Construction - I


Overview

The construction technique for the NIS 18 is similar to other plywood designs. The shape of the hull is defined by the bulkheads. The Tiki 21 and Bobcat were a "stitch and glue" type of boats, where copper wire was used to hold the bulkheads to the hull sections. The joints were then glued in with epoxy and fiberglass tape. This technique is pretty simple and doesn't require a building frame like more traditional wooden boat building. The NIS 18 requires a building frame, so this experience will be a bit different for me. The frame holds the bulkheads while the hull pieces are glued on. Each joint is held together by a cleat -- a piece of trim glued to both surfaces.

Since the boat is 18 feet long, it requires a technique to attach several sheets of plywood together. Even some of the bulkheads are too big to fit within a 4' by 8' section. I've read a lot about scarfing and will use that method to construct the chine logs and rails. But scarfing plywood at the recommended 8:1 angle requires a special tool and procedure that I'm unwilling to undergo. Having used the butt-block method before, I am using it here. It is simple, strong, and very quick to accomplish.

Deviations

I have decided to make a number of slight deviations from the original construction plan. Usually it's because I think it will make my life a little easier.

The first is to glue the cleats for the hull on the bulkheads before putting them on the strongback (the building frame). It's just easier to do and it actually helps eliminate some of the slight warping that is inevitable after each bulkhead is cut. I was careful to make sure that the cleats didn't interfere with the strongback components.

The construction plans call for 300 lbs. of lead as part of the keelson, being thin strips glued and bolted to the hull. This strikes me as being a true pain in the rearmost section, so I am opting to distribute that part of the ballast internally. How this is distributed will be covered later, most likely after I observe the boat in the water.

There a couple of minor design changes I will make. First, make the beam an extra 2 feet...no, just kidding. I am raising the floor of the port head to the same level as the bunk. I don't plan on putting a head onboard; I'd rather have the storage space/seat on that side.

Taking a clue from the longitudinal bulkhead under the cockpit, I'll be putting a similar bulkhead under the bunk. It will serve to strengthen the both the bunk and the hull underneath where it will probably be receiving the most punishment. It also serves to provide more watertight spaces for floatation. The extra weight overall will probably be around 20 lbs.

Progress Report - Start to Apr. 1999

The first stage has been mainly focused on 2 major steps: building the bulkheads and the centerboard trunk. Including the transom there are 8 bulkheads, labeled A through G (the transom doesn't get a letter, it must have flunked high school athletics). Bulkhead B merely functions as the forward cabin end and doesn't extend much below the deck.

These images at right show the bulkheads in order from the bow (at top) to the transom. Bulkheads C, D, and E were too big to fit on a single sheet of plywood, so I had to form them from 2 sections. I chose to match the sections together at the reference waterline which is clear of any structural members.

A note here on waterlines. There are two: the displacement waterline, where the designer expects the boat to rest in the water when properly loaded, and a reference waterline. Kirby has drawn the reference waterline exactly 1 foot above the displacement one to aid in measuring all of the different components and also to lay out the boat correctly on the strongback. This line, along with the centerline, is drawn on each bulkhead on both sides, making further alignments easy.

The slot cut into bulkheads D and E betrays the location of the centerboard trunk. It is located 1 foot to starboard of the centerline. If it was on the centerline, it would not only severely hamper the area in the cabin, but it would also cut into the cockpit. I remember seeing a beautifully kept catboat many years ago at a wooden boat show in Port Townsend. It looked especially sleek with low freeboard and inconspicuous cabin. Unfortunately the large centerboard trunk come up almost to the cabin top! I am thin, but it would take someone even thinner to utilize what little space there was below. Kirby has done a good job here to remedy this problem without much detriment to performance. I don't expect the centerboard to show above the waterline unless the boat is pressed over 35° or more to port.

The funny notch in Bulkhead F is also centerboard related, allowing the block and tackle that raises the board to run back to Bulkhead G. Both F and G show the shape of the cockpit and coaming. The cockpit seat is level, but the floor slopes down to the transom.

Also quite evident from these bulkhead photos is the distinctive sharpie hull shape known for ease of construction.

Here is a closeup of the transom, looking at the inboard side. The glued section is reinforcing for the gudgeons. The 2 holes are cockpit drains. Just visible are the lines drawn for where the cockpit will meet the transom.

This shows both sides of Bulkhead E, the topmost being the forward side. Notice the butt blocks for joining the top half of the bulkhead to the bottom half. It is 6" wide so it overlaps both sides by 3". The vertical buttblock is for reinforcing. I plan to put a big U-bolt on the aft side to use for safety harnesses. I figured that having some extra wood there wouldn't hurt. The butt blocks at the cabintop curve is common to all of the cabin bearing bulkheads.

Looking at the aft side of the bulkhead shows the cleats glued for taking the hull sections. They do not go to the corners because those will be cut out for the chine logs and centerboard trunk bedlogs.

Again, the slot for the centerboard is clearly seen. The other larger hole is to provide access to an otherwise inaccessible space under the cockpit. All the other spaces will be accessible from hatches in the cockpit seats.

This is a picture of Bulkhead D looking at the aft side. This shows the space of the cabin interior. An additional cleat has been glued on the port side to indicate the position of the seat. This is the aforementioned floor that I decided to raise for additional storage space.

I have not cut out the trunk slot completely in this picture so the bulkhead would retain some stiffness. Once the slot is cut the bulkhead is pretty floppy. After it is glued to the trunk, though, the stiffness will return.

These 2 pictures show the centerboard trunk apart and together. The black interior is not paint, but epoxy mixed with graphite powder to give it some sense of lubrication. The interior width is 3/4" and the board will be 1/2" so it won't be that tight fitting.

The extra height on the port side sheet is a small change I made to the building procedure. Since the port side of the trunk coincides with the cockpit, I extended that section to the top of the cockpit seat to give it some continuity and strength.

The hole you see in the aft middle section is where the centerboard control line exits the trunk, and that is also where Bulkhead E is located.

The curve of the hull was measured on one side, and the other side was cut to match. The bedlog at the base of the trunk was not bent, but glued on oversized and then cut to match also.

I had to do all of the glueing of the trunk on my kitchen floor because it was too cold in my garage for the #2 Hardener to set.

The final part of this construction phase is to prepare everything for being assembled on the strongback. Once I glued together the centerboard trunk, I took the liberty of fitting the 4 bulkheads (C, D, E, F) on the trunk. Everything seemed to match up correctly. I didn't see any glaring errors, which is good. I made the cuts for the chine logs in the bulkheads and a few other minor items.

The next step is mostly a mental one. To say goodbye to all my garage space before assembling the strongback. I won't see that space again until I roll out a completed boat maybe a year down the road.


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