NEMAH: The Birth of a Skiff - Part IV
   
 
  NEW
  HOME
  INPUT
  CONTESTS
  FORUM
  STORE

 


NEMAH: The Birth of a Skiff
Part IV: Trimming out the Hull and Fitting the Rig

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

Part I - Part II - Part III

In Part III of this series of articles on the design and construction of the sharpie skiff, Nemah, we went through the process of fabricating her plywood "shell/" Here in the fourth and final part of this series we will complete the hull, as well as fabricate and fit her sailing rig.

A Logical Sequence

At this point m the building process, the plywood shell is structurally complete, but it's still quite flexible, For this reason, we will leave the temporary spreaders in place until the rail and thwart assemblies are fitted. Even with the spreaders in place, the plywood side panels are flexible enough that they can easily be forced out of shape. To prevent any possible distortion of the finished boat, we will need to hold the tops of the side panels in a fair curve while fitting the forward and transom bulkheads to the hull. This can be accomplished by fitting the rubrails to the sides of the hull.

Before bonding the rubrails in place, we can install the "riblets". They are glued and fastened to the side panels with short bronze ring nails driven through from the outside. The rubrails are then glued up and fastened with bronze ring nails driven through the side panels from the inside.

At this point, the breast hook, oarlock blocks and quarter knees are also fitted. The breast hook and quarter knees are glued and fastened to the side panels with bronze or stainless steel screws driven through the knee and into the transom, All screws should be counter-bored and plugged. The oarlock blocks are glued and clamped to the inside of the side panels; no fasteners need be used.

Flotation

With the rubrails in place, the structures that form the fore and aft flotation chambers can be installed. We begin this process by fitting the forward and aft bulkheads at their respective location lines (which were marked on the side panels during layout). These bulkheads are wired in place and fillet bonded to the hull with 3/4" radius fillets on both sides.

After the fore and aft bulkheads are in place, gluing strips are bonded to the bulkheads, side panels, and the transom face to provide bearing for the transom seat and forward deck. Prior to closing up the forward flotation chamber, the towing eye and its reinforcing block should be installed, as shown in Figure 1.

Clockwise from upper left: Figure 1. Bow eye is installed before the top to theflotation chamber. Figure 2. A 3/4 " radius fillet is applied to the inside corners formed by theseatfflotation tanks. Figure 3. The mast step and mast partner. Note the drain hole in the step.Figure 4. The after ends of the inwales iaper into the topside panels. (click thumbnails to enlarge)

The flotation chambers can be foam filled or left empty, as desired. Coast Guard regulations only require foam if motors larger than 2 horsepower are specified.

The transom seat and forward deck (with its doubler in place) are glued and fastened in place with bronze ring nails, and then a 3/4" radius fillet is applied around their entire perimeter (Figure 2), The aft upper edge of the forward deck is bonded to the side panels with three layers of fiberglass tape, as was mentioned in PartII of this series. This helps distribute
the stresses developed by the sail rig
to the rail assembly.

The Inwales

After the forward deck is bonded in place and its reinforcing tape is sanded smooth, the inwales can be fitted. They are cut to length and their aft ends are taper cut to join the side panels prior to installation (Figure 4). They are glued and fastened in place with 1-1/4" bronze or stainless steel screws driven into the side panels aft, and into the inside faces of the nblets in the mid section of the hull. The forward ends of the inwales are fastened to the side panels from station #152-1/2 forward, as shown in Figure 3.

The fore and aft rail spacer blocks can now be glued and clamped in place (Figures 3).

Clockwise from upper left: Figure 5. A simple template can be used to rout the hole for the daggerboard case. Figure 6. The center thwart is notched to clear the daggerboard. Figure 7. A taper jig can be used to taper cut the spars on a table saw. Figure 8. The butt of the sprit is drilled to accept the snotter.
(click thumbnails to enlarge)

The Daggerboard Well

The daggerboard well consists of two side panels, two spreader blocks, and two well beams. It is assembled and trimmed to fit prior to installation. The hole into which the well is fitted can be cut with a saber saw or with a router fitted with a guide bushing, as shown in Figure 5. Note how a temporary pattern is assembled by stapling plywood strips to the hull bottom. The advantage of this technique is that the finished opening will require no further cleanup work.

The case is glued and fastened to the bottom panel with two temporary screws driven up into the ends of the well spreader blocks. Shims are placed between the bottom edges of the well side panels to prevent distortion, and then the well assembly is fillet bonded and taped to the bottom panel.

Two thwart cleats are fabricated from white oak and fastened to the riblets at a point level with the top of the daggerboard well. The thwart itself is fabricated from 3/4" mahogany and is notched to provide clearance for the daggerboard, as shown in Figure 6. The thwart is fastened to the daggerboard well beams and the thwart cleats with bronze or stainless steel screws.

Mast Attachments

The mast is fitted by simply dropping it through the mast partner and fitting the mast heel into the mast step, as shown in Figure 3. The mast step is fabricated from 5/4" white oak and is glued to both the bottom panel and the forward bulkhead. It can be held in place with temporary screws driven up through the bottom panel. (Note that the mast step is fitted with a limber hole.) The mast partner is fabricated from 1/2" or thicker marine or aircraft plywood and is fastened to the forward deck with five #10 x 1-1/2" stainless steel screws.

Finishing the Hull

After the rail assembly and flotation chambers are fitted, the temporary spreaders can be removed and any remaining holes in the hull filled with thickened epoxy. The tops of the rails should be belt sanded and faired into the transom top, and so forth.

