Bulkheads

Added bulkhead, which also serve as the three primary frames. While stitch and glue construction can be done without a frame, it's difficult to control the thin plywood; adding a frame helps. The aft and bow frame will serve as bulkheads for the water tight compartments as well as support the seat. The midship frame will support the mid seat and will be hollowed out to save weight. In Freeship there are several ways to do this. I added transverse planes, copies the intersections with the planks, removed the planes, than built the frames point by point. It's a bit tedious but gives good result. It also gives me good outline from which I can subtract the planking thickness later to get exact frame dimensions.

Freeship allows each layer, which in this case is each part of the boat, to have a set thickness and pounds per cubic foot. I plan to use occume mahogany, with an average density of 27 pounds per cubic foot. I set it as 30 pounds in Freeship with mostly 1/4 inch plywood. That gives me a total on only 49 pounds so far; I'm not done yet, but that's encouraging. This little car-topper looks realistic!

Next I'll add the supporting frames for the seats and start on the developed parts layout on sheet plywood. I hope to order the wood by next week.

Completing the Seat Tops

Decided to double check the hull before trimming the seats because after the seat sides are created, it's very difficult to make any further changes. I tweaked a few things using the scale tool applied to difference layers. I built each layer as a boat part (e.g. shear plant, chine plank etc). Still trying to keep hull weight and the boats purpose for rowing, fishing and photograph in mind, I reduced the bow height to 21", which will be fine for the protected waters where I plan to use this boat. I raised the minimum shear height to 14 inches for good rowing oarlock positions; this required I do some proportional scaling (manually) from the bow to middle of the boat for the sheer and mid planks. Lastly at 4 inches draft the displacement was 260 pounds which was a bit low. Against for rowing without an outrigger, I scaled the overall beam to 41" wide and increased the bottom beam by almost two inches to raise displacement to 300 pounds--a knowing compromise to get better initial stability while probably reducing speed.

I shifted the mid seat aft by a few inches so it was over the center of buoyancy (`6.4' from stern). From prior boats, I know water tends to pool in the end seats so raised the ends by a half inch so they had a gentle slope. Using the point>intersect-layers command identified where each seat intersected the middle plank. Then it was a simple matter of collapsing the points outside the hull so their outside edges fell on the mid plank points.

The results are below:

Next time I'll add bulkheads and the hull weight estimate.

Adding seats

Well I know I've been too busy when I can't get to something I enjoy like messing in boats. Tomorrow Spring starts!

Back to the drawing board (or computer). This time adding seats. I've got two options, build seats as part of watertight bulkheads or if that's too heavy-one with simple seats and canoe floating bags. I'll use freeship to do most of the estimate. I prefer the watertight bulkheads because it's safer because it adds flotation abeam which makes is inherently more stable than float bags or compartments at the end of the boat.  It also serves as storage for dry cloths after recovering the boat.

Most of the advise I see online from Woodenboat (http://www.woodenboatpeople.com) about seats say they need to be at least 7 inches off the floor and about 6 inches below the oarlocks. I've got 14 inch mid ship height, so I'm going to go with 8 inch seats (from baseline). The aft seat will be for my wife, not for rowing so I'll make it the same height so oars can lay flat for short periods. The bow seat shear hieght will be about 16 or 17 inches, so I'm going to make that one 9.5 inches from the baseline. Using the dialog box I chose not to show all the layers (e.g., bottom, sides etc) but leave the control lines so I could continue to see the hull as I placed the seats. Below is the perspective and plan view. Note, I haven't trimmed off the excess where it intercepts the hull. I'll do that next time.

In freeships the easiest way to make seat is add the points away from the boat, select all the points, create a layer/face and finally use transform>move to place it on the hull. I used edge>split to add a curve to the seats; it adds no functional improvement but aesthetically makes for a pretty boat.

Adding a skeg

Most dories have skegs to improve directional stability while rowing--probably a good idea given their flat bottoms. I decided to design a simple one made of 1" stock (3/4") a couple feet long and curved so the wear and tear from dragging the boat around wasn't on one point. This would also allow me to add a protective metal half round if I wanted. Many skegs running deeper than the bottom amid ship, but I decided to make it the same depth. From other boats I've rowed there usually enough water flow upwards towards the stern to provide plenty of grid. Also I've seen several boats with longer skegs damaged after some rough treatment being pulled on shore or hitting something underwater while drifting sideways.   Keeping it small would save weight and less vulnerable. If it doesn't work, I can always laminate another strip on the gentle curve.

