Stability tanks
I have copied below the post I made to the Trawler World list in 2002
when I first installed my stability tank. Since then I have lived
happily on board, cruised moderately open water several times a year,
Anegada Passage, Tortola to St. Croix, in total about 1500 miles in the
intervening four years, and am as enamored with my tank now as the day
I installed it.
My family and I live on board in the worst marina on Tortola, the old
CSY facility in Baughers Bay, Road Harbour. Ferry traffic, open to
anything south of east, it wasn't unusual for us to eat ashore pre tank
when the conditions were bad; now, no need. I occasionally hear the
tank as it is over our saloon and know from the slight noise that it is
active in the harbour.
There are no moving parts, cost less than a paravane system, far less
than active fins and works at anchor or at the dock, which is where we
spend far too much time. When we are anchored out I can anchor where
other boats don't because we just don't roll.
Bob Phillips,
Another Asylum, Tortola, BVI
Hi All,
I have been a digest subscriber for the last two years and am
switching to
active participant as I have just completed a project which will be of
interest to the list. My boat is a sixty foot full displacement
motoryacht
which was built as a custom one off in 1976. I found her in Sydney,
Nova
Scotia and left there Nov. 15th, 1999, delivering her to my home in
Tortola, BVI. During the 2600 mile delivery we experienced most
weather
conditions including south east gales with snow and ice from Halifax
to
Marblehead, dense fog from Long Island through the city, down the
Jersey
coast, up Delaware Bay and down the Chesapeake to Norfolk. We ran the
Intracoastal from Norfolk to Lauderdale in seven, twenty hour days and
then
went directly to Tortola via the Old Bahama Channel through several
tropical waves.
I have been in the marine industry for twenty five years, a mixed bag
of
sailmaking, boat building, professional sailing - I was the division
manager at Hunter Marine responsible for the design and development
of the
Alura 30, which was recently reincarnated as the Mainship Pilot 30 -
and
have owned a number of boats including an Alura 30, which I brought to
Tortola on her own bottom, and an eighty foot Dutch North Sea fishing
trawler for which I had dreams of a yacht conversion. Luckily, the
reality
of owning a vessel that required 165 gallons per oil change and my
wife,
not necessarily in that order, made me see the light and the yacht
conversion turned into an interisland freighter and I found our
current
boat, "Another Asylum"
We weren't far out of Halifax when I decided some form of
stabilization
would be in order and started researching when I reached home. I
looked
hard at paravanes and active fin stabilizers but could neither afford
the
money nor the space for fins nor design the paravane spars to look
natural
on a motoryacht profile. I came across an article in the Journal of
the
Society of Naval Architects and Marine Engineers written by a
professor at
Memorial University in New Foundland, Don Bass, concerning a fishing
boat
accident due to the loss of one paravane and an alternative method of
stabilization. Canadian Fisheries had suspected fishing vessel losses
could
be attributed to loosing one paravane but it wasn't till there was a
survivor of the capsizing of "Straits Pride II" in 1991 that there was
proof. Dr. Bass was given a grant by the Canadian Centre for Fisheries
Innovation, Federal and Provincial departments of Fisheries and
Oceans and
the National Science and Research Centre to analyze the accident and
to
offer alternatives.
Dr Bass found that an athwartships tank with fluid equal to about two
percent of displacement and baffled to stay ninety degrees out of
phase
with the natural roll rate of the boat would out perform paravanes in
most
conditions and be effective regardless of forward motion. Got my
interest
so I contacted Dr. Bass and, after many emails, commissioned him to
design
a tank for my boat. I was lucky in that my boat is close in size to
the
sixty-five foot fishing vessels used for ocean fishing in Canada, for
which
Don has designed and has operating over forty tanks, and I was able to
find
the NA, Dave Munroe, who designed my boat and get a set of lines
drawings.
Besides the lines drawings we needed an analysis of the stability of
the
boat, obtained by inclining. Don's analysis of the information showed
the
optimum placement for the tank was my upper deck just aft of the
flybridge,
which was perfect as it was unfinished space when I bought her and
needed
seating anyway.
