Water Ballast and Small Boats Part 4


ballast weight as far as possible from centerline must surely be the optimum solution? Not necessarily—it depends on various factors other than distance from centerline. Anyone who has ever raced in


I thought my previous column was to be the last of my series on water ballast in small boats, but I have one more thought to share. Tis applies to the ballast tanks used on performance boats like the Mini-Transat (Mini 650), i.e. ballast tanks fitted against the side of the hull and duplicated on the other side. Refer to the previous articles if needed to refresh the basic principles of sailboat stability.


long-distance overnight races on a big boat will know that sometimes it is better for any crewmembers who are not helm- ing or actively trimming sails to sit on the windward seats below decks instead of on the weather rail. In rough weather, with big wind and waves making the spray fly, it becomes uncomfortable and very tiring to sit for long periods at night in the firing line of liquid bullets. Aside from the benefit of having dry and rested crew when they are needed later, when the boat is driving hard to windward that crew sitting to windward below decks will be helping RM a comparable


ballast tank options shown. Approxi- mate CG of the water ballast and center of buoyancy (CB) are also shown. Bear- ing in mind that the weight component of the RM calculation acts vertically downward, high water ballast provides greatest increase in RM when the boat is floating upright. Water ballast is unlikely to be used when upright or at small heel angles even for power-reaching because the performance gain from the increased RM will be offset by increased drag from the added weight. As the heel angle increases, the high ballast moves to lee- ward at a faster rate than the ballast that is placed lower in the boat. For the boat in the diagram, when heel angle gets to about 14 degrees the low ballast will be vertically below the high ballast, so the RM benefits will be identical for both high and low ballast. Beyond that heel


Most designs I see with these tanks


have them placed at the beamiest part of the hull. That makes sense, to get maximum benefit from the greatest lever arm available from the widest part of the waterplane (the surface shape of the ocean on which the boat sits). Te ben- efit of increased righting moment (RM) is comparable to crew sitting on the rail, with the advantage that the weight can be jettisoned when that extra RM is no longer required. Te tanks are normally built into the


corner where the hull meets the deck. That also seems to make sense, to get the most benefit in maximizing RM, but that is not necessarily true. Tanks as far as possible from centerline increase RM by increasing the lever arm through which the weight of the boat acts (RM = weight x lever arm). So, getting the water


10


amount to when they are sitting on deck. This is because the crew weight when sitting on the rail may be on a similar vertical line to the same crew sitting on a comfortable seat in the saloon. Introduce a large swell to the heel angle, causing rolling to leeward on the faces of the waves, and that crew on the rail will be further to leeward than the same crew sitting comfortably on a settee. That means that they may be contributing more to RM on a settee than sitting on the rail. Te same principles that apply to live


crew ballast also apply to static water ballast. When that ballast is positioned at the furthest outboard that it can be placed it will also be as high as it can possibly be at that fore/aft location. Te diagram shows four representative situations, with the low ballast and high


angle the high ballast becomes progres- sively more of a disadvantage compared with low ballast. Te CB is moving to leeward as the boat heels but beyond 45 degrees that high ballast will be moving to leeward faster than the CB is moving. By 90 degrees the CG of the high ballast has overtaken the CB, so the ballast is no longer acting to increase RM, reducing it instead, helping the boat to capsize. Meanwhile the low ballast is still to windward of CB and is helping the boat to get back onto its feet. Te example in the diagram has flared


hull sides, allowing the high ballast to be further from centerline than the low bal- last. If the hull is more slab-sided, then the high and low ballast will be closer to vertical alignment when upright, with greater benefit from low ballast tanks.•SCA•


SMALL CRAFT ADVISOR


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84