Sailing Foiler Design: Foil Assist and Full Flying
I'd like help from everyone interested in foiler design to build this thread into a comprehensive source of references, discussion, ideas etc. I want to copy and paste relevant material by people such as Tom Speer, Greg Ketterman, Dr. Sam Bradfield ,Mark Drela, Steve Clark , Gary Baigent and many others. I'd like to see the thread turn into a resource for all of us as time goes by.
Some topics I think would be worthwhile to expand on:
1) Foil Assist vs Full Flyingwhen, how, why?

2) Foil Assist in Current Applications:
a. DSS
b. lifting foils on rudders( National 12, I14, some cats, etc)
c. Multiple foils for foil assistnot full flyingadvantagesdisadvantages.

3) Lift formulapratical formula for calculating lift at various speeds and areas.

4) Full Flying Foiler Configurationshow and why.

5) Monofoilersapplications of lifting hydrofoils for full flying or foil assist on monohulls from dinghies to 100' + maxi keelboats and beyond.

6) Multifoilersapplications of lifting hydrofoils for full flying or foil assist on multihulls from 12'(or less) to 100 +'.

7) Lifting hydrofoils in cruising applications.

8) Lifting foils in Kite sailing and Boardsailing.

9) Reference books, papers etc.

10) Veal Heel, its applications and the physics involved.

11) Foiler "jumping" including how why and especially reentryprimarily for dinghy, board or kite applications.

12) Marketing Foilers: how?

13) Righting Moment from hydrofoilsapplications, results, ideas.

14) History of foiler design.

15) Building foils.

16) Foil Systemsadvantages, disadvantages, examples etc.
a. Altitude control systems for fully submerged foils.
b. Surface piercing foils.
c. Ladder foils.
17) The future of foiling.
18) Surface piercing foils.
For anyone wanting a practical guide to hydrofoils this is it. It is a book by Ray Vellinga called "Hydrofoils Design Build Fly, published by Peacock Hill Publishing, Gig Harbor Washington ISBN number: 9780982236116
email for Peacock Hill Publishing: patvell@peacockhillpublishing.com
It is written in a clear ,concise manner and teaches you how to calculate foil area and provides a lot of info on hydrofoil types and control systems including a chapter on the Moth foiler.
The formula for lift from page 27: ( get the bookthere is more to understand than just this formula)
L=Vsquared X S X Cl
L= Lift in pounds V= Velocity,ft./sec(1mph=1.47 ft/sec) (1 knot= 1.15mph) S=Surface area of foil projected vertically in square ft. CL=Coeficient of lift, this is a dimensionless number found in tables of wing profiles as in Theory of Wing Sections by Abbott and Doenhoff.
For a 63412 section used on a Moth,for instance, the Cl will vary from .9 at takeoff to 0 at top speed with cruise(neutral flap) at about .3 .
=========
This is one of the best references for airfoils I've found on linestill learning to use it. It shows the airfoil and also gives Lift coeficient etc. http://www.worldofkrauss.com/foils/784
Actually I miss the term 0.5 * ρ in that formula, since lift (or drag) are usually nondimensionalized using a reference area (here: projected wing area) and the stagnation pressure P = 0.5 * ρ * v², which can be derived from the Bernoulli equation.
You can leave this out of course, as long as you stick to one density and as long as you use lift (or drag) coefficients that already include this factor  which is not the case with the coefficients from the graphs in "Theory of Wing Sections" (or in any other book on that topic that I know).
Also be aware that Abbott and von Doenhoff's graphs start at Reynolds numbers of 3e6, which is about three times the Rn of say a Moth foil at 20 knots.
However the book sounds interesting and I'll try to get hold of a copy, so thanks for the hint!
__________________
All the best, Olav GER1045 ____/)_/)/)___
1) Actually I miss the term 0.5 * ρ in that formula, since lift (or drag) are usually nondimensionalized using a reference area (here: projected wing area) and the stagnation pressure P = 0.5 * ρ * v², which can be derived from the Bernoulli equation.
You can leave this out of course, as long as you stick to one density and as long as you use lift (or drag) coefficients that already include this factor  which is not the case with the coefficients from the graphs in "Theory of Wing Sections" (or in any other book on that topic that I know).
2) Also be aware that Abbott and von Doenhoff's graphs start at Reynolds numbers of 3e6, which is about three times the Rn of say a Moth foil at 20 knots.
However the book sounds interesting and I'll try to get hold of a copy, so thanks for the hint!

