Introduction
The use of ice
Fishroom construction
Summary
This note gives advice on the design, construction and operation of fishrooms for inshore fishing vessels. The advice is intended mainly for operators and builders of decked vessels up to about 25 m in length that make voyages of up to about 5 days. Guidance is given on icing practice, and on the uses of insulation and mechanical refrigeration in keeping the catch chilled. This note reemphasizes and augments the information given in Advisory Note 11 ‘Handling inshore fish’.
Cleanliness, care and cooling are all important when stowing the catch, however short the trip. Fish begin to spoil as soon as they are dead, and they go off five times faster at 10°C, the temperature of the North Sea in summer, than they do at 0°C, the temperature of melting ice. Therefore the prompt application of plenty of ice in the right places is vital. If little or no ice remains among the fish at the time of landing, then they have not been iced enough; always use too much ice rather than too little to be on the safe side.
The amount of ice required will depend on the length of voyage, the outside temperature and the design of the fishroom, but at least one part of ice to 3 parts of fish by weight is essential whatever the conditions, if the fish are to be properly chilled. Always use clean fresh ice, and make sure that boxes and the fishroom are clean before stowage begins. Put a layer of ice in the bottom of a box, sprinkle some ice through the fish and put another layer of ice on top of the fish. Use small pieces of crushed ice or flake ice; large pieces mark the fish and don’t cool so efficiently. Don’t put paper over the fish before adding the top layer of ice; the paper will stop the cold meltwater trickling down between the fish to cool them.
Use a box that has enough room for the required amount of fish and for the right amount of ice; don’t overfill the box such that another box stacked on top will squash the contents. Don’t pack the fish so tightly that puddles of stagnant water and blood can form in the box, and lay the fish belly down so that the gut cavities can drain.
Prevent as much heat as possible from reaching the boxed fish by putting plenty of ice between the boxes and the fishroom structure; put ice against the bulkheads and ship’s sides and on the fishroom floor. Keep the fishroom hatch closed whenever possible. The catch can be chilled and kept cool even in a warm, badly designed fishroom provided the fish are always surrounded by plenty of ice.
The main purpose of ice in the fishroom is to remove heat from the newly caught fish and to keep them cool so that spoilage is reduced to a minimum. But some ice will inevitably be used to absorb heat coming in through the fishroom structure and to cool warm air coming in through the hatch. If the incoming heat is not absorbed by the ice, then it reaches the fish and warms them, with the result that they spoil more quickly. The amount of heat coming in can be reduced by good fishroom design and construction, so that most of the ice is available for the prime purpose of cooling the catch.
Insulation
The amount of heat entering a fishroom can be considerably reduced by fitting insulation; the cost of installation and the small loss of fishroom space are normally more than justified by the reduction in ice consumption. For example the amount of extra ice required against bulkheads and ship’s sides will be much smaller.
Complete insulation is recommended for hulls of all materials except wood. The cost of insulating ship’s sides in a small wooden vessel working in temperate waters is hardly worth while; insulation of deckhead and bulkheads is generally sufficient. Where ship’s side insulation on wooden vessels is considered necessary, a ventilated air space between the hull and the insulation should be provided to reduce the risk of wood decay. There is a wide choice of insulating materials, including glass and mineral fibres, expanded plastics, expanded ebonite and cork. Ideally fishroom insulation should be unaffected by water or water vapour, and should be nonpoisonous and nonflammable, in addition to being a low cost heat barrier. Closed cell plastics are more resistant to moisture than open cell plastics, and insulants in slab or mat form are less prone to settling than loose filling; uninsulated gaps can form behind the linings when the insulation settles as a result of constant movement of the ship. On the other hand, loose or flexible materials are sometimes an advantage when insulating small awkward spaces that would be difficult to fill with a rigid slab. Thus a combination of more than one type of insulating material in the same fishroom is sometimes desirable; for example a rigid closed cell plastics slab might be used on a bulkhead, and a flexible glass fibre mat used between beams on the deckhead. The final choice of material and method of installation will depend largely on the type of hull and the materials used in its construction.
Where insulation is to be placed directly against the ship’s structure, for example a wooden bulkhead, it may be advisable to coat the warm face of the insulation with a vapour seal to prevent moisture passing into the insulation. The advice of a competent insulation contractor should be taken when the fishroom is being designed.
