Tuesday, October 22, 2013

Tech Tip Tuesday

Non-Traditional Skin-on-Frame design may be converted from other more traditional designs relatively easily.  The easiest designs to convert are those that have faceted cross-sections.  For someone looking for faceted designs, existing lapstrake and stitch-and-glue designs would be good candidates.   Round-bottomed designs - like traditional canoes tend to be -  can be converted as well, but there is a little bit more trial and error in creating the "flats" along the curve, because you want the stringers to be "fair" curves along the length of the boat.

 Lapstrake Boat

 Stitch-and-Glue Eastport Pram

There are some caveats, however. 

You need to plan for a design where the facets are not very wide - if they are, you may need to break up the "panel" into a smaller width.  By "wide" I mean panels more than 10" or a foot wide.   If the fabric panels are wide and the fabric isn't tight enough, it can sag under water pressure - perhaps hitting a frame - which will slow the boat in the water. 

The other main caveat is that the stringers don't want to have lots of force applied to the hull in the "up" and "down" direction.  This is particularly true of the gunwales or the sheer clamps on the boat.  Excessive forces can cause the hull to bend with more rocker - the fore-and-aft curvature of the hull that is reminiscent of the shape a rocker on a rocking chair - or hogging - a reverse bend where the middle of the keel is higher than the ends.   The other issue is that the hull can distort from the forces.   These forces applied to the hull can be minimized by pre-bending, laminating stringers to the curve, steaming the stringers to shape.

There are some other minor concerns.  One is fabric width.  You may want to be able to build the boat with one single piece of fabric so you don't have a seam at the bottom.  This may not be possible and you may have to stitch multiple pieces of fabric together, but this isn't a disaster or an impossibility. 

One item that should not be ignored - for any boat building -  is that you need to have the correct scantlings for the boat.  Scantlings are the appropriate dimensional timbers for a given size of boat.  This would include the right thickness of plywood - typically 1/2" for small boats - and the correct stringer cross-section.  The boats that we're building have stringers that range from 5/8" square to as big as 1-1/2"x 3/4".   Boats that are designed for paddling - such as a canoe, pirogue or kayak - will have structural elements such as decks or breasthooks, thwarts, floors, coamings and the like to help keep the shape of the boat.  Boats designed for rowing or sailing have other concerns.  For sailing boats, they need to hold their shape when the forces of wind and water are applied to the centerboard/daggerboard and trunk, rudder, mast partners, and mast step.  Rowing craft need to have a strong enough seat and gunwales/oarlocks to resist to forces applied by the rower to the hull.

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