Keels vs dag­ger-boards

Which is bet­ter for a sail­ing cat?

Boating NZ - - Contents - BY PHILLIP BER­MAN

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.

SPEED/LEE­WAY DELTAS

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.

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