In the summer of 2018, Kevin Burns, vice president of design and product development at Sabre Yachts and Back Cove Yachts, was at the helm of a prototype of the company’s first outboard-powered boat, the Back Cove 34O. “The very first 34O [had] twin Suzuki 350s on it,” he says. “We got it up to over 40 knots, and we turned so hard, we broke free the motors and spun it out.”
No one was hurt. “Part of the testing process was seeing what the limits were,” Burns says. “The boat was so responsive compared to an inboard boat with a rudder.”
Thanks to today’s design and construction analysis software, sea-trial stories like Burns’ are few, but the company’s experience speaks to the popularity of big outboards and the challenges builders face when bringing boats to market with them. Today’s outboards make power that was unheard of as recently as five years ago: Mercury has 400- and 450-hp offerings; Yamaha’s XTO Offshore is rated at 425 hp; and Seven Marine models range from 527 to 627 hp.
“We design everything with the anticipation that motors are getting bigger,” says Wylie Nagler, owner and founder of Yellowfin Yachts, which introduced a 54-foot center console powered by quad XTO Offshores at the Fort Lauderdale International Boat Show. “When we determine a boat’s center of gravity, we’re already anticipating what’s going to change five years from now.”
A Steep Learning Curve
Prior to launching the 34O in 2018, Sabre and Back Cove had only built boats powered by inboards or pod propulsion systems. “The sterndrive guys had a head start because they had already been dealing with zero-degree shaft angles,” Burns says.
The typical shaft angle for an inboard-powered Sabre yacht is about 11 degrees. With an outboard that can be trimmed, designers are trying to achieve zero degrees. To make up for the competition’s head start, the Back Cove design team used computational fluid dynamics, a technology that isn’t new but that, when deployed earlier in the design process, helped Burns and his team fine-tune the design.
Using CFD led to the development of what Back Cove calls the “trelace” in the boat’s running surface. The word is short for trailing edge lift surface — a pad in the bottom that starts forward, tapering out of the keel and extending up and aft about 40 inches wide between the inner strakes in the hull bottom. The goal was to raise the installation height of the outboards so they would fully clear the surface when trimmed out of the water.
“The beauty of the CFD was we could try different shapes and compare it to the other simulations,” Burns says.
In addition to design analysis, Back Cove had to change its manufacturing process. On inboard- and pod-powered models, the hulls had been built with a core, while the decks were solid fiberglass. On outboard-powered boats, the bottom core was updated to a higher density, and coring was added to the deck. Stringer heights were made taller, especially where they met the transom. And the transom was new with layers of Coosa composite panels making up the core.
In 2009, Midnight Express Boats, which builds high-performance center consoles, was one of the first companies to put five 350-hp Mercury Verado outboards on a 43-foot boat. As often happens when a company is trying to advance technology, one of the engines came off the boat, and the photo made the rounds on the Internet faster than a celebrity wardrobe malfunction.
“That was a prototype,” says CEO Eric Glaser, who holds a degree in mechanical engineering. “We did the engineering, and I drew special plates to raise the motors. The guy who made the plates didn’t follow my drawings. The way he attached the bolts failed.”
Since then, Midnight Express, which builds boats up to 60 feet, has been among the companies best known for bolting multiple 400- and 450-hp outboards from Mercury Racing onto transoms. The company’s most popular boat is a 43-foot center console that gets quad or quintuple 450Rs.
As the outboards got more powerful, Midnight Express upgraded its transoms with 30-pound-density Coosa coring because of the clamping forces associated with the big outboards. But the builder discovered a weakness. “The biggest issue for us is the bolts,” Glaser says. “That’s the weak point.”
He says the upper bolts are under the most load and that they actually stretch. Midnight Express orders custom-made grade 18-8 stainless steel bolts, and the company doubles the quantity of the upper bolts securing the outboards to the transom.
A Structured Approach
In 2017, Formula Boats built its first outboard-powered model in decades, the 430 Super Sport Crossover. It was powered by quad Mercury Racing Verado 400Rs. Today, the Indiana-based manufacturer has outboard-powered boats ranging from 31 to 43 feet.
When it comes to understanding the punishment of running at faster speeds, Formula had experience from years of building its FAS3Tech models, which had stepped bottoms and were built with a one-piece, molded fiberglass grid that contained the stringers and bulkheads. The design team understood what was required to develop an outboard-powered boat that had the space on the swim platform that people were used to seeing on a sterndrive boat.
“The grids have a heavier layup, and we spread out the shapes more,” says John Adams, who has been Formula’s lead designer for decades. “We just understand what’s going on back there.”
The grid extends past what would be the transom on a sterndrive boat, all the way to the after end of the swim platform. But getting the design right is about more than just the internal structure. “You need to get water to the propellers,” Adams says. “There’s some geometry required to get water to the middle motors.”
On the heels of the bigger boats came the 310, 330 and 350, all of which were existing sterndrive models. “The 310, 330 and 350 all have new hulls,” Adams says. On these models, Formula used brackets to mount the outboards. To accommodate all that weight on the stern, the builder extended the buoyancy farther aft, so the boats remain level while sitting static. Bottoms were designed to improve water flow to the propellers.
The Need for Speed
Randy Scism is the president of Marine Technology Inc. in Wentzville, Mo. His company builds 42- and 57-foot high-performance center consoles, and 34- and 39-foot catamarans, all of which are powered by outboards. After years of competing in offshore powerboat racing, Scism says his boats are overbuilt with carbon and Kevlar, with the expectation of more horsepower being introduced sooner rather than later.
Another high-performance cat builder is DCB Performance Boats in El Cajon, Calif., and the company has three outboard-powered models: the M28, M33R and M37R. The challenge for designers of these boats is the low-profile transoms. Scism and DCB president Jeff Johnston both say they’ve reinforced transoms substantially to support two 450Rs that weigh about 700 pounds each. DCB uses composite Penske Board to core the transoms, and the laminate for the boat includes carbon fiber and Kevlar.
Rather than add weight, Johnston says, DCB added more carbon to lighten its boats and add strength, because the company needs to keep up with the other cats in terms of top speed. “Let’s say the cost was $250,000 for an M28 with 400s. Now it’s $260,000 with the additional carbon in it,” Johnston says.
As outboard technology advances, one thing is certain. “You’re going to e bigger horsepower,” Adams says. “It’s inevitable.”
This article was originally published in the December 2020 issue.