Hi, Olavthe book is worth having!
1) Covered in the bookpages 3135. As to the use of the info in " Theory of Wing Sections", Vellinga says:
"Looking up in the lift table we find the lowest Rn for the NACA 63412 wing section is 3,000,000. Short of lugging our foil section to the nearest wind tunnel for testing, we are forced to use the closest data available, so lets go with Rn=3,000,000. But keep in mind, with our lower Rn, our stall AOA, the CL max, and the drag will be slightly less."
2) As to the formula, Vellinga says: "Because 1/2 p=1, we can simplify the formula to: L=V2 X S X Cl . "
Vellinga has built numerous power hydrofoils over a long period. I think a lot of people would save themselves a great deal of grief by either checking this book out of the library( if possible?) or buying it. It is based on proven technology presented in a way sure to get someone really interested flying sooner rather than later.
"Because 1/2 p=1, we can simplify the formula to: L=V2 X S X Cl . "
OK, if you use imperial units (slugs/ft³ for water density) the approximation and simplification by Vellinga makes sense. With metric units this doesn't work.
I just ordered the book at Amazon and hope to have it next Tuesday.
__________________
All the best, Olav GER1045 ____/)_/)/)___
Thank you for your thoughts on the lift formula that was simplified for my book, "Hydrofoils: Design, Build, Fly". As you point out, I claim "Because 1/2 p=1, we can simplify the formula to: L=V2 X S X Cl . "
You responded: "OK, if you use imperial units (slugs/ft³ for water density) the approximation and simplification by Vellinga makes sense. With metric units this doesn't work."
For our friends outside the American measuring system, I thought about putting in the book a factor for converting the formula to metric units. But an important goal I followed was to keep the math as simple as possible. However, I am pleased to respond to you and provide a factor to convert the formula variables from the American system to the Metric system. The Metric formula would be: L= 4.06 X V^2 X S X Cl.
The 4.06 is the conversion factor. The variables inserted into the formula will be in kilometers per hour and square meters. The resulting lift will be expressed in kilograms. The coefficient of lift is dimensionless and does not change.
The lift formula is taken from the sister science of aerodynamics. Because air is less dense at altitude and denser at sea level, the density is usually included in aircraft lift calculations. Temperature also affects air density. Water varies little in density even with changes in temperature and altitude, like at lake Titicaca, 3800 meter above sea level. One could adjust for salt water vs. sea water, but the difference is small enough that I chose to avoid this unnecessary complication.
The next three posts will give some accurate data(as of 2009 I hope) of three current foilers to give a general idea of what is required for a practical foiler
The Moth was the first sailing bifoiler in history and John Iletts incarnation in 1999 was the first foiling Moth to use a wand as altitude control. Ilett patterned the wand system after Dr. Bradfields system except that he located the wand near the bowbut the princible was identical. The Moth has created a revolution in dinghy sailing with speed that beats all sailboats under 20' in foiling conditions. Pretty incredible for an 11' boat(+gantry).
Thanks to Ray Vellinga's new book"Hydrofoils Design Build Fly" for some information from Table 185, page 219. Also see Bill Beavers paper on the Moth below.
================== LOA 11' + rudder gantryappox. 12.75'
 Beam 7' 2"
 SA:
a. main only= 86 sq.ft.
 Weight:
a. hull+rig= 66lb.(varies a bit)
b. crew= 175lb.(varies from 140200lb)
c. all up sailing weight= 241lb.
 Foils two fully submerged foils, one on daggerboard ,one on rudderarea and loading varies between boatssee Vellinga's book and "The Foiling Guide" by Adam May(below):
a. mainfoil area= 1.09 sq.ft.
b. rudder foil area= .84 sq.ft.
c. mainfoil loading @ 80% of sailing weight= 176.9 lb./sq.ft.
d. rudder foil loading @ 20% of sailing weight= 57.38 lb./sq.ft.
e. nominal angle of incidence of mainfoil= 01.5 degrees
f. nominal angle of incidence of rear foil= 0 degrees( some have adjustable trim)
g. mainfoil flap angle= +(down flap)=30 degrees; (up flap)=15 degrees
h. rudder foil flap angle= adjustable by tiller twist grip/ some boats angle the
whole rudder instead of using a flap.
 Altitude Control SystemAs mentioned above most Moths use bow mounted wands, though some also have experimented with twin bow mounted wands and twin midship wands. A single bow mounted wand is to one side of the CL and therefore altitude can change tack to tack. Twin wands eliminate this problem. No experimentation yet reported in manual altitude control.
================= W/SA(weight in pounds divided by SA in sq.ft.)=2.8 SA/total foil area(both sidesnot struts)= 22.28

