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Pick a Configuration

When examining the differences between rafts, some are immediately obvious. Aviation rafts come in three basic shapes: round or nearly so - hexagonal, octagonal or decagonal, square and rectangular ovals. "Round" rafts encompass the majority of aviation rafts. The only exceptions are the Survival Products rafts and RFD's Navigator which are square and the BFGoodrich rafts which are rectangular with rounded ends. The shape can have some bearing on comfort and livability and we will get to that later. Perhaps more important is the effect shape can have on sea worthiness.

Of particular concern is what happens if the sea anchor (also known as a drogue, which is more technically correct) is lost or improperly deployed, neither of which are a particularly uncommon experience, as documented by numerous survival stories and studies by maritime safety organizations. Rectangular rafts, and to a lesser degree square rafts, are more likely to settle in a wave trough, dig into the water and capsize while a round raft will tend to carrousel rather than capsize. Rectangular rafts are particularly prone to being capsized in conditions where they end up broadside to a wave. Without a functioning sea anchor, it is only a matter of moments until a rectangular raft turns sideways to the swells and waves.

Round rafts also seem to ride waves easier, with little of the fishtailing, bending, and twisting common to rectangular rafts. Octagonal and decagonal rafts are inherently more stiff than round, square or rectangular shapes, the spliced sectional construction adding strength and rigidity when done properly. SPI's square shape is better than BFGoodrich's rectangular shape with regard to seaworthiness, but neither is as good as a round (or hexagonal, octagonal or decagonal) design in my opinion.

One Tube or Two?

Another obvious distinction between rafts is whether they are built with a single tube for floatation or if they use two tubes stacked on top of one another. There is generally a significant difference in freeboard between most double tube rafts and those with only a single tube. While it makes little difference in calm waters, in any sort of sea condition other than calm, the more freeboard you have, the better off you are.

Besides the very important questions of adequate freeboard and greater protection from the sea, most double tubes also offer a much more comfortable backrest for passengers in the raft, whereas the single tube rafts were deemed to be very uncomfortable. The higher sides become a much more important factor in high seas when it is necessary to brace oneself against the violent motion of the raft. Reversible designs such as the Hoover and RFD rafts, with the floor between the tubes instead of at the bottom, eliminate this advantage.

Raft Test 2000: Freeboard

Redundancy

There is another even more vital area where single versus double tubes comes into play. This is the matter of redundancy. We are discussing inflatable rafts that, while pretty damn tough, are still subject to puncture by an number of means, both from within and outside the raft. Even in closely supervised training sessions, rafts are sometimes punctured. In one incident, a student shot an aerial flare right through a raft tube, despite an instructor standing by giving directions. In a survival situation you have to accept the real possibility that a puncture is going to occur (or perhaps, a partial inflation), whether by accident or as a result of natural causes.

To accommodate this possibility two factors must be taken into account. The first is that there must be some means to ensure that if there is a puncture, then some buoyancy remains. This is addressed in the TSO by requiring at least two separate inflation chambers. Whether by having multiple tubes like in a double tube raft or by dividing up a single tube raft internally, puncturing one buoyancy chamber does not sink the whole raft. With but three exceptions, the manufacturers provide this very basic measure of redundancy.

Winslow's single cell RescueRaft, SPI's single cell unapproved rafts, the RFD Navigator, and EAM in their 8-person and smaller rafts of their non-TSO'd EAM line are woefully deficient in this regard because they are constructed with but a single chamber. Lose that and you are back in the water. I consider this a grievous design flaw. By the way, nowhere in its literature does Survival Products make mention of its single chamber design and anyone not familiar with the subject isn't likely to notice anything amiss. When queried on this point, Donna Rogers, vice-president of Survival Products, described this "feature" as simply "a way to keep weight and price down" and fill a market niche. Revere's rationale was essentially the same. EAM indicated they were simply trying to be competitive with SPI. I consider the unapproved single cell Survival Products rafts, the RFD Navigator and the single cell EAM rafts unacceptable due to this design deficiency.

The TSO specifically accommodates both twin tube (Type I) and single tube (Type II) construction. However, one aspect of the TSO seems at odds with most of the Type II single tube construction we saw, based upon our evaluation. This is the requirement (TSO C70a 4.2.2) that ". . . the liferaft will be capable of supporting the rated number of occupants out of fresh water in the event one chamber is deflated." I cannot imagine how the Survival Products, Hoover and EAM single tube TSO'd rafts in this review pass this requirement.

In these rafts the single tube is typically divided in half with vertical bulkheads within the tube, like a donut broken in half. When one chamber of the single tube raft is punctured or deflated, all the persons in that half of the raft are going into the water. What you are left with is essentially a half a circle of tube open to the water across the diameter with the deflated half floating in the water. It is all but impossible for all the survivors to either fit in the one half remaining afloat or even if they somehow managed to fit in, for them to remain "out of fresh water" (the TSO C70a 4.2.2 requirement) which has poured into the remaining half. Even a partially deflated half presented problems for our volunteers, swamping the raft. The possibility of finding themselves in such a predicament in open water was distressing to all involved. The remaining inflated portion of the tube does provide buoyancy and a base from which repairs can be made, but it is difficult at best.

I consider these designs to have a critical failure mode and seriously question how the EAM and Hoover rafts can possibly pass current TSO requirements, despite the approvals received (though they do pass the earlier original, less stringent requirements in this particular regard, which is all BFGoodrich ever claimed of their old designs). Since the manufacturers claim these rafts meet the latest TSO, I asked the FAA about the matter. Based on my description, they also questioned how these rafts might meet the TSO. Unfortunately, it is not much of a priority with them and no further investigation has ever been undertaken. While the Winslow RescueRaft II and GAST life rafts are also of this design, with the same flaws, they are not TSO'd.

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Revision: 002 October 6, 2000
 

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