Inflatable sailboats, or RISBs

Inflatable (also called pneumatic) structures, known for their light weight to volume ratio, have not the place in sailboat construction they really deserve, despite the recent advances in materials and construction methods that contributed to their widespread application in powerboating (RIBs). I wonder if there will come a day when inflatable sailboats, rigid or not, will compete their much heavier all composite companions. Are there some shortcomings prohibiting their use in sailboating ?

 

Lack of wide acceptance, and the inability to form a particular desired shape. Both of these are changing as RIBs become more common and fixed-shape technologies like Zodiac's H2P air panels find more uses. An inflatable's high initial stability and relatively high freeboard, compared to something like a Laser or Sunfish, would make for easy sailing. But how do you step the mast....
But a lot of the RIB/inflatable's advantages don't really apply to sailboats. A big part of an inflatable's appeal is that, when planing at 35 kts in six-footers, the boat can flex to absorb the impact of a wave. Sailboats don't slam around like planing powerboats do, and so don't get much benefit from this.

 

The Catapult catamaran is a sailing inflatable. And it's in production.

There are a whole host of shortcomings to inflatable structures, all stemming from the fact that you can't have any local compression stress without buckling. That severly limits the shapes available to you and the loadings they can endure.

For a monohull, you can't provide any form stability with an all-inflatable structure that has a single tube. Multiple tubes joined together or internal webs form creases that increase wetted area and drag.

Hull rigidity is very important to performance - hence the widespread use of carbon composites - but an inflatable hull has very low rigidity at safe and reasonable inflation pressures.

The inflated volume takes away from internal volume, leaving no room for habitation in anything larger than a day-sailor.

It takes time to inflate and deflate the structure.

The surface texture of a typical inflatable fabric is not as smooth as a composite skin - more drag.

Minor damage that causes a hole compromises the entire structural integrity.

When you take into account the need to work around the limitations of an inflatable structure, it's not necessarily lighter than a rigid one. Note the aluminum tubes running along the tops of the hulls to take the compression load from hull bending. I doubt that the Catapult is any lighter than a composite catamaran of the same size.

 

Modern Inflatables

All that may be true, but it's becoming less so every day.

The typical inflatable material is a core of some load bearing strata such as polyester or nylon which is laminated and "wetted-out with the air holding material be it neoprene, hypalon or PVC.

In the last several years there have been developments in Kevalr re-enforced core materials that allow much higher pressures to be run with the same safety as those used in PVC material, for instance. This means that the boats can become more rigid and therefore, more resistant to the loads being induced by a typical sail rig. It also allows a faster hull in the water with less material deflection. We're not talking about extraordinary pressures here, just the difference between the normal whitewater raft inflating pressure of 6psi and the Kevlar material pressure of around 10psi.

Another development, drop stitch material, is allowing flat panels to be constructed which can also take large pressures, giving the surface of the panel enormous rigidity. When shaped panels are arranged much like a those of a multichine hull, the nflatable craft can take on much better hydrodynamic properties.

When all the potential draw-backs, as listed above by Tom, are brought a little more into line with these modern materials along with proper design concepts, these boats have the potential to approach the capabilities of their rigid sisters.

Then there's the single most powerful characteristic of any inflatable boat, they can be deflated and stored in a very small space compared to other hardshell craft. With the escalating costs of owning a boat in a marina, this technology is going to be more in demand in the future. I've been using inflatable amas on my sailing canoes for about ten years now and while a set of hardshell amas could be faster in the water, they will never be able to store in the hull of the canoe and allow me to go paddling.

The owners of these types of air filled boats typically do not ask for them to be fast or ultra sexy. They are looking for utilitarian boats within an economical envelope that will still allow them the potential for getting out on the water.

Will you see an inflatable boat beating a composite hull in a sailing event? Not in the near future, but you will see many more recreational boats going in this direction as new developments present promising potential solutions.

 

Another issue, relating a bit to Tom's point, is efficiency. A Zodiac typically requires 20-40% more horsepower than a fibreglass boat of equivalent total displacement, to reach the same speed. That shock-absorbing flexing also absorbs a lot of the propulsive power. With the very limited power available from a sail, these losses become more important.

 

Rigid inflatable powerboats (RIBs) managed to bypass many of those above stated shortcomings. They are now produced by the thaousands. They combine the rigidness of plastic parts with the lightness if inflatable ones, each used at the specific part of the hull where its advandages are more wanted. From my window I see tenths of such boats, whose happy now owners sold their old expensive heavy plastic boxes and discovered the benefits of these light, fast, economical, trailerable, seawothy and insubmersible powerboats. There is a sector of the powerboat market, up to 10-12 meters long, that is completely dominated by the RIBs. I wonder if there is anything more than the well known addiction to traditional looking sailing crafts that is prevailing sailboating from similar developments.

 

Not any more. They weren't selling. Against a conventional A class Catamaran it was heavier and slower. The principle advanage was tolerance to small bumps.

As far as racing dinghies and catmarans are concerned there really isn't any great amount of hull shell that isn't structural to a greater or lesser extent. So a RIb approach really wouldn't have very much to gain. At best you might be able to replace part of the side tank/deck structure aft of the centreboard with a tube, but by the time you've factored in all the complexities of attaching the tube firmly and securely any weight saving is pretty much gone. Its not like a RIB where you are basically attaching non-structural topsides to a very low freeboard hull shell - in the dinghy if the freeboard is not structural it ain't there...

One nice exception, very many years ago in the UK, was a childs boat called a Gremlin. This was an Opimist sized boat with exactly that - a basic hull shell with inflatable topsides. On that sort of craft the loads were neither here nor there and it worked well enough. Good for bumps between the kids too I imagine. However for one reason or another the Optimist squeezed it out of the market...