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KAHUNA
Questions and Answers
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Q. Why is the draft of
Kahuna only 4' 7"? Is the boat tender? At
what angle of heel will
she be at in 15, 25, 35 knots of wind?
I've kept
Kahuna's draft moderate as I've found -- after sailing
many thousands
of miles on a boat with a raft of 6'+ -- that there
were many times
that I wished for less draft.
The
question regarding whether she's
tender, and how
much she will heel, raises
two important issues: ultimate stability
and power to carry sail. Both
power to carry sail and ultimate
stability are determined by the
boat's hull (and for ultimate stability,
cabin) shape and the location
of the center of
gravity. But a boat that is
very stiff
(initially very stable) may not in fact
have the type of stability
curve that's desirable for a cruising
boat. A barge and a
catamaran are good examples of vessels
that derive all of their
stability from their hull form, and are
very stiff initially, but are
lacking in ultimate stability. In
general beamy boats will
show greater initial stability, but
have a smaller range of total
positive righting moment, than an
otherwise equivalent narrower
vessel. Draft is a good way to gain
stability, and a narrow,
deep boat can have a very good
overall stability range (some are
positive through 180
degrees), while being
somewhat
tender under sail. In
addition, too much draft (or more
accurately a very low CG), combined with
a narrow hull, can produce a
very quick motion that is uncomfortable
at sea. In essence one
does a balancing act between all the
factors, striving to deliver a
boat that will be stiff (but not
excessively so), show a healthy range
of positive stability (say 135 degrees
or better), and have reasonable
draft (to give access to a wider range
of cruising grounds).
So to Kahuna: she
has positive stability through about 140
degrees.
I evaluate
stiffness using two methods. Both serve to
compare a design with
others, rather than produce absolute figures of
anticipated heel at
given wind strengths (There are too many simplifying
assumptions for
accurate prediction) The most useful is
probably
the Wind Pressure Coefficient, which
compares righting
moment to heeling moment (righting
arm x disp)/(sail
area x heeling arm), at 20 degrees of heel. The
ratio for Kahuna is
1.32; some 20-30 years ago this would have been
considered quite stiff, while today
this is more in the
normal range. The other method (
Dellenbaugh Angle) produces
a heel angle for the boat assuming a wind of 14
knots The
Dellenbaugh angle for Kahuna is 16.8 degrees, which
is again stiff by
standards of some 20-30 years ago, and fairly
typical today.
As to heel at 25 and 35
knots, that depends on many factors. I would
expect to
reef when the wind builds to around 20 knots, and
to reef again
when the wind exceeds 25-28 (remember that wind pressure
varies as the
square of the wind speed, so a 28 knot wind has about
twice the force
of a 20 knots). At 35 knots you'd be looking
at carrying a
double reefed main and a staysail.
Q. What
is a suitable
marine diesel for Kahuna?
Many engines are suitable.
I've repowered my own boat (35', 15,000 lbs) with a
marinized 21
hp Kubota, with excellent results. Many companies now
offer marine
diesels based on small Kubota engines,
and both initial price
and parts are very reasonable. Otherwise, many other
small (20-25 hp)
marine diesels will work fine (in terms of weight and
space). I'd
suggest discussing engine reliability and serviceability
with a
mechanic you trust.
Q. I am a bit uneasy (
and
will be until the boat actually gets into some rough
weather) of the
comfort of ride and speed. One of the reasons I
want a boat of
this type design is to come through rough weather
well, that is to say
not bounced about like a cork or floundering and on
it's ear becuase
she's too tender. Could you compare Kahuna to any
production boats?
The chart
below lists (in addition to basic dimensions, D/L and
SA/D ratios) the
following ratios for Kahuna:
Comfort ratio:
Developed by
Ted Brewer. Larger numbers indicate a smoother, more
comfortable motion
in a sea way.
Capsize risk:
the lower the better.
Boats with values over 2 are not considered
safe offshore.
