Left: one of the designs to beat for the new Class40 scows coming on stream in the coming months, Sam Manuard’s Mach40.4 is built by JPS in La Trinité – three examples are sailing with two more already well advanced. Eric Levet of the Lombard office suggests that while the Mach40.4 is very quick reaching, with less aft rocker she is stickier VMG running than David Raison’s TJV-winning MaX 40. Above: the new Pogo 40S4 is designed by Guillaume Verdier – as are the rest of Pogo’s current open class line-up. Verdier studied the current scow shapes carefully before opting for refinement rather than reinvention. All the new designs aggressively exploit the chamfer permitted on the sheerlines, although a deck-beam rule prevents the savage cutaway upper bow sections of recent Imocas
‘Drag,’ he explains, ‘is always broken down into friction drag and pressure drag – more usually labelled wave drag. ‘On a Class40 friction drag dominates up
to about 8kt, before the pressure drag takes over reaching its maximum in (and propor- tional to) the speed range between 10 and 18kt. Then it quickly falls away again as we reach planing speed. ‘With that generous rocker the MaX 40
of Raison experiences a drag penalty at speeds between 12 and 18kt. But above 20kt the rocker ensures a lower wetted surface and therefore lower drag. ‘The challenge for the Lift 2 was therefore
to combine a low-pressure drag at speeds between 12 and 18kt, to be fastest when VMG sailing in light and medium winds, but then quickly lift the hull out at higher speeds for better behaviour in a seaway and to reduce drag further above 20kt. ‘Numerous comparisons, but remaining
with similar forward shapes,’ he continues, ‘have allowed the optimisation of our aft keel line to minimise drag at a comparable nose-up trim. Two parameters are decisive here: the depth of immersion of the transom on the one hand and the rocker of the aft keel line on the other. ‘The MaX has a medium stern immer-
sion and plenty of rocker aft. But we iden- tified a completely different optimum, with a higher transom but with a negative rocker (ie a concavity) on the keel line.’ But this principle of using an inflection
in the keel line should, Huetz emphasises, ‘be handled with considerable care… ‘Get your approach wrong and it can
lead to the opposite effect to what we seek, pushing the nose down; there is a compro- mise to be found. It’s all a matter of how far to push the transom in, and where to place the inflection.’. [Interestingly, co-designers
Daniel Andrieu/Guillaume Verdier success- fully introduced just such an aft rocker inflection on their IRC Sun Fast 3300, simi- larly aiming to shed hull drag in the 12-16kt speed range]. Huetz continues, ‘The slope of the for-
ward lines, ie the angle of entry to the water in a vertical plane, is also critical. The objective is to produce a positive “rearing-up” by unloading the boat’s nose without a spike in hull drag further aft. The deeper/lower your hull is the more water you push,’ adds Huertz. ‘The more tangen- tial you are to the surface the more you slide over the sea, even managing to catch a few balls of air passing beneath the hull!’ The objective remains the same when
heeling, but the problem becomes more complicated because the Class40 forbids (actually limits) round noses by placing a measurement point with a maximum width on deck taken 2m behind the bow, that is to say slightly in front of the start of the waterline when the boat starts to trim up. Lombard’s team solved the problem by introducing a kind of hump in the hull profile, which gives the boat a unique silhouette when seen from the side. The Class40 is governed by a rule box,
and the Lift 2 fills out that box to the milli - metre, exploiting the maximum width allowed of 4.5m. The design stays very wide along the waterline, the objective being to achieve the most powerful boat in the class… even though a race like the Route du Rhum is essentially downwind. But according to Lionel Huetz, what can be overlooked is that upwind and reaching conditions dominate the first few days of this race, and it is the boat that has man- aged to take the lead and hoist its down- wind sails the earliest that will arrive first in Guadeloupe, even if it is not intrinsically
the fastest boat sailing VMG angles. In comparison the new Pogo 40 is less
radical, less muscular, a generally ‘softer’ shape all round. The design was entrusted to Guillaume Verdier, who had already designed the last two Mini 6.50s of the Structures Shipyard in Brittany: the Pogo 3 Series and the Pogo 4 Proto foiler. ‘Guillaume was onboard to work with
us but on the strict condition that a proper CFD and VPP study was carried out for research and optimisation,’ says Erwan Tymen, naval architect and head of the Structures design office. ‘He wanted to follow the route of technological refine- ment… not dramatic revolution. Luckily that suited us very well too!’ According to Tymen, ‘The experience of
the Minis has shown that the design of scow hulls is not as simple as it may some- times appear. In the Mini Series division Raison’s Maxi 650 does not win every- thing… far from it. The Pogo 3 “skiff” has continued to win races, including the last Mini Transat. ‘Within the Mini class the scow designs
you see are still subtle, surprisingly so to some people. A scow that performs well when reaching is an easy thing to make, but it’s costly on wetted surface area and this wetted surface punishes you in down- wind VMG conditions.’ However, unlike the Mini 6.50s, in the
Class40 the dominant driver of design is the Route du Rhum solo race, where after those first few days most of the course is still downwind. ‘And in CFD studies on flat seas it’s very difficult to design a scow that goes as fast downwind as a boat with a more conventional pointed nose,’ says Tymen. ‘To make real progress we had to start
out trialling around 20 candidate hull types, slowly narrowing down to about
SEAHORSE 49
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 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120
