DESIGN PROBE
AN ANALYSIS OF HULL FORMS
AND OTHER DESIGN ELEMENTS

BY KEN HANKINSON, N.A.
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Highlighting the differences between a displacement and a
semi-displacement hull
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    The design premise of both the Fleming 53 and Krogen 54 is to offer long-range, offshore cruising capabilities in a seakindly, seaworthy vessel.  Considering also that the boats have similar lengths and displacements, one might think their hull forms would be similar as well.  In certain respects, they are; but the aft sections of their underbodies show some distinct differences that markedly affect performance and fuel efficiency.
    Comparing each boat’s body plan, the
forefoots are much the same.  Although the Krogen 54 is round-bilged and the Fleming 53 hard-chined, the chine on the Fleming 53 fades out and actually

 

disappears from about Station Two forward to the stem.  Both boats show a fairly deep and fine forefoot with generous flare in the topsides.  While such a shape may be inappropriate on a faster planing vessel, for slower vessels like these that usually operate at or just above displacement speeds, this hull form assures directional control, a soft ride, and ample reserve buoyancy to keep water off the decks in high seas.
    What distinguishes the two designs is the form of the after two-thirds of the underbody.  On the Krogen 54, naval architect James Krogen opted for a pure, full-displacement hull – double-ended with round bilges, it

 

resembles a motorsailor and can be economically driven.  With this hull type, the tradeoff is a limit on hull speed equal to a factor of 1.34 multiplied by the square root of the boat’s waterline length (or about 9.3 knots, although Krogen claims the boat can reach a top speed of 10.5 knots and a cruise speed of nine knots with the single Lehman 225.)
    Conversely, naval architect Larry Drake (long-time southern California boatmen may recall the numerous wood/ply Drake Craft boats he produced during the 1950s) selected a hard-chined, semi-displacement hull for the Fleming 53.  This shape allows speeds beyond the displacement range to about 13 knots, according to the literature, with a cruise speed of 11 knots.  Although these numbers represent a 30-percent faster speed than the Krogen can achieve, they require virtually double the horsepower, supplied by pair of 210-hp, naturally-aspirated Caterpillar 3208s.

Fleming 53
 

    A pair of 375-hp 3208TA Cats is optional, but would be overkill in my opinion because any appreciable gain in speed is unlikely.  (The company estimates an increase in cruising speed to 14 knots, and a top speed of 17 knots.)
    Readers may wonder how a body plan indicates a boat’s speed and performance capabilities, apart from showing that one design is round-bilged and the other of chined form (with a distinct break at the bottom/side junction).  Basically, boat speed capability depends much on the distribution of underbody volume, especially how fine or full the boat is in the ends, and the overall shape of the bottom.  Looking at the body plans, in particular the after underbodies depicted to the left of the centerline, note that sections of the Krogen 54 have more space between them than on the Fleming 53.  Also, the Krogen’s aft sections are more similar in their angle to the horizon (deadrise, dihedral, and rise of floor are all descriptive terms for this angle.)
    These elements indicate more lengthwise curve in the hull shape.  In other words, when viewing the hull from the side or profile, the longitudinal lined called buttocks swoop up at the stern and terminate above the waterline.  Considering also that the last station aft (Station 10 here) is out of the water, we can deduce that the hull is double-ended (at least below the waterline) with volume concentrated more amidships.  Hence the boat is most surely of displacement form – speed will be limited per the formula noted before, but such a hull is easily driven and probably seakindly.

On the other hand, bottom sections on the Fleming 53 are much closer together and their dihedral angles vary, decreasing aft toward Station 10, indicating a flatter bottom

  Krogen 54
 

here.  In profile, buttock lines would be straighter, especially aft, and end below the waterline, indicating a transom stern.  These characteristics, along with the fact that Station 10 is well immersed, indicate greater hull volume aft, a necessary component if speeds beyond the displacement range are intended.
    Can one tell how much more speed will result from such a shape?  This is a complex issue since the differences between semi-displacement and true planing hulls are not as clear-cut.  Much has to do with the rise of certain buttock angles in the aft run of the hull lines.  The body plans of true planing boats show sections with little or no space between them and well-immersed transoms.  Space precludes going into much detail here.

    One indicator that high speed is not feasible is the presence of a deep skeg on both boats.  This feature gives directional control and protects underwater gear, but can add considerable drag.  In short, we can pretty much assume that neither boat will reach true planing speeds, defined as three times
the square root of the waterline length.
   

