The True Story Behind Weight and Vessel Strength

The truth is, that weight in a vessel only guarantees two things: reduced fuel economy and assurance that if buoyancy is lost, she will go to the bottom faster!

So, why do people associate weight of a trawler, or other vessel type, with strength? Because back in the “early days”, the market consisted predominantly of shallow, flat-bottom coastal craft with insufficient internal volume for the fuel and supplies needed for long-range voyaging. These craft had very low D/Ls (links back to FD pillar page Spelling-out the Full Displacement Hull Form). To help separate the wheat from the chaff, Captain Robert Beebe published very useful minimum values of D/L, below which the vessel was said to have insufficient “heft” or carrying capacity for long-range voyaging. Therefore, D/L was used to verify that a vessel had sufficient heft instead of being used to rate hull efficiency. Heft was rightfully considered to be a good thing in that context. And, back then, since most boats’ D/L ratios were way under the minimum for long range, the higher the D/L the better. Beebe and other experts agree that the minimum D/L should be around 260. With many modern passagemakers in the 350+ range, that makes those in the 260-310 range seem “light” and some builders’ marketing guys have tried to capitalize on this incorrect assessment.

So, how exactly can one build a boat that is lighter than others, yet one that is stronger and more fuel efficient? How do you build a boat that is seakindly and seaworthy, capable and livable?

For starters, begin with a clean slate. The builder of a cruising boat will want the naval architect to create a hull that flows optimally through the water, dissipates rather than absorbs, reacts to the effects of sea conditions, and provides a comfortable and safe ride in the conditions for which she is designed.

Designing a new boat is not done piecemeal and many decisions and measurements affect multiple characteristics. Sometimes builders will create a new model just by lengthening a mold (Links to stretch a boat post). While a less costly solution, it abandons the architectural integrity of the original design. Like any formula, if you change some of the variables without changing others, you will get a different result. This is inescapable.

Aramid laminate at transom edgeTo provide strength, stem-to-stern fiberglass girders are closely spaced to ensure distribution of hull loads to the bulkheads and encapsulated mahogany girder inserts allow equipment to be attached for maximum strength and reliability. Lead ballast can be cast to conform to the keel contour and concentrated low in the hull, so less ballast material is required and weight is reduced.

Some builders will use iron, which is less expensive, but it is also less dense than lead. For the same weight, iron takes up 44 percent more space than lead. In a keep cavity of given volume, iron ballast won’t achieve the same righting characteristics as lead.

A builder can also make use of cored laminates in the deckhouse and topsides to reduce weight, resulting in a lower center of gravity. A side benefit of the coring is thermal insulation, making these vessels more comfortable in cooler weather. They are also easier to cool in warmer weather.

Cut out - stern thurster - showing aramid fiberAnother way to provide strength without just making the hull thicker is to reinforce it with an aramid fiber. Kadey-Krogen, for example, uses Twaron, which is the same fiber used to give body armor its bullet-proof capability. Twaron is five times stronger than steel and up to 60 percent lighter than ballistic steel—yet another example of weight no being related to strength.

In addition to the safety benefits, these construction techniques can result in a more fuel-efficient vessel—one with up to 40 percent better fuel economy than a more heavily built trawler. In real world numbers, that’s 900 versus 1,260 gallons on a 3,000-nautical mile passage.

The next time someone tries to tell you that heavier is stronger, give them this basic lesson on boat construction!

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