Spelling-out the Full Displacement Hull Form 

If cracking and decoding unfamiliar boat terms happened like a student desperately fishing for answers during a spelling bee, a follow-up question to hearing “full displacement hull form” might be, “The term’s usage in a sentence, please?!” To which a correct answer would be, “Kadey-Krogen Yachts builds full displacement trawler yachts.”

That seems clear enough, right? Only if you know about the seaworthy trawler brand that is among the oldest boat manufacturers involved in the fuel-efficient, full displacement pleasure craft business, it is. But even so, there is confusion over what “full displacement trawler yacht” really means. Let’s break that phrase down into its parts, beginning at the end, and then working backwards.

Starting with the word “yacht.” It is synonymous with boat, vessel, cruiser, craft…you get the idea…and may suggest big in size and exceptional quality. That one was easy. Now take “trawler.” Since we covered that term in our LIVING ON A BOAT: A Boat Buyer's Guide to Choosing the Right Boat for the Right Purpose, let’s just agree that a “trawler” is a vessel with which you can live and travel for an extended period and perhaps has the look reminiscent of a commercial fishing vessel. This leaves the words “full displacement.” Not all full displacement vessels are the same—that would be like saying all snow is the same. Yes, all snow is cold, falls from the sky and is white, but after those characteristics, snow begins to be defined by measurable parameters that in layman terms are words like wet, dry, heavy and fluffy. After we get past common full displacement characteristics such as fuel efficient yacht, ocean-capable boat, and slower, we can differentiate between full displacement hulls based upon two form fundamentals: Longitudinal symmetry and displacement-to-length ratio (D/L).

  1. Longitudinal symmetry refers to the degree to which the stern shape matches the bow shape. Consider a barge. A barge is highly symmetrical and seaworthy but does not have ideal longitudinal symmetry and therefore is not very seakindly. A straightforward Kadey-Krogen description of ideal longitudinal symmetry for a liveaboard trawler yacht, is a symmetrical hull form with V-shaped aft sections and sharp waterlines at both ends. In contrast, some trawlers have flatter aft sections.

hullform-symmetry-2-1Why the V-shaped aft sections and sharp waterlines at both ends?

  • The fine entry has superior wave cutting ability, making it more efficient compared to blunt, stout-looking forms. The fine entry also yields a softer ride, which means less pounding in head seas.
  • Symmetrical forms track better in a following sea. The V-shaped sections aft slice following seas rather than surfing them, making for a safer and more comfortable ride.
  • The aft V-shaped sections also offer less resistance and drag than the broad waterlines of asymmetrical hulls. This results in better fuel economy.
  • Symmetrical forms roll less. An asymmetrical hull with an immersed transom and/or relatively flat aft sections will tend to roll more than a symmetrical hull in a following or beam sea due to simple physics. The leverage (upward force) that wave action has on those flatter sections is greater than on a hull that is more rounded and/or tapered. The upward force on one side creates a downward force on the other side and voila, you have roll—and the cause of that vessel to veer off course (yaw).
  1. 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 must carry (displacement) on a per-foot of waterline basis. Lower ratios tend towards lean forms with fine ends, like Kadey-Krogens, and higher ratios tend towards full-bodied and less efficient forms with blunter ends.

Kadey-Krogen Design: Low D/L ratios 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 allow for higher displacement speeds and streamlined sections (less hull drag) result in a hull more easily driven through the speed range. A more easily driven hull form thereby improves fuel economy and allows for the use of smaller engines. The improved fuel economy reduces the weight of fuel that must be carried for the desired range of cruising (it is 3,000 nautical miles to cross the Atlantic and Pacific oceans and other bodies of water), and the use of lower horsepower engines reduces the weight of propulsion machinery. The weight reduction 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. 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. In addition, Kadey-Krogen hull laminations include an aramid fiber like those used in bulletproof vests. Solid, baby! So, Kadey-Krogen yachts are strong, yet lighter in weight. This is accomplished by using proven structural design techniques and conventional lightweight materials. It is important to understand that low D/L does not mean lightweight, or a less substantial structure, or a long and narrow form.

Head to our next blog post: Why then, do some people still stand by the idea of associating weight with strength? 

Naval architect James S. Krogen said, “To produce a successful design, you must make an honest determination of how the vessel ‘really’ will be used and then prioritize every design decision to favor that outcome.” In the case of a Kadey-Krogen full displacement trawler yacht, that outcome is a seakindly, live-aboard, ocean-crossing yacht.


To receive notification of blog updates, subscribe here!