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Standard Trailer Modification

Construction Notes by Steve Norris, Laser #168221
norr@midwest.net

March 18, 2000

Before taking delivery of my new Laser, I had been warned about hull deformation resulting from storage right side up on a conventional trailer. However, storing and trailing it bottom up would complicate single-handed launching, and the other readily available alternative, a specialty trailer such as the Kitty Hawk (which suspends the boat by the gunwales), involved a larger monetary outlay than I was willing to make once shipping and assembly was added to the initial cost. What I describe here is a labor intensive but less expensive alternative that still allows easy single-handed launching: a two part cradle mounted on a standard trailer.

The lightweight galvanized trailer that my local dealer (who is 100+ miles from my small town home) happens to sell is a Haul-Rite made by B & M Manufacturing Co. of Camdenton Missouri, and which retails for about $450.It has fixed bunks about five feet in length and a long tongue. The winch stand (winch not included) is fully adjustable, and the bunks can be moved in or out by bolting it to different pairs of holes drilled in the cross braces.  They can also be adjusted up or down .It turned out to be ideal for the modification I planned.  (A trailer with tilting bunks could be modified to mimic the Haul-Rite by substituting carpeted 2x4s set on edge; alternatively, the below-described frame could be attached directly to the trailer frame, with a little drilling.)  I carried the plan out in two stages: stage one for transporting the boat home from the dealer, deck down; stage two for storage and transportation to and from the lake, deck up and cradled.

Stage One: The Frame 

Stage one involves building a simple frame that can be quickly and securely attached to the trailer, with a view toward having the Laser make its maiden road trip from the dealer deck down.  I used construction grade 2x4 lumber to build it.  The frame consists of a 60" board across the bunks, and a matching board forward above the tongue.  The pair are connected by two 8' boards running lengthwise snugged up against the inside surfaces of the bunks.  The cross-boards are screwed down to the longitudinal boards so that the cross-boards are 88" on center.  That happens to be the distance between the aft cockpit bulkhead and the center of the mast-step, which are the two most reinforced parts of the hull, with the possible exception of the bow stem.  Important: Position the forward cross board so that it is flush with the ends of the longitudinal 2x4s.  Securing an additional short cross-board under the longitudinal boards directly beneath the forward cross-board raises the height of that board so that it is even with the aft cross-board.  That, at least, was the result on this particular trailer with the bunks in the lowest position, which is directly  on the trailer frame.  The lower cross board at the bow end is screwed to the 8' 2x4s from the bottom, after it has been drilled for a "U" bolt and the "U" bolt has been installed (see next paragraph).  Finally, I covered the cross-boards with 4" thick foam rubber.  The foam is the kind one would use for a sofa cushion, and I obtained a large block of it for $5.00 at a local wholesaler who looked like he was on the verge of going out of business.  Expect to pay more.  The result, in any event, is a very soft deck-down ride. The frame is attached to the trailer at the aft end by metal angles screwed to the under side of the cross-board and to the outside of the bunks using 2" coarse thread drywall screws.  The forward end is secured to the trailer tongue by drilling the lower cross-board for an appropriate size square "U" bolt.  Be sure to use two cross bars on the bolt: one under the tongue, of course, to connect the legs of the bolt; the other under the "U" end on top of the board, to keep it from driving into the board and splitting it when the nuts are tightened on the legs. The beauty of this frame is twofold.  First, you can pre-assemble it at home, including positioning the hardware, if you first obtain the relevant trailer measurements, the critical ones being the distance between the bunks, their height relative to the tongue, and the cross-sectional dimensions of the tongue so that you buy the right "U" bolt.  You can then transport the assembled frame by car top or pickup truck to your boat dealer.  Second, once installed on the trailer, a very minor modification allows it to receive the two part cradle once the latter is constructed. When transporting the boat deck down on the above-described frame, I suggest placing the tie-downs directly over the cross-boards so as not to stress the hull.  Run the lines through a length of foam pipe wrap, using the hole meant for the pipe.  This will not only keep the line from abrading the bottom of the hull, but will also inhibit fore and aft slippage.  

Stage Two: The Cradle  

To construct the cradle, you will need a half sheet (4ft. x 4ft.) of 1/4 inch exterior or marine plywood good both sides.  Its OK if it has knots on one side as long as they are plugged.  Cut the sheet in half into a pair of 2ft. x 4ft. sheets. Then firmly clamp the sheets together so that when you cut the curves into them, you end up cutting identical pairs.  You will also need the following: some additional 2"x4" lumber, some of which you will rip into 2"x2"s; 1 1/4" and 2" drywall screws; some 1 1/4" lattice; good all purpose glue, waterproof or exterior construction grade; and about four to six pieces of closed cell foam tubular pipe insulation designed to fit around 1/2" copper pipe. The next step is to measure the cross-sectional hull shape at the mast step and aft cockpit bulkhead and transfer the measurements to the plywood.  There are two ways to do this.  

