difficult line to draw. It has to account for hull depth, bottom width, freeboard, draft, waterline length, and ultimately, displacement! Here is where you stare at the drawings in American Small Sailing Craft until the lines are burned into your brain.

In drawing Nemah, I first established her hull depth amidships, and then drew a preliminary load wateriine in pencil. Knowing that I wanted the heel of both the transom and the stem to stand clear of the water, I was then able to lay down the chine line by "eye," but not without a point of reference.

Because Nemah is to be of plywood composite (sewn-
seam) construction, and no frames or building jig is to
be used, the exact shape of each plywood component
must be determined prior to assembly.

The exact point of the chine line's aft termination is easily established by drawing the transom in profile at whatever angle one chooses, but the stem heel is a bit more difficult to locate. The most accurate way is to pencil in several additional stations near the stem in plan view, and then determine the height at which the side panel will touch the centerline for each station. By plotting several of these points on your profile drawing, you can establish the line of the stem. The point at which the stem line crosses the chine line is the stem heel. I draw in an additional station at this location and record it on the table of offsets.

Because Nemah is to be of plywood composite (sewn-seam) construction, and no frames or building jig is to be used, the exact shape of each plywood component must be determined prior to assembly.

To derive panel widths mechanically, take off the sheer and chine point locations from the end view drawing, by measuring down from a line drawn perpendicular to the side angle of the boat. (On Nemah, these measurements can be taken directly from the end view sheer line, but this is usually not the case.) Bottom panel widths are taken from the table of offsets and layed out from the panel centerline.

Expanded stations must be determined for each panel separately, as their curves are not equal. True station distances for the bottom panel can be lifted off the profile drawing by simply measuring from station point to station point along the chine line. To determine the amount of expansion for each station of the side panel, we have to resort to a little simple trigonometry: If we take the distance between each station as measured perpendicular to the station lines in the end view (distance "A" in Figure 3) and apply the following formula to it, we will arrive at a fairly accurate
figure.

The expanded station distance is equal to the square root of the sum of the end view station distance (A) squared and the profile station distance squared.

The widths of the transom at sheer and chine can be determined from the table of offsets, while its true height can be taken directly from the profile drawing.

In Part II of this series, we will consider construction materials, interior layout, and the development of a sailing rig for Nemah.

Visit Tracy's website:
http://www.tracyobrien.com/