Crunch Time
Lines
LOA: 6.2m
LWL: 5.9m
Beam: 0.9m
Design waterline: 0.08m
Freeboard amidships: 0.22m
GMT: 0.55m
After rowing it for much of 2020 I decided that this CLC Annapolis Wherry Tandem at 141 lb (21lb for the rig and 23lb for the add-on I devised to increase the freeboard a little bit) was just too heavy to be pleasant.
So I set out to make a lighter boat that rows just as well - and maybe a little faster, too. Meet Crunch Time, a 20'4" boat for the same drop-in sliding seat rig weighing 81lb (21lb for the rig), made of fiberglass over styrofoam.
The first step was to find comparables and model them - this is a rowing shell and the wherry, and the rowing shell stretched laterally to give it similar initial stability. One of the main goals of this project was to make a boat which did not compromise on the characteristics that make the wherry so great in a choppy harbor and not a boat confined to flat water. Crunch Time has similar length, breadth, freeboard, and initial stability.
Using FreeShip+ I modeled these hulls.
Using Michlet I analyzed the drag on these hulls.
Then I used Godzilla, which is michlet's hull optimization mode, to produce new hull shapes with similar stability optimized for minimum resistance at 3.0 m/s
After several different attempts I came up with this hull shape which michlet claims is about 15% lower resistance than the wherry and about 90% of the GMT.
Above the waterline I wanted to keep the same overall dimensions as the wherry, as well as a vaguely similar shape. Michlet's results were pretty unequivocal about rocker (I tried adding some several times) so I went with a pretty sharply curved stem - I don't like the look of a plumb stem, so the 3" or whatever of curve here shouldn't have too much effect on performance. I also made a mistake here in that I made the boat the same length at the waterline as the wherry, rather than overall. Considering the wherry's heavily raked transom, long stem and rocker, vs Crunch Time's moderately raked transom, sharply curved stem and lack of rocker, this resulted in a boat about 2 ft shorter overall.
I wanted a wineglass transom, but the hull shape given by godzilla was more of a canoe shape. In order to get the wineglass transom I wanted, I put a chine in at the design waterline which disappears about 1/4 of the way forward. With the model made, I decided to 3D print it so I could really look at it. The print failed near the bow but this was good enough for me.
I decided the very raised bow was a bit too much, so I lowered it a bit. Next step was to generate offsets so I could make the molds. I wrote a program to generate a table of offsets from an .stl file, since the offsets generation in FreeShip outputs garbage. Then I traced the station molds out full sized on paper. I chose 18" mold spacing, since I wasn't sure if I could get away with any bigger spacing.
Here are the even numbered molds cut out of plywood. I didn't bother beveling.
I made up a stem and stern knee out of marine ply and a styrofoam transom. Set everything up on that strongback - had to extend the strongback a bit since this boat is longer than that canoe. Mold spacing was different from the canoe that was last built on this strongback. Here the molds are just clamped in place, later I screwed them in.
I used 1/2 x 3/4" solid wood strips for the sheer strakes and a 1/2" x 1" marine plywood strip for the bottom. Glued that all together and started adding 1/2" x 1" strips of styrofoam cut out of a sheet of insulation board. One of the biggest mistakes was choosing to make the foam only 1/2" thick. I should have made it 1" thick using the whole thickness of the board, and made much wider strips. That would have saved a lot of effort and made for a stiffer boat.
I experimented with several different methods of attaching the strips. Initially I was putting epoxy on the whole edge of each strip and holding them together with tape or by nailing to each mold. By the end I had settled on stitching the strips together with nails, nailing to the molds as necessary, then after setting up several strips, smearing on a bit of epoxy on the outside to hold them together. Every strip was glued to the stem with epoxy and glass bubbles filler.
When the strips approached the design waterline (3-1/8") amidships, I pulled the next strip down to the waterline at the stern, in the hopes of avoiding making the severe twist on short strips there later.
The resulting triangular gaps were filled in with short strips. The football in the middle was filled in around 8 strips wide. I struggled to keep the football flat with its short strips of styrofoam. Next step was to add lots of filler (epoxy + glass bubbles) in an attempt to smooth the inside surface. My ability to sand was limited since the only structure at that point was styrofoam.
Then with the inside vaguely smooth, I put on a layer of 6oz fiberglass cloth. I used a bunch of bits of scrap.
Then I added three strips of carbon fiber tape because I felt like it. Unfortunately I didn't think about where to put the bulkheads before putting the carbon fiber in, or else I would have spaced them better. The bulkheads were cut 1" foam. Then a second layer of 6oz glass.
Next I made these frames. These serve to hold the sliding seat rig and are more heavily built than the rest of the boat, 1x1 douglas fir across the boat, 1/4" plywood underlayment on each side, and the rest foam.
I made this jig for fiberglassing the camber into a sheet of foam with relief cuts. I should have overbent a bit, since the decks ended up a little flat.
After making the decks I cut them to size and then glued them in - they rest on some little blocks in addition to being glued around the edge. They are just about strong enough to stand on.
Then lots of fill coats of epoxy+glass bubbles, sanding, priming, sanding, painting, sanding, etc. And the resulting finish is... not nice because I had no way to get the boat smooth before glassing without any structure except foam. I completely finished the inside before flipping the boat. The color scheme is whatever we had half-cans of left over, or Neapolitan ice cream.
With structure on the inside I was able to smear lots of filler on the outside and sand the hell out of it. Here I tried from most to least brown: phenolic microballoons, System3 fairing compound, West 405 Microlight fairing filler, and glass bubbles (and some combinations thereof). I didn't notice any substantial difference, except that the other stuff is a little bit lighter than glass bubbles for the same volume.
At this point I started getting worried about weight so there is just one layer of 6oz glass on the outside.
Filled, primed, and painted banana yellow, the final weight was 60lb for the bare hull. The rig adds another 20lb.
Launch time
I'm looking for a source for this tool