My wife and I and our two teenage daughters live aboard and keep our
boat
at the rollyest marina on Tortola, the old CSY marina in Road
Harbour. Not
only are we subject to the wakes from all the traffic in the outer
harbour,
which is also wide open to any weather from southeast through
southwest,
there are also the Peter Island ferries operating almost every hour
from
the dock next door. There were days, pre-tank, when you couldn't set a
glass down on a table and several meals were eaten on the dock. All of
that
is over. We haven't seen even five degrees of roll at the dock since
the
tank was installed and don't much care where we anchor anymore when
we are
out for the weekend.
We just returned from fifteen days of cruising to Anguilla, St.
Maarten,
St. Barts, St. Kitts and Saba with two crossings of the Anegada
Passage,
the eighty mile wide passage through which water flows from the
Atlantic
to
the Caribbean and vice versa. We had eight to twelve feet with twenty
five
to thirty knots true going to Anguilla with the seas twenty to forty
degrees off the bow and rarely rolled as much as twenty degrees. We
had
the
same conditions on the beam from St. Maarten to St. Barts and
experienced
the same.
At this point I need to stop and explain to the list that while I
have no
commercial relationship with Don Bass at this time, I am discussing
with
him the possibility of marketing stability systems. While the tanks
are
simple in operation with no moving parts there is a lot of complex
engineering work before a project is complete. Stability tanks have
been
in
use since the 1920's and almost all tankers are stabilized using cross
linked trim tanks known as U-tanks. A company named Maritime Tank
Systems
has been designing tanks for commercial vessels, known as Flume
Tanks, for
years and there are a number of other companies around the world
doing the
same thing, although almost exclusively for commercial vessels. To our
knowledge, my boat is one of the first pleasure vessels this small to
be
outfitted with a tank.
Within the next few months I will be doing testing with a vertically
referencing gyro interfaced to an analysis programme and be able to
provide
precise roll reductions for a given sea state. While my tank was
designed
and tuned for ocean voyaging Don is doing simulation work on tanks
more
generic in shape and sized for coastal and inshore cruising. I have
lots
of
information and will send additional posts concerning stability and
actual
tank construction and cost. Don't hesitate to ask questions.
Bob Phillips, Another Asylum, Tortola, BVI
Robert,
I found your post very interesting. I have a Scout 30. I know it's small
for this kind of thing but it's a full displacement hull & likes to roll.
If something this simple would tone the rolling down it would sure be great.
The boat displaces 5 tons with 20% of that ballast. I would love to see a
diagram of one of these systems to get a better idea of what's involved.
Especially the baffle system.
Thanx for any info,
Carl
----- Original Message -----
From: "Robert Phillips" bob@doylecaribbean.com
To: passagemaking-under-power@lists.samurai.com
Sent: Sunday, September 03, 2006 8:04 PM
Subject: [PUP] Stability tanks
I have copied below the post I made to the Trawler World list in 2002
As some of you know, I am in the midsts of building a 53' trawler that I
engaged Michael Kasten to design (Arild took care of the electrical design)
[kastenmarine.com/valdemar52.htm]. I first wanted active stabilizers, but
then developed questions about their reliability on long passages,
especially after the NAR. Then I got the the price tag for active stabies
(and the necessary hydraulics) and puked. Knowing that I couldn't fix the
dang things if they broke found me searching for alternatives.
I e-mailed Dr. Bass and Michael about the flume tanks. I have posted below
the two most detailed of the e-mail responses -- one from Dr. Bass (I
believe that Dr. Bass' fees have gone up a little) and one from Michael.
It is clear that you cannot just slap the tanks in place and go from there.
You'll need someone to determine stability and the like.
In the end I went with paravanes and twin keels -- but the flume tanks did
intrigue me. I was unwilling to give up the necessary space. When you
added the necessary, additional design and stability calculation costs into
the mix I concluded that there was no clear cost advantage of flume tanks
over paravanes.
anyway, perhaps the below e-mails will be of assistance. You will note they
are now nearly two years old. Michael was responding to a bunch of
questions i posed on the subject. Hope this helps.
Alan
Tampa
From: dbass
Date: 11/22/04 18:31:20
To: 'Alan Wagner'
Subject: RE: Stabilizer Tanks
Hello Mr. Wagner,
Thanks for the photos. She looks a fine vessel.
