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Walking on Water

Adding a submersible swim platform to your boat has many benefits, one of them being quite miraculous.

Submersible lifts are often vented to facilitate vertical movement.

Submersible lifts are often vented to facilitate vertical movement.

Folks enjoying the summer of 2017 around the picturesque Connecticut River hamlet of Essex were shocked to see Power & Motoryacht Editor-In-Chief Dan Harding walking on water. Magazine editors typically claim omnipotence—was Harding the first one to actually demonstrate it? Or is he just unnaturally buoyant? Or was it maybe a French thing? After all, Beneteau loaned the magazine a slick Monte Carlo 5 for the summer, and Harding’s seeming miracle took place in the vicinity of the boat’s swim platform—a submersible swim platform, in fact.

OK, OK, yes: It was Beneteau that was behind, or rather below, Harding’s aquatic perambulations. He explains, “One of the standout features [of the Monte Carlo 5] was the submersible swim platform. Standing at the grill and cooking up hot dogs while cooling my feet in the clear water … is a memory I’m missing now that winter is upon us. Of course, it also made launching the tender or kayak a welcome chore.”

The submersible swim platform—or hydraulic stern platform, or hydraulic lift, or platform lift—is much in vogue today and makes the swim ladder redundant. Rather than haul one’s mass up three or four stainless-steel rungs onto dry fiberglass, with the platform lowered to water level one can slither back aboard like a sea lion at Marineland. Or stand, toes awash, and cook some burgers, as did Harding. Submerge the platform all the way and even a kid can drive a jet-powered RIB or PWC right into the chocks, touch a button, and lift it out of the water. (Outboard-powered tenders demand more caution; the prop of even a 10-horse motor can do a number on a fiberglass platform.)

Many production boats have a hydraulic platform as an option, but you needn’t buy a new boat to get one. Chances are you can bolt an add-on against the transom you own now, if your boat’s over 30 feet or so and has inboards or stern drives, and a convenient way to get onto the platform—a transom door, for example, or a transom door a boatyard can install for you. A submersible platform that extends an integral, molded-in deck is ideal. (Outboarders don’t despair: A platform is on the horizon, or at least on the drawing board, for you, too. More on this later.) But there are many factors to consider before placing the order, and since it’s a costly project, it’s smart to take time to look at every aspect.

Sea Ray 300 SLX

Sea Ray 300 SLX

Picking a Platform

They look complex, but submersible platforms are basically multi-jointed arms, usually fabricated of marine-grade stainless steel, bolted to the transom and capped with the fiberglass platform itself. Each arm is raised and lowered by a hydraulic cylinder, and is cleverly designed to keep the platform level as it rises and falls. The hydraulic pump is mounted inside the boat, along with the control boxes, wiring, and so forth. Installation isn’t a do-it-yourself job.

A number of reputable aftermarket fabricators build submersible platforms that a competent boatyard can install; some companies will install the platform, too. Florida Bow Thrusters, for example, offers one-stop-shopping. Company President Tom Gillikin says he has a number of fiberglass molds on hand to build platforms for many production boats, or he buys the platform from the boatbuilder. He then mounts it on a hydraulic mechanism from Latham Marine, General Hydraulic Solutions, or Nautical Structures. Gillikin says he installs platforms primarily on boats 40 feet and up.

Customers can also buy directly from Florida Bow Thruster’s suppliers. General Hydraulic Solutions’s website says it has built, shipped, and installed more than 3,400 hydraulic platforms in its 35 years of operation, and claims to be the world’s largest OEM supplier. Nautical Structures is well-known for building first-class davits, cranes, and passerelles, but it has also built a slew of hydraulic platforms for boats from express cruisers to megayachts. And there are others: TNT Lift Systems has built more than 2,500 hydraulic lifts for yachts from 28 to over 150 feet, according to its website. SeaLift in Cocoa, Florida, uses a patented design with a single cylinder and arm on the boat’s centerline, rather than one near each end of the platform.

Using just a single cylinder has several advantages, says Stephen Johns, the founder of SeaLift. There are fewer moving parts to maintain versus a dual-cylinder platform, the up/down motion is smoother, and the system is more reliable. Dual cylinders tend to run at different speeds, he says, so the platform doesn’t stay level athwartships while moving. A single cylinder removes that issue. Since there’s only one cylinder and two hydraulic hoses outside the transom, maintenance is easier, too.

And, adds Johns, most dual-cylinder lifts actually have four cylinders: two to raise and lower the platform, and two locking cylinders. When the platform is raised all the way, the locking cylinders engage some kind of mechanism—hooks, latches, cams, whatever—to lock the platform securely in place. The dual-lifting, dual-locking design dates from the 1990s, when hydraulic platforms first became popular, says Johns. But the four cylinders mean eight hoses and the associated connections to inspect and maintain. Instead of a locking cylinder, SeaLift uses a patented mechanical safety lock that’s similar to one used on aircraft landing gear—it engages automatically, has no hydraulic connections and, according to Johns, requires no additional maintenance.

Platforms tend to be heavy, especially when paved with teak.

Platforms tend to be heavy, especially when paved with teak.

What About Lifting Capacity?

Platforms are typically rated for lifting capacities between 1,500 and 3,000 pounds—even small hydraulic cylinders can lift a hellacious amount—but the real-world capacity isn’t a function of hydraulics, but of boat size and transom strength. Overloading a boat, especially with weight concentrated abaft the transom, is a very bad idea. Too much weight not only stresses the transom, but can also create handling and trim issues, and risks swamping the platform, the tender, and maybe the mothership, in following seas or when dropping off plane.

