Professional BoatBuilder - February-March 2018

FEBRUARY/MARCH 2018 11

concave relative to the plane of symmetry of the hull when

viewed horizontally parallel with the plane of symmetry of

the hull, and circular convex relative to the plane of sym-

metry when viewed vertically parallel with the plane of

symmetry of the hull, with the hull outer surface area extend-

ing in a vertical sense from a lower area extending in a direc-

tion that tangentially intersects the plane of symmetry or

optionally a plane extending parallel with the plane of sym-

metry on the same side of the plane of symmetry as the dual

circular curved outer surface area and upwardly in a direc-

tion tangentially intersecting the waterline plane of the hull

or a plane extending parallel to the waterline plane.”

In simpler terms, the ÖK Hull features a narrow keel at

the center of the running surface, perhaps half the length

of the waterline. Its concave sections transition into the hull

with a series of ever-

wider convex shapes.

Kris tinsson says he was

inspired by the work of

Fredrik Ljungström and

his “arc-of-circle radius”

hullform.

Asked to amplify,

Rafnar’s Thorsteinn

Sig urbjörnsson says,

“ e ÖK Hull is scal-

able and is therefore

not limited to any par-

ticular set of dimen-

sions. The ÖK Hull

design is based on a

particular formula that takes into account the weight and

dimensions required and scales the hull for that particular

utility.  e ÖK Hulls for the boat series that we produce

now, Flengur 850 and Lei ur 1100, were designed speci -

cally for those models.  e Lei ur 1100’s length overall is

11m and beam 3.2m [36.1' and 10.5' ].  e Flengur’s overall

length is 8.5m [28' ], whereas its beam is 3m [9.8'].  e Flen-

gur provides a larger stable platform, making the boat an

ideal workhorse, whereas Lei ur’s dimensions are better

utilized for speed and precision handling, with a narrower,

leaner body and a sharper keel, which allows for tighter grip

in turns. We have designed and built 12m and 15m [39' and

49 '] vessels on the ÖK Hull, and in addition have designs

for catamarans as well as 24m and 38m [79' and 125']

yachts.

“ e ÖK Hull is in essence a displacement hull with

planing hull characteristics, despite the boat never actu-

ally planing unless trimmed to do so. Instead, the water that

hits the bow is forced along the bottom of the hull, where

the keel grabs it. At the keel, a suction, or vacuum, e ect is

created, which ensures that the keel stays in the water at all

times and that the hull does not get airborne and slam down

again. A combination of almost planing and creating water

resistance allows the ÖK Hull to reach high speeds,

remaining steady in the water.”

Claimed advantages are: reduced slamming in waves;

superior handling without sliding in turns; “no wake behind

vessel,” thereby reducing drag; and excellent stability. Web-

site videos tend to support the claims, showing the boat

staying level in boisterous seas, plowing through waves

rather than rising and falling.  e University of Iceland con-

ducted tests in which the ÖK Hull was compared to a deep-

V hullform; the measurement employed was the number of

hazardous conditions at the bow based on ISO 2631-1:

for Whole Body Vibrations (WBV).  e measuring device

was a Dyena Acceleration Recorder.  e result: just six

instances for the ÖK Hull compared to 112 for the deep-V

cra , prompting Rafnar to claim “20x less slamming than

competing hulls.”

Rafnar is producing two recreational boats in its 60,256-

sq- (5,600m^2 ) facility: the 28' Flengur 850 Tender RIB with

Hypalon tubes, and the Lei ur 1100 Tender, also a RIB.

Both hulls are recon gured as workboats for multiple ser-

vices, such as search and rescue, coast guard, law enforce-

ment, and patrol. Twin outboards with up to 800 hp (

kW) for the larger vessel, and an inboard engine are o ered.

Concepts have been developed for superyachts and fer-

ries as well. There have been some lively exchanges on

boatdesign.net regarding the ÖK Hull—some writers are

impressed by the studies, and others are skeptical of all

claims, with a few saying it looks like a wet ride.

Rafnar ehf, Vesturvör 32b, 200 Kópavogur, IS, tel. +

525 2320, website rafnar.is. —Dan Spurr

RAFNAR

This rendering of an upside-down

ÖK Hull offers a clear view of the keel

and running surface.

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