Skip to main content

We Predict the Winner

Will Alinghi’s catamaran, A5, or BMW/Oracle’s trimaran, BOR 90, be the faster boat in the 33rd America’s Cup Deed of Gift match? There are strong opinions on both sides.Despite the secrecy that is part of every America’s Cup, we used data collected on both boats by SAIL and then applied it to the Wolfson Unit’s velocity prediction programs (VPP), in particular the

Will Alinghi’s catamaran, A5, or BMW/Oracle’s trimaran, BOR 90, be the faster boat in the 33rd America’s Cup Deed of Gift match? There are strong opinions on both sides.

Despite the secrecy that is part of every America’s Cup, we used data collected on both boats by SAIL and then applied it to the Wolfson Unit’s velocity prediction programs (VPP), in particular the WinDesign VPP developed in conjunction with Clay Oliver’s Yacht Research International.

Although the actual sailing weights of these boats are well guarded, weight is an integral part of the VPP analysis. For reference we scaled a C-Class catamaran, another class that has no weight restrictions, up to 90 feet. Interestingly, the estimated sailing weights for both A5 and BOR 90 compare favorably with the scaled up C-Class cat, even though the C–Class cat has just half the beam and sail area (Table 1). Although the estimated lighter weight of the powerful BOR 90 seems counterintuitive given the simpler two-hull platform of A5, the calculations show that this is indeed the case.

In fact, lighter weight is not necessarily an advantage upwind because weight provides stability and power to carry sail. However, water ballast, rumored to be used aboard both boats, will provide BOR 90 with added stability upwind, much like a crew trapezing from the windward hull of a C-Class. And when it is no longer needed, the water can be dumped, which moves the advantage back to the lighter boat.

We used boat geometry and the VPP’s internal hydrodynamic and aerodynamic models to derive baseline upwind speed polars for BOR 90 when sailing with water ballast in the windward ama (Figure 1). When true wind speeds go up to 14 knots—which is just below the 15-knot maximum allowed by the Notice of Race that existed in early December—the VPP suggests that BOR 90 can sail to windward at twice the true wind speed (TWS) and can reach velocity made good (Vmg) speeds that are about 50 percent higher than true wind speed.

Because of the shallow resistance curves and the strong link between boatspeed and apparent wind speed, the true-wind angle for the optimum Vmg—represented by the top of the curves—is not well defined and the apparent-wind angle hardly changes as the boat bears away and accelerates. This means that the performance potential for both boats will depend on how well each team has learned its boat’s optimum sailing angle for a given wind strength.

Our analysis using BOR 90’s soft-sail rig shows her flying a hull in just 8 knots of true windspeed, even when she is carrying water ballast. Above that windspeed, the VPP controls the sail forces to limit heeling moment, which means the crew will have to control the sails very carefully to keep BOR 90 on its feet. An interesting comparison comes from the 32nd Cup match, which was also sailed off Valencia, but in monohulls. When true wind reached 9 knots, those crews also had to start controlling their sails’ power. But in that case they wanted to optimize sail efficiency rather than limit heel angle, because unlike a multihull, the righting moment kept increasing with heel.

Apparent wind angles for both these boats are remarkably similar across a wide range of wind speeds and true-wind angles, because once they bear away, they sail so fast that their apparent wind is always well forward. For BOR 90, predicted apparent wind angles vary between 14 and 16 degrees, which makes it very close-winded. This is another interesting contrast with the America’s Cup Class monohulls that sailed with apparent-wind angles of around 17 degrees. Although both multihulls do sail upwind at narrow apparent-wind angles, their optimum upwind true angles of 40–45 degrees are relatively wide compared to a monohull because their boatspeeds are so much higher relative to true-wind speed: In other words, foot fast upwind rather than point higher and go slower.

Once a baseline performance is established, the VPP can assess other differences between boats. If, for example, A5 really is heavier than BOR 90, can it sail successfully upwind against BOR 90 without water ballast while BOR 90 sails with water ballast?

In 8 knots TWS, the VPP shows BOR 90 sailing on its leeward ama and the catamaran flying a hull. Because both are sailing with the same weight, they have similar drag, but BOR 90 should have an advantage of 5–10 seconds a mile because the increased water ballast converts directly into power and speed. A5 can also add water ballast, but since BOR 90 can add more (it has three hulls, not two) it should maintain a relative advantage. In seven knots of true-wind speed, performance is not limited by stability and BOR 90 can sail without water ballast and still fly a hull when A5 may not be able to do so.