1. Direct transfer method.  Leave the boat deck down on the trailer.  Mark the center of the long edges of the plywood and have someone hold the plywood marked edge down and parallel to the deck with the mark touching the keel in line with the aft cockpit bulkhead.  Measure the vertical distance between the lower edge of the plywood and the deck joint directly under it.  Make a tracing tool by sharpening one end of a 1" x 2" board to a point and at the other end drilling a hole just large enough to hold a pencil.  The distance between the hole and the point should be the same as the distance just measured.  Glue or tape a pencil in the hole so that the point comes through the 1x2 just enough to be able to mark the plywood.  Now holding the plywood parallel to the deck, and holding the tracing tool vertically at a right angle to the edge of the plywood, its point touching the hull at one side of the boat and the pencil touching the plywood, move the tool across the hull directly transferring its curve to the plywood.  Make sure to keep the tool absolutely vertical as it follows the curve of the hull.  Invert the plywood and repeat the procedure at the mast step. 

2.  Measurement method.  Again, leave the boat deck down on the trailer, with the trailer on a reasonably level surface like a garage floor.  Measure the distance from the floor to the starboard and port sides of the transom to make certain that the deck is parallel to the floor.  Then place a drywall T-square (or make a large square, 3 feet by 4 feet), short side on the floor, against the gunwhale in line with the aft cockpit bulkhead.  Using a small block or piece of carpet on the inside of the gunwhale lip to protect the fiberglass, clamp the T-square in place.  Next, place a long level along the hull-deck joint at the transom.  Mark the level with a felt tipped pen at the center of the bubble.  (This establishes a virtual level parallel to the hull and floor, neither of which may be truly level.)  Now place the level across the curve of the hull in line with the T-square.  If it the level is metal, place a small piece of paper under it to protect the hull finish.  Move the outboard end up or down on the T-square until the bubble centers on the mark you made earlier.  Then clamp it to the T-square.  Using a soft pencil (if your square is wood) or fine felt pen (if using a metal drywall square), draw a line across the standing part of the T-square along the underside of the level.  Now, mark the level at 2 inch intervals measuring from the keel line out to the gunwhale.  Finally, using a plumb bob and line (carefully so as not to scratch the hull), drop a plumb line from the 2 inch marks to the hull, and mark the hull at those points with a felt tip pen.  (Ordinary writing pen marks will wipe of with a damp cloth. For permanent ink, use lacquer thinner.)  If your floor is seriously off level, then use a carpenter's square to find the points on the hull directly under the 2 inch marks on the level.

Now un-clamp the level, and place its inboard lower corner at the first 2 inch mark.  Move the outboard end up or down until the bubble centers on the mark you made earlier, then draw a line on the T-square along the underside of the level.  Repeat this operation for all of the 2 inch marks.  Let "0" be the line on the T-square which is down closest to the lip of the gunwhale.  Measure the distance from "0" to the next line up, and then from "0" to the second line up, and so on.  Write down all these numbers.  They will be used to plot the shape of your stern cradle.

Repeat the entire operation described above at the mast step.  The resulting vertical measurements will define the curve of your bow cradle.

Now transfer the measurements to the plywood, working from one of the long edges for the stern measurements and from the opposite edge for the bow. Find the center of one of the long edges of the 2x4 plywood sheets you clamped together and mark it.  Label it "k" for Keel line. Measure out, in both directions from the mark, as many full 2 inch intervals as you were able to mark on the hull between the keel line and the gunwhale lip.  Then, line up a square with each of the 2 inch intervals in turn, and draw a line completely across the plywood to the opposite edge.  Now using the largest number you recorded for the stern cradle shape, make a mark that number of inches in on the line drawn from "k".  Label that mark "0".  Repeat the process to mark positions 1, 2, 3, etc. measuring in from the edge on either side of k, using the rest of the measurements in descending order of length.  The series of marks will define the curve of the hull under the aft cockpit bulkhead.  Use a soft pencil to draw a smooth curve through the marks.  Now repeat the process starting from the opposite edge of the plywood and using the measurements taken from the hull beneath the mast step.  Cut out the crescent-shaped pieces from each side of the plywood, then cut the plywood in two lengthwise, making sure that you leave equal wood on either side of the cut, between the cut and the deepest point on the curved cuts.

Test the fit of the plywood sections before going any further, and shave them where necessary.  Make certain that the deepest section of the curved cut contacts the keel area.  It is all right if the sections above the water line and near the rail flare out a bit and do not contact the hull.  It is not good if the curve is tight at the topsides or turn of the bilge while not making contact with the keel.  You want to end up with slightly gentler support at these thinner sections of the hull.