I have written to my client asking permission to let you have his email
address.
I dont know for certain but you are probably looking at somewhere between
1000 -2000 lb of water in a tank system. That should not make a lot of
difference to the draft. Obviously if the vessel has insufficient stability
putting that amount of water up high would not be a good idea. Generally
these boats have sufficient stability. One important point is that there is
a safety feature with the tank system and that is the ability to dump the
water in the tank very quickly should the need for extra stability arise
(for example if a list is developed, due to damage or the ingress of water
etc). Im sure that Michael will take care that all stability criteria are
satisfied. In fact that is one of the things that I always check myself
before designing a tank.
My cost of carrying out the computer simulations to design the tank is $4000
Basically I provide the basic dimensions of the tank, baffle configuration
together with operating instructions concerning recommended water levels.
These dimensions are based on consultation with client and boat designer.
Im not sure what Michael would charge to incorporate the tank into his
present design.
Don Bass
From: Michael Kasten
Date: 01/07/05 20:40:39
To: Alan Wagner; AlanWagner - Law Office; Deborah Wagner
Subject: Stabilizers... and Brief Note on Electrical
Hi Alan and Deborah,
[non-flume portion of e-mail deleted]
Flume Tank(s)...
I do not ordinarily pander flume tanks unless actually asked to do so.
Possibly this is just to avoid being thought of as a weirdo, though that may
be an entirely futile endeavor...!
To the average Joe, flume tanks are somewhat on the lunatic fringe. What's
there to like about them...? After all, they don't cost much; they don't
require maintenance; they don't use any power; they don't cause added
resistance; they don't stick out so they can't break off; they can't get
stuck or jammed; you don't have to monitor them or fix them; they work
equally well whether the vessel is moving or is at anchor; gizmo
manufacturers can't profit from them... they just have to be a totally
crazy idea... right...?
As to your questions: here are several thoughts - possibly inconclusive -
but possibly able to outline a strategy....
- Do these things actually work?
Yes they do.
You can observe a few of the relative merits of different approaches to roll
attenuation that I've researched at kastenmarine.com/roll_attenuation.htm.
The percentages of roll reduction given in various research papers are most
commonly quantified in terms of amplitude, however roll period, decay, and
roll accelerations are also important.
In particular, roll accelerations are the most significant component in
terms of incidence of seasickness / discomfort, even though it is commonly
imagined that amplitude is the culprit.
A flume tank can have its greatest benefit in terms of reducing
accelerations by reducing the amplitude and / or lengthening the roll period
thus reducing accelerations - basically inducing a "softer" roll, but one
which may on occasion (depending on sea state, etc.) have a larger amplitude
A flume tank achieves this by two means:
a) raising the center of gravity thus degrading stability, and
b) sloshing out of phase with the natural roll period of the boat.
Raising the CG is okay to a point, but must be considered carefully in terms
of its effect on overall stability. Several thoughts follow...
Although there are not any required stability standards for yachts built
and used within the US, I consider the stability criteria imposed on
commercial vessels and passenger vessels to be a minimum standard.
For offshore power yachts, I impose the basic stability criteria used by the
IMO (International Maritime Organization), which is also used by a number of
other regulatory bodies for commercial craft. I further impose the slightly
more restrictive initial GM criteria used for fishing vessels (which have to
operate in all weathers with highly variable loads).
If it is at all possible to do so, I prefer that a power yacht additionally
exceed the IMO's more recently developed extended weather criteria. For
relatively small yachts that have large windage areas (i.e. many small power
boats) this criterion can present some difficulty, mainly because
mathematically the equation used by the IMO creates an overly pessimistic
view of the exciting forces.
Specifically, when calculating windage and the resulting upright heeling
moment, the equation takes the product of the assumed force per unit of area
times total area times the lever arm. When a vessel heels, the lever arm is
reduced and the area that is presented to the wind is also reduced. To
account for this the heeling moment at heel angles other than zero is
mathematically expressed as being the unit force times the total area times
the lever arm times the square of the cosine of the heel angle.