Downwind

The predictions for BOR 90 in Figure 2 show that optimum downwind Vmg speeds, with a Code O, come at apparent wind angles of 22–28 degrees. (The VPP can account for the improved lift generated by a Code O type of sail used at narrow angles.) Because of its sail area and ability to fly a hull downwind in 7 knots true, BOR 90 might be as much as 20 seconds/mile faster than A5 in 8 knots of true wind. This advantage disappears at 12 knots true, and in higher windspeeds A5 appears to be quicker. However, BOR 90 can regain an advantage of 5 to 10 seconds a mile by adding water ballast. Just as in the upwind mode, whenever A5 adds water BOR 90 can add more. So anytime these boats are flying a hull, upwind or down, the VPP gives the advantage to BOR 90.

Even though the two reaching legs used on the 39-mile triangular course required for the second race will be a little tighter than the optimum downwind gybing angles used for the 20-mile windward-leeward courses called for in the first and third races, BOR 90 should retain the power and speed advantage similar to that established for the downwind Vmg.

Because both boats will be flying a hull in most conditions, there will also be some aerodynamic differences. For example, even though there is additional windage from BOR 90’s amas—representing a loss of about 0.5 seconds/mile—the main hull also acts as a seal between the sails and the ocean, a feature that A5 does not have. This loss of seal can be simulated in the VPP by reducing the span of A5’s rig. A redution of just three feet can make a speed difference of up to 5 seconds/mile, or 1.5 minutes on a windward leg. Both boats are so close winded that their rigs can produce the very high efficiencies that come from having large spans.

BOR 90’s wing mast and solid sail has less sail area than its soft sail, but it is reportedly much more efficient, especially in light airs. It is also reported to be able to quickly produce more camber coming out of a tack and that can help it to fly a hull more quickly. Downwind, a wing mast can produce more lift than a soft sail, although with no sail area restrictions, both teams can use large Code O headsails. In fact, A5 has reportedly increased its bowsprit to accommodate larger downwind sails.

The basis for these predictions could be wrong, of course, and with boats as large, powerful and complex as these two are, anything can happen. But if the engineers and builders have done their job and the BOR 90 team can keep the rig in the boat and avoid other structural breakdowns, they stand to prevail over A5, and possibly by a considerable margin.

Related

01-LEAD-AnchorChain

At the Helm: Anchoring Instincts

In 2015, our friends Lee & Rachel Cumberland were onboard their Tayana 37, Satori, tied to a mooring buoy in a Bahamian anchorage when a severe line of thunderstorms rolled through. The rare weather phenomenon known as a “derecho” wreaked havoc throughout the Exumas with ...read more

C38_BAVARIA-227

Boat Review: Bavaria C38

Following a change of control and reorganization in 2018, Bavaria Yachts, one of Europe’s biggest builders, tapped a new design team and started updating its entire range of boats. The first new “C-Line” Bavaria, the C42, designed by Maurizio Cossutti and Alessandro Ganz, was a ...read more

01-LEAD-DSC_1255

BoatWorks: The Rig and the Crane-Barge

During the three months my little ship lay in Belfast, Maine, I had three friends. The first was a schooner bum I’d met sailing in Florida who now worked for the shipyard next door to where I had just bought my boat, Teal, a 1963 Tripp 29, sight unseen. He would hook me up with ...read more

01-LEAD-18-Running-before-strong-winds-en-route-to-Molokai

Cruising: Hawaiian Island Hop

We didn’t get off on the right foot sailing into Hawaii. It was our own fault, of course. We should have known better. It’s never a good idea to assume that just because procedures were a certain way one year, they will be the same the next. It was an especially bad idea given ...read more

DUFOUR_470.JM-LIOT-15

Boat Review: Dufour 470

Annapolis may be the sailing capital of America, but if you looked around the United States Sailboat Show last fall, you would have no choice but to conclude most sailboats are now built in Europe. The Dufour 470 is a good example of a modern French performance cruiser. DESIGN & ...read more

01-LEAD-IMG_6563

Close Encounters: Captain Sarah Schelbert

I met Captain Sarah Schelbert back in 2019 while on the boat trip from hell aboard a seaworthy but poorly run Triton 28 in the western Caribbean. I was trying to help the owner sail his boat back to Florida from the Rio Dulce, in Guatemala. Outbound from the river basin, we had ...read more

02-Voice-of-the-Oceans---sailboat-Kat-11

Raising Their Voices

Many of us who are cruising sailors have been sailing mid-ocean or walking along a perfect beach in the middle of seemingly nowhere, only to be appalled at the amount of plastic trash we find. Few of us, however, have taken that disheartening reality and turned it into a ...read more

IC37racingonSunday-Photo-by-Paul-Todd

IC37 North American Championship

This past weekend saw 20 IC37s off Newport, Rhode Island engage in fast and furious one-design racing with the win going to Peter McClennen’s Gamecock. “It’s huge,” said McClennen of the win. “I think of the one-designs of this club going back to the New York 30 [built in ...read more