logoHome Button  

Strength of Materials

QUESTION: Of what interest (if any) is tensile strength testing of wood or plywood, for use in a boat?

ANSWER:  Any area of a boat where the structure effectively works as a beam, will be subject to tensile loads at surfaces away from (and perpendicular to) the applied load and with compression close by.  That covers most of the boat in various ways.  Even a panel (like a bulkhead or hull side) that is loaded say by a persons’ foot or by water pressure, will be subject to a tensile load on the far side.   It plays a critical role for all the skin panels underwater and also for panels loaded by body weight or foot load.  The latter includes the cockpit floor, cockpit seats and the side skin above the cockpit floor, where foot pressure on the side while sailing at some angle, will apply tensile load to the outside skin.    This sketch gives examples.

It also applies to the top, bottom and sides of main box cross beams, as they can be loaded in various ways ...upwards from ama buoyancy, downwards from crew weight on an ama, or fore and aft due to collision with waves and docks etc.   With any one of these loads, one side of the box will be subjected to a tensile load, but each loading will move this to a different side, so for the design ALL box sides must be designed for a tensile load as well as a compressive one, as when one side of the beam is in tension, the opposite side is in compression, as the neutral axis is somewhere between the two surfaces.  

Deck beams are a special case.  When one treads on a deck, there is a high tensile load on the wood fibres at the underside of the beam.  Because the deck ply has far more X-section area than the beam itself, the neutral axis is typically not far below the underside of the plywood . .. and as that is far from the underside of the supporting beams, the stress at the bottom of the already spaced-out beams can be very high.

This is also somewhat true of side framing and is one reason why I personally prefer rather shallow frames, so that the stress is kept lower and shared more by the total panel.   For example …. supposing you add a frame that's 3" x 3/4" on edge, then the max.stress at the unsupported 3/4" edge will be 3" from the ply and be very high.  But if you lay the 3" x 3/4" flat against the plywood, the stress will be less, and the frame less likely to crack ..., but yes, the panel will flex more, so obviously there must be a limit to panel size.  Sometimes this flex is beneficial to absorb shock, such as on underwater bow panels. For the inside of some of my designs, I actually use flat strips of 2" wide fiberglass laid vertically (or diagonally) to give additional tensile support to the plywood.  This still allows the panel to flex, but with greater resistance to failure, as there's no frame to crack or be overloaded … and, space is saved and maintenance easier.   But few designers share this concept and most traditional ones still use hard framing with multiple stringers.  Strong and rigid, but for me this introduces too many hard spots and areas where a panel can shear - such as at the edge of a hard frame that is attached to a boat's side.

In the case of deck beams, the local load can be quite high (due to toes and knees etc), so although I use conventional beams, I laminate them with relatively weak lightweight cedar near the low-stress neutral axis area but add a 10-15mm hardwood at the lower edge for the high tensile load.  We can further add to tensile resistance, by adding a strip or two of UNI glass or carbon-fiber 'tows' (straight unwoven lengths of fiber) along the underside (see sketch).  This is akin to the heavy steel strap or rod that large barns often have added below their main beams.


Also, the whole body of a boat ... particularly a multihull, works as a beam from one end to the other.   Both waves and rigging plus the position of crew weight, are trying to 'bend the box’ and no doubt succeed to a small non-apparent degree.    So at one moment, the deck is under tension and within seconds, it's the bottom.  Large, sophisticated ocean raceboats are sometimes fitted with strain gauges to measure these fluctuating loads on a prototype, but for small boats, we learn from experience what will work.

So yes, knowing the tensile strength of a material IS important and it’s also worth noting that for most materials (though with some exceptions), materials with higher tensile strength, typically have a higher resistance in other stressed directions.

As important as it is, it’s certainly not the only figure important to analyze material performance.  Each application can favor different aspects .. such as flexibility, manner of fracture, surface durability, water absorption, impact resistance etc, including of course, weight, cost and material availability. 

mike/                                                                                                                                                      February 2018

"New articles, comments and references will be added periodically as new questions are answered and other info comes in relative to this subject, so you're invited to revisit and participate." —webmaster


"See the Copyright Information & Legal Disclaimer page for copyright info and use of ANY part of this text or article"