This design is becoming increasingly popular and has proved to be highly effective and weatherproof.
How to make a square raft
The square raft design has proved to be highly effective and similar structures are in use in many reserves.
Here is a step-by-step guide to constructing a square raft:
- Construct a framework of 25 x 150mm boards or similar. Nail the flooring across the top of the frame, leaving the margins open to take vegetation, and nail duckling ramps to one end of the raft. Use galvanised nails.
- Turn the raft over. Staple close-mesh galvanised wire netting across the bottom of the raft, leaving the central part free to hold the flotation blocks.
- Place 150mm thick polystyrene blocks in the uncovered centre of the frame. Hold the polystyrene in place with diagonal boards nailed across the frame.
- Turn the raft the right way up. Cut out blocks of rush, willow etc to fit into the margins of the frame. Fit anchor bolts to two opposite corners. Fix a nesting box or basket if required. You can cover the raft with some gravel. Finally, tow the raft into position and anchor it firmly.
A heavier variation
This second raft style is very successful when attracting terns to nest.
Bare shingle is required for the nesting but a completely exposed raft results in high chick mortality. At about one week old, tern chicks leap overboard at the slightest disturbance. This can be prevented by providing them with small shelters to hide underneath.
- Drill the sleepers as indicated in the diagram, using a brace and a bit, and bolt them together with eight 250mm coach bolts. Drill and fix anchor bolts in the end sleepers.
- Drill and bolt the cross members to the side sleepers. These are required to make a rigid structure and to resist the upward pressure of the floats.
- Nail the side battens into position. These help hold the shingle in place.
- There are two ways to floor the raft. One is to trap plastic-coated chain link fencing, covered in heavy-duty polythene, under the cross braces. Staple the fencing firmly to the sleepers. Alternatively, nail old garage doors or other suitable sturdy timber to the cross members and spread the flooring with a layer of concrete to help keep the shingle in place.
- Float the raft. Unless you have mechanical help, placing approximately 0.8 cubic metres of polystyrene blocks under the raft for flotation will require a number of water-hardy volunteers.
- The amount of polystyrene needed varies with the weight of the raft, so trials are necessary. Provide some extra flotation to compensate for the shingle, which is added afterwards. The polystyrene stays in place between the sleepers due to its buoyancy and should not need fastening.
- Spread a layer of shingle over the flooring.
- Fix ramps or walls to the raft's sides, place a shelter on it, tow it into position and anchor it by means of bolts in the end sleepers.