 

    Although the Fleming 53    can also operate at displacement speeds (seven to eight knots), the Krogen 54 won’t be penalized as much by the added drag of an immersed transom.  What does the penalty amount to?
    The Fleming 53 literature notes that extra tanks are available amounting to a 1600-gallon fuel capacity.  When underway using a single engine, the range at 7 ½ knots is nearly 2900 miles at 90 percent capacity.  By comparison, on the Krogen 54, 1100 gallons (allowing for a 100-gallon reserve) at the same speed gives nearly a 3000-mile range.  The Krogen’s 33-percent lower fuel consumption is an important difference in costs for anyone considering an ocean crossing or extended voyage.

    An argument in favor of a chined semi-displacement hull, though, is a gain in transverse stability due to hull dynamics that develop once displacement speeds are exceeded.  When operating at lower speeds, though, either due to sea conditions
or to

to increase range, the chined form may have a quicker, less-comfortable motion compared to its round-bilged brethren like the Krogen 54.  In addition, the double-masted steadying sail rig on the Krogen 54 will dampen much of this motion.  Nevertheless, for open-ocean distance cruising, I’d prefer active stabilizers on either of these boats (see The Western Boatman, “Stabilizers,” June 1989).
    As for layouts, there are some inevitable compromises, just as with the type of hull used.  Although each vessel has a
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Although both boats are intended for long-range voyaging, they differ significantly in the design of the aft two-thirds of the underbody.
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pilothouse with flying bridge, the Krogen 54 offers a better view aft.  On the other hand, the Fleming 53 offers more features such as a larger sundeck with more seating.  Furthermore, the Fleming 53 includes full walkaround decks and protective dodgers, including a “Portuguese bridge” forward, while the Krogen 54 forsakes passage to port for interior deckhouse space due to the saloon being offset to this side.
    Typical on Taiwan-built boats, the builders offer plenty of interior layout options.  Although not listed for the Fleming 53, I’d prefer a layout option that locates the owner’s stateroom amidships (as on the Krogen 54) rather than in bow (the stock layout), which probably would not be as comfortable for sleeping while underway on an overnight passage.  The guest stateroom to port is large enough for a double berth, but the companionway prevents this stateroom from being enlarged.  I’d also like to see a single engine option as I’ve recommended for other boats discussed here in the past.

    Experienced readers who have been boating for awhile may find the Fleming 53 familiar. While basically the same vessel as Falmouth Yachts’ Fleming 50 (with three feet added to the cockpit), more notably the boat was inspired by the old Alaskan 49,

 

designed years ago by Newport Beach naval architect Robert Dorris for American Marine.  Falmouth Yachts principals Tony Fleming and Anton Emmerton were once associated with American Marine and didn’t want to see the concept die with the demise of the Alaskan.  They have made some slight alterations, the most obvious being the flying bridge, which the Fleming’s predecessor didn’t have.
    Some construction differences between the Fleming and Krogen are worth mentioning.  The Fleming 53 uses thick, conventional, hand-laid fiberglass in its hull, reinforced by hat-section longitudinal and transverse stiffeners.  Such construction, though perhaps low-tech, is proven, reliable, durable, and virtually idiot-proof from a builder’s viewpoint.

    On the Krogen 54, construction methods are roughly similar, but sandwich principles are used, including rigid-elastic Airex PVC core in the hull, and rigid Divinycell PVC in decks and superstructure.  Some traditionalists are skeptical of core construction, but when properly done, it increases impactresistance, prevents condensation, adds
buoyancy,

 

and insulates against heat, cold and noise.  Krogen has considerable experience with the system and the fact that they stick with it indicates success and acceptance.
    I was impressed with the attention to sound and vibration control taken on the Fleming 53.  The use of Aquadrive constant-velocity joints between the shafts and engines allows soft mounting of engines, virtually eliminating concern about proper alignment, while also reducing vibration and noise.  In addition to lead-lined, foam insulation, the engine room also has hatch covers that dog down against neoprene gaskets.  Such details are not cheap, but add considerably to cruising comfort and peace of mind.
    If having a vessel with a soothing motion at sea and the range of seven-league boots is essential, the Krogen 54 easily satisfies such demands.  Yet some cruising yachtsmen with less time on their hands may feel the cost savings in fuel is not worth the time lost at a slower cruising speed.  If that’s the case, the Fleming becomes the more attractive choice.

 
Reprinted with permission from Western Boatman magazine.