Sketch the trailer, considering size and weight of the boat and power of the tow vehicle. Cut a 3-inch "C" channel steel beam into two 20-foot pieces, six 2-foot pieces and two 1 1/2 foot pieces. Saw off the ends of the 1 1/2-foot and 2-foot beams, using a hand grinder so they can form a 165-degree angled joint. Weld the 1 1/2-foot and 2-foot pieces in symmetrical pairs to form a 110-degree elbowed joint to fit the hull shape.
Weld the 4-foot elbowed joint between the parallel 20-foot beams from the beginning of the parallel beams and distributing them across 15 feet, with the elbows forming a hull shape. At the 15-foot point, bend the parallel 20-foot rails and weld the two 1 1/2 foot pieces, now welded into a 3-foot 110-degree angled bar, to the 20-foot rails. Bend the rails, using a pipe clamp, to form a triangular shape with an open tip. Weld a 35 1/2-foot steel bar across the middle of the triangular section.
Make a T-shaped structure using a 6-foot-by-2 1/2-inch square tube center member and a 3-foot-by-1 1/2-foot steel crossbar. Weld the crossbar, halfway across the triangle with the stem at the tip of the triangle, to form a surface for the coupling assembly. The coupling attaches the trailer to the vehicle. Turn the trailer over on its correct side and hoist it on support objects. Weld a 1/2-foot round bar along the sides to strengthen the trailer bed.
Install the axle kit onto the undercarriage by fastening the axle to the spring, using U-bolts. Attach the springs to the undercarriage by fitting it to a sticking piece of metal called a hanger and securing with bolts inserted through the two parts. Attach the hanger to the undercarriage by welding it onto a metal plate. Use the hanger to mount the spring. Repeat the same procedure for the other side. You can also buy a pre-assembled axle kit.
Weld the coupling assembly and keel rollers. Keel rollers are rubber rollers that the boat slides over during loading and unloading. They are located in the middle of the V-shaped bars between two metal bars, 5 inches in length. Install the wiring system and the braking system as per the manufacturer's instructions. Weld the fenders onto two protruding bars of steel welded on the undercarriage frame. The fenders should have a 4-inch clearance from the tires.
Paint the trailer, install the signal lights and fix the bunks, four bars of steel running along the trailer from top to bottom. Cover them with strips of indoor carpet to provide a soft surface for the hull to rest on and to prevent the paint from chipping. Make a lockable compartment for the spare wheel and fix the tires. Load a boat on the trailer and drive some distance for testing.