A blast from the past, RETRO-ROCKET skims across the glassy surface of Minnesota’s Lake Minnetonka. At 10′ 2″ LOA, a 5′ beam, and 12″ draft (standing still), this pocket-sized hydroplane was built by Rob Sotirin for his son, John, to run in the waters off the family’s Shady Island home.

Named for the boat’s old-time style and its speed, RETRO-ROCKET is a throwback to the small outboard-powered hydroplanes that were popular during the 1940s, ’50s, and ’60s. After World War II, plans for do-it-yourself builders were available through boating and woodworking magazines, sparking an interest among returning veterans, and others, who were looking for on-the-water excitement at a reasonable price. Best of all, these boats were of a size that could easily be constructed in a home workshop.

RETRO-ROCKET was built from Glen-L Marine Design’s plans for Super Spartan, a three-point hydroplane. Constructed in marine plywood, RETRO-ROCKET weighs less than 100 lbs and darts across the water, mostly on a cushion of air. A three-point hydroplane has two forward sponsons, plus the aft end of its hull, which is flat. When on plane, only the sponsons and the aft bottom surface touch the water.

At an overall length of 10’2” and weighing just 100 lbs, Glen-L Marine Design’s Super Spartan looks like she’s ready to race but is mostly intended to provide summertime fun on calm waters.

Rob built the Super Spartan to keep pace with the interests of his children. “My son John is now 13, and I figured he would be ready for a little excitement. We live on an island, so the kids spend a lot of time on the water.” At the outset of the project, Rob took time to carefully read and understand the plans. “I found it useful to have them posted right next to the ‘operating table’ so that I could turn around to refer to them every step of the way,” he said. “This is such a small, light boat that there isn’t much in the way of materials or cost. Glen-L does a nice job of providing step-by-step pictures of the construction process.”

Facing another Minnesota winter, Rob began work on the hydroplane in his basement workshop. Thinking ahead, he constructed a full-sized mock-up out of scrap wood to assure that he would be able to get the boat up the stairs and out the door when his project was complete. By the following summer, the boat was ready to launch.

RETRO-ROCKET’s bottom is made of 1⁄ 4″ marine fir plywood and the deck is 1⁄8″ mahogany plywood. He used two layers of 3⁄4″ solid mahogany for the transom. The stringers and cockpit cowling are also made of mahogany. Rob remarked, “The idea is that with very little hull weight, a small motor will accelerate the boat quickly and bring it up out of the water [on plane].”

Rob purchased Glen-L’s fastening kit, which is specific to the Super Spartan and includes silicon-bronze screws and ring nails. He assembled the boat’s lower hull stringers and transom with 3M-5200, assuring that seams were watertight yet allowing the hull to remain somewhat flexible. He bonded the less vital deck pieces with a polyurethane construction-grade adhesive, which is less expensive.

RETRO-ROCKETPhoto by Rob Sotirin

RETRO-ROCKET’s stained-and-varnished 1⁄8″ mahogany plywood deck is pleasing to the eye. Strategically placed racing stickers hide seams where plywood sheets were butted to one another.

Well before Rob installed the deck, he sealed, primed, and painted the hull’s forward interior. Water that gets inside the boat can collect in the sponsons, so it’s a good idea to make sure they are protected to discourage rot. He stained and finished the aft interior (which is visible) with four coats of varnish, along with most of the outer hull. The cowling pieces were left natural to provide a two-tone effect and allow sufficient contrast with the lettering. Rob strategically placed a large racing number (even though his son doesn’t race) to hide the butted seam between two pieces of deck plywood. “I took a lot of extra care so that the whole boat could be varnished,” Rob added. “This made for a lot of critical attention to fit and finish. However, if you planned to paint the boat, you could be a lot less fastidious.”

As RETRO-ROCKET neared completion, Rob was concerned about the comfort of the driver, who must kneel on the cockpit sole when the boat is underway. “It’s important to put something down to absorb shock and stay dry,” he said. “At a local surplus store I found just the thing—the long black foam wrist rests used with computer keyboards. I bought 20 of them, and that’s what you see lined up on the floor [sole] of the boat.”

As a practical matter, Rob recommends that strong stainless or galvanized eyehooks be built into the boat’s hull before the deck is attached. This allows owners to store the boat by hanging it in the garage over a car. “With the motor off, a couple of people can easily lift [the boat] off the trailer and pulley it up to the rafters,” he explained.

Stock boat trailers require some modifications to accommodate the Super Spartan. The hull is basically flat, so Rob built some bolt-on attachments to his trailer that provide the bow the support it needs, while keeping the back end from sliding sideways. He can use the same trailer for several small boats this way.

Photo by Rob Sotirin

The simple design of the compact-sized Super Spartan let Rob Sotirin complete this racy little hydroplane over the course of a Minnesota winter in his basement workshop.

Flat water is critical to the safe operation of RETRO-ROCKET, which is only allowed to run on relatively smooth days. Underway the “shovel nose” bow is only inches above the lake’s surface, and a wave could cause the boat to “submarine,” or dive under the water. The other extreme is that the boat can go airborne. To keep this from happening, the driver must lean forward, especially when accelerating. “It’s easy once you get the feel for it, but can be dangerous to the uninitiated. No one is allowed to drive without a proper understanding of how to handle her.”

Glen-L calls for a short-shaft outboard motor up to 35 hp to power the Super Spartan. Rob chose a 1960s-vintage Merc 200 (20 hp), which he found through word of mouth. “I spent some time getting it ship-shape with new paint and a few new parts, but it’s reliable and fits the boat very well,” he said. “I found other parts like the steering components, gas tank, and remote controls online. I find that 20 hp is more than enough to scare you. It does an easy 40 miles per hour with my son at the controls, and that’s plenty fast for both of us.”

Though RETRO-ROCKET looks like she’s ready to race, she’s really intended for family fun—providing summer-time thrills on calm waters. She certainly achieves that goal.

Super Spartan by Glen-L Marine Design is an ideal beginning project for adults or youths (with a little help from dad or mom). Construction of the three-point hydroplane, using sheet plywood planking, is straight-forward, requiring no building form.

This Boat Profile was published in Small Boats 2009 and appears here as archival material. Boat plans for the Super Spartan are available on Glen-L’s website.