Hull Form Displacement

The displacement-to-length ratio (D/L) indicates whether a given displacement is carried over a long waterline length or a short one. It reflects the load the vessel has to carry on a per-foot of waterline basis. Lower ratios tend towards lean and slippery forms with fine ends, and higher ratios tend towards full-bodied and less efficient forms with blunter ends. The lighter the load per foot of waterline length the better the economy and hence the better the range will be.

Low D/Ls result from either long waterlines or streamlined underbodies, or a mix of each. Both characteristics serve to improve hull efficiency and therefore fuel economy. Longer waterlines permit higher displacement speeds and streamlined sections result in a hull more easily driven through the speed range. For example, if a given displacement is stretched over a longer waterline, two things happen: The LWL increases (higher hull speed) and the ends get finer and sectional areas less full, i.e. streamlined (less hull drag). Of course, reducing a vessel’s fully loaded weight (displacement) will lower D/L and improve economy but the fully loaded displacement requirement is usually already more or less locked in by the voyaging requirements (living quarters, fuel supply, provisions, etc.).

This long waterline low D/L approach (those in the 260-325 range) can be compared to a bank account earning compounded interest. Reducing D/L yields a more easily driven hull form thereby improving fuel economy and allowing for the use of smaller engines. The improved economy reduces the weight of fuel that must be carried for the desired range and the use of lower horsepower engines will reduce the weight of propulsion machinery. All this weight reduction now results in even greater fuel economy resulting in more weight savings resulting in even better economy, and so the benefits compound.

You may be asking, what about strength? Isn’t strength compromised by this weight savings approach? Not in a Kadey-Krogen, which one could argue is actually stronger. Why? Because while the boat is solid fiberglass below the waterline, the coring material is vacuum bagged in place everywhere above the waterline. Utilizing core materials and the vacuum bagging process creates a stronger and lighter weight result.

So Kadey-Krogen yachts are strong, yet lighter in weight. This is accomplished by using proven structural design techniques and conventional lightweight materials.

Interestingly, D/L has had an opposite and incorrect application in the evaluation of long-range trawlers, and many inaccurate conclusions can be made using this ratio. It is important to understand that low D/L does not mean lightweight, or a less substantial structure, or a long and narrow form. How did these misconceptions come to be?

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. 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 260-325 range seem “light” and some builders’ marketing guys have tried to capitalize on this incorrect assessment.