Friday, August 26, 2016

New Design Model, Part II

With an outboard engine, all the power and 300+ pounds of weight rest on the engine support; thus, that area has to be very strong.  The engine support itself should be 2" thick; weakness can easily result if that thick, solid plywood support isn't adequately integrated to distribute its stresses to the surrounding structure.  My solution has been to recess the engine support into the stern of the boat.  In this way it is linked to the last three frames of the boat, to the keel, and hull sheathing.  With a fiberglass-wrapped, bonded hull structure, forces are dispersed into the entire hull structure.  The hull extensions behind the transom provide buoyancy for the weight of the engine, act as trim tabs, and aesthetically help blend the size of the engine into the overall boat shape.
A splash well has been included in the support design.  This allows the engine to be raised and tilted forward while allowing a rear deck and splash protection for the cockpit.  The engine support is angled at 14 degrees, the splash well bottom is angled to drain to the rear, and the well sides flare from bottom to top, providing more space for the engine itself than the lower unit.  None of this is "cut to fit"; with the hull being designed mathematically, all dimensions can be pre-cut very exactly.  The angles in this structure actually make it easier to put together; even without adhesive, the pieces are wedged together in a tight fit.

I have had time this weekend to fully sheath the model (my wife is out of town), and I am very satisfied with the result.  I'll post some photos; what do you think?

Wednesday, August 24, 2016

Creating a Model of a New Design

Boat design has been on hold.  After getting back from our trip to northern Europe, I cleaned, sanded, wiped-down and re-varnished the topsides on my current boat.  The boat needed to look nice for a boat show at Grand Lake, Colorado.  Following that, we went water skiing at Lake Granby (8500' altitude).  We had three adults and a dog (and picnic lunch) in my 18' boat and were pulling a slalom water skier all with the 75 hp. Evinrude.  I'd like to think that it takes an efficient setup to handle such a load.

Now I am back to creating a 3' wooden model (1/6.25 scale) of what could be my 9th boat design, if I can justify building it full size.  Functionally, it may not be much different than my current boat, but I hope to provide some pictures to demonstrate a more curvaceous, attractive hull shape.  The building steps for a model are similar to full size building, and I will describe the steps as if it were a full-size build. 

The first step in building is always to build the frames and curved anterior keel section.  This is important as it will form the skeleton of the boat.  Dimensions must be accurate, and joints need gussets to be strong.  For the model, I made the frames extra beefy, cut intact out of plywood to insure strength and accuracy.  I draw a pattern for each frame.  The design of frames must anticipate everything that comes afterwards because the frames help support the deck, seats, fuel tank, battery, shelves, steering... all the details that follow.  Instead of hard oak, I prefer to use softer, larger dimension lumber, such as Douglas Fir, because I want larger surface area joints for adhesive bonding instead of screws.  Bonding distributes stresses more evenly.  Each frame should be marked with the mid-line and a common reference horizontal line to be used for frame alignment.  The pile of resulting frames still looks like an odd pile of lumber, not giving much hint of the hull to come.

A rigid strongback form is created to position all the frames in sequence and alignment.  This is where you first get a glimpse of the boat's shape.  The strongback needs to aid in holding each frame in horizontal, vertical and longitudinal alignment.  The frames can be accurately aligned using a laser beam; eyeballing or using a tight string can work also.  A clever design addition might be to make the strongback so that it can be tilted port and starboard to improve access to various portions of the hull during building.  Then the keel, chine, and sheer are added.  Other longitudinal elements are added: keelson, cockpit edging, foredeck plank, etc. to add more rigidity and further define the hull shape until the strongback is no longer needed.
With all the major elements in place, the hull next needs to be faired so that all parts flow together as a foundation for a smooth hull sheathing.  Using a developable design allows visualization of the "ruling lines" which radiate from a conic projection or run parallel in a cylindrical projection.  Laying a straight edge along these ruling lines makes fairing easy.  Once the fairing is completed, patterns for the hull sheathing need to be made.  Stiff paper or thin (and cheap) plywood work well.  Rosin paper, used with flooring underlayment, has worked well for me.  The staging of the outer layer of planking can easily be drawn out on these patterns.  I was pleased to see that the hull patterns for the bottom and topsides are somewhat linear, rather than banana or "S" shaped as in some designs.

Resulting patterns are then laid out on plywood for the inner sheathing.  For this scale model, 1/8" plywood was used and bent around the frames smoothly; 6 mm. marine ply should adapt easily to the full-size hull without excessive clamping pressure.   When cutting out the plywood from the pattern, I leave a little extra to allow for inaccuracy.  Then I soak the plywood in a basin of water for several hours to make it more flexible; clamp it in place on the framework until it dries; mark and trim the excess; and finally the plywood is ready for final bonding.  On a full size hull, the plywood is in two sections for ease of handling which is scarfed together in place on the hull.