Roll acceleration:
Calculated in
units of gravity (Gs), from a formula by C.A.Machaj,
who suggests that
malaise starts at .1 G. Lower is better.
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Boat
& Designer
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LOA
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LWL
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BEAM
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DRAFT
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DISP
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BALLAST
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SAIL
AREA
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SA/DISP
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DISP/LWL
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COMFORT
RATIO
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CAPSIZE
RISK
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ROLL
ACCELERATION
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PERIOD
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Kahuna
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32.33
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28.75
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10.625
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4.5
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15550
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6000
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624
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16.0
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291
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34.2
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1.64
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.044506
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3.8
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In essence no small boat
will be really comfortable in rough weather, but a
relatively heavy,
not too beamy boat like Kahuna will definitely fair
better than
lighter big volume hulls. In bad weather Kahuna
will behave as
boats should: by taking care of the crew. She's really
an attempt to
combine traditional looks and motion with modern
construction methods
and improved performance.
Q.
Kahuna can be built framed, non-framed with epoxy
sheathing, or
coldmoulded overlay with veneers.
Which is
the strongest ? What is the advanage of veneers over
epoxy glass?
Surely laminated frames give more strength than non-
framed method? I
would appreciate some insight in this area.
As designed
Kahuna uses either
1) strip planking w/ veneer overlays,
2) strip planking w/ heavy cloth+epoxy sheathing (inside
and out), or
3) cold molded veneers (no strip planking at all). None
of these have
traditional framing, but all use the bulkheads, locker
dividers etc as
framing (this is done by securing these to the hull with
biaxial tape
and coved epoxy). Another option (but this is not
detailed on the
plans) is to use more traditional framing (either
steamed or laminated
frames) in conjunction with strip planking (with or
without veneer
overlays, or cloth sheathing).
What is
strongest? Probably 1), the strip + veneer overlays.
This produces an
extremely strong hull, and one that is least likely to
experience
problems as a result of movement in the strips. You
could use laminated
frames with this method, but bonding in bulkheads etc
will also result
in a very strong hull. The biggest difference is
probably in the
flexibility you get in terms of the arrangement:
laminated frames can
be set up at each station, and then leave you free to
adjust the
interior arrangement to a greater degree than you could
if building
with bulkhead framing. One advantage of using strips +
veneer overlay
(with or without frames) is that you can build in as
much of the
interior as you wish while the boat is not yet planked,
which can much
simplify the process. That can't be done with method 2
above (a hull
that must be sheathed with cloth on the interior).
Q. I am very very attracted to the gaff rigged
version, I
want to try to avoid having to buy tons of custom
rigging ( Lewmar,
Harken etc).I also want the rig to be maintainable in
third world
countries. I also guess the mast section is stronger
because of its
shorter length and lack of spreaders, not to mention
lower centre of
gravity. I have never in my sailing life sailed a gaff
rigger and my
only concern is windward ability, as I am considering
engineless, and
also ease of handling for a single hander ( reefing in
particular).
Also which would be better balanced for self steering?
A. Gaff
rigs win out over Bermudian hands down when it comes to
cost,
maintenance (esp. in remote places), simplicity, lower
CG, etc. If
properly designed and set up they don't give up much at
all in the way
of windward performance, at least compared to cruising
boats (racers
are something else), and of course are much more
efficient downwind.
Handling a gaff rig is somewhat more complicated,
because of the gaff
and dual halyards, but careful set up can usually deal
with this. I've
sailed many miles with both and would encourage you to
try the gaff rig.
Q. I was wondering if a quarter berth, aft of the
chart
table, underneath the stbd cockpit seat, would be a
possibility? I am
very fond of this arrangement as a sea going berth
away from the main
saloon area, I am willing to sacrifice cockpit storage
for this.
A. A
quarter berth could be fitted in. The chart table would
end up just aft
of the STB settee, and then the berth.
