Naval Architecture Articles

General

It is of vital importance that there be provided efficient means of protection for all openings to the hull and superstructures, for the protection of the crew in heavy weather and for the rapid freeing of water from the weather decks. The regulations to ensure compliance with these considerations are grouped together as Conditions of Assignment.

By the terms of the International Load Line Convention (ICLL, 1966), freeboard may not be officially assigned until the ship has been inspected and a qualified surveyor representing the Administration of the flag country is willing to certify that the Conditions of Assignment have been met.

These Conditions of Assignment must not only be complied with initially but they must be at all times maintained in satisfactory condition. Their vital importance is recognized in the regulations which call for annual inspections to be made by the assigning authorities' Surveyors to ensure that, in fact, they have been maintained in satisfactory condition for the continued validity of the Load Line Certificate.

In the Convention (ICLL, 1966) repeated in the Regula¬tions (46CFR42), are the specific formulas for minimum height of openings, strength, deflection, etc. for each of the items covered by the following subsections. They are described here in general form so that one realizes that a significant portion of load line review and freeboard assignment is dependent upon the watertight integrity of the hull, and the weathertight integrity of the ship's topside area.

Hatchways

Most important, because of their size, are the cargo hatchways. Standards are set forth in the regulations for the construction, heights of coamings, the covers, and the fittings of all exposed hatchways on the freeboard and superstructure decks. Hatchways inside superstructures must meet standards which depend upon the type of closing appliances fitted on the access openings in the end bulkheads.

The requirements for hatch coaming heights and hatchway covers, and their supports, comprise a standard of strength and protection. Coamings may be reduced in height, or eliminated altogether, in association with gasketed metal covers, subject to the approval of the flag administration. While the regulations specify strength criteria, the rules of most of the classification societies contain formulas for hatchcover and beam design which provide equivalent strength.

Machinery Casings

Machinery space openings on the exposed portions of the freeboard deck or superstructure decks, or within open structures, must be provided with steel casings, with any opening fitted with steel weathertight doors. Openings in required machinery casings must have specified minimum sill heights. Machinery access hatch openings are to have permanently attached steel weather-tight covers. Exposed machinery casings of Type A ships cannot have direct access from the freeboard deck to the machinery space.

Other Openings in Deck and Shell

Other openings include ventilators, air pipes, hull piping, and air ports.

Ventilators on exposed positions on the freeboard and superstructure decks leading to spaces below the freeboard deck, or to enclosed superstructures, are to be fitted with coamings of minimum height, depending upon the location, and with provision for temporary means of closing.

Air pipes from ballast tanks or other tanks below the freeboard deck, which extend above the freeboard deck or superstructure deck, are to be provided with a permanently attached means of closing.

In the sides of the vessel, the numerous small openings required present a problem in maintaining the intactness of the vessel. Each overboard discharge pipe leading from spaces below the freeboard deck must have an automatic non-return valve with positive means for closing from an accessible position above the freeboard deck, or in some instances, two automatic non-return valves without positive means of closing may be allowed if the inboard valve is always accessible in service. Scuppers or sanitary discharges from superstructures or deckhouses may also be required to have similar protection.

Portholes in superstructures on the freeboard deck or in the hull are required to be fitted with hinged deadlights. Any other openings in the shell below the freeboard deck such as gangway or cargo ports must have closures designed to ensure watertightness and structural integrity.

Miscellaneous Conditions of Assignment

Guard rails or bulwarks, gangways, lifelines, or other means must be provided for the protection of the crew in its operation of the vessel and for getting to and from their quarters. Deckhouses used for the accommodation of the crew are to be of adequate strength. Where bulwarks on the weather portions of freeboard or superstructure decks form wells, ample provision is made for freeing the decks rapidly of water and for draining the wells. The draining area re¬quirement is based on the length and height of the bulwark. In ships with no sheer or in those fitted with trunks restricting free flow of water across the deck, adjustments are made by increasing the freeing port area requirements to a degree.

Information to be Supplied to the Master

The master is to be furnished with sufficient information in an approved form satisfactory to the administration of registry. This is to enable the master to arrange for the loading and ballasting of his ship in such a way as to avoid the creation of any unacceptable stresses in the ship's structure. In some cases, based on the length, design, or class of ship, the administration may consider this requirement unnecessary. In addition, the master is to be supplied with sufficient information in an approved form to give him guidance as to the stability of the ship under varying conditions of service.

Timber Deck Cargos

Timber deck cargo refers to a cargo of timber carried on an uncovered part of a freeboard or superstructure deck. Such cargo may be regarded as providing the ship with certain additional buoyancy and a greater degree of protection against the sea. For that reason, ships carrying a timber deck cargo may be granted a reduction of freeboard calculated basically in accordance with the requirements for Type В ships with additional conditions stipulated relating to construction, stowage of the timber cargo, stability, protection of crew, and access to machinery and other such spaces necessary for the safe operation of the ship.

In the previous article we have touched the very basics of the shipboard load line. In order to assign a load line properly, it is necessary to compare the design to a geometric ship of the Standard form. The concept of the Standard Ship with definite geometric proportions was evolved early in the discussions for standard freeboard.

Board of Trade Standard Ship

From a historical standpoint, the 1906 Board of Trade Rules (Board of Trade, 1906) in England used a Required Reserve Buoyancy to establish desired winter freeboard for both steamers and sailing vessels. This Reserve Buoyancy referred only to the intact weathertight ship and was deemed necessary for safe seakeeping. The freeboard to be assigned was such that the percentage of the total volume of the hull above the load line was equal to that required in the table.

