Foil-assist configurations tank testing paper

Just came across this foil-assist paper that i haven't seen linked here so far:

https://www.researchgate.net/public...achts_Hydrodynamics_Stability_And_Performance

cheers,
David

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Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
HYDROFOIL CONFIGURATIONS FOR SAILING SUPERYACHTS: HYDRODYNAMICS, STABILITY AND
PERFORMANCE

J-B R G Souppez, Solent University, UK.
J M M-A Dewavrin, Gurit, UK.
F Gohier, Bénéteau Group / ENSCBP University, France.
G Borba Labi, Principal Power, Portugal.

SUMMARY

Hydrofoil-assisted racing monohulls have undergone significant development phases in the past decade, yet very little scientific data has reached the public domain: an increasingly critical issue as the superyacht industry is now looking at the implementation of foils onto leisure vessels. Consequently, three contemporary configurations, namely a Dynamic Stability System, a Dali-Moustache and a Chistera have been towing tank tested to present the first complete characterisation of the hydrodynamic efficiency, quantification of the added dynamic stability and eventually the resulting impact on sailing performance. Furthermore, the considerations inherent to the design and installation of hydrofoils onto superyachts will be detailed. Building on extensive experimental work, this paper provides a comprehensive assessment
of current design options with both technical and practical guidelines and recommendations to improve performance.

 

I had a problem with this statement.

"Under the limitations presently considered, the configuration without any foils appears more hydrodynamically efficient. Nevertheless, despite creating more resistance, the Dali-Moustache and the Chistera would contribute to reducing the leeway angle; this could permit the vessel to sail a shorter distance on an upwind course."​

First, leeway angle has no effect on the distance sailed upwind, because the heading angle is not fixed. What matters is the lift/drag ratio. Two boats with the same lift/drag ratio will take the same course through the water, with the boat making more leeway having its bow pointed more to weather. Since the no-foil configuration had the least drag, it would have the best upwind performance when sailing with the same side force.

Second, the configurations were compared on the basis of equal side force. This is not really a fair way to do it, because of the additional heeling moment provided by the foils. The crew will take advantage of the heeling moment by increasing the side force from the rig. I think it would have been better had the upwind force line been adjusted for each configuration in proportion to the additional righting moment, and the lift/drag ratios compared. I expected the Dali foil, in particular, to come off quite differently in the ranking using this approach. However, given the pre-optimization VPP results, perhaps not.

I was initially confused by the dashed lines showing the "original" performance in Figure 14, thinking they were the no-foil configuration, until I realized they were the pre-optimized performance. That explained why the dashed lines differed for each configuration. The optimization must not have been a matter of selecting the best way to use the tested configurations, but instead made large changes to the configurations. Given that the final results were based on configurations that were different from those tested, it would have been very useful had the paper included a comparison between prediction and test data for the configurations tested. That would have calibrated the prediction methodology and lent credence to the final estimates. The paper should also have shown the optimized vs baseline geometries.

It's interesting that the Chistera configuration, which appeared to be the weakest foil arrangement in the testing, proved to be the best performer in the optimized VPP. I think that shows the importance of a good VPP when designing hydrofoils.

 

Dear Tom,
As regard the Chistera configuration of the Figaro 3, you can be interested by the arguments of its NA Vincent Lauriot Prévôt (quote #35 of this thread here below) , and by my tentative comparison, using a VPP, of the Figaro 3 with and without foils (in that case the weight difference estimated to 120 kg is put in the bulb) and with a Melges 32 ( 8 crew and 2 crew cases), in quote #43 of this same thread. The most clear advantage of the foils seems to be downwind with wind > 20 Knots. In the other cases, foils could give some advantages in terms of trim, of pitch damping, ... that a simple VPP cannot show.
From the first races with Figaro 3, the best sailors were at 7,1 Knots upwind by wind 12-14 Knots and ~ flat Sea (so Fn ~ 0,38) , my VPP estimation giving 6,6 Knots. During the final night of the Solo Concarneau, they were downwind with wind over 20 with gusts up to 30. : the winner Armel Le Cleac'h said he had surfs at more than 20 Knots, Eric Péron (the 3rd) recorded a speed max of 24 Knots (interviews in French here after).
New Figaro 3 launch and first sail https://www.boatdesign.net/threads/new-figaro-3-launch-and-first-sail.59086/page-3
Interviews of sailors : SOLO CONCARNEAU - TROPHEE GUY COTTEN http://soloconcarneau.blogspot.com/

 

… here attached again my tentative comparison with some corrections (wrong sub-tiltles in some graphics are corrected). Considerations on the lift/drag ratios of both the hull (where lift = buoyancy) and the foil(s) are instructive and lead to 2 general cases I think for foils assistance :
Case 1 , simple effect : for boat low speeds (typical of upwind sailing or when light winds), the lift /Drag ratio of the Hull (~ 35 to 70) remains a lot higher than the one of the Foil (~ 10 to 15). So a positive reward, if any, of the Foil use can rely only on the gain on righting moment RM, a slight reduction of the heeling arm (the lateral resistance of the foil is closer to the water surface than the one brings by the keel wing), a better trim of the boat, … as long as this more power can overcome the extra drag of the foil. These conditions are not fulfilled (in my estimation) for Figaro 3 speed less than 7 knots.
Case 2, double effect : at sufficiently high boat speeds ( > 12 Knots for the Figaro 3) , the lift/Drag ratio of the Foil becomes higher than the lift/drag ratio of the « remaining displacement » hull, meaning that the foil provides the double advantage of more RM and less drag. Here, you are sure to a have a positive reward.

 

Another "they just don't get it" remark.

"The heave has been neglected in this instance has it was previously shown to be beneficial for typical sailing Froude numbers (see Section 4.4), and a significant reduction in displacement would not be expected on superyachts, as it is on some of the small and light racing crafts."

Of course a reduction in displacement is expected. They are going to use less ballast if they don't need to generate as much righting moment with the hull. RM when you need it, without carrying the weigh around all the time, is the prime motivation. Superyachts are draft constrained and don't typically sail at as high a heel angle as smaller boats. Physics works in their favor here since hull RM scales faster than heeling moment, But if the foils scale too, they are actually more of an advantage vis a vie ballast weight when applied to bigger boats than they are to smaller ones. The foils can be configured to some low drag configuration when off line. Using a foil to go to windward doesn't make the boat go faster upwind, it allows it to be designed to go faster off the wind.

Another crucial error is one touched on by Dolfiman above. There is a coupling between where the sideforce is applied, and the associated heeling moment. It is different for each of the three foils, as well as for the reference hull. This has to be accounted for. It is a first order term in any meaningful VPP algorithm. Sail performance parameters from a VPP include twist and reef adjustments for each sail combo. These are based on performance objectives for any given heading and conditions, on constraints such as maximum stresses in the rig components, and on the heeling moment/hull RM balance. You have to get the heeling moment calculation right. VPP reported performance will be very sensitive to this.

 

This approach can't even begin to work. Hopefully, it is a matter of not really describing what they did. But taken literally, if your foiler hull's resistance figures correlate well with the DSYHS regressions, you are doing something very important very wrong.