Here is a video tutorial of steps 1-5! Hull Cross Sections in Solidworks demo
Open the CAD model of the hull in SolidWorks.
Extrude the the cross-sections of the hull so it looks like the image below.
Use an extruded cut to remove half the hull.
Here is a video tutorial of steps 6 and 7! Cleaning Up Hull Cross sections in AutoCAD
Open the DXF in AutoCAD
Use ROTATE to rotate the cross-section upright. Draw a centerline down the middle of the cross section. Label your cross-sections! Use OFFSET to offset the curved outline inward by the thickness of the stringers. Our stringers were 0.25” thick. See the image below. In red is the original outline (too big!), (things highlighted in red were deleted).
Play tetris to condense all the cross sections onto one sheet of paper. The cheapest poster printer at FedEx is 36” wide and infinite length (paper is on a roll), but you pay by the square foot. The image below is meant for a 36” x 125” sheet of paper.
Return to the SolidWorks model, and view the submarine in a plan-view:
Produce a DXF of the plan view and open it in AutoCAD. Add a centerline and lines that match the spacing of your cross-sections. These are to locate the cross sections.
Acquire sheets of plywood. The tabletop should be thick enough so it doesn’t flex if you press on it. Suggest using at least ⅜” thick plywood. Any wood, OSB, Plywood, MDF, is OK.
Suggest scavenging for wood (it’s free!). If you have questionable morals, they usually leave wood out on campus loading docks. (hey if they didn’t want it to disappear, they shouldn’t’ve left it outside!)
The table should be longer than the length of the hull by a couple feet (for a foot of table at the nose, and a foot of table at the table), you can combine 4x8’ sheets of plywood to achieve the desired size. It should be wider than the sub by at least 6 inches on either side.
The tabletop needs to be as flat as possible. Suggest selecting wood that is not warped. You can reinforce it with straight 2x4’s oriented vertically underneath. This is how we did our reinforcements:
Screw down through the tabletop into the reinforcing 2x4’s. We suggest screwing down with drywall screws every 8 inches or so. Make sure the screws are long enough to go sufficiently deep into the 2x4’s.
Add a reinforcement sheet of wood on the bottom, this effectively makes the table an I-beam (super strong), and prevents it from twisting and flexing.
Acquire large sheets of wood for the cross sections. Can be OSB, plywood, MDF, anything. Thickness should be > 0.25” and < 1”. We found a 0.75” thick 4x8’ sheet in a dumpster. Definitely fair game!
Print out the cross section PDFs and the plan-view PDF at FedEX. Use the cheapest printer they have.
Cut out the cross-sections and plan-view
Along the centerline, tack the plan view onto the 4x8’ sheet using nails or screws. We did one screw at the nose and one at the tail. Tacked at the nose:
Tacked at the tail. If the sub is longer than your sheet of wood, use a second piece of wood to accommodate the tail.
Cut the plan-view cutout at the edge where the wood sheets are butted together.
Tack the tail section to the second piece of wood.
Remove the plan-view cutout and the screws tacking it down. Mark the spots where the screws were tacked to the board.
Use a marker to connect the dots (i.e. the markings you just made), to create a centerline.
Tack down the cutout at the same locations as previously. Use a spray adhesive like 3M-77 to glue the cutout to the board
Cut out the plan-view a quarter inch outside of the guide line. Do not cut directly on the line! We used a jigsaw.
Use disk sanders to shave off the quarter inch offset from the previous step.
Now cut out the cross-sections from the large sheet of paper. If you can align the bottoms of cross-sections with machined edges on the boards, do so. A store-bought rectangular board has four machined edges (the edges of the board). These edges are very straight and perpendicular to one another.
Glue the cross-sections down to the boards
Cut the cross-sections out of the wood boards. Cut outside the guide lines with a quarter inch offset, same as previously.
Use a disk sander to remove the offset from the cross-sections
Measure and mark points to create a centerline on the table you constructed earlier. Use a straight edge to connect the dots and create the centerline.
Line up the centerline of the plan-view cutout with the centerline on the table.
Make sure the tip of the nose is at least 1 foot from the edge of the table
Make sure the sides of the cutout are at least 6 inches from the side edges of the table.
Screw the cutout down to the table.
Trim off the excess table area using a circular saw. We trimmed it down such that the side edges of the table were 6 inches from the cut-out.
Begin the process of attaching the cross-sections by cutting out small blocks of wood and screwing them to the base of the cross-sections.
Line up the lines on the cross-sections with the lines on the plan-view cut out so they are located accurately.
Screw the small wood blocks to the plan-view cutout to attach the cross-sections.
Acquire wood to make the stringers. No lumber stores in San Diego sell pre-cut stringers (as of 2020). Need to cut them DIY out of wood planks, it’s important to use a soft wood like redwood or pine, so it bends around the cross-sections. We used 0.5” x 5” x 8’ planks. Make sure the planks have little to no voids! Voids = bad
Cut the pine planks into as many stringers as you can. We set our table saw guide to cut 0.25” thick, 0.5” wide stringers.