The IMO wind and wave criteria do not take advantage of this latter credit
(cos^2 angle), so end up being overly (in my view) pessimistic in terms of
actual heeling forces. In other words, rather than taking credit for the
heeling force being diminished as the vessel heels (as is actually the case)
the IMO presumes that the heeling force remains constant at all angles of
heel. Mathematically and empirically this is incorrect.
The rationale used by the IMO for assuming a "constant" heeling force is to
make a generous allowance for "dynamic" sea conditions. In order to pass
the criterion, the vessel must have less than a given heel angle at
equilibrium, assuming that "fixed" windage moment. This does not logically
make sense. Nevertheless it is the only method that the IMO has so-far
developed and despite it being rather imprecise, this is the criterion we
strive to meet.
Other regulatory bodies and navies of the world often use the cos^2 angle
credit, but may instead use a more severe wind force. In my view that is
more logical and more accurate. However since the IMO criteria are used for
recreational craft within the EU, we've taken the IMO criteria to be the
prevailing standard.
There are of course many components of stability, such as a vessel's range
of positive stability, roll moment of inertia, etc. All of our boats have
their stability evaluated in the design stage, first by a thorough weight
and CG calculation, then via rigorous hydrostatic analysis. To date, all of
our power yacht designs are fully self righting. In other words, they do
not have any region of negative stability, and therefore have a much greater
range of positive stability than most sail boats.
What do I think about the stability issue as it relates to flume tanks for
the Valdemar...??
The stability question is very boat-specific. Before knowing what to advise
I will first have to complete our weight calcs to determine the preliminary
CG, then run a preliminary stability analysis to determine what margin we
may have to play with in terms of satisfying the IMO criteria. The basic
criteria are hardly ever an issue (easily passed with good design) - only
the extended weather criteria (not as easily met).
If we have a generous margin in the basic criteria, and 100% positive
stability, we will probably also have a good margin and 100% stability with
a flume tank. The extended weather criteria then becomes the point of
decision.... While we can in all likelihood introduce a flume tank and still
meet the basic IMO criteria, it is much less likely that the extended
weather criteria could be met (which can be difficult to meet even without a
tank...!). For most boats it is windage (i.e. lots of cabin area that is
located high up) that causes this, and also because the IMO imposes such an
overly severe mathematic penalty.
For Valdemar, while there is a fair bit of windage, it is not located high
up, so there is a good chance to meet the artificially strict IMO extended
weather criteria without a tank. With a tank - much less likely.
Is that important...?
I think it is entirely a matter of degree. We have to first discover the
inherent stability of the vessel, which we will do shortly during our design
development process, then we can evaluate the excess margin of stability we
have to work with. We can then better understand the relative benefits of
incorporating a flume tank, the purpose of which is to intentionally degrade
stability.
One point to mention is that on the flume tank we worked out with Professor
Bass, we introduced a "dump valve" which basically allows the tank contents
to be dumped if extreme conditions might warrant that action, thus restoring
the stability completely, but eliminating any roll attenuation.
The above is a much longer note than I'd intended, but even if it is
inconclusive at this point, it is a thorough overview of the issues having
to do with the stability question. Basically, even though we will determine
the vessel's stability with great precision (with and without the tank), the
relatively imprecise nature of the criteria in common use end up making the
final judgment of adequacy somewhat fuzzy.
- Is the water too much space to take from the boat?
Possibly so... but it might be workable.
For the Coaster 40, we were asked to create a variation on the original
design that made use of a flume tank, but we did not have the option of
increasing the height of the pilot house (canal clearance). In order to keep
the tank from becoming a visually imposing structure, we placed the tank
athwartships right above the galley (see the profile and layout images of
the original vessel at www.kastenmarine.com/coaster40.htm). As it turned out
we were able to simply extend the house sides upward and aft, and extend the
pilot house top aft to the mast, completely hiding the tank from view.
This imposed a definite limit on the space available for the tank...! Thus
Professor Bass' comments: "On small vessels, a well designed tank will
considerably reduce the short, sharp motions associated with high frequency
responses to waves, so even if the roll reduction is quite modest, the tank
will improve the comfort levels of the boat. I think this would be the case
for the tank system I designed for a client of Michael Kasten. That boat was
really quite small (40 ft) and the space for the tank quite limited."
