Fig 1a
Fig 2
Fig 1b
Figure 1a: streamlined airflow over a wing with tip vortices; Figure 1b: the vortex sheet and downwash on a complete wing section. Figure 2: the induced angle of attack has been reduced from the actual angle of attack as a consequence of the effects of downwash
into the water to form the perfect seal with no need of a vortex other than when the boat inevitably flies a little too high. Wetted surface area would go up but this
idea is not without merit. Because the extra wetted surface area would be long the addi- tional viscous drag would not be as high as if it was contained in a short surface. Drag coefficient (Cd) reduces with
rising Reynolds number (Re). Re, in a given fluid, is the product of length of run and velocity, so that long surfaces have comparatively high Res. One way of look- ing at this is, because the boundary layer grows in thickness with the length of the run, the surface becomes better lubricated and frictional drag, per unit area, reduces. Looking at the drag at each point along
the length of the run from the bow, the drag coefficient (Cd) will become smaller the further from the bow it is measured, so that the overall Cd will be less than for a short surface. Another conclusion is that wetted area in the aft part of the boat is generally less draggy than at the bow. But in this instance it means that, though
the wetted area of such an arrangement might be quite large the associated drag will not be anywhere as great, and the advantages of totally closing that gap may be larger than any increase in drag. Perhaps we can put some numbers to it that might reveal if indeed it might be an advantage… Assuming that the part of the skeg in the
water is a flat plate there will be very little wave drag and no pressure drag and we can perhaps ignore them. The extra drag will thus be frictional and easily calculated – although, due to the very low aspect ratio of this skeg and thus the amount of possible crossflow on the surface plus spray drag, a bit of an educated guess is required on what drag coefficient (Cd) to use. But, using a figure of 0.001 and an area of 4.0m2
of 30kt (15.432m/sec), using the formula: Drag=0.5*density*Cd*area*velocity2
(20m*0.1m*2) and a speed :
Drag=0.5*1025*0.001*4.0*15.43222 = 976.401n. By contrast the aerodynamic induced
drag of the rig, which closing the gap will reduce, at an apparent windspeed of approximately 50kt, is derived as: Induced drag = lift2 velocity2*Pi*span2
/0.5*density* So, assuming a rig force of 29,435n, 54 SEAHORSE
(righting moment/heeling arm) a density of 1.225 and a span of 30m then the rig induced drag will be: 29,4352
/(0.5*1.225*25.722 *3.142*302 )
= 756.19n As induced drag varies inversely as the
square of span, doubling the effective span by completely sealing the gap under the hull reduces this figure by four times to 189.05n. The saving in induced aero drag will be about 567.07n so the immersed depth of the longitudinal strake would have to be less than the 100mm used in the calculation, say 600mm to just break even. In reality the small gap under the hull
and the vortex developed to seal that gap will increase the span, but it will be some- what less than double and will come at some cost in drag. Whether a complete sealing of that gap, which will double the span, will pay will therefore depend on just how near to doubling the span the vortex gets and the cost in vortex drag. It does seem very borderline but would
indicate that, provided the strake or skeg was no more than a flat plate, for the lower 60mm or so it might just be worth it, and slight touchdowns, only involving the lower part of the skeg, would not increase the drag by much, which might make height control a little less critical. Luna Rossa 2024, even though she has
a V rather than a U-shaped skeg and thus does not need a flat-plate vortex generator, nevertheless has one on the bottom of the skeg. Was that for the reasons discussed or is it to just create a more powerful vortex? Looking at the challenger races, I think
that Luna Rossa and Patriot were ‘poten- tially’ the quickest boats there. I am, how- ever, still rather puzzled by the cockpit placement on Patriot. I can see that moving the crew weight forward increases the download on the elevator, but surely the cyclors could still be moved outboard where there is greater hull depth to accom- modate them and where they may have been able to cycle more productively? I wrote earlier when formulating this
article that ‘While talking of cyclors it did always seem somewhat comical having four guys pedalling like mad with, appar- ently, no direct connection to what the power is doing.’ Fortunately the AC38 Protocol dispenses with an archaic concept in favour of a modern electric solution.
One final thing. These boats sail so close
to the water that the hull is probably producing some ground effect downforce (ie suction that will increase the effective weight of the boat and thus its righting moment). Ground effect wears off rapidly with
height above the ground (in our case water) and reductions in span. In our case span is small and the height above the water is large, so that ground effect forces will be small; yet proper shaping of the underside of the hull will effectively camber it in the correct sense to produce downforce, albeit at some cost in hydro - dynamic drag when in the water. However, doing so in a conventional
manner will produce a pitching moment which, because the camber is upside down, will be a nose-up moment – the last thing you want because it will have the effect of moving the longitudinal centre of gravity (LCG) back. This will reduce the downforce that the elevator is producing to balance the forward LCG and thus the increase in right- ing moment this produces. You could end up just swapping elevator downforce for canoe body downforce with no net gain (and a loss with the hull in the water). However, by using a reflex camber line,
that pitching moment can be reduced or even eliminated and then the benefits of canoe body downforce will be in addition to the elevator downforce and righting moment, and performance maximised. If ground effect downforce was used a
chine at the edge of the canoe body would produce a beneficial vortex that would have the effect of increasing the canoe body span, and thus the effectiveness of the ground effect. Britannia does appear to have such a
chine on the edge between the canoe body bottom and topsides aft of the swing arms, and the forebody does seem shaped to better produce an airfoil between deck and underbody. This is one part of the boat that makes sense to me and I like. It seems the rest of the package worked pretty well too, but not quite well enough… and she’s still no oil painting. If she’d won she would have been pretty. Or was it because she wasn’t pretty that she didn’t win? They do say that if it looks right it will
go right. Unfortunately, to my eyes at least, she never looked quite right.
q
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