Just as a started a noticed a problem. I'd designed the transom to a point. My first task was to make it the same width as the skeg. This isn't very hard in Freeship as long as you're careful to preserve the surfaces as you extrude edges, remove or add point and edges. Here are the results:

To add the skeg I selected the three aft bottom center edges and extrude them vertical by about 4 inches. Like this:

I set the vertical height of all the bottom points to zero, adjust the most aft so it slopes in line with the transom and round the two corner points. Next I extrude the whole skeg transverse by 3/8" which is half the final thickness. That completes the skeg (see below). I left the control points in view so you can see how the rounded corners work.

Next time  I'll be designing the seats, and thinking about the framing and bow construction.

Curvature progress

 Last time I discussed the flat spots in the curvature. Control point by control point I went through the chine. The curvature tool and ability to move the view into any perspective helps here. After a few hours things are much better. First the dory using the curvature tool again.

The curvature is much more uniform along the length of each chine.
Here's an idea of what the hull looks like, with some color this time.

Converting to Freeship....problems

Imported the lines from Hulls into Freeship. I allowed internal stations to be imported so now there are 10 stations.

I pull up the linesplan, which shows all the plan, profile there are at least two obvious problems.

1) The stem is misshaped. There's too much of a bulb at the lower chine.

2) The bottom plan view (looking down) is odd and hump where Hulls station 4 used to be instead of a smooth transition from bow to stern.

To confirm what I'm seeing isn't an optical illusion from the lines, I select the edges and curvature change (purple in Freeship). . To confirm that, I choice perspective view, and select all the chines. Now I can see the double hump in curvature and slack curvature near the bottom mid-frame; not only that but all the chines show a similar slack.  This is probably a problem of how it imported from the 5 station shape and interpolated it into the 10 stations of Freeship. I've got work to do....

modifying the dory...making it mine :-)

A note about getting Hulls to run on a modern machine.

I've been playing with Hulls for about ten years used it to design another small boat, and as a recreation even during my time in Iraq because it ran on my netbook. I'm running 64bit Windows 7 Home Premium doesn't have a built in emulator. I think Hulls is a 16 bit program so there was no easy way to get it to run. To make a long story short, I installed a free program called VMWare Player, which is a virtual machine program, dug through my closet for and old copy of XP professional and loaded it up. Windows XP and Hulls work really well.  Here's the web page for more about VMWare Player, http://www.vmware.com/products/player/

--

As much as I like the dory, it was really too heavy and more boat than I needed. 

Every boat is a compromise. What I wanted to do is nudge this excellent design towards features I wanted while decreasing some features that I didn't need.

My first priority was to lighten the boat, I narrowed the hull by two inches and reduced the bow freeboard by four inches. That's a lot and risk ruining the shear line, so I'll need to be careful.

I wanted better initial stability so widened the bottom by about 4 inches. Hulls makes it easy to rescale the entire table of offsets based on changing length, height, or beam.  

A few aesthetic concerns. To make the boat look even more like a swampscott dory, and dispel the plywood slab sided look, I added another chine. To do this I just took the mid height between the existing 2nd chine and the shear line. I also added slit round.  

As much as I like dories, I'm not a big fan of wedge shaped dory tombstone transoms. Using the added chines, I rounded the transom sides. 

To compare the two boats: 

FSK Dory

LOA: 12' 3”

BOA: 3' 6”

LWL 2'
BWL 2' at 300 pounds which is boat plus one person

Height of the bow: 24 3/4”

Height of stern: 17 3/4”

Height of mid shear: 14 1/2”

Weight: 120-150# (estimate from traditional construction)

Lynx Dory

LOA: 13' 6”

BOA: 3' 4”

LWL: 11' 7”

BWL: 2' 10”

Height of the bow: ~21”

Height of stern: ~17”

Height of mid shear: ~15”

For the next blog post I'll convert into freeship.