If we had taken the design right from the very start with the concept of
using a flume tank, we would in all likelihood have been able to dedicate a
much larger envelope for the tank, and we would have also made the vessel
more burdensome (more displacement). The extra displacement would have
allowed the tank to be carried more gracefully in terms of overall load, and
would have also produced a vessel with a more gentle roll to start with.
Both would have been a big advantage.
As it was, we were faced with adapting an existing design to the use of a
flume tank, which imposed definite limits on the tank, its weight, its width
etc. and ultimately on its overall effectiveness.
- Since you were involved in this, could I ask what your experience was?
I found Professor Bass to be exceedingly competent and highly professional.
When the design and analysis were completed, Professor Bass provided us with
a thorough analysis and a written report of what performance he expected
from the tank in terms of roll reduction.
I would be absolutely delighted to have the opportunity to work with Dr.
Bass again.
- Since it was not one of your mentioned options, do I surmise that from
your perspective it was not worth the effort?
I think it's an open question. It's a matter of what can be achieved in
terms of spatial arrangement vs. the tank's roll reduction effectiveness vs.
resulting stability, and it's also a matter of the simplicity of a totally
passive system vs. the complexity and expense and maintenance / reliability
of active stabilizers. The tank design and incorporating it into the vessel
design was far less costly than buying, installing and maintaining active
stabilizers... and vastly simpler overall.
- Or not really ocean capable?
See stability commentary above - basically yet TBD.
- Are they worth considering?
Very possibly so. Here are a few thoughts...
For a new vessel starting from a blank sheet of paper, I very definitely
think that a flume tank can be designed to be very effective at roll
attenuation, and can also be extremely cost effective. The presence of the
tank will impose limitations on the layout, and possibly also impose upon
the aesthetics of the yacht.
Can the IMO extended weather criteria could be met in a totally new design..
? Maybe.
Per Professor Bass, many of the fishing vessels using flume tanks never have
had their IMO basic stability compliance checked - with or without the tank
(it is not legally mandated, so very few would bother), much less their
compliance with the IMO extended weather criteria....! Per Professor Bass,
some vessels even have trouble with the IMO basic criteria.
What about for Valdemar?
First we would assure maximum compliance with IMO. In order to plan it, we
would first be looking for the best tank location - that being the biggest
primary hurdle. Two possible locations come to mind...
- On the house top. This would be the most effective, since it could be
nearly the full width of the vessel and the highest. It would also be the
most visually imposing, but maybe could be made to have the appearance of a
large deck box, or maybe a seat...? If located entirely above the house top
we would still try to stay within the maximum canal height we have been
working with.
To do so, possibly the tank could possibly extend from Sta. 14 to Sta. 17
for the full width of the house top, and up to the canal limit. In plan
view it could possibly be arranged in a large "H" so that the two vent boxes
could still be as planned. This would make use of a larger area than we had
available on the Coaster, would keep the weights relatively amidships, etc.
- Partly on top, partly below the top... This might allow the canal height
limit to be preserved, but would invade the headroom in way of the pilot
house guest berth. Feasibility to be determined...
Once we have settled on an acceptable location and the overall envelope, we
d have to see if Professor Bass could work within that envelope effectively,
and what the weight of water would actually be.
Once Professor Bass has determined the feasibility, we would re-do the
stability analysis in order to consider the effects of the tank.
If workable, then Professor bass would take the updated stability
information, finalize the tank design, and prepare a report of its expected
performance.
With that information, we can then finalize the tank details in the design.
What next...?
If this is an interesting concept, I recommend that we first finish our
weight calculations and our stability analysis. With that (and without
having expended any time on the actual tank concept), we can propose a tank
envelope and solicit a preliminary opinion from Dr. Bass as to its
workability and potential effectiveness. If we have a workable envelope and
a favorable response from Dr. Bass (and an approximate weight of water in
the tank), we can then make a preliminary determination of stability with
the tank. If it turns out to still be a workable notion - then I say go for
it.
Those might seem like a lot of ifs... but at that point the inquiry will not
have imposed much cost. I think it is at least interesting enough to
investigate - and potentially quite beneficial...!
Please let me know your thoughts...
All the best,
Michael Kasten