Mirabaud is still the largest bifoiler in history. Thanks to the generous help I received from Thomas Jundt, designer and owner of the boat this is accurate as of 2009. Awaiting details of the 2012 version with a new taller version of a wing sail and more total sail area.
================== LOA 26'
 Beam 17'
 SA:
a.big rig= 355sq.ft.
b.small rig=258 sq.ft.
 Weight:
a. hull+rig= 374lb.
b. crew= 528lb.
c. all up sailing weight= 902lb.
 Foils two fully submerged foils, one on daggerboard ,one on rudder:
a. mainfoil area= 3.77 sq.ft.
b. rudder foil area= 3.77 sq.ft.
c. mainfoil loading*@ 80% of sailing weight= 191.4lb./sq.ft.
d. rudder foil loading* @20% of sailing weight= 47.85lb./sq.ft.
e. nominal angle of incidence of mainfoil= +.5 degrees
f. nominal angle of incidence of rear foil= 0 degrees(trim+1 degree,2 degrees)
g. mainfoil flap angle= +/ 12 degrees
h. rudder foil flap angle= +/ 12 degrees
*Note from Thomas:
a. at takeoff(8.5 knots) loading is 50% main foil; 50% rudder foil
b. at 23 knots all load on main foil
 Altitude Control SystemMirabaud uses twin wands set about halfway between the bow and the mainfoil and in 2010 will use, for the first time, a manual control system that bypasses the wand for direct crew control of the mainfoil flap particularly in rough conditions.
================= W/SA(weight in pounds divided by SA in sq.ftbig rig)=2.54 SA/total foil area(both sidesnot strutsbig rig)= 23.54
The Rave multifoiler was designed by Dr. Sam Bradfield in the nineties. It uses three hydrofoils with dual independent wands that allow the leeward side to lift up and the windward side to pull down. All the RM for the boat is derived from the foils and not crew movement. Foil loading below is calculated based on a .5lb/sq.ft pressure@ a 10'CE spread evenly between the main foils. Loading goes up considerably as the boat sails in more pressure.
================== LOA 16'
 Beam 16'
 SA:
a.main +jib= 195sq.ft.
b.main,jib and screecher= 292sq.ft.
 Weight:
a. hull+rig= 380lb.
b. crew= 175lb.
c. all up sailing weight= 555lb.
 Foils three fully submerged foils,two forward ,one on rudder:
a. mainfoil area= each 1.77sq.ft.; total 3.4 sq.ft.
b. rudder foil area= 1.77 sq.ft.
c. mainfoil loading*@ 80% of sailing weight= 176.48lb./sq.ft.
d. rudder foil loading* @20% of sailing weight= 58lb. /sq.ft.
e. nominal angle of incidence of mainfoil= +2.5 degrees
f. nominal angle of incidence of rear foil= 0 degrees
g. mainfoil flap angle= +/ 20 degrees
h. rudder foil flap angle=+/ 20 degrees
*Note 
Foil loading at .5lb/sq.ft. pressure (approx. 8.69 knots/10mph) / angle of incidence measured from static waterline. Altitude Control System The Rave uses dual ,independent wands that not only control altitude but control righting moment as well. Some Raves have been successfully sailed/raced with manual altitude control.The boat features retractable foils and also has two "flight" settings to reduce draft where required.The wands retract with the foils and are attached to the foils.
================= W/SA(weight in pounds divided by SA in sq.ftbig rig)=2.84 SA/total foil area(both sidesnot strutsbig rig)= 18.36
The Rave , Hobie Trifoiler, Skat(40'Rave) and Dr. Bradfields new 18' Osprey all use dual independent altitude control systems.
Here is a description of this type of system by Greg Ketteman, designer of Long Shot and the Hobie Trifoiler:
HYDROFOIL SAILBOATS IN GENERAL / HYDROFOIL CONTROLLED STABILITY
"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."

