3SD

17'8" (5.4 m) and 17'1" (5.2 m) three sheet dories

Since I published the plans of the Little sister dory in 2003, I've been thinking about even better use of plywood.

My aim has been producing dory plans, that would produce a good looking dory, that would be as big (in terms of length and displacement) as three sheets of plywood would make possible, and would make as nearly 100 % use of plywood as possible. Remember, I started this hobby with the one sheet design concept Prism, that makes a full 100 % use of a single sheet of plywood.

So here it is now. A new dory design, 3SD, standing simply for "Three Sheet Dory" (or "three standard deviations", meaning that this dory is that far from an average three sheet dory, that seems to be a tiny 15' 6" or so boat ;-).

Be that as it may, this dory reaches about 91.9 % plywood utilisation. When You cut three sheets of plywood according to these plans You are left with so little waste plywood, that You can't even make oar blades out of it.

I call that economical and ecological.

In my opinion the 3SD is good looking. Some might argue, however, that she has too much of sheer "for modern taste".
That may be so, but the strong sheer can be seen as traditional. In the following line drawing the profile of the 3SD is compared with the 1884 Portsmouth U.S. Navy shipyard dory presented by John Gardner in his "Dory Book".

• The green lines are those of the 3SD profile.
• The blue lines are those of the 3SD profile scaled up a tiny bit to match the overall length of the 1884 Navy dory (18' 10").
• The red lines are those of the 1884 U.S. Navy dory.

To tell You the truth, the strong sheer is a consequence of keeping the top of the boat side completely straight when flat ;-)
And I want to keep the flat side piece as straight-sided as possible. To avoid unnecessary lofting, cutting and waste plywood.

Two sizes, two ways of joining the plywood pieces

Two ways of building this boat are presented.

• Butted sides and bottom, where pieces are connected with fiberglass tape butt joints. This way of building produces a 17' 8" (5.4 m) boat.
• Scarphed sides and bottom, where pieces are scarphed together using 3" (76 mm) scarph joints. As this method of building "wastes" some of the total plywood length in the scarphs, the boat produced will be slightly shorter, 17' 1" (5.2 m).

Cutting the plywood

The pieces for a butted boat are cut out of three sheets of plywood like this.

• The green pieces present the right, starboard side of the boat, the inner face up. Piece number 1 is the bow piece, number 2 is the middle piece, and number 3 is the stern.
• The red pieces present the left, port side of the boat.
• The yellow pieces make the bottom.
• And the transom is laminated out of the two grey pieces.
• Note, that the plywood sheets 1 and 2 are mirror images of each other, so they can be cut together in one go. If you do that, take care that the sheets are extremely well bonded together so that they have no change of slipping relative to each other while making the cuts.
• The transom pieces are displayed in a position for best grain alignment, grain running along the middle of the transom. In case you want to use a clear finish. For an opaque finish it would be one cut less to cut the pieces in such a way, that one side of the transom lies along the plywood edge. Also leaves you with slightly larger left over pieces, maybe useful for something. Oar blades, for example.

The pieces for the scarphed version correspond to the ones for the butted version, except for the provisions for the scarph joints.

• The bright yellow zone indicates the area where wood is removed on the upper, visible surface of the plywood.
• The vertical lines near the left edges of the individual sheets indicate the area where wood is removed from underneath the plywood.

Note: The sides are assembled "one side of the plywood out, one side in". In case Your plywood has one good and one bad side, You can have the inside of the boat all good, or the outside of the boat all good, depending on Your taste. I'd probably have the good side in, because that's the side I'd be looking at most of the time. The fish may look at the bad side.

Measurements for the left boat side, inner side up. This would correspond to the red pieces of the above picture, the pieces connected. The plywood seam lines are indicated with blue vertical lines on the drawing.
The plain measurements are for the butted version, the measurements in parentheses for the scarphed version.

Measurements for the boat bottom. The plain measurements are for the butted version, the measurements in parentheses for the scarphed version.

