1976 "Sprint Hull" 1/16-scale

Radio Control Boat Modeler, Dec 2001 by plane, Pickle-fork hydro

DRIVELINE

The motor-mount assembly is built as a separate unit. Use 1/16-inch aluminum angle stock to make the mounting bracket. Make the motor pan from a 1/12-inch piece of 1/32-inch-wall steel tube. The stator ring off an old Mabuchi 550 motor works well for this, or buy mild-steel tube from a machine supply store where you'll also buy the 1/8-inch drill rod for the drive shaft. With the motor bolted to the aluminum bracket, CA it, the pan, a 1/6-inch plywood motor-mount base and the endcap together. Be certain there is no gap between the motor and the pan, but don't glue them together. Cut an opening in the sump for the pan to extend below the bottom by 1/4 inch. Glue this assembly in the boat so that it lines up with the shaft (shown on the plan).

Build the strut by soldering 1/16-inch-thick brass sheet to a 1-inch-long, 7/32-inch-diameter tube. The drive tube is made of 61/8-inch-long, 3/16-inch-diameter brass tube. Solder a 1/2-inch piece of 5/32-inch-diameter brass tube lube fitting over an 1/8-inch hole drilled through one side of the tube, 3/4 inch from the end. CA 1/2-inch-long, 5/32-inch-diameter bushings into each end. This assembly goes through the slot in the bottom of the hull; the strut blade slips into a slot in the bottom and in the transom. Everything is offset 1/4 inch to the right.

Grind small flats for the setscrews on the 7 1/2-inch-long, 1/8-inch-diameter drill-rod drive shaft. Use an 1/8-inch, Dumas double U-joint, then add Octura 1/8-inch thrust washers, a drive dog and a silicone prop nut. Line this assembly up with the motor shaft, tack-glue it into position, check the prop location, and if it's correct, CA it into place. Balance the propeller, lube the shaft and run the system; everything should be smooth and quiet.

Cut a turn fin out of 1/16-inch aluminum angle stock. File, grind, or sand the blade into a wedge shape. Mount it on the back of the right sponson with a no. 6 sheet-metal screw, angling it inward about 5 degrees.

FINISHING

Brush a coat of model dope under the deck, and be sure to work it into all the hard-to-reach spots. Without adding weight, this will make the wood water-resistant where the paint doesn't cover it. Prepare the hull's exterior with balsa filler or epoxy thinned 50:50 with alcohol, and sand it smooth with 320-grit sandpaper. Or, just spray Pactra Formula-U over the bare wood. Formula-U is a durable polyurethane finish that will not embrittle with age. Spray on four or five light coats at 10-minute intervals-inside as well. Finish with a wet, glossy final coat, and if this coat doesn't run or show "orange peel," you've finished. If it has flaws, wetsand it with 400- and then 600-grit sandpaper, and rub it with automotive polishing compound and wax. Do this only after waiting a couple of weeks for the polyurethane to fully harden.

You're now ready for lettering and decals. To see photos of actual racing boats from 1972 through 1990, visit www.thunder boats.org. Bill Fulmer at www.flightbox.org/customcut will cut adhesive-backed vinyl lettering or patterns to your specifications, and he charges only for the materials and shipping.