Keels vs dagger-boards
Which is better for a sailing cat?
My company in the US – Balance Catamarans – designs and builds cats with both keels and dagger-boards. Our vessels attract a range of buyers – some focus on strength, space, safety and comfort (cruisers), others are prepared to sacrifice comfort, ease of operation and payload to go faster (racers).
A few customers asked for our 16m 526 cat to be fitted with keels rather than dagger-boards. They felt the keels trumped the performance advantages of boards. We were happy to comply – but not to guess at the performance differences between the two configurations.
Instead, we heeded Lord Bryon’s advice to evade the “kant and hyperbole” (the language of the day at boat shows!) and hired the Wolfson Unit for Marine Technology and Industrial Aerodynamics (University of Southampton) to do a comparative study of the configurations on two identical 526 cats.
I think you’ll find results interesting. There are a few points to note: • The 526 is a light, narrow-hulled epoxy performance cat and the keels we install are considerably finer, deeper and more carefully shaped than those on mass production designs • The study was tackled with the dagger-boards in the ‘fully-down’ position on all points of sail to reduce the study’s complexity. Off-thewind, with boards raised, the 526 would be marginally faster than the study indicates due to the reduced wetted surface • The disparity between keels and boards on a mass-production cat versus a performance cat would be exaggerated considerably compared to the study • We erred on the side of caution in setting our lightship weight for this study at 12,500kg. The actual lightship weight is 12,215kg for the keeled 526 and 12,261kg for the dagger-boarded 526. Performance, of course, is never the only factor in the game. It’s important to the pros and cons of boards/keels beyond speed. Overleaf is a basic overview.
The Wolfson study returned the predicted boat speed and leeway angle for all combinations of true wind speed from 4 to 25 knots, and true wind angles from 40° to 180°.
We can compare the two configurations in terms of speed and leeway at major points of sail, specifically beating, close-, beam- and broad-reaching. Beating, the two designs are predicted to perform as follows:
TABLE 1: BEATING COMPARISONS
The dagger-board design, on average, can be expected to sail 1.5 knots faster and 1.75° higher.
Table 2 shows that with close-reaching the dagger-board design has less of an advantage than with beating:
TABLE 2: CLOSE-REACHING COMPARISONS
While close-reaching the dagger-board design is expected to sail 0.5 knots faster and 0.75° higher.
As the true wind angle approaches 90° for a beam reach, the keel and dagger-boards become more closely-matched.
TABLE 3: BEAM-REACHING COMPARISONS
When beam-reaching, the dagger-board design will sail slightly higher with a leeway advantage of about 0.25°, but its 0.3 knots speed advantage in 5 knots TWS will fade to a 0.5-knot deficit in 16 knots TWS. At this point of sail the performance of the two designs is equal – the differences in leeway and boat speed are negligible.
Finally, while broad-reaching the following speed/ leeway is expected.
TABLE 4: BROAD-REACHING COMPARISONS
In summary, when broad-reaching the dagger-board design regains a slight advantage, as it maintains an additional 0.5 knots of boat speed. Leeway differences are again negligible.
Typically, it seems, the dagger-board design not only out-performs the keel design but also out-points it, make less leeway close to the wind. This gives the dagger board design a substantial advantage for racingoriented sailors.
All boats are a blend of compromises and trade-offs.
While the Wolfson data gives a fair comparison of the two designs at specific true wind speeds and angles, to fully understand the performance trends and enable better predictions between them, we should consider the entire range of wind conditions and all points of sail. This is best done with speed and leeway deltas, where the differences between the two designs are calculated and analysed.
The graphs on this page display average speed and leeway deltas. The calculation process works like this: a positive value for the delta indicates an advantage for the daggerboard design, and a negative value favours the keel design.
For example, a speed delta of +2 knots and a leeway delta of -1 degrees means that the daggerboard design is 2 knots faster than the keel design under the same conditions but would sail 1 degree lower than the keel design.
Considering Figures 1 and 2, the three TWS ranges used are from 4 to 7 knots, 8 to 12 knots and 14 to 25 knots and will be referred to as light, medium, and strong wind conditions respectively.
An analysis of the Average Speed Deltas shown in Figure 1 – taking a beam-reach (90° TWA) as the point of sail – shows that in light winds the dagger-board design will perform better than the keel design as the blue line (4 to 7 knot TWS range) indicates a positive delta of 0.4 knots at 90° TWA. So we can expect the dagger-board to out-perform the keel by about 0.4 knots under these conditions.
Counter to this, while still on a beam-reach but when moving from a low wind condition to a high wind condition, the keel design begins to perform better. This is now taken from the green line (14 to 25 knot TWS range) and gives a -0.6 knot reading. So the dagger-board design is now expected to be about 0.6 knots slower than the keel design under the new, higher TWS conditions.
FIGURE 1: COMPARISON OF AVERAGE SPEED DELTAS
Figure 2 is used in the same manner as Figure 1 but shows the leeway angle of each design rather than boat speed. Focusing again on a beam reaching condition, the leeway deltas indicate that the daggerboard will always out point the keel design as all three lines (TWS from 4 to 25 knots) indicate. positive vlaues of 0.3, 0.35, and 0.1o at 90o TWA.
FIGURE 2: COMPARISON OF AVERAGE LEEWAY DELTAS
Overall, Figure 2 shows that the dagger-board design has a significant gain in leeway over the keel design at all wind speeds and angles. This reaches a maximum when beating (TWA ±4045°) of 2° and declines to zero when running (TWA ±180°).
In terms of average boat speed deltas, Figure 1 shows that the dagger-board maintains a higher boat speed when sailing in light and medium winds (red and blue lines in Figure 1).
In stronger conditions (green line) the dagger-boards yield better boat speed when beating but the keel surpasses this between TWA of 60°-145°. Keep in mind that the study was conducted with the dagger-boards fully-down at all times. In reality, the dagger-boards will be raised in the 60°-145° TWA range. As such the study slightly underestimates the speed of the dagger-board design over this TWA range.
Overall, the dagger-board out-performs the keel in both speed and leeway in the vast majority of sailing conditions – it’s considered the better design choice if ultimate performance is the objective. While this study is specific to the Balance 526, corresponding trends can be expected in performance cats of similar size and design.
In the final analysis, all boats are a blend of compromises and trade-offs – and includes features such as helm design, sheeting system, rig layout, bimini design, bed design and location, engine placement, etc. There is no ‘right’ or ‘wrong’ boat – and certainly no perfect boat. Only a boat that’s a better fit for a particular sailor and his/her budget, cruising agenda and specific aesthetic proclivities.
I will say that, based on the five 526 cats we have sailing today, with over 60,000 miles of sailing behind them, those owners with keels seem just as happy as those with boards. BNZ
ABOVE The Balance 526 catamaran – available with keels or dagger-boards. MAIN IMAGE Most production cats are equipped with keels – for a bit more performance you might consider dagger-boards.