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 phase with the first pendulum. This would result in no rolling as one pendulum cancels out the other.
Dave’s objective was to search for a design for a passive tank for roll reduction. Professor Don Bass, at the Marine Institute in St. John, Newfoundland, had designed a tank for our friend Bob Phillips on Another Asylum. Bob is very pleased with the roll reduction he has achieved. 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. We then did a stability/inclination test with weights on the deck edge which gave us data points for Swan Song’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. We looked at several composite materials 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’x12’ panels. These were used to fabricate the tank which measured 4’x12’x16” high with five 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. As the water is now racing to the low end, which when it gets there is the high end again. As usual “timing is everything”. The location, size and spacing of the baffles determine the effectiveness of the tank.
We are really pleased with this very “cost effective” solution to our stabilization problem and have traveled 700 miles since we installed it. Our maximum roll may have been 15 degrees between St Barths and Antigua in 10-12’ beam seas – 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 are well worth it.
Nancy Terrell has lived in the Caribbean for 20 years, is an international free lance writer, and holds an MA Degree in Literature. Swan Song is her 6th boat during 40 years of sailing.