Anti-Roll Tanks – A Simple way to Stabilize
By- Nancy Terrell
REPRINTED FROM ALL AT SEA - February issue 2007
One of the biggest problems we faced, when reconstructing our classic trawler Swan Song, was that of stabilization. Because SS was built in 1974, as a motor yacht, she had no fins underneath the hull or paravanes extending from the pilot house. If there is anything I detest on a boat it is rocking back and forth with everything crashing. There had to be a solution.
All boats roll, as we know, from side to side. What is less obvious is that every boat has both a natural roll period and roll damping rate which is always the same - it never changes. If you think of the roll as a pendulum, just moving from side to side, the theory of the roll tank is to add a second pendulum which is moving 180 degrees out of faze with the first pendulum. This would result in no rolling as one pendulum cancels out the other.
Dave’s objective was to develop a design procedure for a passive tank for roll reduction in rough seas. Professor Don Bass, at the Marine Institute in St. John, Newfoundland, had designed a tank for friend Bob Phillips on Another Asylum. Dave contacted Dr. Bass who required a lot of specific data about Swan Song. As we had no design drawings or information from the original naval architect, we had to provide the measurements. What a riot – with me aboard and Dave in our dinghy, Leda, we measured the hull, as well as the underwater hull, in one foot vertical increments and five foot horizontal increments. These data points, when input into Don’s computer program, allowed the generation of an accurate 3-D wire-frame model of the hull surfaces.bbWe then did a stability/inclination test with weights on the deck edge which gave us data points for SS’s stability. Lastly, the roll rate and roll damping rate were determined and were also supplied, along with a nominal displacement waterline length and waterline beam.
Knowing that the tank is most effective if it is full beam of the boat, we selected a couple of possible locations for installation. Our first choice was on top of the pilot house as this would be out of the way. We gave Don the height off the water and the width available. Don’s software then took all of this data and produced the dimensions of a tank in that location. We then modified these dimensions to make full use of commercially available building materials. Don, in turn, took those dimensions and modified the tank design to accommodate these materials.
Our choice of construction materials was based upon the need for a tank that is light in weight but structurally rigid and cost effective. Several composite materials were looked at that could be made into the size that was required and shipped to the BVI. Nida-Core in Stuart, Florida, won the bid. They manufactured 25mm sheets of Nida-Core bonded with 38 ounce fiberglass on both sides in 4 X 12 foot panels. This was used to fabricate the tank which when measured was 4’x12’x 16” high with six T-shaped baffles 18” on either end of the tank. The empty weight of the tank is about 250 lbs.
SS’s tank, as designed by Don, is filled with 1550 lbs of sea water, which is about 6 ½”; this water initially moves from a static position towards one side or the other as SS starts to roll. The baffles slow the water movement – as the boat reaches its limit on one roll it will start to roll back to the other side, like a sine wave. The water, however, is again held by the baffles, on what was the low side, as the roll continues it becomes the high side.
We are really pleased with this solution to our stabilization problem. We have traveled 700 miles since we installed it and have had no problems – this is a real compliment to Dr. Bass coming from me as I like all of my “frou-frou” out and in place. Of course, there are some things you just have to move while underway but for the most part, picking a good weather window, the ride is smooth and stable. This project took a lot of time to plan and to build but the results were well worth it.
From the top down
The first two pictures show the building of the tank by BVI Painters under the direction of Chris Spencer. You can see the way that the baffles are arranged.
The next pictures show the instillation on the boat itself, done by Dave. If anyone would like more information on this please e-mail Dave at - [email protected]
On the right we have the finished tank as seen from the side. For a full picture please go to the first page - IT REALLY WORKS !!!!