Firebird Daggerboard, by Toby Richardson
DAGGERBOARD DESIGN HISTORY
When Martyn Smith designed the Firebird in the mid-eighties, he noticed that the 60 foot trimarans were being challenged by the 53 foot rival, Exmouth Challenge. The shorter boat used a dagger board that was proportionately bigger than that of other boats, making it particularly good to windward. The Firebird uses a similar ratio of sail area to board area, resulting in a remarkably fast boat.
Designed to take enormously high sailing loads while still being relatively light, the boards have never failed from sailing despite some extremely wild antics. The aspect ratio - how long and thin to make the dagger boards - also contributes to the balance between performance and structural integrity. While a high aspect ratio, very long and narrow, would provide the best efficiency, the longer the board, the further the center of pressure is away from the hull. Longer and narrower boards would require very sophisticated and expensive construction and would be fragile hitting the bottom. These boards have semi-elliptic tips to minimize end losses in the same way as for the rudders; the elliptical tip also provides lift that is almost free of additional drag.
BOARD DESIGN AND HULL DURABILITY
When it comes to grounding, the case and surrounding hull are designed to be stronger than the board so that if the Firebird hits a rock, the board breaks, not the hull. The board can endure grounding into a soft bottom and more. One boat ploughed through an oyster bed by mistake; the drag from the board tip cutting through the oysters was so great that the helmsman feared a capsize. The bottom of the board looked like it had been sandblasted; the oysters had removed all of the gel coat over the bottom twelve inches. However, the structure was undamaged and only needed re-finishing before going back into service. Boats that have repeatedly broken boards have sustained no hull damage; a simple board change and Firebird is back out racing.
The dagger boards consist of a thin fiberglass skin over a lightweight foam core. To take the high side loads (approaching 1 ton acting 20 inches below the hull), bands of unidirectional carbon fiber are placed in the thickest part of the section, where they will be most effective. These bands are of maximum thickness around the bottom of the hull where the bending load is greatest, and they taper off towards the ends of the boards to save weight. Because of the high shear stress between these thick layers of carbon, the core in this region is a relatively high-density end-grain balsa. The laminate in the leading and trailing edges is thickened to make the boards much more robust when handling (otherwise the edge might collapse if you dropped a board), and there is some unidirectional glass put in these edges to make the board take reasonable grounding.
DAGGER BOARD PLACEMENT
The boards are angled to keep the accommodation as clear as possible. While ideally they would be on the hull centerline and vertical to do so would completely block the hull. So they have to be moved to one side and exit the bottom of the hull as squarely as possible. On Firebird the boards are on the outside of the hull. On the inside of the hull it would be a slight down force. However, on the outside, the board generates a small amount of lift (actually quite a bit of lift as the boat heels, flying a hull.)
It has been asked why the boards are not asymmetric. It's true that such a section would be more efficient, especially if given just a little positive incidence. The problem then is that you can only sail with the leeward board down, to have both down together would mean them fighting each other and putting the drag right up. Such an arrangement is fine for an all-out race boat but for anything more casual would just make for a lot of work. Imagine going out for a casual sail and having to tend the boards all the time. So with symmetrical sections there is slightly less lift, but when the Firebird is not racing, it can sail with the dagger boards down the whole time.
When racing, going to windward, have the leeward board right
down and raise the windward one so that only the tip is exposed below the hull
to keep the slot filled, thus avoiding considerable drag. That means swapping
them over when tacking; however, if tacking frequently it is probably better
to keep both down and concentrate on sailing the boat. For sailing off the wind
you need considerably less board area. As the tip of the boards have to be below
the hull at all times to avoid drag, the leeward board only needs to be perhaps
1/3 of the way down. A tip when sailing in shallow water is to mark the boards
so that they protrude about three or four inches below the rudders and not to
raise them above that mark. That way if you sail into water that is too shallow
the boards will ground before the rudders, avoiding possible damage and giving
you the chance to turn around and sail into deeper water.
Contact: Still Water Design, 1 Winnisimmet
Street, Chelsea, MA, 02150, by e-mail: email@example.com,
Credits: Photos by Lorin Alusic and Claus-Christian Plass