The Design Requirements
This page is a brief introduction to the requirements set forth prior
to the design of the 25' Tug yacht, Boojum.
Please also see the article, An Overview
of a Few Common Roll Reduction Strategies presented at this site.
Aside from the primary design requirement, "She's got to be Cute!"
Boojum's other owner-imposed requirements necessitated the eventual keel
configuration given to the boat. Here is a partial list of those
requirements, many of which may at first seem to be in conflict with one
another, but which in the end have been nicely resolved in the final
design:
A limitation on overall length for a passagemaking
"trawler-yacht" of 25'
A limitation on beam imposed by a requirement for trailering
the vessel anywhere in the US without a permit
That the vessel sit upright on the mud; on the deck of a
ship; or on a flat bed trailer without requiring a cradle
Full 6.5' standing headroom in the Pilot House
That the vessel carry nearly all the comforts of home and no
small amount of electronics
That the vessel carry enough fuel for a 3,000 mi offshore
passage (thus the fuel tankage size and location)
That the vessel pass the full IMO offshore stability
criteria (thus the ballast quantity and location)
Excellent roll dampening (thus the overall size of the keels
and the distributed masses within them)
Enhanced directional stability in such a
short-overall-length vessel (thus the long symmetrical foil
shape; the drag given to the leading edge; and the lack of
toe-in or toe-out)
Some amount of performance under sail with the (albeit
somewhat small) get-home sail rig
This is a partial list of the rather strict design requirements
imposed by Boojum's owners, Charles and TC Vollum. After no small amount
of research and no fewer than some 60 or more iterations of the hull
model, the above requirements were best met with the eventual design
configuration.
It should go without saying that another vessel design, starting with
a different or less rigid set of requirements, would naturally have
emerged with a rather different configuration. This is a good
illustration of the task of Custom Boat Design: to take the stated
design goals and bring about an integrated solution to the requests...
The Design Choices
Short of actual tank testing, Boojum's twin keels have been designed
using the best information available. In that sense they are
experimental. However as you will see, they have been carefully designed
to fulfill the tasks required of them. Here are some of the issues
which were considered, and the solutions which have been offered in the
final design:
The twin keels on Boojum are absolutely huge in proportion to the
size of the boat, and also by comparison to the usual concept of "bilge
keels."
Boojum's twin keels have been provided for a variety of reasons. As
described above, they are for the purpose of being able to sit upright
on the mud, to sit upright on a flatbed trailer, or to sit on the deck
of a ship or on a marine railway without a cradle. In places with large
tidal ranges and shoal water estuaries the use of twin keels is
justified on that basis alone.
The keels on Boojum will additionally provide stability and lateral
resistance for sailing. The get-home sail rig is very small for the
boat, so performance will not be stellar. Nevertheless, she should be
able to make her way under sail for the purpose of self-rescue,
especially with a good strong wind.
The twin keels hold Boojum's fuel, so help to lower the center of
gravity and also help to spread out the vessel's masses.
The twin keels contain the vessel's ballast, for which purpose they
could not have been made any smaller and still contain the required
amount in the correct location. The ballast amount was determined by the
requirements of adequate stability per the IMO offshore stability
criteria. The ballast was divided equally between the two keels
for the purpose of spreading out the overall mass of the vessel
athwartships in order to increase the vessel's roll moment of inertia.
Using this arrangement, the entire mass of both lumps of ballast will
have to be put into motion whenever the vessel rolls.
The effect is similar to that of a gyroscope which has its mass
distributed toward its perimeter. The effect is augmented by the
entrained mass of sea water which must move along with the keels in
their boundary layer.
Studies done by the US Navy have indicated that a roll reduction on
the order of some 30% is possible with passive twin keels depending on
the speed of the vessel, sea state, etc. The US Navy tests were done on
rather larger vessels than Boojum. In proportion to vessel size the Navy
vessel's "bilge keels" were much smaller.
On Boojum, the twin fins are relatively large and they extend farther
from the vessel's roll center than do most "bilge keels" on power
vessels. We are therefore expecting excellent results in terms of roll
reduction.
Of course, the twin keels provide a much greater overall wetted
surface, so will contribute to the overall drag of the vessel. However,
in consideration of the possibility that the use of paravanes would
contribute much greater drag, and that the paravanes will now be able to
be smaller, and will need to be used far less often, we expect that the
net result will be greater overall passagemaking efficiency. In
addition, tank tests on other vessels having twin keels have shown some
"wave-cancellation" effects provided by twin keels. In other
words, there is a tendency for the displacement of the twin keels to
help "fill" the 'midship wave trough at speed.
The relative benefits of this "wave cancellation effect" seem to
depend on placement of the keels and their size.
Boojum's twin keels have been designed using a low drag, symmetrical
foil shape. The foil chosen is in the NACA 0012 family for the sake of
being able to sustain high angles of attack (as the hull rolls) without
stalling, therefore creating less turbulence and less excess drag.
Flat plate "bilge keels" on the other hand, if given the same the
same profile, would have nearly the same amount of wetted surface as a
foil, but would have no grace whatever for moving through the water. A
foil shape provides far less turbulence under way, so less energy is
dissipated into the water. The foil shape also contributes some amount
of lift, which will be of use for Boojum's "get-home" power: the sails.
Due to the twin keels, Boojum has greater wetted surface overall (the
dominant drag factor at low S/L ratios) but as mentioned, there is also
the potential for fairly effective wave cancellation, a benefit at
higher S/L ratios (wavemaking being the dominant drag factor at higher
S/L ratios). Being a very small vessel, and therefore having a low
absolute speed potential, it is anticipated that Boojum will be driven
at the upper S/L ranges quite frequently, and that the wave cancellation
effects may prove to be a significant benefit.
The twin keels are located as far aft as possible without the vessel
falling on her nose while on the mud. The twin keels have been aligned
parallel to the centerline and have been swept back to a large angle.
All of these features were chosen in order to minimize any potential
steering anomalies.
Based on the evidence from prior vessels which have made use of twin
keels, we expect that Boojum's twin keels will actually help the vessel
track more steadily.
The Prediction
My hunch, after no small amount of research, conjuring, scheming,
modeling and re-modeling the hull lines, is that the result will be
excellent. Being quite experimental, whether these strategies have
been successful will only be known for sure during sea trials, and after
a good number of sea miles...
The Result
Boojum, now launched, has proven the above concepts admirably.
In other words, the efforts expended have paid off, and the vessel
behaves as intended. Tracking is excellent, and performance under
power is as expected.
Performance under sail is also as expected, i.e. less than
stellar...! This would be enhanced considerably by the addition of
more sail area, particularly forward. This would be best achieved
via the addition of a mast forward in order to improve balance.
Roll attenuation due to the presence of the bilge keels is of course
impossible to quantify without having another vessel of equal
proportions with a single keel... a difficult proposition at
best... The subjective impression however is that roll decays very
quickly.
Roll attenuation is augmented by the presence of a riding sail, and
of course by paravanes. In combination, the result has been
described as being comparable to the stability offered on a sailing
vessel under a full press of sail. In other words, quite steady!
More Information...
For a discussion of some of the research sources used for Boojum's
keels, and a discussion of roll reduction in general, please see my
article, An Overview of a Few Common Roll
Reduction Strategies. Also, please check out the 25' trawler,
Boojum.
