True to Form

by Kurt M. Krogen                                                                                        Unedited Version

A Perspective on Full and Semi-Displacement Hulls
Published in Power & Motoryacht-July

In the trawler and motoryacht markets the question of hull forms seems to be at the forefront in many buyers minds. With our related companies’ involvement in the design and production of displacement yachts, we hope to provide some objective insight in to these hull forms and their distinct characteristics and limitations.

In the full displacement hull category we find some are round bilged, some are hard chines and others with a combination of these forms. The common ingredient, however, can usually be found in the after third of the underbody. Typically the full displacement hull’s underbody will resemble that of a traditional deep draft sailboat with a long ballasted keel, a round bilge and with the transom lifted well out of the water.

One interesting example of a displacement form of the hard chined variety can be found on the Manatee 36’, a popular house trawler designed in the Old Dutch tradition. Her beam is carried well forward to a bluff bow, however, with her concave sections forward she tends to slice rather than push an excessive bow wave. Hard chines add stability underway and her keel keeps her on track while protecting the rudder and propeller.

The intent of this design was to offer a spacious and economical coastal live-aboard yacht with more emphasis on function than from. While not intended

 

as an offshore vessel, her bottom shape serves her well, offering efficient operation and stability in light to moderate sea conditions. Although she proved quite seakindly, with her lack of ballast and full bodied superstructure, the Manatee 36’ was never intended to match the offshore capabilities of her big sister, the Krogen 42’.

The brain child of the late Arthur Kadey, the Krogen 42’ was designed as a long-range offshore pilothouse trawler yacht, displacing nearly 40,000 lbs. My father, James S. Krogen, utilized a round bilged full displacement hull form to achieve the blue water passagemaker capability, which Kadey sought.

Looking at the body plan (the cross sectional view at various stations) you may note she has a rather fine forefoot or bow section and is rounded at midships. Her buttock’s lift out of the water going aft, coming to a point which defines the lower transom. My father was quick to note that not all round bilged displacement yachts are created equal and emphasized the importance of form stability. Upon closer inspection of the body plan one can see that while her shape is rounded just below the waterline, you will find she boasts relatively flat sections as you come around the turn of the bilge towards the keel.

This shape offers greater resistance to roll as opposed to some other semi-circular or fully rounded displacement types as it redistributes underwater volume creating a wider waterline beam, which serves to enhance initial stability as opposed to relying on the increased ballast to achieve a comparable range of positive stability. The result of this design is a seakindly, stable and efficient hull form that only requires 60 hp from her standard

 

Manatee 36': Manatee proved to be a livable and lovable craft.
Her unique hull form helped make it possible.

135hp diesel to push her at over 8 knots in spite of her 17.6 ton displacement.

While full displacement hulls typically have a greater range of positive stability (the ability to recover from a roll), can carry considerably more gear and are usually more efficient at lower speeds than their semi-displacement cousins, they are limited in hull speed which for most displacement hulls is a factor of 1.34 multiplied by the square root of the waterline length (or about 8.4 knots on the Krogen 42). Although most full displacement vessels can be pushed a bit beyond this theoretical hull speed, it usually comes at a significant increase in fuel consumption as the higher speeds cause the wave length to increase so that the after-body drops into a hollow and the stern begins to squat. Added power at this point is wasted, as it will only create a bigger trough for the after body to settle into. So for those who want more speed, but not necessarily a longer L.O.A, it may be time to consider a semi-displacement hull form.

A case for semi-displacement
One analogy likens a full displacement hull to a swimmer, and a planning hull to a water skier. At the end of a towline at slower speeds a swimmer fairs better, at higher speeds the

swimmer wallows unmercifully. The semi planning hull usually has characteristics of both and can run fairly  efficiently at lower speeds as well as at the higher speeds. Depending on the design objective the semi displacement hull can favor either end of the spectrum. How much the fore and aft lines are rockered is a significant determining factor. An important characteristic in favor of the semi planner is the transverse stability that develops, due to hull dynamics, when moved beyond displacement speeds. While a properly designed semi displacement hull can obtain the favorable characteristics of both forms, the best of both worlds if you will, a poorly designed semi planner can have the opposite effect.