All trim work should be given a coat of penetrating sealer. Some or all of the trim can be left natural, as desired. On the prototype, I opted to paint the majority of the hull, leaving only the transom, rubrails, quarter knees, thwart assembly and mast partner bright. All brightwork was finished with gloss spar varnish. Single component urethane marine enamels were used on all the painted surfaces. A small amount of Grumbacher cobalt drier was added to both the varnish and the enamels prior to application to speed drying, as these finishes are often quite slow in drying when applied over epoxy coated surfaces.

Fabricating the Rig

Nemah's sailing rig consists of five major components: a 12-foot mast, a 12-foot sprit, a daggerboard, a rudder and tiller assembly, and an 80 square foot spritsail.

The mast is fabricated from a 2" square by 12' long piece of clear fir or spruce. The blank can be laminated from a number of pieces, if required. The top 2 feet of the blank is tapered on two adjacent faces to form a 1-1/2" square, when viewed from the end (Figure 7). The blank is then chamfered on all four corners to form an octagon. The heel is tapered slightly all around to fit into the mast step, and a shallow groove is cut around the mast head to provide a secure place for lashing on the sail throat.

The sprit is fabricated from a 1-1/2" square by 12' long piece of clear fir or spruce. Each end of the blank is tapered on two adjacent faces, with the taper cuts running out at 3 feet, leaving a 1-1/4" square at each end. The sprit is then chamfered on all four corners to produce an octagon, as with the mast. The top 1" of the sprit is reduced to 3/4" in diameter, leaving a shoulder to catch onto a rope loop fitted to the peak of the sail. The butt end of the sprit is drilled to accept the snotter, as shown in Figure 8.

Both mast and sprit are given a coat of clear sealer and several coats of spar varnish.

Above left: Figure 9. Oak cleats, fastened to the quarter knees, are used to turn the sheet back to the cockpit. Right: Figure 10. The sail is lashed through a small pad eye to provide tension on the luff. (click thumbnails to enlarge)

The daggerboard is cut from 3/4" x 12" hard mahogany, and is fitted with a cleat at its upper end. The lower leading edge is given a 6" radius and the entire underwater profile is faired off to a streamline section. The daggerboard is held in place by a short length of shock cord, as shown in Figure 6. The rudder can be cut from a length of 3/4" x 10" hard mahogany, and is radiused and streamlined as described above. It is fitted with bronze or stainless steel pintles, and will require a keeper to prevent its floating out of the gudgeons. The tiller is fabricated from two lengths of 7/16" thick white oak, with a 13/16" mahogany core, the whole unit being epoxy bonded together. The forward end of the tiller is tapered to a comfortable thickness, and all edges are given a 1/4" radius. A 1/4" bronze or stainless steel carriage bolt and self locking nut are used to assemble the tiller to the rudder (Figure 9).

Daggerboard, rudder, and tiller can all be finished bright or painted, at the builders option.

A pair of oak cleats should be fabricated and fastened to the quarter knees, also shown in Figure 9. These provide a simple method of routing the sheet from the quarters back into the cockpit.

Besides the bow eye, pintles and gudgeons, and rowlock sockets, the only other hardware requirements are a jam cleat to secure the snotter to the spirit (Figure 8), and two small pad eyes, which attach to the mast to prevent the sail lashing from riding up the mast (Figure 10) and the snotter from sliding down the mast,

Performance

I have sailed Nemah in a variety of conditions, from the light air of Olympia's Boston Harbor, (where the rowing and sailing photos were taken), to the gusty winds and choppy conditions of the Ilwaco channel at the mouth of the Columbia river. On Seattle's Lake Union she easily overtook a generously canvassed traditional sloop while sailing close hauled in a 6 to 8 knot breeze!

To windward, the loose foot sets well, and actually is not much different than a boomed sail, except that you can control the shape to a greater degree with the loose foot. Downwind, the sail bellies out, owning to the lack of a boom, but as the sprit gives it shape at the peak, the sail performs quite well, and doesn't tend to roll as much as most cat rigs do on a run. The rig's low center of effort is quite noticeable; even in a gust. I have been able to keep her on her feet without having to sit up on the rail.

Under oars, Nemah is a delight. Rowing solo, the hull trims a bit down at the bow, which keeps her quiet and on track. With one or two people in the stem, she is well balanced, moving easily and going where she's pointed. I'm quite pleased at her speed under oars; she carries well between strokes and is not unduly hampered by the wind. I had an occasion to row her dead to windward with board down and sail luffing while in the Ilwaco channel, covering just under a mile in 20 minutes.

At about 135 pounds without her rig, she would take two adults to car top, but can easily be handled by one person from a trailer or the bed of a pickup. I've got the prototype on a trailer fabricated from an imported trailer kit. I fitted it out with bunks running across the width of the frame and bolted on a 10-foot tongue made of a nice piece of dry fir. The whole towing package weighs under 300 pounds, and can be disconnected from my mini pickup and rolled down to the water's edge if no ramp is available.

Working out the design of Nemah, and building and sailing the prototype has proven to be both rewarding and enjoyable for me. I'm certain that should you choose to build Nemah for yourself or your family, you'll also find the building to be simple, yet satisfying, and the sailing to be fun.

Plans and instructions for Nemah are $55 from Ken Hankinson and associates or from:

Tracy O'Brien Marine Design
156 Bunker Creek Rd.
Chehalis, WA 98532
(360) 748.4089
fax (360) 740.0504
(tracy@tracyobrien.com)

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