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Materials & Structural Strength

Hull Materials

There will always be debate among builders, designers and owners about the best materials for boat hulls. In general for ocean going trawler yachts three materials are commonly used:


For each of these materials claim and counter claim are continually made by builders who promote them. In trying to make sense of these claims we should weigh up what is the most desirable from the point of view of the owner. The main concerns must be all of the following considerations;

Build Cost
Durability and Maintenance
Propulsion and Running Costs

Design Regardless of the hull materials used if the design of the vessel is poor in other respects no matter what material the hull is made from the design is still poor. Poor design features may be:

Stability Appearance
Accommodation layout
Propulsive characteristics
Steaming Endurance

Build Cost It is extremely difficult to make a cost comparison between finished vessels of fiberglass, steel or aluminium because typically the hull cost is only 25-30% of the total build cost. Also Fiberglass boats are generally production boats against steel and aluminium which are generally custom built. Production fiberglass boats should be the cheaper option.

What we do know is that aluminum hulls, for the same design, are at least 40% more expensive than steel hulls. This translates into a finished vessel cost of at least 10-15% more than steel.

On a comparison of custom vessels, steel would be the most economic with fiberglass and aluminum approx. 10-15% more expensive.

Strength Vessels built of any of the three materials can be designed, without difficulty, to withstand the normal sea forces imposed on them.

So what is meant by strength? Perhaps a better comparison should be made for “off design” situations. It is a statistical fact that most mishaps at sea are not caused by a vessel being overwhelmed and broken up in a seaway but damaged from stranding and fire.

Of the three materials steel is the most resistant, by far, to stranding damage. Steel hulls are almost immune to puncturing damage and the structural integrity of a steel hull is also immune to fire damage.

Aluminum hulls are somewhat less resistant to stranding damage due to the lower specific strength of the material. There is also a tendency by builders to make aluminum hulls lighter than they should be due to the very high material cost. Aluminum hulls also have poor resistance to fire due to the low melting point of the material.

Fiberglass hulls have poor resistance to stranding damage and even in a minor stranding extensive damage can be caused by abrasion. Fiberglass hulls have no resistance to fire damage and total loss through fire is not uncommon.

Durability & Maintenance

No material is immune to degradation by the elements.

Fiberglass hulls suffer over time from penetration of water into their laminates and fading of gel coats.

Steel hulls rust if their protective coatings are damaged and may suffer from electrolytic corrosion in some circumstances.

Aluminum hulls can also suffer from electrolytic corrosion.

It has been our observation that for a new vessel hull with properly installed machinery and systems, maintenance over the first five years of service is minimal. So what are the long term hull maintenance requirements?

Fiberglass Hulls

The owner must eventually face the prospect of repairing underwater damage due to penetration of water into the laminate. This involves complete removal of the gelcoat layer along with any delaminated areas, drying out, repairing and recoating with an epoxy resin system. Topside gelcoats eventually fade and must be repainted.

Steel Hulls

Over time steel hulls suffer from rusting due to isolated damage to the protective paint system. Periodically finish coats must be repainted due to fading, typically every five to six years. Rarely if ever does the entire paint system require renewing. Electrolysis damage is caused by electrical system leakages and is therefore avoidable, and such damage is now becoming rare.

Aluminium Hulls

Hull maintenance will require only periodic repainting for cosmetic reasons. Electrolysis damage is caused by electrical system leakages and dissimilar metal combinations in the structure and attachments and therefore should be avoidable. On the whole aluminum hulls should have the lowest long term maintenance costs.


Steel trawler yachts may be as much as 30% heavier than an equivalent fiberglass or aluminum vessel. This means that their propulsive costs may be that much higher. However, heavy displacement in an ocean going trawler yacht has other advantages which must be weighed against this higher propulsive cost.

Heavy displacement vessels have better sea motions along with better “staying power” in a seaway.

In addition to this there is some antidotal evidence that steel vessels have lower insurance costs. Underwriters perceive them to have a lower insurance risk.


Once again this factor is difficult to quantify. Prospective owners should certainly take this into account when considering a purchase. We suspect the overall design has a bigger influence on resale than the hull construction material.


The determination of scantling sizes and plate thickness for a particular hull design can be approached in two ways:

Empirical methods
Classification Society Rules

In the past for small craft empirical methods were used by designers to determine hull scantlings. In our opinion it is preferable to use one or another of the Class Society Rules and Regulations. These rules are very comprehensive and based on realistic sea state conditions. The owner can be assured the design is to a known standard.

Typically Class Rules are divided into three size classes for small craft.

Yacht Rules up to 40metres (131ft) Ship Rules up to 24metres (78ft) Ship Rules 24-65metres (213ft)

The Yacht Rules cover vessels built of wood, steel, fiberglass and aluminum. The Ship Rules cover vessels built of steel only.

For practical construction the Yacht Rules up to about 60ft are to light for trawler yachts. We use the Ship Rules in all our trawler yacht designs.

The Ship Rules are written for a navigation notation “Deep Sea” with various reduction factors for the following lesser notations:

Coastal Waters Sheltered Waters River Waters

For ocean going trawler yachts we always use “Deep Sea” notation.

The Rule frame scantlings and plate thickness are derived from specific sea pressures calculated for various parts of the hull, decks and superstructures. These pressures are dependent on the length, breadth and draft of the hull in question.

For example the following is the Rule pressures derived for our W79 hull:

Bottom forward 982 lbs/ft2 Topsides forward 420 lbs/ft2 Topsides amidships 260 lbs/ft2 Main Deck amidships 208 lbs/ft2 Main Deck forward 260 lbs/ft2 Collision Bulkhead 375 lbs/ft2 Eng. Room Bulkhead 230 lbs/ft2 Deckhouse Front 568 lbs/ft2 Deckhouse Sides 280 lbs/ft2 Boat deck 83 lbs/ft2

Note the very high pressure allowance for the bottom forward. This reflects the slamming pressures experienced in this area in a seaway. Also of note is the wave impact pressure allowance for the deckhouse front.

Two hull framing methods comply with the Rules; longitudinal frames or transverse frames. We always use close spaced transverse framing in our designs because the shape of the hull is more easily defined. Hull shape will be discussed in more detail further along.


If a vessel is to be classed with one of the Classification Societies plan approvals have to be obtained before construction commences.

This plan approval process not only involves detailed drawings of the hull itself but must include material specifications, welding schedules, drawings of machinery layouts, propeller shaft assemblies, rudder assemblies, pumping systems, electrical systems along with numerous other details. For example our W48 design requires submission of a minimum 60 detailed hull construction and system drawings. The approval of these drawings and construction of the vessel in compliance with them should give the owner supreme confidence in his vessel.


In the case of vessels built to class the Classification Society lay down strict specifications for the type of resins, aluminuim alloys or steel to be used in the hull construction. The builder must have good record keeping systems for all the material test certificates used in the hull construction. All materials are then traceable to the source of manufacture.

©2005 T.C. Watson & Sons Ltd, Naval Architects, Whangarei, N.Z
All rights reserved.

Last Updated (Saturday, 29 August 2020 02:06)