 

3.  Cradle assembly.

a. The cradle sections are assembled by screwing the matching pairs of cut plywood to a simple frame made of 2" lumber.  The main frame members run up the outboard edges of the plywood and across the bottom (See Diagram).  A half dozen shorter pieces of decreasing in length are inserted to add stiffness between the outboard members.  Cut these diagonally at the top and position them so that the slope roughly matches the curve and is about 1/4" below it.  I used 2"x4" lumber to frame the stern cradle section, with the small edge of the 2"x4" pieces against the plywood, so that the result was a cradle with a 4" cross-section.  I used 2"x2" lumber to frame the bow section.  The result in either case is a rigid and light weight sandwich, open at the concave curved side, with the interior frame members or "ribs" visible.  Before going any further, drill at least four 3/8" holes through the bottom frame member of each cradle section distributed along its length in between the vertical ribs.  This step is critical in the case of the stern section, since it must be submerged to launch the boat, and must be able to drain quickly when pulled out of the water.

b.  The next step is to finish and pad the cradle sections.  I used 1 1/4" lattice to cover the top edges of the interior frame members.  First I soaked lattice strips of the approximate length necessary to cover the entire concave curve.  I then bent them into place, and screwed them to the tops of the frame members, making sure to countersink the screw heads.  When everything had dried, I sliced the tubular pipe insulation in half lengthwise, and cut the halves to fit the arc of the cradles.  The best instrument to cut foam is a very sharp filleting knife.)  I used two insulation halves side by side on the stern cradle, and one on the bow cradle.  The pipe insulation halves are then glued, round side up, to the lattice.  You may need to hold them in place while the glue sets.

Once the glue has set, place the cradle sections on the hull.  In all likelihood, the sides of the arc will be a little too tight, preventing the deepest part of the curve from contacting the hull at the keel.  Just shave off enough foam at the critical places until the cradle fits snugly, with equal pressure all around.  Or, if there is a gap between the hull and the foam above the turn of the bilge, then add more foam.  Once you are satisfied with the fit, carpet the arc of the cradle sections.  I used indoor-outdoor carpet about 1/4" thick.  I bent the carpet down, slitting it every three to four inches, and screwed it to the sides of the cradle using 1" drywall screws and washers.  Again test the sections for fit to the hull shape.  If they pinch at the thin sections of the hull, or fail to make contact at the outer ends, remove the carpeting and shave or add foam accordingly.

c.  Now remove the boat from the trailer, setting it deck down on a couple of 2x4s temporarily, and fasten the cradle sections to the frame cross members using metal angles, as follows.  Place one of the plywood sides of the forward section against the forward edge of the forward cross members with its bottom edge directly on the trailer tongue.  Use steel angles to fasten it to the upper cross-member, and screw it directly to the lower cross-member using 3" drywall screws straight through the horizontal rib of the cradle section.  Again using steel angles, attach the stern cradle section against the stern edge of the stern cross member.  >Now loosen the "U" bolt under the bow cradle section and remove the screws fastening the stern cross-member to the longitudinal 2x4 and the screws connecting the cross-member to the sides of the bunks.  Now reposition the entire frame by moving the forward cradle section and attached longitudinal 2x4s aft, and the aft cradle section forward, until the cradle sections are 88' on center (the distance between the aft cockpit bulkhead and the mast step).  Replace the screws and re-tighten the "U" bolt.

d.  You will need to varnish or paint the exposed wood on the cradles, especially the stern section, since it will need to be submerged to float the boat off.

e.  With the assistance of a couple of friends, carefully lower the boat, right-side up, into its fully assembled cradle.  Once it is positioned so that the mast step and aft cockpit bulkhead are positioned over the cradle sections, move and adjust the height of the winch stand so that the rubber bumper is firmly against the upper sections of the bow stem, but not touching the lip of the hull to deck joint.  I use a tie-down to the winch stand by taking a short length of line through the bow eye and over the sides of the deck.  Then I run two lines over the deck, one at each of the cradle sections.

f.  Be sure and take note of the position of the automatic bailer.  In all likelihood, part of it will be covered by the aft cradle section.  This is all right, as long as you remember to close it before pulling the boat up onto the cradle, and slide the boat aft a few inches before draining it while on the trailer.  If you need to store your boat outdoors where it must drain unattended, then I suggest moving the aft cradle section forward slightly so that the bailer can open.  Since the aft sections of the boat are relatively flat, you may be able to do this without having to dismount the carpet and shave foam.  I garage my boat, so I have not run into this problem.

 

page created 03/18/2000
last updated 03/23/2000

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