As builders of full displacement trawlers for many years, Kadey Krogen recognized that a fair percentage of buyers, although appreciative of the attributes of displacement hulls, due to their cruising needs (usually time constraints) had a desire for additional speed and did not want to be confined to the limiting factors of full displacement. In the early stages of the development of the Krogen 48 trawler yacht, we were persuaded to seriously consider a semi displacement hull form. However, based on the results of our survey of nearly 400 owners (admittedly a slightly bias

crowd) they overwhelmingly favored full displacement, thus reconfirming our original thinking. Hence, we proceeded to build the Krogen 48’ model, expanding the tradition of the Krogen 42’s successful displacement hull form. We felt, as did our owners, that the compromises made for the additional speed were not warranted in this application. (See Krogen 48’ Trawler displacement versus semi displacement design comparison survey).


Krogen 42': A salty and seakindly vessel with incredible range, thanks to her form stabilized ballasted full displacement hull.

   
 

Krogen 48 Trawler
Displacement vs. Semi Displacement Design Comparison Survey

By James S. Krogen

Let's presume you are in the market for your next boat. Look over the differences and then check the appropriate box below.

Characteristics Full Displacement Semi-Displacement
Displacement 56,000 lbs 50,000 lbs
Max Speed 10 kn 13.5 kn
Ballast 5000 lbs Zero
Range of Positive Stability 100 plus 70 approx
Length/beam/draft same same
Accommodation same same
Single screw yes yes
Fuel 800 gal 800 gal
Cruise speed 9 kn 12 kn
Trim at cruise 5.1 gal/hr 21.1 gal/hr
Horsepower at cruise 90 370
Fuel comp at 9 kn 5.1 gal/hr 7.06 gal/hr
Range at 9 kn 1412 miles 1029 miles
Seaworthiness excellent good
Seakindliness excellent good
Steering in following sea good fair/plus
Roll characteristics equal equal
Ability to carry extra Gear (weight) excellent fair
Wake at cruise small to medium large
Engine noise medium louder
Boat, base cost 100% 108%
1.     I'd only go for a semi-displacement with the 12 knot cruise
2.     No way will I switch - I vote for full displacement
3.     I could accept either on you manufactured but I prefer _____ full _____ semi.
4.     Briefly what are your major objections to the one you didn't choose.

The survey did not, however, deter us in our pursuit to meet the desires of those clients with a need for more speed. Upon reviewing my father’s design portfolio, we came upon a rather intriguing 65’ commuter design dating back to the mid 1960’s. With her narrow beam and light weight construction, a very fine deep V bow section transitioning into a shallower V midships and a very shallow V aft, in addition to round bilged sections for the length of her underbody, she promises to be an economical semi displacement form.

by the shortening process. Most notably is the addition of a hard chine aft which runs forward from the transom and diminishes to form the round bilge sections which occupy the middle third of the underbody. A deep V forefoot with bell shaped sections with  bell shaped sections moving aft was utilized to enhance wave penetration, while her stealthy keel drops just below the running gear to provide needed protection and directional stability. With the addition of a spray chine on her topsides running from her plumb stem to the waterline amidship, while not provided to add dynamic lift, it proved effective in its intended purpose to deflect spray. The overall result of this form is an exceptionally dry, smooth running semi displacement hull that is easily driven up to 20 knots with her standard pair of 350 hp 3116 Caterpillars burning less than 23 gals per hour at her 17 knot cruise. At displacement speed she also proved to be economical, easily maneuvered and sure-footed, but hard pressed to rival the 1.7 gal per hour found on the full displacement 42’.

In summary, while both the 65’ and 49’ Express designs are fine examples of efficient seaworthy semi planning forms they were never intended to match the passagemaking capabilities of the ballasted, full displacement Krogen 42’ and 48’ trawler yachts. It may also be prudent to note that due to the nature of yacht design, these performance characteristics may not be common to all yachts within these categories. Moreover, it should be understood that while good hull form is important, the complete design included, but not limited to arrangement, construction, machinery, tankage in particular placement, can have considerable impact on a yacht’s performance regardless of the bottom shape. So, in your search for your ideal yacht, be sure that not only does the hull have the attributes you seek but that she is properly designed, engineered and true to form.

With some styling modifications influenced by my brother Jimmy, (J. Morrow Krogen N.A.) in particular the addition of a prominent plumb bow, the 65’ inspired the formation of the Krogen Express Yacht Company. While the 65’ never went into production, it helped to offer greater market distinction from the Kadey Krogen heavy displacement trawler line and lead to the successful introduction of the Krogen 49’ Express pilot house model.


Krogen Express 49': A timeless performance cruiser,
bringing the past gracefully into the future.


The Krogen 49’ Express was one of my father’s last of over 700 designs produced before his untimely death in late 1994. A traditional power cruiser, as we prefer to define her, continued the lean and light philosophy of the 65’ but incorporated subtle changes necessitated