NEMAH: The Birth of a Skiff - Part II
   
 
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NEMAH: The Birth of a Skiff
Part II: From Lines to Structure

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

Part I - Part III - Part IV

In Part I of this series of articles on the design and construction of the sharpie skiff, Nemah, I outlined the preliminary development of this design.
Working from a notion of what constitutes a "boat," lines were developed for a hull that will be well suited for its intended uses. In Part II of this series, we will develop a sailing rig for Nemah, and then consider how
to select and apply various structural techniques to create a boat that is both strong andlightweight

The Loose-Footed Sprit Rig

There are several sailing rigs that produce minimal heeling effect. These would include the gaff rig, the sprit rig, various types of lug and lateen rigs, and even some low aspect ratio fully battened jib-headed rigs. In selecting a rig for Nemah, I was influenced by several considerations, including length and number of spars, complexity of the rig to set up and handle, relative performance, appearance, safety, and cost.

Sprit rigs offer several advantages for boats of the type and size of Nemah:

  • The four-sided shape of the sail itself eliminates the narrow triangular areas that do little work, as in many three-sided sails
  • The sails are generally lashed to the mast, eliminating the expense of running rigging and sail track.
  • The rig offers the most driving power in relation to spar length of any known rig
  • The short, stiff irast does not need to be stayed and is easy to step and unstep without assistance

On the minus side, the sprit must be properly set to be efficient, and the rig cannot be reefed in the conventional manner, (Sprit rigs are usually reefed by folding the peak of the sail down and lashing it off to the mast, and then resetting the sprit to a grommet set in the leech for the purpo e.)

The point on the hull where the sheet is made fast while sailing close hauled, needs to be far enough off the centerline of the hull, so that the sail can be sheeted down hard and still be able to draw; hence the wide transom. In addition, the clew of a loose-footed sail needs to be enough forward of the stem of the boat that the sheet can be secured at a point
where it places equal tension on both the leech and the foot of the sail. This usually requires that the mast be stepped well forward in the boat, which in turn gives two advantages:

  1. It moves the center of effort of the sail well forward, which is needed to counteract the apparent weather helm that loose-footed sails demonstrate
  2. it frees up a generous area forward of the rowing thwart.

Traditionally, the masts of loose-footed rigs were sharply raked aft. This has the effect of keeping the pull of the sheet at an optimum angle through a greater range of points of sail, as well as shifting the weight of the mast aft.

In drawing Nemah's rig, I selected a preliminary length of 12 feet overall for both mast and sprit, and a target sail area of 80 square feet. Mast rake was determined by eye and the mast drawn in. I next drew in the foot, with consideration given to head clearance and so forth. At this point, I pencilled in a peak angle and found a likely peak height by moving the sprit around on the drawing until it formed an even-sided triangle with the top of the luff. I then sketched in the leech and checked to see if the clew angle and location would allow the sail to be properly sheeted in, close-hauled. (The sheet angle in profile should roughly split the clew angle.)

At this point I checked the sail area to see if it was coming out near my target area. (To determine the area of a four-sided sail, first divide the sail into two triangles, figure the areas of each, and then add the two together.) If the sail area had fallen much outside of my target area, I would have had to adjust the length of one or both of my spars. Small adjustments can be made by moving the peak around and then relocating the clew to maintain the sheet angle.

Daggerboard Location

The location of the sail's center of effort in relation to the hull's center of lateral resistance determines the "helm" of a boat; that is, whether you will need to "pull or "push" on the tiller to maintain your point of sail. Generally, you want a bit of "weather helm" both for safety and because a "lee helm" causes the rudder to work against thecenterboard, reducing your ability to point.

The center of lateral resistance is the point at which the total of the sideways resistance of the hull, centerboard, rudder and skeg is in balance. This point is generally adjusted to fall 10% to 20% of the hull length aft of the center of effort of the sail, depending on the rig. In practice, this relationship varies with weight distribution, the point of sail, the depth of extension of the centerboard, and so forth. The trim can be "fine-tuned" by adjusting the rake of the mast, which moves the center of effort fore and aft.

Because Nemah is designed to be operated under power, she must meet U. S. Coast Guard level flotation requirements. Her wood hull provides a portion of the requirement, but some additional buoyancy is needed.

