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Subdivision & Collision Protection

At this point we should touch on Classification, because from here on many points of design and construction come within the Rules of Survey if the vessel is intended to be “classed”.

The classification societies are curious institutions. They are non-profit non-governmental incorporated societies who publish rules, standards, guides and other criteria for the design and construction of marine vessels of all kinds. They review designs and carry out surveys during construction to verify compliance with such rules. They issue certificates of survey on completion if the vessel has been built in accordance with their Rules and Regulations. Governments also grant them powers to issue Loadline, Tonnage and Safety Certificates for Merchant Shipping of all kinds operating in their Registry.

The principal Classification Societies that issue rules for Motor Yachts are:-

ABS – American Bureau of Shipping

L.R. – Lloyds Register of Shipping

BV – Bureau Veritas
Each of these societies publishes Motor Yacht Rules for construction in steel, Aluminum, GRP and Wood. These Rules and Regulations have been developed over more than 100 years of trial and error, research and statistical analysis. They provide a good guide to designers and builders for design and construction even though it may not be intended to be “class” the vessel. It is from these Rules that we shall base our comments on Subdivision and Watertightness. Structural Strength, Machinery and Electrical Installation, Steering Gear and Equipment Installation.

The decision to build to class should be made at the design stage because, from our experience, obtaining a class certificate after construction (CAC) is a very difficult proposition and at best the certificate will be a slightly inferior class.

What are the advantages of classification?
For the owner classification has great benefits.

(a) He can be certain that the vessel has been built to a recognized standard by properly certified personel after plan approval.

(b) The hull materials will be of an approved and certified type.

(c) The Classification certificate will be recognized by insurance companies and will result in lower premiums.

(d) The resale value of the vessel will be enhanced.

What are the disadvantages of classification?

(a) The build cost is higher.
(b) Annual fees are payable.
(c) The vessel is required to have annual surveys.
(d) The vessel has to be dry-docked every 2 1/2 years for a full underwater survey.

What class notations are available?

The Classification Societies are reluctant to grant full Class notations to private motor yachts under about 78ft mainly because it is not economic for them to do so. They will however undertake plan approvals, survey the vessel during construction and issue a letter of attestation on completion stating that the vessel has been built according to their Rules and Regulations. There are no further surveys or fees payable after this time. If subsequent to this the owner converts the vessel to “ply for hire” or for other commercial purposes a classification certificate can be applied for based on the original letter of attestation. We recommend that owners obtain at least this degree of class for all vessels under 78ft. It should also be borne in mind that if owners have future intentions of chartering their vessels many Government regulations are also applicable which are outside of the scope of degree of class mentioned. All these need to be considered at the design stage.

For vessels over 78ft we recommend that a full Classification Certificate be obtained. For example our W79 Trawler Yacht is delivered with the following class notation: -

Bureau Veritas - I 3/3E YACHT-/s ·MACH STB

I 3/3E: indicates the hull is designed and built to the highest BV standards applicable.

E: means the equipment (anchors and cable) meets the applicable requirements of the rules.

YACHT-/s: indicates the service notation and hull material (steel).

·MACH: means the vessel machinery has type approval and is installed to BV satisfaction.

STB: an inclining experiment has been carried out on the completed vessel and stability file prepared and approved.

In addition to the above it is well worth considering for vessels over 78ft, obtaining M.C.A certification. This certificate will allow the vessel to be chartered from Red Ensign States and thereby greatly enhance eventual saleabililty.

In conclusion it can be seen that some degree of class approval is beneficial for an owner of a trawler yacht and should be seriously considered. Beware of vessels advertised as “built to Lloyds Rules” or similar statement as this is a misleading and meaningless term. Few vessels built without plan approval and inspection during construction would be able to pass the rigorous classification process.


Subdivision means the degree of longitudinal watertight subdivision of the hull. Every vessel should have the following minimum longitudinal standard:

(a) A watertight bulkhead set back from the stem between 5% and 8% of the waterline length.

