Raft Systems

The increasing number of deep water shellfish aquaculture sites entails a greater emphasis on highly efficient and productive off-bottom culture systems. Considered on an area unit basis alone, raft culture systems are among the most productive in the world. A typical raft might measure 24 by 24 feet (7.3m) covering an area of 576 square feet (53.3m2). Yet this small area, only 1.5% of the area of an acre (0.5% of a hectare), might produce 50,000-100,000 marketable oysters per year!

Rafts are used to suspend several different types of culture systems. Before constructing a raft, the grower must take several factors into consideration:

  • What will I grow?
  • What methods will I use to grow them?
  • Do I need to use the raft as a work platform?
  • How often will I need to inspect or grade and sort the crop?
  • How will I harvest from the raft?
    How dense will I grow the crop?
  • How much flotation will I need?
  • How will I anchor the raft?
  • Can it withstand currents, wind, waves?
  • How much does it cost to build?
    How long will it last?

Rafts are usually built by the grower and some are still constructed using immediately available materials, i.e. logs. However, considering the value of the crop hanging from the raft, it makes sense to ensure that rafts are durably constructed. They will need to withstand the severest weather, the weight of hundreds of dozens of mature oysters, and often serve as safe work platforms for handling or harvesting. To keep the costs down, the grower can build a raft from off-the-shelf materials.

Growers who plan to build rafts, especially for rough water conditions, should keep in mind the following principles:

  1. FLOTATION: Rafts must be able to support the weight of planned oyster capacity at harvest with adequate margins for safety and unplanned events. Harvest may have to be postponed. Continued growth of oysters and fouling have caused rafts to sink, representing heavy losses to growers. Block foam is now the standard flotation material either 18″x24″ or 24″x24″ in 8′ or 24′ lengths. Raft users usually recommend that the foams be cleaned of fouling at harvest which must be done either by a diver or foams must be removed cleaned and replaced.
  2. STRENGTH and FLEXIBILITY: Must be able to withstand significant wave, wind, and current activity. Rafts must not be constructed too rigidly or the forces of currents and waves may cause rafts to break up. Rafts should be able to “Z” significantly and still return to the original right angles without compromising the strength and durability of the raft.
  3. STABILITY: Rafts must be constructed in such a way as to dampen the effects of waves and minimize the transmission of the surface motion of waves to the suspended stock.
  4. FUNCTIONALITY: Rafts must function as efficient and convenient platforms for stocking rafts (loading trays or stringing with cultch or tubes), tending and harvesting the stock. There should be sufficient flat and wide surface to serve as a walkway and crossmembers wide enough to facilitate kneeling. Rafts should be designed to facilitate easy access to strings at harvest or other times, to minimize labour requirements at harvest, afford efficient and safe working conditions, and, increasingly to be accessible to mechanical harvesters already in operation.
  5. DURABILITY: Rafts should endure 5 or more years, but this will depend largely on the site in which they are they are located and the natural forces to which they are subject.
  6. CAPACITY: Rafts should be constructed so as to maximize productivity where oysters can be hung at maximum sustainable densities and depths. Actual levels of production per raft will vary depending on site conditions.

Rafts are used for different stages of the culture of oysters, clams and mussels. Rafts are not suitable for scallop culture, which, because of low stocking densities and sensitivity to motion, are usually reared on sub-surface long lines.

Rafts for Tube or String Systems

  • The Redonda Raft
    Redonda Seafarms, which operated at deep water sites around Redonda and Cortes Islands, redesigned conventional rafts to withstand the sometimes severe conditions experienced on its more exposed sites. The rafts at this site are well placed to capture nutrients from currents and upwelling but they are also quite exposed.

Rafts for Tray Systems
Growers are now using trays for nursery rearing of juvenile clams and oysters, and for single oyster growout. The two types that follow have been developed for very high density oyster growout in trays but will serve equally well for tray nursery rearing. Click on the thumbnail images below to find details of the rafts’ construction.

  • Odyssey Shellfish Ltd.
    The rafts below were designed and built by Odyssey Shellfish Ltd. to accommodate suspended tray stacks (88 stacks per raft) and be able to absorb the forces of waves and currents.
  • Desolation Sound Oyster Company
    This is an example of a raft used by Desolation Sound Oyster Company (Cortes Island) for tray oysters.

Anchoring Rafts
The majority of raft systems comprise a series of rafts in a row. If there are several, perhaps up to ten, rafts in a series, anchoring must be very secure to prevent movement or drift of the rafts. Water movement resulting from waves and currents exerts enormous pressure on the system, particularly on a raft fully stocked with mature animals. Strings or tray stacks can be deflected significantly from a vertical hanging position.

Therefore, anchorage is critical and rafts can and have floated away, dragging one-ton concrete anchor blocks along. One of the best options, if it is possible, is anchorage using shore pins (steel rock pins) to fix one length of the row of rafts to a secure immovable place. (see more about shore pins in the Long line pages.)

Remaining deep water anchor points can be done using 1-2 ton concrete blocks. If shore anchoring is not possible, the series of rafts must be anchored at least at the four corners and, if necessary along lengths of the raft system. Main anchors are 1-2 ton concrete blocks placed at correct points.

Fixing the lines from anchor to raft is generally done by attaching a length of chain to the anchor. Commonly with the Redonda type raft this may be 16 feet of 4-inch-link black iron (boom) chain, terminating in 1-inch poly-lined thimbles to which poly ropes are attached and these are tied to the rafts. The rope used is normally 1-inch polypropylene line, preferably black (as this color withstands UV degradation the best). Under exposed conditions, it may be advisable to run two lines from the anchor point to the rafts. The purpose of the double line is to offer both security for the rafts and to be able to move the anchor back to its place if it has been dragged by the raft. One line can be tied to the towing vessel, while the other remains fastened to the raft.

Rafts are usually anchored in rows of up to 10 rafts each with anchors on either end. Along the line, rafts are roped together and securely tied at three points on each raft (along the 25foot side) such that the spread between the rafts is no more than one meter.

Other operations utilize somewhat different anchoring systems. Odyssey Shellfish Ltd. is using 6-8 feet of chain from the anchor block, a 1 1/4″ thimble and 1 1/4″ poly-steel rope, followed by 6-8 feet of chain again which attaches to the raft. This avoids the problem of having the ropes chafe on the raft edges.

Anchor ropes will “sway” in the currents and slacken at low tides. One method of stabilizing the anchor ropes is to attach concrete weights. Used collapsed spherical floats can either be filled with concrete or used as forms for this. Plastic 5-gallon pails filled with concrete or blocks will also do the job. Alternatively floats may be used to tension the lines.

Odyssey Shellfish developed a raft design that is available to view. There are five files listed below.

Model
Raft Details
Raft Main Dimensions
Oyster Rack Connector
Dock System Components