Interior Layout

Once we are satisfied with the lines of the hull, and the mast and daggerboard locations have been determined, we can develop an interior arrangement that will provide for safety, passenger comfort, and structural integrity. Because Nernah has been designed to be operated under power, she must meet U.S. Coast Guard level flotation requirements. Her wood hull provides a portion of the requirement, but some additional buoyancy is needed. By fitting an enclosed seat across the width of the stem, we can supply the needed swamped buoyancy, give additional stiffness to the transom, and provide dry seating (see Figure 1).

To provide buoyancy forward, we can partition off a portion of the hull from just aft of the stem heel forward. If we reinforce this chamber sufficiently, we can step the mast to it, eliminating the need for a separate mast partner.

The location of the rowing thwart in small boats is always a dilemma. Do you position it for best fore-and-aft trim when rowing solo, or with a load? In Nemah's case I have opted to move the thwart forward of the ideal solo
location for two reasons: (1) it is better to trim a bit bow heavy when lightly loaded than to trim very stern heavy with a load on, and (2) under power, the operator will need to be located fairly far aft, so any passengers should be seated a bit forward of the center of buoyancy to bring the hull into trim.

Structural Considerations

Boats are subjected to a variety of forces, both in use and during storage, We must account not only for forces generated by the rig under all points of sail; but for forces applied to the rail structure under oars, for the thrust of the engine and its associated twisting forces, for the stresses applied to the daggerboard well in a grounding, for the localized loading applied to the hull bottom when someone steps into the boat, and so forth.

My approach has been to consider these forces from two standpoints; resisting distortion that could affect performance, and preventing structural failure. While these two factors are certainly related, they are not one in the same. Consider the case of the $39.95 rubber boat under oars the "hull" will flex to such an extent that the efforts of the oarsman
are almost totally negated, while the structure itself is in no danger of
failure. Another example of this problem is the loss of sailing performance noted when the forestay slackens on a sloop, distorting the shape of the jib. Structural distortion can also have an emotional effect. Noticeable "give" when you step into a boat or "flexing" of the rails under oars can make you uneasy at times.

In open boats such as Nemah, the two areas where distortion and/or structural failure are likely to be a problem are both related to the rail structure. The first is hull twist, particularly under sail, and the second is rail flex under oars. Traditionally, frames and bulkheads have been used to maintain hull shape, although in practice they do little to resist twisting
forces unless used in conjunction with an adequate rail structure. The rail structure must be stiff enough so that twisting forces are resisted by the triangular assembly whose sides are formed by the rails and transom. (This makes more sense if you realize that hull twist translates to gunwale movement fore and aft, and that the connection of fairly stiff rails at the bow will do much to arrest this movement) In relatively light boats, this can be accomplished by installing a trussed rail assembly (see Figure 2).


Figure 2

With trussed rail structures that are made up only of blocks, such as those used in some canoes, twisting forces caused by the use of oars can still cause a noticeable flexing of the rails. To counter these twisting forces, I have adopted the practice of fitting short ribs between the inwale and the side panel. These "riblets" usually terminate about four inches above the chine. Besides adding stiffness in the area of the oarlocks, these ribs are useful for distributing the load of the thwart (see Figure 1).

The bending loads on these rail structures are greatest in the middle and decrease towards the ends. For this reason, it is acceptable to decrease the separation between the rubrail and the inwale as it approaches the ends of the hull, and to actually terminate the inwale just short of the quarter knees and breasthook (see Figure 3). This simplifies their fitting to a great extent.


Figure 3

Since Nemah's mast is to be supported by the forward rotation chamber panel structure, some consideration has to be given to transferring the sail's forces to the rest of the hull structure. The simplest way to do this is to locate the aft edge of the chamber close enough to the rail structure that the fiberglass tape bonding the chamber to the hull is integrated into the structure of the rail itself (see Figure 4).

This relieves the side panel of the need to absorb a lot of stress in a small area.

Plywood

Because we have given careful attention to the distribution of stresses throughout the boat, we can safely use 1/4" marine plywood for the sides of the hull. Three-ply fir would be satisfactory in this instance, while 4 or 5-ply mahogany would be a bit stiffer and somewhat less likely to suffer damage from collision.

By scarphing the bottom panel with its face grain running athwartships, 5/8" 5-ply fir marine grade plywood will be satisfactory. I generally sheath the bottom panel inside and out with fiberglass; outside for abrasion resistance, and inside for resistance to impact damage.

In Part III of the series we'll discuss bonding materials and techniques, and then go through the actual process of fabricating a plywood composite "shell."

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