Composite Rudder Shelf - (Bertram 31)While the diagrams shown here are specific to the 31 Bertram, the basic principle is the same for any boat with twin rudders.
Start by making up a composite blank to work with. This can be done with either plywood or PVC Foam Board, (see PVC Foam Board as an alternative to plywood), and using either polyester resin or epoxy. Make up a long enough blank composite to cut out all elements of the design. If you are going to run the shelf totally between your two outboard stringers, then you will need approximately 24" in length than the measurement between the stringers.
If you are going to use plywood, 2 layers of 1/2" will be sufficiently strong enough for a 30' class power boat. If PVC Foam Board is chosen, 3 layers of 3/8" is recommended. Each layer of core will be separated with a layer of 24 oz. fiberglass roving. In either case, the entire composite will be wrapped with 1708 biaxle fiberglass fabric.
Once the treated plywood is tacky, add a very wet layer of 24 oz. roving to one side of one piece of plywood and place the other piece on top, line it all up, and screw the two pieces together. One screw every 4 square inches is fine. These screws will be removed after the resin has cured, so common deck screws are fine.
If you elect to use PVC Foam Board, first hand sand all 6 faces with 24 grit paper, leaving no un-scratched surface. Thoroughly Clean the surfaces with acetone, and while still sticky, brush on a coat of catalyzed resin. Again, allow it to tack up before proceeding. Once tacky, add a wet layer of 24 oz. roving to one side of two pieces of the PVC. Stack the three pieces so that each layer of core is separated by roving. Align and screw together. Remove the screws after the resin has cured.
Using epoxy rather than polyester: If you elect to use plywood as your core, epoxy will be be your best resin choice since epoxy is a genuine adhesive, but will not bond properly with PVC. For PVC, only use polyester resin.
After you've bonded your composite stack with the roving, round over all four long edges with a 3/4" round over bit & a router. With 1708 biaxle fabric, glass one of the faces, allowing the fabric to drape fully over the two long edges. Don't attempt to wrap the cloth to the under face or round over. As soon as the glass is leather hard, a razor blade can be used to trim the excess fabric. Allow the glass to cure and sand down the trimmed edges. Flip the composite over and repeat the process for the remaining side.
After the glass has fully cured, you may elect to pre-refine the surfaces. Lightly sand with 80 grit paper and apply a liberal coat of catalyzed resin. Allow to cure and sand again lightly. The surface is now ready for gelcoat if polyester resin was used, or paint if you used epoxy.
Cut and assemble your parts as per your particular design. If perpendicular elements are to be used, they should be dadoed in and glassed in at the joints.
Once the shelf, (shelves), have been fabricated and dry fitted, they should be permanently bonded into place with thickened epoxy resin. Vertical elements can be simply through bolted with 3/8" stainless steel bolts, nuts and washers.
If possible, it is recommended that the shelf be a continuos run from outboard stringer to outboard stringer. If this is not practical, be sure that the free 90° joints are reinforced on both sides...
Once the shelf is bonded into place on the boat, you'll need to calculate the holes for the rudder shafts to pass through. The easiest way to do this is to slide the rudder shaft through the rudder port with the packing nut or flange in place on the rudder tube. Push the rudder up to the underside of the shelf and check for alignment. Measure the distance from the forward point of the shaft to the forward edge of the shelf. Mark this measurement plus 1/2 the diameter of the shaft onto the top of the shelf. This will be the center point of the hole.
The hole should be drilled to 2" to allow plenty of room to center the shaft for final placement of the flange bearing. Flange bearings are self aligning so it not critical the the shelf surface be exactly square with the properly aligned shaft. If you elected to use plywood for your core, drill your holes to accept 2" PVC pipe. Cut a section of the PVC pipe to be the same length as the thickness of your shelf and epoxy it into the hole, thereby permanently sealing out any water that could possibly get to your wood. (See Sleeving Through Bolt Mounts...)
Below is a basic diagram of rudder shelves for a Bertram 31:
Below are the shelves that were made for Tommy Drake's "Island Girl" in 2001. The core used here was a high tech composite material and vacuum bagging was employed, but the resulting structure is the same as described above.
Below are a few images of the shelf in Jim Bailey's "Buddy Boy":
Have fun! Feel free to download and print this article, but please don't use it on a website without linking it to Bertram31.com.
Capt Patrick McCrary
834 Scott Dr., LLANO, TX 78643
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