True To Form 

You canít choose a hull form until you know what the design is capable of doing.

By Kurt Krogen

There are basically three types of hulls to consider in your next boat: A displacement hull goes through the water, so its speed is limited by its waterline length. A planing hull travels more or less on top of the water, so its speed is limited primarily by the available horsepower. A semidisplacement hull is a compromise designed to combine the displacementís stability and seakeeping with the higher speed of the planing form. Deciding which is best for you involves as much an examination of what you plan to do with your boat as what the hull form is capable of doing.

Displacement Hulls
Displacement hulls come in many shapes, some with round bilges, hard chines, or a combination of the two. What nearly all share is the after third of their underbody, which typically resembles the underbody of a traditional deep-draft sailboat. It has a long, usually ballasted keel for directional stability at slow speed and to protect the running gear. This form is usually at least partially round-bilged for better efficiency and a gentler roll, and typically features a transom well out of the water for better directional stability in a following sea.
    However, not all displacement hulls look like this. Take the Krogen Manatee 36, a "house trawler" no longer in production.

 




 

The Krogen 42 (above) is a full-displacement hull form designed for blue-water passagemaking. Note the fine forefoot for good wave parting and the rounded midships, which provide good roll resistance. In comparison, the Express 49 (below) has a hard chine running from the transom to about the middle third of the underbody. This gives it its semidisplacement characteristics and speeds to 20 knots.

 





Although planing hulls are designed for high speed,shape can determine how well or poorly they perform.

The goal here was a spacious coastal liveaboard with the emphasis on function. Thus her displacement hull form has hard chines to add stability underway and concave forward sections below the waterline that let her slice through the water rather than push a large bow wave. The Manatee also has a generous beam carried well forward of amidships, creating a bluff (blunt) bow. The design works, but despite her efficiency and stability in light to moderate seas, she was never intended as an offshore vessel.
    Compare her to the Krogen 42, a long-range, offshore, pilothouse trawler yacht displacing nearly 40,000 pounds. My father, James S. Krogen,

 


Although a displacement form, the Manatee's hull features hard chines for additional stability and concave forward sections that let her cut through waves instead of push them, producing a smoother ride.

 

defines the lower transom. By redistributing underwater volume, my father gave this shape greater roll resistance than many full displacement forms, plus a wider waterline beam, which produces good positive stability. The result is a seakindly, stable, and efficient hull that only requires 60 hp to push her at more than  8 knots Ė in spite of her 17.6-ton displacement.
    But while a displacement hull typically has a greater range of positive stability (the ability to recover from a roll), provides more interior volume so the boat can carry considerably more gear, and is usually more fuel-efficient at lower speeds, its hull speed is limited to 1.34 times the square root of the waterline length (about 8.4 knots on the Krogen 42). Those

 

gave her a round-bilged, full-displacement hull and a fine forefoot (bow section), yet she is rounded midships. Her after sections are out  of the water, coming to a point that

 

who want more speed but not more length must consider a different hull form. 

Semidisplacement hulls
A few years ago, in response to buyers who wanted more speed, we searched my late fatherís design portfolio, where we found an intriguing 65-foot commuter.  She features a narrow beam and a very fine, deep-V bow section, transitioning into a shallower V midships and a very shallow V aft. Yet with round-bilged sections the length of the underbody and lightweight construction, the design promises good efficiency.
    While the 65 never went into production, it did lead to the Krogen 49-foot Express pilothouse. She was based on the 65 but with a hard chine aft, which runs forward from the transom and diminishes to form the round bilge section in the middle third of the underbody. A deep-V forefoot with bell-shaped sections moving aft enhances wave penetration, and a keel extends just below the running gear. A chine from her

 

plumb stern to the midship waterline adds no lift but deflects spray.
    This form has proven exceptionally dry and smooth-riding. It is easily driven at a 17-knot cruise. Another advantage, common to semidisplacement hulls, is increased transverse stability due to hull dynamics above displacement speed. Itís also economical, maneuverable, and sure-footed at displacement speed, but nowhere near so as the full-displacement 42. And therein lies the trade-off between displacement and semidisplacement. Neither the 65 nor the 49 were designed to match the blue-water capabilities of their full-displacement cousins. 

Planing hulls
Although planing hulls are designed primarily for high-speed operation, they can operate in displacement and semidisplacement modes (shape can determine how well or poorly they  perform). Regardless, thereís usually a penalty in efficiency and

 

seakindliness in the lower speed ranges.
    As you can see from these examples, there really are an infinite number of hull forms Ė not just three. To decide which is right for you, you must examine your needs and desires and realistically consider the kind of cruising you will be doing. Also note that while hull form is important, the complete design (including but not limited to space arrangement, construction details, and the selection and the placement of machinery and tankage) has considerable impact on a yachtís performance, regardless of her bottom shape. That done, it will be much easier for you, your designer, and ultimately your builder to create a hull that is appropriate, properly designed, adequately engineered, and in all ways, true to form.
    Kurt M. Krogen is a lifelong boater and President of Kadey Krogen Yachts and Krogen Express Yacht Company.




Reprinted with permission from Power and Motoryacht magazine.