Dynamics of Hull Stress

Dynamics of Hull Stress Power boat hulls are essentially modified rectangles with a shallow vee on the bottom. When a boat falls off, or slams down off a wave, the bottom impacts the water and suddenly stops its downward movement. This sudden stop sends shock waves up the hull sides that are then transmitted to the deck and any upper structures that may exist. In the meantime, while the hull suddenly stops its downward movement, everything inside the hull wants to continue on downward, creating even more stress.

When the hull impacts the water, the resultant stresses work to cause the hull to want to buckle transversely and longitudinally. The impact with the water is never uniform along the length of the hull so that one end, or one side, of the hull is more stressed than the other. One effect is to try to break the boat in half like snapping a stick in half. The other effect is to bow the hull sides inward or outward, the effect of bending along the horizontal plane. Yet another is twisting or torsional stress along the entire length of the hull.

In actual operation under heavy conditions, the hull sides of most boats will deflect to greater or lesser degrees depending on how well it is designed. This is the result of impact loading, bending and torsional loading on the hull caused by high velocity over waves, porpoising and so on. If you've ever wondered why so many boats have rub rails falling off and weak and damaged hull/deck joints, you probably thought that this was primarily due to hitting up against dock pilings. But the real reason is that many boats have poorly designed hull/deck joints that are simply lap joints screwed together. It is the stress transferred from the hull bottom to the hull sides and thence to hull/deck join that causes the screws that join these parts together to break loose. Putting screws into fiberglass is a terrible means of making connections. Screw joins are simply too weak to work effectively.

So it is that the deck - and the superstructure that is often integral with the deck, i.e., are molded as one piece - are not only part of a unified structure, but also absorb much of the load initially induced on the hull. This also accounts for much of the damage and cracking found in and around deck structures, and why on many boats windows, doors and hatches and portholes just never seem to stop leaking. The whole structure is working so that no amount of caulking, bedding and gasketing can ever stop the leaks because they just open up again

These are the effects of stress on the exterior boat hull and structure. But the stress doesn't end there for we've not yet considered the hull framing system. The framing system consists of stringers, bulkheads and frames in more conventional construction. Yet increasingly builders are seeking to reduce costs and streamline production by eliminating much of the detail work involved in the framing system. They are doing this by again utilizing the principle of monocoque construction which takes the form of premolded "liners" or so-called 'grid liners," a premolded combination internal framing system and accommodation components. And rather than bonding these parts together with conventional tabbing or taping, instead they are being glued together with some sort of adhesive putty.

Although the use of liners has been around for a long time, the combining of a framing system with a liner is new. And as any experienced surveyor can see, it poses some obvious problems, but that's a subject I'll deal with in Part II. In the meantime, the conventional stringer, bulkhead and frame system is the method used by about 98% of all boats over 30 feet.

Stringers In power boats, stringers provide the majority of the longitudinal hull resistance to bending in the vertical plane. The apex of the vee at the bottom or keel adds additional strenght. This is qualified by whether the deck is also designed to give the hull longitudinal rigidity. Depending on design, some decks, particularly on motor yachts with very short decks and lots of windows, are so small as to add very little additional strength. On the other hand, the typical flybridge sport fisherman with its long foredeck, relatively small windows and strong house sides, adds a great deal of rigidity to a hull. So it is that we can now understand why there is a lot more to the strength of hull than just the framing system. In monocoque, or semi-monocoque construction, the whole structure must be considered. And it is precisely here that so many untrained "designers" who lack a solid background in engineering, make their mistakes.

Mistakes involving stringer design and installation are legion, about which a whole book could be written. And yet the principles for creating an effective stringer system are very simple and easy to achieve. Surely there are not many designers or builders who do not understand this. Or are there? Problems usually arise as a result of other design and marketing considerations. Typical examples are when a designer wants to create a small boat with 6'6" headroom or wants to install unusually large engines. The machinery spaces, which are not subject to appearance and marketing considerations, are usually sacrificed.

In order to get the 6'6" head room or make high profile engines or other equipment fit, the principles of proper stringer design are often sacrificed. In other words, the principles of sound hull design get sacrificed for marketing considerations and the surveyor needs to be constantly aware of this fact. Its the primary reason why, in this day when all is known how to build a good boat, bad boats are still being built. Give the customer what he wants, even if the product is going to fall apart.

The principles of good stringer design are simple. They must run uninterrupted from one end of the hull to the other. They must be of adequate height to width ratio, i.e., structural modulus, to resist impact loading on the hull skin, be of sufficient strength to carry the engine load, be stabilized against lateral movement if high profile, and be securely attached to the hull so that they don't break loose. The profile, or top of the stringer, should run in a straight line. If there are any changes in the profile, then special design reinforcements must be added.