The thickness of fishroom insulation on vessels working in temperate climates should be not less than 50 mm. Where the structure is likely to be very warm, for example an engineroom bulkhead, a thickness of 100 mm is often advantageous. The thermal conductance of an insulated fishroom for temperate climate should be not more than 0·7 W/m2 °C; in other words the insulation should be good enough to permit a flow rate of not more than 0·7 W through each square metre of fishroom surface for each degree of temperature difference between the outside and the inside of the fishroom. For example, an engineroom bulkhead, 6 m by 3 m, between an engineroom at 25°C and a fishroom at 1°C should transmit not more than 0·7 × 6 × 3 × (25-1)W= 300 W when properly insulated.
An inner lining is necessary in an insulated fishroom to protect the insulation from physical damage and to prevent as far as possible water penetration; linings are never completely watertight. The lining alone, without insulation but with an air space behind it, can considerably reduce the amount of heat entering the fishroom. On wooden ships, the air space must be ventilated to prevent wood decay. Wooden linings should be treated with a suitable preservative.
Mechanical refrigeration
Although insulation reduces the flow of heat into a fishroom, the temperature of the air in the fishroom may still be more than 5°C or even more than 10°C when the outside air and sea temperatures are high and when the hatch is open for long periods. A mechanical cooling system can be used to keep the fishroom air temperature between 1 and 2°C, thus reducing unnecessary ice meltage. The amount of refrigeration needed will depend on the size of the fishroom, the amount of insulation and the outside temperature, but for a well insulated 20 m boat working in temperate waters a capacity of 3-4 kW is usually sufficient.
The cooling plant can be used to cool the fishroom before ice is taken aboard, and to reduce wastage of ice on passage to the grounds. It will also give some added protection during fishing by combating warm air coming in through the hatch and thus allowing the ice on the fish to do its job. It must be remembered however that the presence of a mechanical cooling plant is no excuse for using any less ice on the fish; only ice in close contact with the fish in a box will have any marked effect on the temperature of the fish.
Air coolers are of two types, those which circulate the fishroom air by means of a fan, and those which rely on gravity circulation. The gravity type is preferable because it gives a more even fishroom temperature and more comfortable working conditions. The gravity cooler is usually a grid of plain pipe covering the fishroom deckhead and sometimes extending down the sides and bulkheads. The pipe, typically 25 mm diameter, should have ample cooling surface, at least 1 dm²/W, and should operate between -3 and -5°C. The fishroom air temperature should not be allowed to fall below 0°C, otherwise the ice will not be free to melt and the fish may become partially frozen.
The fishroom temperature may be controlled either manually or thermostatically, but the control must be accurate; the thermostat range should be 1-3°C. A remote reading thermometer should be used where necessary to display the fishroom temperature, in the wheelhouse for example, at all times. The sensing element of a thermometer or a thermostat should be located under the deckhead, roughly midway between bulkheads, below the level of any pipe grid and not too close to a hatch. The bulb should be protected by a perforated shield.
A plain pipe gravity cooler can operate for long periods without defrosting, usually throughout the full trip; fan coolers can be equipped with an automatic defrost. The condensing unit, that is the refrigerant compressor with its condenser, is usually housed in the engineroom. The condenser can be cooled either by air or water but, since an air cooled condenser can become overheated unless separately ventilated, the water cooled type is to be preferred. The condensing unit power supply should be independent of the main engine. A separate internal combustion engine can drive directly both the compressor and the condenser sea water pump, or electric motors can be powered from a separate generator. A chilling plant that extracts heat from the fishroom at a rate of 3 kW requires a power supply of less than 2 kW to drive the refrigerant compressor. Fuel consumption will be highest in warm weather when the plant is running for long periods, and will be about 20 litres a day for a 2 kW engine.
Refrigerants 12 and 22 are commonly used in plants of this size; there will be very few leaks in a properly installed plant and only occasional servicing will be necessary. The compressor should have pressure switches to protect it against failure, and the lubrication system should be capable of working in rough conditions.
1. Use plenty of ice in the right places.
2. Insulate the fishroom, either partially on a wooden vessel or completely on others.
3. Mechanical cooling can help to make better use of the ice, particularly on trips lasting 4 days or more.
4. Don’t rely on mechanical cooling and insulation to
chill the catch; only ice will do this properly. Insulation and refrigeration
are aids to good fishroom practice, but they can never supplant ice as the means
of cooling fish and keeping it cool.
