Redesigning the New Dinghy

With the maximum dimensions of the dinghy measured in place on the deck, it is time to finalize the design of the dinghy.

Using a hull design program from Carlson Design, I was able to place specific stations and allow the program to produce a fair line. This program also allows me to evaluate the maximum displacement of the craft and evaluate where the center of buoyancy will lie. The program is available for free from Carlson Design.

http://carlsondesign.com/projects/hull-designer/

Follow the link above to the webpage where the calculator can be found and downloaded.

Clicking the link below the yellow arrow will download the file which can be opened to run the program. The program works wonderfully, and provides a wealth of information in just moments. 

With the maximum and minimum measurements, I was able to design a dinghy that fit within these tolerances. I then received approval from Maddie and got the go-ahead to build such a dinghy.

The measurements were then transferred back onto the dinghy and verified on the deck, making sure that the chimney will not be obstructed and that the hatch will be fully covered. 

Building the Backbone: Test Fitting

When I built Tooth, I never actually measured the space on the deck. I loaded Tooth onto the deck and realized that it didn't actually fit between the mast and dodger. I felt like a fool for not actually measuring the space available before building the dinghy. To avoid repeating that mistake, I am test fitting the backbone on the deck before further construction.

The dinghy needs to serve as our dinghy, but it also needs to cover the salon hatch (which leaks) and not cover the chimney. Since fitment is not a simple length requirement, I wanted to see it on the deck and make sure everything had plenty of space.

I tried positioning the backbone in various positions, measuring to see which position would give me the most displacement capacity. My requirements are it must cover the hatch and not cover the chimney. To avoid covering the chimney, the dinghy will be mounted backwards on the deck, stern facing forward and bow facing aft. 

Two positions shined as the most feasible, straight, with a carrying capacity of 897 pounds, and angled with a carrying capacity of 1091 pounds. The angled position would allow the bow to be bulkier, which is where the difference in displacement is made, but the stern needed to be a bit narrower. The bulkier bow would not be big enough for cargo but would offer more resistance when working into waves. The straight position offered a more slender bow with a narrow angle of entry, allowing the bow to cut through waves more easily and a wider transom which will offer more cargo space in the stern.

The final decision was actually based off of aesthetics, and not function. If the dinghy were set at an angle, it would look odd. Because of this, we decided to build the dinghy based on the straight orientation.

The keel was separated into 1 foot sections and the maximum distances for beam were measured. These distances would not produce a fair hull, but they are the maximum size that would fit.

Now that I know that the backbone fits on the deck and I can finalize the design and finish building the rest of the dinghy!

Building the Backbone: Part 2

The stem and keel are connected via a finger joint, which is strong, but not strong enough. All the force of waves hitting the bow will stress this joint, trying to push back into the boat. To further strengthen the stem, a knee is placed to transfer the load to the keel.

To fabricate this piece, a large cutoff was placed over the stem-keel junction and skewed at an angle, allowing us to create a slightly larger knee than if we used the board set ninety degrees to the joint.

A pattern was drawn on the board, giving me a guideline for the pattern to cut out of the board. I don't want to use the portions of the wood with large knots, so I sketch out a pattern that will cut out the knots and just leave clear clean wood with good grain orientation. The faying surface of the knee is verified to be square, ensuring that the stem will remain plumb. The finished knee is much smaller than the original sketch, as all the bulk simply adds weight to the bow and takes up interior volume.

Building the Backbone: Part 1

The backbone of a boat is the keel, stem, and stern post. These three timbers will be the key pieces that will support the entire boat, this is why they are referred to as the "Backbone". 

The stem and keel are made out of laminated douglas fir, with the grains book matched and set in towards each other. This will give the timbers very good dimensional stability and strength.

Most stem-keel junctions are either butted or connected via a gripe. I decided to further strengthen this connection by adding a finger joint. 

The bottom of the stem was cut to match the front of the keel. This will give me a lot of surface area where glue can act to hold the joint in place. The joint will be further reinforced with bronze fasteners. During the next few stages of construction, the keel and stem will be left over sized, allowing us flexibility in scrubbing wood that is not needed at a later point in the construction process.

Keel Construction

The keel is the next component to be made for the dinghy. Its final length is planned to be 6 feet long, so I am making an 8 foot long timber out of douglas fir which will let me cut it down to the needed dimensions.

Just like the stem, I am fabricating this timber out of two planks of quarter sawn lumber. Just like the stem, the keel is cut out of a flat sawn board that was next to the heart. When I ripped the board down the middle, the two remaining pieces are quarter sawn and will be much more dimensionally stable. Book matching the boards will also increase the dimensional stability of the keel as the grains will oppose each other and prevent any warpage from taking place.

To ensure that the boards will match up perfectly, I need a very tight joint. Running the boards on the jointer provided me with a smooth face that will provide the best gluing surface. The problem is my jointer is rather small and the keel timbers are very large, making this a difficult task. 

After the faces were smoothed, I still had this one bump. I decided to plane this down by hand instead of taking off an entire layer of wood again.

Reducing the high spot, the two faces were able to meet much closer. This makes for a tighter joint which will result in a stronger bond. Ideally, the two boards would be perfectly perfect, but the truth is, this is on an end with knots, so it will end up being cut off during later construction of the dinghy.

After planing the bump in the board, the two faces meet much more evenly. The little gap will be removed when the two boards are clamped together.

The excess glue that squeezed out doesn't need to be cleaned since the entire face will be cut off when the keel is shaped. The squeeze out is actually wanted, as it lets you know that the entire face is covered with glue. If you glue the two boards and no glue squeezes out, you can assume that either you had the perfect amount of glue or not enough glue. More likely than not, there was not enough glue and the resulting dry spots will not be glued together, creating a weak bond and a location for moisture to collect. The moisture that will collect between the faces will cause the boards to rot from the inside out. 

An important note with clamps is to alternate the sides the clamps are on. If you put all the clamps on the same side, the combined pressures will induce a twist into the boards. This can cause distortion in the glue up, producing a bowed product. By alternating the direction the clamps are placed will help cancel out this tendency and produce a straight glue up.

Now the assembly can sit and cure for the next day or two before I take it out of the clamps and clean up the sides on the band saw, just like I did for the stem.

With the stem and keel glued up, I can begin positioning them to create the gripe that will hold these two pieces together. While construction will seem slow in these stages, once the backbone is completed, the dinghy will appear rather quickly from the pile of lumber.