On Powerboats - The science behind the swim platform

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A well-designed platform makes a boat safer, more enjoyable and can contribute to seaworthiness

A well-designed platform makes a boat safer, more enjoyable and can contribute to seaworthiness

Swim platforms. There’s a subject we all take for granted and rarely put a lot of thought into. They give you a place to hop onto the boat when tied up stern- or side-to to a floating dock and serve as a handy staging platform for water sports like swimming, wakeboarding and skiing.

But the swim platform also has a role to play when it comes to making your boat safer and more enjoyable to use. And it can affect seaworthiness and performance. Let’s take a closer look at this multifaceted hull extension.

Platform design

Structurally, there are basically two types of swim platforms, the first being integral to the hull. Actually, the integral type is an extension of the hull and deck parts that are bonded together at the hull-to-deck joint. This is the strongest kind of swim platform because it is a continuation of the hull and relies on no external bolts or brackets for support. You’d have to rip the hull apart to dislocate it, so the integral variety is going to be quite damage resistant.

The other kind of swim platform is the bolt-on type. This is just what it sounds like: a separately built fiberglass part that is bolted at its forward end to the transom and with brackets (usually metal) below that add support and hold it up. The discussion about the inherent strength of the integral type isn’t meant to imply that the external bolt-on platforms aren’t strong, just that the integral platform doesn’t rely on good engineering, with bolts and brackets all properly sized and spaced, and a laminate schedule behind those bolts and brackets stout enough to withstand their point loads.

Another variation on this theme is the combination integral/add-on platform that when combined project the desired distance aft. This design works well when the builder (and hopefully you) wants a two-level surface. For a dayboat that’s used a lot for water sports, the two-level design seems to me ideal, as you can sit on the upper/forward section while putting on your board or PDF and then scooch aft and drop into the water.

Length

The length, or distance fore and aft, of the swim platform is important for a number of reasons. Length, along with width, determines the square footage of the surface. The more surface area, the more room you’ll have when boarding the boat, donning scuba tanks, or whatever else people do on swim platforms — like swimming. Obviously, bigger is almost always better when it comes to livability, but there are practical limitations to size. One is slip length, and since the marina likely will charge you for the overall length of your boat — which includes the swim platform — your platform length will be the gift that keeps on giving.

From my perspective, though, there’s one thing alone that should be driving minimum platform length, at least in a sterndrive-powered boat, which is the most common propulsion when it comes to swim platforms. That’s the distance the raised lower unit projects abaft the transom. The aft edge of the swim platform should project at least several inches past the propeller when the lower unit is fully raised for a very simple reason. If your children — or anyone else, for that matter — are sitting on the swim platform, you want them to be well clear of the prop when they ease off and drop into the water. As you raise the lower unit, it will stick out farther aft, so that’s why I specify that the platform project past the raised lower unit. It’s also the position the drive will often be in when the boat is beached or anchored in shallow water.

If you own a boat with a platform that doesn’t project beyond the raised lower unit, and you use your boat for swimming or towing boarders and skiers, there are two options, as I see it. Make sure your passengers know about the hazard, or have the platform extended. If you opt to have it extended, I’d recommend an open, or slotted, surface designed to minimize water pressure on the platform when it submerges, which is likely when the boat is backing hard or a wave washes over it (more on this in the surface solidity section below).

Another length consideration has to do with maneuverability, especially when backing into a slip. You’ll have to get the boat lined up a little farther away from the slip when docking, and this might make a difference if you have limited maneuvering room. What helps when backing a boat with a swim platform is a transom door or walkthrough on the same side as the helm station so you can see the end of the platform as you’re backing down. You’ll save a few hard landings if you can see the position of the dock or pilings relative to the end of the platform. There’s not much you can do about this if you can’t see the platform due to boat design, except maybe mount one of those little flags in either aft corner.

If you own an inboard fishing boat, the main length consideration is the interference caused by the platform when you’re working a fish around the stern. From a fishing perspective, no platform is best, but for many fishing/cruising boats they are considered pretty much indispensable.

Finally, the length of the platform dictates the lever arm developed when weight or force is added to the aft end. The longer the platform, the greater the lever arm, so the platform and hardware scantlings have to be beefed up to take the greater forces imposed. Because of this, the builder must be careful about the height of the platform off the water as its length increases, since the lever arm increases. Also, the platform will tend to be deeply submerged more often, due to wave action and the delayed buoyancy reaction as the wave that immerses the platform reaches the hull.

Along with length, it also helps if the platform is full-beam, or nearly as wide as the hull at the transom. When you’re boarding the boat from the side, the closer the platform width is to the transom beam, the less of a gap there will be between the dock and the platform. Some platforms are as much as a foot narrower than the hull, which may make it a little easier to back into a slip but also makes boarding the boat a little more difficult.

Height off the water

There’s quite a variation in swim platform heights from one boat to the next, from a few inches off the water to well more than a foot. Since a conventional inboard boat has no running gear projecting past the transom, the platform height can be as low to the water as the builder wants. The sterndrive-boat builder, on the other hand, is restricted by the height of the top of the lower unit.