The required buoyancy was least for the shortest vessel, 20.4 percent at 22 m, and increased to 35.8 percent becoming maxi¬mum at a length of 183 m. The required extra buoyancy for sailing vessels was 1 percent to 2 percent higher than for steamers. However, in lieu of making a complete volumetric calculation up to the freeboard deck, the designer was permitted to use certain tables of winter freeboard provided by the Board of Trade based upon a standard length to depth ratio (L/D) of 12.

Freeboard reductions of a very small order were allowed for summer weather. On the other hand, an arbitrary addition of 50 mm in winter time for the Mid North Atlantic area was required.

In addition to the regular reserve buoyancy due to the basic freeboard amidships, the regulations also prescribed a standard sheer curve adding buoyancy at the bow and stern. This buoyancy was considered effective in promoting the seakeeping properties of ships in heavy weather.

The freeboard for a given length and depth also varied slightly according to the "coefficient of fineness" which was defined (Board of Trade, 1906) as the ratio of all under freeboard deck volume to the product of L x R x D.

ICLL 1930 Standard Ship

The Standard Ship of the 1930 Convention had:

• a L/D ratio equal to 15;

• a fineness coefficient equal to 0.68;

• a table of freeboards increasing with length of ship;

• a standard sheer; • a standard camber of the main deck;

• a minimum percentage length of superstructure;

• a required forecastle for tankers.

In the International Convention on Load Lines 1930, the coefficient of fineness was specially defined only in English units as follows:

where d1 figure was the mean molded draft at 85 percent of the molded depth. Subsequently, in the ICLL, 1966 the title Coefficient of Fineness was dropped and the correction is now called the Block Coefficient correction.

ICLL 1966 Standard Ship

The Standard Ship of the 1966 Convention (ICLL, 1966) is similar to the 1930 standard ship except for the camber requirement which was dropped and the forecastle requirement which was removed in favor of a minimum bow height for all manned vessels. The fineness coefficient was redefined as the block coefficient, as previously mentioned.

Some types of ships less than 100 m in length are expected to have a weathertight superstructure on at least 36 percent of their length which will add buoyancy and form a righting moment to resist extreme rolling. These ship types with superstructures covering less than 35 percent of the length must accept added freeboard.

The "Load Line" is actually a formally accepted term which has been given to a mark normally located amidships on both sides of a vessel in order to show the limiting draft, i.e. the one to which the ship is allowed to be loaded. The limiting value shall be obtained through measuring from the freeboard deck, usually the uppermost continuous weathertight deck of the ship, down to the load line mark. Subject distance is referred to as the ship's Freeboard. The load line mark itself is one of the essential requirements of the ICLL, 1996, standing for the International Convention on Load Lines 1966; in addition, same is required by the Public Law 93-115, 1973 in the United States.

The arrangement of this mark will be governed by the regulations that are issued by the government of the Flag's country, for example the USCG in America. The ICLL, 1966 requires the Administration of each country accepting this convention to provide all facets of load line examination and control. Since it involves continuous knowledge of the ship throughout its life, countries with no inspection staff often delegate portions or possibly all load line activities to classification societies.

Since the load line regulations apply to almost all ships and embody a complete review of the general seaworthiness of the vessel, it is therefore important that the designer consider not just the desired freeboard but all facets of safety governed by load line regulations early in the preliminary design. The associated calculations, legal assignment, and marking of the minimum allowable freeboard plus the overall seaworthiness evaluation are intended to make sure that the ship:

• has adequate structural integrity for the intended voyages;

• has adequate hull stability for the intended service;

• has a hull confirmed to be watertight from keel and up to the freeboard deck and weathertight above this deck;

• has a working platform located high enough from sea surface, thus allowing for the safe movement on the exposed deck in heavy seas;

• has adequate hull volume and reserve buoyancy, above the waterline - this guarantees that the ship will not be in danger of foundering/plunging when in a very heavy seaway.

The above five basic rules have been the guiding principles for many decisions made during the past century by classification societies and national administrations relative to the proper minimum freeboard for ships of all kinds. The history of official load lines is well documented by the discussions and papers noted in Board of Trade (1906), Norton (1942), and Ryan (1967) for the further guidance of those who wish to review the development of load lines.

The rules for determining the correct freeboard on any particular vessel are not scientifically exact. The freeboards determined under the international rules for vessels on international voyages evolved within the last 100 years. They are completely empirical and were initially based upon experience on vessels up to 91 m in length projected to 137 m in length. After the initial freeboards were set down as national legal requirements in Europe in 1890 they were amended from time to time and the tables extended gradually to lengths up to 229 m in 1915 when vessels still were generally well below that length.

Internationally, there have been only two load line conventions. The first was hosted by the UK back in 1930 and the second one was held in 1966 under the auspices of the UN agency for marine safety, which is the Inter-Governmental Maritime Consultative Organization (IMCO) located in London.

The five basic rules of load line philosophy mentioned earlier are embodied in an agreed set of regulations published in the International Convention on Load Lines (ICLL, 1966) currently in force. In America, the load lines were originally established by Congress in 1929 for foreign voyages and in 1935 for coastwise and Great Lakes voyages (Public Law, 74-354, 1935). Currently, the ICLL, 1966 is implemented by Public Law 93-115 dated October 1, 1973 which is reproduced in the United States Code 46 USC 86. Subject regulations implementing this law are detailed in the Code of Federal Regulations (46 CFR 42) which constitutes an almost verbatim copy of the technical Annex of the International Convention. All new ships 24 m or more in length which make an international voyage are required to be assigned load lines under this law. Warships, fishing vessels, existing vessels not exceeding 150 gross tons and pleasure yachts are exempted.