Use a nail gun to attach the stringers to the cross-sections. Start with the middlemost (or topmost) stringer going right down the middle.
If the stringer can conform to the curvature of the nose without cracking, great. If it cannot, you may need to offset it a cross-section or two from the tip of the nose. See the picture below:
Start nailing more stringers down. As you add more, you will need to start again in a different location. Due to the nature of the shape, it’s impossible to perfectly line up all the stringers side by side.
To fill the gaps between two groups of stringers, cut the ends to make wedges that fit nicely together. We used a razor blade to cut the wedges because the pine wood was soft enough.
Several hours later . . . Make sure to add stingers all the way down to the table
Some gaps are okay, see the nose in the photo below. At this point we started calling it a plug.
Use a hammer and punch to hide any nails that are sticking out. Nail heads sticking out will ruin the sandpaper in the following steps
Use power sanders to round down the areas where stringers butt up against one another. We suggest using 40 grit sandpaper with power sanders and sanding the whole plug.
Use a jigsaw at the nose and tail to trim off any stringers that are sticking out. Cut the stringers back to the nearest cross-section.
Construct forms at the nose and tail that are sealed off around the table and the plug. These function as containers to put pour foam into.
Pour “pour foam” into the forms. We used a two part pour foam that we bought at Fiberlay. insert how-to video
Wait a while for the foam to cure fully (we waited overnight). When it is hard, trim it roughly to size using a wood saw. Make sure it’s slightly larger than you want it to be.
Sand down the nose and tail with power sanders + 40 grit sandpaper. Be careful not to oversand and keep the power sander moving so you don’t introduce flat spots.
Vacuum the plug to get it clean for fiberglass.
Acquire the thinnest fiberglass weave you can, and drape it over the plug. Trim off the excess around the perimeter.
You will likely have to patch different sheets of fiberglass together
Paint the resin into the fiberglass. We used paint brushes, paint rollers, and one part UV-cured resin (donated to us by SunRez). If you are using two part resin (e.g. from home depot) be sure to add your catalyst and work quickly!
You may have to spend extra attention on the nose and tail because of the sharp contours, try to get the glass to lay as flat as possible (i.e. no air voids)
Trim the excess fiberglass so that no fiberglass is stuck to the table (i.e. it’s all on the plug).
If you are using UV-cure resin, cure it with UV light!
Use power sanders + 40 grit paper to sand down any fiberglass strands sticking up
Use squeegees to paint bondo on to the plug. Remember to catalyze it and work quickly! Cover the whole thing so no wood is showing. We used two gallons for this operation.
Use acetone to clean your tools between every batch of bondo. If you don’t, hardened pieces of bondo will chip off your squeegee and get caught in the bondo you’re applying
Paint the table with bondo, at least six inches from the base of the plug. We suggest painting the entire table with bondo (not picture here) to get a large flange going.
Wait until the bondo gets hard enough and start sanding it with power sanders + 60 grit sandpaper. Sand until the whole thing feels smooth.
We can now start getting rid of high points and low points. Spray a thin layer of spray paint over the sub. As you sand the paint off, the high points and low points will reveal themselves.
Before you begin sanding, keep in mind there is a way to sand methodically so you don’t introduce low frequency waves into the plug surface. You want to sand in long diagonal motions. Pick one half of the plug (long-ways), start at one end, and sand at 45 degrees (bottom to top) uniformly down the full length of the plug. See the picture below to help visualize:
There are two things going on: surface smoothness and surface fairness. A fair surface has no low/high points, but could be rough. A smooth surface is very smooth but could have low/high points. We want to achieve both fairness and smoothness. Get multiple people doing it to make it go faster (this step should take at least a few days)
An example of a high point (the wood underneath has revealed itself), try to sand less on these spots.
An example of a low point (blue spray paint disappears around low point). Fill these with bondo and sand again.
Use a long sanding block to reveal low frequency waves. Low frequency waves cannot be revealed using power sander. Sanding blocks take longer than the power sander but they are necessary to fix low frequency waves. See the diagram below:
Make sure to sand horizontally along the bottom
Trim the corner of a squeegee to make a rounded tip
Create a corner fillet by putting bondo along the bottom and running the rounded squeegee over it
If you haven’t done so yet, coat the entire table with bondo
Keeping applying/sanding off layers of spray paint. If you fill the low points each time, it should get progressively smoother/fairer.
Circle the low points in sharpie to make them more visible
Towards the end of the sanding cycles, you may wish to apply a skim coat to fill the finer low points. Skim coat is a finishing filler used to achieve better smoothness.
When fairing the hull is complete, it should look similar to the photo below:
Spray the entire plug with a thick coat of Duratec high build primer. A wide nozzle is required for the spray gun. Wet sanding the high build primer can create a mirror-smooth finish.