You may have noticed, that the sides and the bottom were both symmetric end to end. Just for simplicity :-)

Measurements for the transom. The plain measurements are for the butted version, the measurements in parentheses for the scarphed version.

Measurements for frames.

The angle of the stem is 63 degrees at the sheer, tapering to 42 degrees at the bottom.

Resistance curves at 150 kg / 330 lbs total displacement

Compare these with the resistance curves of the original Little sister dory. The wave making resistance is lower now, so the new 3SD should be slightly lighter to row at speeds above 3 knots or so.

Rt (violet curve) = total resistance
Rv (red curve) = viscous resistance (friction)
Rw (blue curve) = wave forming resistance
Rh (pale blue curve) = resistance created by transom stern
Full speed scale = 4.0 m/s = 14.4 km/h = 9.0 mph = 7.8 knots

Building sequence

The building sequence is roughly like this:

• Cut the sides, bottom and transom out of plywood.
• Cut a stem out of suitable wood.
• Do the butts or scarphs, and laminate the transom.
• Mark the frame position lines A, B and C on the inside of the sides and bottom.
• Connect the sides to the stem at the bow.
• Build the frames A, B ja C out of plank.
• Glue and screw the middle frame B between the sides in such a way, that the line B on both boat sides coincides with the middle of the frame.
• Connect the stern ends of the sides to the transom.
• Insert frames A and C between the sides.

At this point do not attach frames A and C permanently. This is just a temporary arrangement to measure the angle of the sides at the frames to make a good fit.
Position the frames A and C in such a way, that the front frame A is just behind the line A (red in the drawing), rear frame C is just ahead of line C (red in the drawing).

• Measure the distance between the frame and the side (blue arrows).
• Transfer the measurement over to the other side of the frame (green arows).
• Remove frames A and C.
• Cut off the marked (orange) part of the frames.

Attach the chamfered frames permanently.
The front frame A in front of line A (red), the rear frame C behind line C (red).

• Attach the bottom.
• Attach risers inside the frames, for seats to rest on.
• Attach seats and oarlocks.
• Paint her.
• Use her.

Suggested seating arrangements

The 3SD is primarily suitable for one to three people. Using more than three seats makes good sense, however.

For best directional stability in windy conditions it is often best to have the boat:

• Slightly bow-heavy rowing against the wind. The slightly raised stern acts as a wind vane helping to keep the bow into the wind.
• Slightly stern heavy when going with the wind or wind on the side.
  • When rowing with the wind this loading keeps the bow from diving into the back of waves.
  • When rowing in a side wind, the combined forces of the wind and the boat movement through water tend to try to turn the boat into the wind. The stern heavy position acts to ovecome this tendency.

The more seats You have, the more possible loading combinations.

A suitable oar length for both suggested oarlock positions would be in the range 8' 4" (2.5 m) to 8' 10" (2.7 m).

A prototype

A quick and dirty prototype was built. Not by myself, but someone wanting to build a "quick-and-easy to build" boat for himself to use for fishing as long as a boat built that way would last.

	As can be seen a prototype boat yard does not need to look much like a boat yard :-)
	No saw horses, just a couple of garden chairs, and a couple of short lengths of two by four. Probably a better way of building would be to build the frames first, then attach the sides to the frames and transom, then add the bottom. As described above.
	The plywood hull completed.
	Frames inserted into the complete plywood skin. But as written, this is probably not the best building sequence.
	The hull from another angle.

"There's no picture of the finished boat" the prototype builder told me.
"I was in a hurry, and had to leave. Building took three long days altogether for one man."
"The boat gave a nice ride, it was very light to row. The strong head wind I had during the maiden voyage did blow the boat around a bit when I was alone in the boat. There was, however, no fear of capsize."

Well, yes, that's the flip side of light displacement, shallow draft and light rowing. There is very little lateral resistance to the wind.
Add more weight or add an external keel. Both would add lateral resistance. But at the expense of shallow draft and light rowing.
Boatbuilding is nothing but compromises ;-)