(b) The engine room enclosed within watertight bulkheads.

This gives a four-compartment subdivision and is the absolute minimum degree of subdivision that should be entertained in trawlers of any length. For larger vessels 78ft and above where the engine room is amidships an extra bulkhead should be fitted in the aftpeak and where the engine room is aft an extra bulkhead should be fitted between the forward engine room bulkhead and the collision bulkhead.

For vessels of normal form this degree of subdivision will make the vessel immune to foundering if the hull is penetrated in any one compartment, prevent flooding of the vessel in the event of collision and confine fire to one compartment.

The three watertight bulkheads mentioned in (a) and (b) must be exactly that, watertight from bottom to deck. The collision bulkhead must have no openings whatever be they bolted or dogged or not. The only penetration permitted is for suction pipes and these must be fitted with valves bolted directly to the bulkhead in accessible positions. Access to the Forepeak must be from a deck manhole or hatch.

The engine room bulkheads must not have any access openings except bolted covers. Cable and pipe penetrations must be through proper watertight glands. Often we see designs where engine room access is through a bulkhead opening. From a safety point this is bad practice. Engine room access should always be from the deck.

To preserve the integrity of the longitudinal subdivision particular attention has to be paid to piping systems that pass through these bulkheads. The general principle is that a fire in the engine room or spaces either side shall not destroy the watertight integrity of these spaces. This means that all piping in the engine room that pass through these bulkheads must be of fireproof material. This rules out materials such as P.V.C piping in engine room systems. All such piping must be steel, copper or stainless steel.

Many trawler yachts have some degree of vertical subdivision. This means they have a double bottom fitted in parts of the hull. This is a useful feature because these spaces can be used for fuel or water tanks and also gives protection to the hull from stranding damage. There are no Class or Statutory requirements for double bottoms in Trawler yachts. Passenger Ships and some classes of Merchant Ships however are required to have double bottoms throughout their length.


Watertightness refers principally to the protection of openings in the hull, deck and superstructures. A firm principle should be that that any such openings that become immersed with the vessel healed to 60 degrees should be able to made fully watertight. The following is a list of openings that fall into that category and the protection required: -

(1) Portholes in the hull must have permanently attached deadlights of equivalent strength to the hull. The portholes must be glazed with toughened glass in metallic frames. Hull portholes should never be fitted less than 20” above the load waterline.

(2) Deck Hatches should be mounted on coamings at least 12” high and be fitted with metallic covers and rubber seals. The securing dogs should operate from both sides.

(3) Skylights should be mounted on coamings at least 12” high and preferably 24” high. The glass should always be toughened safety glass mounted in metallic frames. Permanently attached, hinged metallic deadlight covers must be fitted. Proprietary plastic or polycarbonate skylights are not suitable for ocean capable trawler yachts.

(4) Water tight doors. Any watertight doors in the deckhouse that fall within the 60-degree angle of heel must be of the watertight type with proper dogged closures and rubber gaskets. The sill height should not be less than 12”. The doors should be equivalent in strength to the deckhouse structure.

(5) Deckhouse windows should be positioned so that they fall outside the 60-degree angle of heel. Where this is not possible it is prudent to supply portable aluminum storm covers for their protection. Windows should be glazed with toughened glass with a minimum thickness of 3/8” and mounted in aluminum frames. Proprietary plastic or polycarbonate windows are not suitable for trawler yacht application.

(6) Ventilation Openings: Engine room ventilation openings should be positioned so that they fall outside the 60-degree angle of heel line. They should never the less have watertight hinged covers for isolating the engine room in case of fire. They should stand on coamings no less than three feet above deck and preferably be positioned at the height of the boat deck. Similar considerations apply to other ventilation openings.

Hatch/skylight arrangement manufactured in accordance with Classification Rule. Note the height of the coaming, the permanently attached deadlights fitted with rubber seals and access hatch (2nd from left) with securing dogs that operate from both sides.

Last Updated (Saturday, 29 August 2020 02:01)