By Gordon Lacy MaSc., PEng.
Naval Architect, Structural Engineer

Why Engineered Boats?

Unlike spacecraft, airplane or automobile design, boats have been around since long before the profession of Engineering existed, yet somehow were built and did their job. There are many boats around to this day that perform very well without the benefit of any engineering and are beautiful to look at as well, so you say why do we need engineers?

A traditionally built wooden vessel consists of many independent planks connected to a series of transverse frames (either sawn or steam bent) by either metallic fasteners (wood screws or nails usually), or in some very old vessels, by wood dowels or even lashings made from leather or other material. The whole vessel is made from a collection of pieces that are connected by some type of mechanical fastener. The net effect of this type of construction is that the whole vessel can never be considered as behaving as a single structure. If the vessel is propped up on dry land only at the ends it will sag in the middle over time. Each piece of the boat is effectively independent of every other. Stress is not transferred between members very well. The end result is that the wood around the fasteners takes the biggest stresses. Planks are effectively sized by the holding power of a screw or nail, and by the distance between frames.

Additionally all traditional vessels operate at displacement speeds. At these lower speeds, the loads on the hull structure are usually fairly low and amount to hydrostatic pressures. Rules of thumb or simple tables such as Herreshoff’s scantling rules suffice.

Enter new materials such as steel or fiberglass or new wood adhesives, suddenly we have the ability to make a vessel behave as a unitized structure or monocoque. Now when the vessel is propped up on its ends on dry land or suspended between two waves in the ocean, it doesn’t just sag in response to the load, it either holds up without sagging, or it breaks at the site of the highest stress. In addition, powerboats were developed that could “plane” on top of the water where failures begin to occur in their bottoms due to the extreme slamming loads imposed on the wood planked structures. Suddenly it becomes important to engineer the structure.

One perfectly good, but time consuming and expensive engineering method is to make a boat purposely too light, then take it out and pound it through big seas to see where it starts to fail. Then reinforce those areas and try again. In theory one should arrive at the minimum design in this manner, assuming you had enough time, and you had truly made every member in the structure slightly too light so it would just start to fail. The fact is some boat companies have (and do) operate with this seat-of-the-pants style of development. The other way to operate is to always make everything too big so that nothing ever breaks. In this way, of course, your structure will always be too heavy.

The real case is generally somewhere in the middle. The typical non-engineered structure will have many structural members which are too large for the actual environmental loads, and a few that are too small. By understanding the loads that are imposed on the boat hull, the manner in which those loads become stresses in composite structures, as well as something about the materials science of composites, one can design the whole vessel so that every part of the boat has the same strength. In this way the vessel can be optimized to have the least weight/greatest strength and/or highest stiffness/best fatigue resistance, and lowest cost for a particular choice of composite materials.

 Monocoque design

The whole of the NAVA 38 structure combines to contribute strength and resistance to operating loads. This engineering method is known as Monocoque design or unitized structure. This means all major components of the structure carry a portion of the overall load. Components individually may seem light by conventional standards but the sum of these elements combine to achieve full strength, allowing operating forces to be absorbed and dissipated throughout the structure. Parts like floors, walls and cabinets all contribute to the overall structural integrity. With earlier designs these parts would just go along for the ride causing redundancy and extra weight in the structure.

 The bottom line

Modern materials and methods allow the engineer to build stiffer, stronger structures that are much lighter than conventional methods of boat construction. An engineered boat optimizes strength while lowering weight. This provides greater speed, or lower fuel consumption as well as lowers manufacturing costs due to a reduction of consumed materials and reduced labor to install them.