A low platform is easier to get back aboard from the water. If you have the upper body strength, you can hoist yourself up without using a ladder, like a swimmer in a pool. Boats with very low platforms, however, are meant for calm-water use on lakes and inshore, protected waters because the lower the platform, the more it’s going to be exposed to immersion.

An offshore cruiser should have a higher platform since it will be exposed to seas that can easily wash over the surface, especially when backing into or laying stern-to to seas. That’s because the boat doesn’t pick up buoyancy and start to lift until the wave reaches the hull, and by that time it’s traveled right past, and possibly over, the swim platform. Also, keep in mind that if you fill the boat’s fuel and water tanks, and add a load of people congregating aft, your platform might be 6 or 8 inches closer to the water.

If you use the platform as a cradle for a tender, the height of the water is important because it affects the ease with which you can launch and recover the boat. The lower it is, the easier it will be; the higher it is, the more protected the cradled boat will be from wave action. So like in most matters, moderation in height is key to a satisfactory design.

Surface texture

The surface of the platform will have different priorities depending on the type of boat. If it’s a family dayboat used for towing or swimming, the platform is going to get a lot of use by people in swimsuits. So comfortwise, the non-skid finish should be such that you are able to sit down or board without chaffing your own transom, so to speak.

The rubberized non-skid used by an increasing number of boatbuilders works great for this purpose. The material affords a good grip on your Topsiders or bare feet, and it’s smooth and comfortable against bare skin. Also, it’s easy to clean, unlike fine-grit non-skid. If the fiberglass non-skid on your swim platform is too rough — or too smooth, for that matter — you can buy rubberized self-stick sheets to cover the old surface.

Large cruiser swim platforms typically have the same diamond tread non-skid as the cockpit, since it’s an aggressive surface underfoot that matters most, not swimmer comfort. These and other boats are sometimes offered with an imitation teak application, which works well enough as non-skid and also is comfortable to sit on. Just make sure that on sunny days the surface isn’t too hot before you sit.

Surface solidity

While a solid surface is fine for inshore, calm-water boats, this design is a recipe for trouble when you mix in rough water. It presents an unbroken surface to water pressure, and there’s no way for this pressure to be relieved. If a platform surface is broken up by slots or holes, solid water can flow through, greatly relieving the stresses and strains on the platform and hull structure. It’s like the difference between scooping a sieve or a pot through water; the sieve moves easily, since much of the water flows through it, while the pot puts up much more resistance.

Some of the best swim platforms are made of laminated teak with 30 or 40 percent of the surface area consisting of freeing ports or slots. These are designed to give you a comfortable working and walking surface — the gaps are too narrow to catch a toe or twist an ankle — while allowing plenty of water to pass through as the boat pitches or waves break over it, relieving pressure on the platform and its mounting hardware.

So for an inshore dayboat, the low, solid platform is fine, but look for the higher, slotted or perforated platform on an offshore cruiser or fishing boat.

Cleats

I’ve become a firm believer in having a second set of stern cleats at the aft end of swim platforms that project 3 feet or more past the transom. These cleats will be at deck level, so dock lines won’t create nearly as much of a tripping hazard as lines tied off to conventional stern cleats up on the gunwales. Also, with the stern lines tied off all the way aft, you can have a fair lead to the dock cleats, whether the lines are crossed or leading forward as springs.

If moored side-to to a dock in an area with big tides, being able to tie off the stern several feet farther aft also will make it easier to adjust the lines so the boat stays parallel to the dock. (This is a subject near and dear to my heart, having grown up on Cape Cod, Mass., with its 10-plus-foot tides). Of course, you’ll want either recessed, fixed cleats or pop-up cleats back aft to prevent injury and to avoid creating another trip hazard.

Boarding ladder

The swim platform is usually where you’ll find the boarding ladder, and ideally it should be visible from the helm when the transom door is open so the skipper knows what’s going on back there. This goes without saying, but the boat should never be in gear when someone is on the platform, and the engine should be shut off if anyone’s in the water. This is to avoid injury from the propeller, of course, and also to avoid the risk of carbon monoxide poisoning under the platform.

The ladder should be deployable from the water, so if anyone falls overboard they can easily climb back on board. This means that any hatch covering the ladder should be easy to lift, and the ladder then folded down for use, by a swimmer in the water. It should project into the water three or four steps so the average person, fully clothed and with unremarkable upper body strength, can easily climb back up. Of course, ladder length depends on platform height, as a three-step ladder on a low platform might project as deeply into the water as a four-step on a higher platform. Also, there should be a handle to grab onto as you climb.

Conclusions

Wow, who would have thought there was so much so say about swim platforms? Let these insights help guide you when shopping for your next boat. Many of these elements represent compromise, but good boatbuilders will arrive at a well-considered, moderate solution, creating a platform that does many things well, with your interests and needs as a boater clearly in mind.

Eric Sorensen was the founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at eric@sorensensguide.com