# Dynamic ballast

Discussion in 'Sailboats' started by Dolfiman, Jan 17, 2018.

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### DolfimanSenior Member

In a previous thread reminded here below, we had an interesting discussion about what we called a "dynamic ballast", i.e. to substitute all or fraction of a ballast-bulb by a foil hinged to the tip end of the keel and providing a downforce.
MX Nova Winged Keelboat Concept https://www.boatdesign.net/threads/mx-nova-winged-keelboat-concept.51259/page-2

I had promised to carry out a numerical exploration, on how such foil can reduce the heel angle, can delayed the reefing and affect the speed (taken into account the extra induced drag linked to this negative lift).

About the method and main assumptions :
- A numerical hull (inspired by a Melges 32 for which 8 crew sit windward is needed to exalt the boat full potential) is built with the speadsheet application Gene-Hull, from which data and parameters can be used to feed a VPP.
- A simplified VPP is also built for 2 typical sailing cases, upwind with true wind angle (twa) between 39° to 43° and downwind with true wind angle 140°, and validated for 3 boat displacements (according to three bulb ballasts) by comparison with the results of a Maxsurf VPP. This VPP aims to be a tool for comparison between various configurations with and without foils.
- A foil assembly is proposed which can orient the foil in the direction of the leeway when heeling occurs, and so avoid as much as possible that side forces act significantly on the assembly. Consequently, foil is assumed horizontal and its downforce lift is assumed vertical.
- This assembly as well as the variable trim of the boat tend to slightly increase the foil incidence when the boat heels, but otherwise no active command of the foil incidence is taken into account in this simulation.
- The numerical simulation concerns 3 initial displacements cases, for which 2 foils surfaces are tested. Full results regarding speed, heel, displacement anf foil drag are discussed.

About the main conclusions (with this simulation specifically) :
- The induced drag of the foil is usually not negligeable enough to avoid a reduction of the speed. So the use of such « passive » foil (i.e. without active command of its incidence) is relevant only for others objectives than speed.
- When the objective is to have a minimum displacement, e.g. a boat with a low ballast ratio in the range of 24%, we can compensate the lower ballast by the dynamic downforce of a foil providing equivalent heel angles, but it is at the cost of 10% to 17% speed loss in upwind conditions and less than 9% in downwind conditions.
- When the objective is to increase dynamically a ballast ratio from an initial value of e.g. 40%, a foil can give a reduction of the heel angle (but especially in downwind condition, up to 7°, e.g. 14° instead of 21°) and a delayed reefing. The heel reduction effect as well as the speed loss depend of the foil surface. When using a small enough foil, we can have a full positive result, i.e. both heel and speed gains, in downwind conditions when wind speed is over 16 knots, and quasi no speed loss in upwind conditions (0,1 to 0,2 knots VMG).
- An heavier bulb, for a higher ballast ratio (e.g. 51% instead of 40%), when comparing to the above small foil option, leads to : less speed but lower heel angle when wind < 8 knots, less heel angle and equivalent speed for wind 8 to 16 knots, better speed (upwind) / lower speed (downwind) when wind > 16 knots.

Axis of investigation to improve the efficiency can be :
- the use of a foil with an active device for the lift adjustement (flap) can improve these results in particular for the light winds < 10 knots where the speed loss is usually maximum while the heel reduction is not crucial.
- to add sails area when adding a foil, to compensate the speed loss instead of searching for less heel angle.

#### Attached Files:

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philSweet and Doug Lord like this.
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### tspeerSenior Member

Good job! You've provided a great example of how to go about investigating and resolving new boat design ideas.

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### DolfimanSenior Member

Dear Tom, many thanks for your kind message !

The fact is that in upwind condition, it seems impossible to have a better result in term of speed when exchanging 1 kg of bulb ballast by one kg of downforce lift. But downwind with good winds, when boat speed is over Froude 0,6, with a small enough foil you can boost the RM at low cost in term of drag and then get an extra speed.
That could be explored too for a trimaran equiped with a daggerboard and such foil, i
ncluding for safety purpose in case of wind gust. A
ctually the 2 last Ultims launched, Gitana 17 and Banque Populaire IX, have already the device for such tests.

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Don't forget Maserati-when in her "foiling mode" she uses an adjustable lifting foil on her daggerboard........
Maserati's daggerboard foil:

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### DolfimanSenior Member

Yes Doug, and I don't forget that you support this concept since long.
We should learn a lot on its relevance/ efficiency in the coming 2 years when some foiling reports will be made public.

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Dolfiman, there is a history in multihull design of the use of downforce particularly on the Hobie Trifoiler, Rave and Osprey where the foils are used to develop most of the RM. Newer multihulls like the S9, IFLY and Whisper also use dual independent wands but because of their relatively narrow beam rely on crew movement for the majority of RM.
This is Greg Kettermans comment on his Hobie Trifoiler System-if you haven't already seen it you might find it interesting:

HYDROFOIL SAILBOATS IN GENERAL
"Hydrofoil boats can be categorized into two categories; 1) Incidence controlled hydrofoils* and 2) surface piercing hydrofoils. The difference lies in the way the boat maintains the proper altitude above the water surface. A surface piercing hydrofoil boat maintains proper height by varying the amount of foil submerged. The boat raises up as the speed increases and reduces the amount of foil submerged and therefore the lift. The boat finds equilibrium at the proper altitude. An incidence controlled hydrofoil sailboat has a mechanism that controls the angle of attack of the foil to maintain the proper altitude. It is generally believed that surface piercing is simpler, but incidence control is more efficient. In reality, it is the method that works with fewer problems that is simpler.
From the beginning it was felt that incidence control was better suited for a sailboat even though most of the existing hydrofoil sailboats were of the surface piercing type. There are many advantages of the incidence controlled foils; however, the most important is what I call the DLA (dynamic leveling affect). This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited.
Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise. The dynamic leveling affect not only produces a dramatic increase in top speed, but is also responsible for all the other key features that this stability provides.
The other major advantage of the incidence controlled foils is they are less affected by the waves and other surface affects. Drag and losses associated with the surface are the major reason incidence controlled foils are more efficient.
All hydrofoil sailboats have problems with ventilation; however, surface piercing foils have larger problems, because the foils are piercing the surface at a smaller dihedral angle which makes it easier to ventilate."

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* On the Trifoiler the entire foil was moved to control RM, lift and negative lift hence the term "incidence controlled foils". On the Rave the incidence was generally fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil. Lift and negative lift on a Rave foiler is generated by the wand (designed by Dr. Sam Bradfield), a surface sensor(dragging in the water) and attached directly via linkage to a flap on each main foil. The wands are independent just like the trifoiler "incidence controlled" foil sensors.

Last edited: Feb 9, 2018

7. Joined: May 2009
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Dolfiman, do you have the names of the reports that may be made public or any more information on them? Thanks for your work in this area!!

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