Developable Surface Boat Designs

While notches for the stringers were pre-cut in the frames and stems, many of those cuts need to be modified.  The notches were "square-cut" as if the stringers simply passed through at right angles, but, except at amidships, the stringers curve inward and upward as they approach the stems.  So the notches need be be beveled to accept the stringers in a fair curve; a total of 120 notches.  It slows down the process, but provides good exercise as it needs to be done carefully by hand.


It has been said that using developable techniques to design boats tends to result in boxy-looking, uninteresting shapes.  But by using multiple chines, and the inherent accuracy and fairness of the resulting curves, I think that the result can be quite harmonious.

Two longitudinals added so far; two more to go.  I will also add a second strip at the sheer, the inwale.  Beginning to look boat-like, but much work lies ahead.

The inwale and a third stringer have been added.  I have an epoxy allergy (rash), so before mixing a batch, I put every clamp near by, set out the spatula, the applicator, pre-mark every matching location, clear out unneeded objects, put on gloves, etc.  With many small joints to bond, I pre-coat matching surfaces and give the epoxy time to soak in before assembly.  After assembly comes cleanup of any excess squeezing out of the joints.  Thin coats of small quantities cause little exothermic reaction, so the resin takes many hours to harden.  If I warmed my shop up beyond 66 degrees F., of course, the setting would be quicker.

The overall length at this point is 179.5".  The curved length of the sheer is 187.04"  The half angle of the bow is 21.8 degrees.  I used an algebraic equation and projections to provide all the offsets for the hull shape.  I integrated that formula (calculus) to find the curved length of the sheer at each frame & stem location.  I differentiated (again calculus) that formula to find the slope at selected points.  Then I used the inverse tangent function (trigonometry) to find the angle in degrees to set my chop saw.

People say, "Math isn't important; I never use it."  Of course, you don't use things that you have little knowledge of.  I have little use for knitting; perhaps it is because I know nothing about it.  The math I just mentioned is not difficult if you are accustomed to doing such things.

Next to-do is fairing the hull.  Not real fun to do; it is careful work in which the progress is not dramatically evident.  But, it is the foundation for what comes next: making patterns for the sheathing.  Will the shape be nice and linear or have a banana or "S" shape?  I still have not made a final decision on sheathing.  Each alternative has its advantages and limitations.

We are readying for a month-long trip overseas; there may not be much progress in the near future.

My guide boat design is symmetrical at both ends.  It makes for simplicity in building and results in needing only one oarlock location.  Calculating the location of each frame along the sheer curve is best done by measuring from the midpoint toward both ends.  (For reference, the equation used is listed in my blog post of Feb. 11, 2015, Sail Design)

X-dimension at keel:  5.75    17.75    29.75    41.75    53.75    65.75    71.75    83.75    89.75  
Sheer dimension:       5.78    17.85    29.91    42.19    54.67    67.43    73.92    86.99    93.52  (inches)

Most of the frames are set in place on the keel in this photo.  This was just practice to see if everything fit as planned.  The keel sits on paint buckets and most frames are held in place with bungee cords.

If you look closely you can see some wood wedges under the paint buckets.  The keel has been checked for straightness and twist and two battens, marked for sheer lengths, have been clamped in place.  The frames have been mated to the keel with epoxy.  This entire setup is somewhat self-aligning.  I used a square to check that the frames are vertical; no adjustment was required.  If this simple set up didn't do the job, I would have been forced to create an entire strongback to force alignment.

These are the two stems mated with the end frames using epoxy.  The next step will be to bond these stems and the final two frames in place on the keel.  With ten frames already in place, and clamped sheer battens marked for the proper lengths, it shouldn't be difficult to get these bonded in the proper positions.  Then, three more battens/stringers will be added to each side.  At that point it should look rather boat-like.  Two future steps where I anticipate difficulty (time consuming) are fairing the hull and fitting the garboards. 

The hull is starting to come together.  You never have too many clamps for boat building.




It looks much like a wide canoe, but the 44" beam will allow the use of oars.  The 18 degree deadrise creates a narrow waterline width while allowing a low seating position.

The stem pieces came together perfectly using the calculated half-angle of 21.8 degrees and the half length for the curved sheer stringer of 93.52".