Half frames are located at stations 4 1/2, 5 1/2, 6 1/2, and 7 1/2 as shown on the Framing Plan. The half frames provide support for the ballast keel. The structural arrangement is such that the primary frames at stations 5, 6, and 7 support the loads from the ballast keel as well as the hydrostatic loads on the hull. The planking supports hydrostatic loads between primary frames and loads from the ballast keel at the half frames. The half frames transfer the ballast keel loads to the planking. Note the planking only spans 500 mm between primary frames, so the hydrostatic load on the planking is relatively small in way of the keel.

I had originally designed the frames and half frames to be laminated out of Douglas fir, but after considering the cost and material wastage of milling lumber to 1/4" thickness, I decided to look for suitable material pre-milled to 1/4". What I found was pine lattice molding available at 1/4" thick by 2 1/2" or 1 5/8" wide. They have tons of it at the Solana Beach Dixieline, and it's good clear wood free of knots. Of course, any boat builder will tell you that pine should never ever be used in a wooden boat. Fortunately, I have developed an immunity to boat builder dogma and am able to assess the suitability of pine for my laminated frames and half frames based solely on the properties of the material.

One problem with using pine as a boat building material is it is susceptible to rot. The laminated frames and half frames on my boat will be encapsulated with epoxy resin. Problem solved. Another problem is pine is a relatively easily damaged soft wood. The pine on my boat will only be used on interior structural components and not be at risk for impact with heavy objects. Problem solved. Yet another problem with pine is it is not as strong as other boatbuilding woods used for transverse framing. I plan to increase the hull sheathing to (2) layers of 12 ounce 45 degree bi-axial to increase the stress on the sheathing and reduce the stress on the frames and half frames. Problem solved.

My research into pine lattice molding that is available at the lumber yard found that it is either Radiata pine, Ponderosa pine, or both. Ponderosa pine is the weaker material, so my scantling calculations are for Ponderosa pine. The available material is listed as 1/4" thick but is actually more like 7/32" or 5.6 mm thick. The allowable stress for Ponderosa pine is 29 MPa; the calculated stress for the half frames as built is 28 MPa. Just right. The half frame is treated as a composite beam consisting of the half frame itself and the hull sheathing, with a 19 mm gap (planking thickness) between the half frame and the sheathing.

The half frames are laminated on the loft floor which is covered with a clear plastic drop cloth. The clamping angles are positioned to allow for some springback, and I have included a springback table on the Shop Drawings page. I used a 4:1 block and tackle to pull the pieces into the clamping angles. I have since covered the skids with clear plastic tape over the blue tape because the blue tape tends to stick to the epoxy. The first half frame was dry-fit to make sure the 4:1 purchase and clamping arrangement worked.

Waxed paper is used to keep glue drips off the loft floor, even though it is already covered with plastic. After the laminated part is pulled and the sides are planed smooth, it will be placed on the loft floor so buttocks and waterlines can be marked for bevel locations.

Once both sides of the half are laminated, the next steps are to cut in the bevel, assemble the half frame, and coat the whole thing with epoxy.

The half frame pieces are fit and assembled dry on the loft floor, temporarily held together with dry wall screws. I used a gap filling mixture of epoxy, milled fibers, and silica to glue the half frame assembly. The half frame was re-assembled for the glue up with the same dry wall screws used to clamp everything together. The drywall screws will be removed and replaced with hardwood dowels after the eopxy has cured.

At this point I decided to add a unidirectional E glass cap accross the tops of the floors to compensate for gaps in fitting the filler pieces. The uni caps are sized assuming the glue joint between the filler piece and the laminated half frames is ineffective. The floors are what hold the ballast keel in the boat and there can be no quesion of their structural integrity. I also decided to use one row of large keel bolts down the centerline instead of two rows along the perimeter of the top of the lead ballast. This will save time and simplify the ballast keel installation. These design updates are indicated on the Laminated Half Frame Assembly included on the Shop Drawings page.