1976 "Sprint Hull" 1/16-scale

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

CONSTRUCTION

Pickle-fork hydro

plane

Over the last couple of years, I've had a lot of fun with 1/16-scale, round-nose, electric hydroplanes and moved on to a next-generation hydro: the 1976 Ron Jones Sprint Hull. This design can be configured to resemble more than 20 boats that raced between 1972 and 1990. As drawn, my plan is very nearly true to the scale of the U-12 Bud, but I made a few concessions to use electric power.

This is a light, low-profile, pickle-fork hydro. The tunnel has a sump for the batteries, and a motor belly pan extends below it. Together, these features produce a deep tunnel with a shallow profile and a low center of gravity. They reduce the prop's angle of attack and even help to cool the motor. The drive is offset to reduce propwalk when running at full power. Non-trip tunnel faces and ride pads with a low deadrise angle help to smooth the turns and reduce straightline drag without adversely affecting stability.

HULL CONSTRUCTION

The 3/32-inch balsa stringer, bulkhead and sheet construction gives a very high strength-to-- weight ratio; the bare wooden hull weighs only about 5 ounces and will not flex in heavy chop or when making tight turns. Balsa absorbs energy well, so any damage that results from "close encounters" will be confined to the point of impact. This boat will run well with 6 to 8 cells and many motor/prop combinations. If you use an economical radio and electronic speed control (ESC) you can be throwing big rooster-tails for around $300.

With low-tack contact cement, stick the paper templates to sheets of good-quality firm balsa. (Double the number of sheets under the templates, and you will have parts for two boats for only a little more effort.) When you cut out the parts, cut slightly outside the lines, then sand to the center of the lines.

Transfer the centerlines and other marked locations from the templates to the parts.

Assemble the tunnel by fitting the sump standoffs on top of the tunnel strips, and then bevel the corners. Dampen, heat and bend the front of the sump strip to fit the standoffs and glue it in place. Lay the nose planks (you can substitute 3/18-inch model aircraft leading edge) and the control plate on a flat, 5-inch-wide building board and assemble the stringers, transom, and bulkhead C on the plan. Check that these parts are level, square, and flush with one another. Tack-glue the transom and bulkhead C, and then finish gluing everything from front to rear. Don't glue bulkheads B and C into the sump; they will be cut away later.

Bevel and glue the rear non-trip panels and the deck strips. Add the sponson bottoms and then the side panels by dampening, heating and bending them to match the bulkhead's curves; then glue the panels into place and trim the edges. Set the deck on the bulkheads, check the fit and trim as needed. Using thick CA, glue the deck down from the deck strip centerline to the main stringer, front to back. When the CA starts to cure, glue the other side. Lift the deck away from the framework, put glue around its edges and press the deck down. Add 1/32-inch plywood doubters to the transom and back of the right sponson.

Cut out the centers of bulkheads B and C. Sand or plane a 45-- degree, 3/16-inch-wide, non-trip bevel on the inside of the sponsons and an 1/8-inch bevel on the edges of the sump. Glue 1/32-inch balsa strips to the sponson ride pads. Inside the hull, fill any gaps you see at the seams of the deck, bulkheads, bottom and main stringers with thick CA so that bilge water from the engine compartment won't be able to leak into other sections. Add the front, center, deck section (the rear goes on after the antenna and rudder pushrod sleeve have been installed). Sand the deck, sides and bottom with 180-grit sandpaper, then use 320-grit to keep the edges sharp. Add thin CA to the outside comers of the hull to harden them. After that, the basic hull is finished.

HATCH AND DECK DETAILS

Build a hatch cover of 3/32- or 1/8-inch-thick balsa by wetting, heating and curving it to match the deck; it will sit inside the deck on top of the deck strips. Trim its edges so that it fits snugly. Sand the top of the hatch cover so that its curve matches the deck's curve. Carve the cockpit and motor cowl out of a block of foam. Mark the top and side profiles, cut it out, and then sand it to the final shape.

Seal the hull completely to keep out water. When coming up on plane, a pickle-fork hydro sends bow waves toward its center, and any opening around the hatch will allow enough water in to end your run almost before it begins. I use colored vinyl tape to seal and secure the hatch.

If you are modeling a full-size boat that ran with an exposed engine, make a dummy one of foam or balsa by cutting the basic engine pieces shown on the plan. Sand the pieces to their final profiles, and glue them together. Make exhaust stacks out of drinking straws or 1/4-inch-diameter dowel.

Cut the tail fins and wings to shape out of 3/32-inch-thick balsa, and then sand them and cover them with TopFlite MonoKote. Attach them so that if they're knocked off, the deck won't be damaged. And build spares; you will need them! A 1/12-scale sport or military pilot makes a realistic driver. Cut a section of airplane canopy (or plastic soda bottle) to the shape of a windscreen.