A platform for even a smaller boat typically weighs several hundred pounds on its own; put a tender on the chocks and the forces from the combination of weight and leverage can be considerable. Throw in high-speed running in rough water, and a weak transom may not be able to take the strain. Florida Bow Thrusters’s Tom Gillikin says his crew will often reinforce a boat’s transom to better withstand the stress; nevertheless, it’s exceptionally important to use common sense when loading the platform, he adds.

How much weight is too much? Gillikin suggests a maximum of 1,000 pounds for a boat of 40 feet or so, even if the platform is rated for more capacity, but many experts are more conservative. TNT Lift Systems guidelines suggest 500 pounds max load on boats from 33 to 39 feet, 650 pounds up to 48 feet, 800 pounds up to 58 feet, and 1,000 pounds up to 68 feet. (TNT assesses boats larger than 68 feet individually.) Nautical Structures publishes a list of boats it’s built platforms for, and the recommended tender weights, on its website. For most boats in the 40- to 50-foot range, the company recommends 850 pounds or so, and 1,200 for those 50 to 60 feet. Larger boats can handle more weight, but it’s not only a function of length; other factors come into play, too. For instance, the optional submersible platform on the Sea Ray 510 Sundancer is rated for 600 pounds, while the one on the slightly shorter, narrower, and lighter Monte Carlo 5 is rated for about 770.

This rendering gives a fair idea of how the S.A.F.E. design works.

This rendering gives a fair idea of how the S.A.F.E. design works.

Submersibles for Outboards?

Submersible platforms can be fitted to conventional transoms, integral-platform transoms, boats with propshafts, and boats with stern drives. But outboards? Not so easy: How do you work around the motors? Opacmare is the Italian builder of some very sexy swim platforms, installed as factory options by many boatbuilders worldwide. For example, the Transformer platform aboard the Evo 43 we recently tested nests in the extended cockpit sole and opens to become a dive platform, a boarding aid, and a submerged swim platform with steps. (It’s not for carrying a tender, only people.) And now the Opacmare design team has devised a submersible platform for outboard-powered vessels. We haven’t seen one on an actual boat yet, but the drawings look interesting. It’s called the S.A.F.E. platform.

The S.A.F.E. design is complex, and the hull has to be custom built to accept it, so it’s not an add-on. Passageways on either side of the outboard or outboards lead from the cockpit onto the platform, which lives aft of the motors. Instead of hydraulic arms bolted to the transom, the platform is supported by a lifting apparatus housed in the hull sides. When the platform is raised to cockpit level, the outboard powerheads project above it, but there’s still clearance for the full range of steering, and for joystick maneuvering. The platform must be raised above cockpit level to permit the motors to tip up. Electronic cut-outs prevent starting the motors when the platform is submerged, or lowering the platform when they’re running. According to Opacmare, the S.A.F.E. platform can be adapted to single, double, triple, and quad engines on monohulls, catamarans, and other boats up to 50 feet.

Really though, most submersible swim platforms are less complex than the S.A.F.E., but provide lots of convenience nonetheless. Add one and divers, swimmers, and watersports enthusiasts will like the easy entry and egress from the briny that results (ever notice how dive tanks get heavier every year?), and nobody will miss the chore of hauling the tender out of the water. Driving it onto the platform is so much easier. But the best feature, we continue to think, is the ability of everyone to use the platform to walk on water. After all, you don’t have to be a magazine editor to appear omnipotent.

Verve 40 pivoting teak swim platform

Verve 40 pivoting teak swim platform


The Tender Trap

How much does a yacht tender weigh, and how big a tender can fit upon a swim platform? The rule is to pick a tender that’s at least 2 feet shorter than the beam of the mothership. That leaves enough clearance for docking misadventures without damaging the tender. At least with any luck. So, an express cruiser with a 14-foot beam—the Monte Carlo 5, for example—should carry a tender that’s 12 feet or less on the platform.

The true limiting factor, however, is weight, not length. The Monte Carlo’s submersible platform is rated for about 770 pounds. That’s capacity aplenty for an outboard-powered RIB. AB Inflatables’s Nautilus 11 DLX (11-foot LOA x 5-foot 10-inch beam) weighs 440 pounds. Add a 111-pound, 20-horsepower Yamaha outboard and some gear and it’s still under the limit. And there are many nice RIB tenders that weigh less than the Nautilus, and don’t need as large a motor.

Of course, it’s easier to launch and retrieve a jet-drive like the Ribjet 10 Sport tender. You can just submerge the platform and drive the boat on and off. The 10 Sport weighs 570 pounds dry, including the 60-horsepower Rotax jet package. I figure with fuel, PFDs, anchor, and other gear we’re at 650 pounds. Still OK.

But what about a PWC? A typical model can weigh 700 or 800 pounds. For instance, Yamaha’s 10-foot 3-inch Waverunner EX weighs 577 pounds, and the 11-foot Waverunner VX weighs 708 pounds. Both weights are dry, so add as much as 100 pounds more for full gas tanks. The EX makes the cut, but the VX is too heavy. (Note that the nominal platform weight limits include a very big safety factor three or four times what may be necessary, and sometimes more), but we recommend going by the book.

When planning to add a submersible platform with the intention of carrying a tender, don’t just see if it’ll fit on the platform. Take into account total weight, too. And even on calm days, always cover the tender and remove the plug to keep out sea and rainwater. Water is heavy, and a half-full RIB will overstress the platform in short order. In rough weather, it’s often smarter to tow the thing.

This article originally appeared in the February 2018 issue of Power & Motoryacht magazine.