* 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.
Foil Assist is when a hydrofoil or hydrofoils are used to reduce the displacement and wetted surface of a boat without causing the whole boat to fly. Generally, "foil assist" does not require an altitude control system. Foil assist is used on multihulls( Orma 60's, Mod 70's, Banque Populaire V, A Class and C Class cats, Nacra 20, Sea Cart 26, Marstrom 32 and many more) and monohulls( I14, National 12, Quant 28(DSS), Welbourn 25(Brace,Brace,BraceDSS), JK 50 cruiser racer(DSS), and more.
===============================================
Most radical of all has been the introduction of curved foils below the hulls that act like wings, producing vertical lift and reducing drag by raising even the leeward hull out of the water. Lift equals speed; speed begets more speed. "Sometimes you are even 100 percent on the foil  the whole boat on one single point," says Vincent Lauriot Prévost, the Brittanybased designer behind seven of the multihulls that competed in the Jacques Vabre. "Six years ago, foils took 30 percent of the displacement, and the boats were sailing at 28 or 29 knots," he says. "This year it's 70 percent, with speeds of up to 39 knots, which means you are sailing on the very sharp edge of a knife. We are pushing the limits of the machines." Flying at 45 miles per hour on a foil means the boats are as precariously balanced as a tightrope walker. If the foil loses lift for any reason  turbulent water, say, or a floating log  bad things happen. Fast.
================== And from an Interveiw about the Mod 70'sreplacing the ORMA 60 tris:
Excerpt from interview by Lia Ditton with Vincent Prevost on SA frontpage:
MOD 70 LD: The Multi One Design or MOD 70 is the socalled new one design, which will replace the ORMA 60 fleet. You must be very excited to be the designers for this project. What new ideas about multihulls were you able to exercise with this class?
VLP: The new idea is a very pragmatic one: conceive a multihull, which is the synthesis of 20 years of evolution and development. A new generation of hull shapes, more seaworthy platforms, increased safety factors for structure reliability, all that staying in the same type of power as the last ORMA 60 projects.
LD: “A fast, modern design with foils” says your VPLP website. Am I correct in assuming that if these foils are designed to support the same percentage of displacement as an ORMA tri, a single foil could lift up to 70% of the boats weight in "normal" sailing?
VLP: Correct.
LD: At what point would you say a multihull becomes a ‘foiler’ rather than ‘with foils?’
VLP: Good question! No one has ever asked me that! Maybe a foiler would apply to ‘Geant’ whose main rudder was equipped with a T foil with adjustable angle of attack to get rid of hull buoyancy at the stern.
LD: Standardizing and establishing a one design of this size with such a varied design brief (offshore, inshore, full crew, short handed) must have presented a great deal of technical challenges. What sort of design boundaries has the MOD 70 project pushed for you?
VLP: The MOD 70 design brief is clearly for offshore races with 6 people on board. Inshore races will happen but it is not the primary goal of these boats.
LD: Wave piercing bows seem to be the zeitgeist. Can they be seen as one approach to addressing two different problems: reducing pitch for the purpose of stabilizing airflow over the sail plan and moving reserve buoyancy forward in big seas to reduce the chance of pitch poling? If we assumed a fixed LOA then wouldn't the later of these two benefit by adding reserve volume to the extremes thus allowing a more powerful hull and rig for the length?
VLP: These kinds of bows are possible when, as for the MOD 70, hull length is not limited by any box rule. The volume distribution forward of the front beam to counteract pitch poling is distributed more aft and the extra length allows us to have a finer angle of entry, reducing drag when passing through waves.
LD: Isn't the hull length of the MOD 70 limited by the box rule?
VLP: The idea was to design a boat safer than an ORMA, more in terms of structural reliability than in high speed behavior, notably the longitudinal stability, all of that by keeping the load numbers of an ORMA (same range of rig size, same deck gear, same appendages etc…) Increasing the reliability implies adding structure and weight at the same time. To keep the same load values, we have reduced the beam size to end at the same righting moment.
About longitudinal stability, we replaced the bowsprit with a main hull bow extension to reach the same overall length, which corresponds to adding 10 feet. This extra main hull length at the front, combined with the 5 feet increase in the length of the floats adds some important buoyancy forward to delay the time when the boat could capsize. So you are right! I made a shortcut. It is not a box rule, all boats being identical as monotypes.
I didn't know that Geant, now Vodafone, had an inverted T rudder setup  she certainly didn't have one when assembled and launched here, maybe the crew has tried it out since. Michel Desjoyeaux was/is always pushing the boundaries (was frustrated that the MOD 70 fleet won't allow him to experiment) so it's not surprising. Gitana X (the ORMA with the X beam design) also had T rudders but that boat was the dog of the fleet, (although designed by a collection of brilliant designers) mostly because she was too heavy, I think. Maybe also, too many superb brains clashing during the design.
Technical info from 2010 on Doug Halsey's Broomstick foiler:

This is the fourth in my series presenting the technical details of as many foilers as possible.
So far:
1) Mirabaud post 10
2) Rave post 11
3) Moth post 9
 Broomstick

Video compiled in 2011 here: Onboard Video  Hydrofoil Trimaran Broomstick
This boat is a surface piercing foiler designed and built by Doug Halsey and I thank him for his generous contribution of this information.
LOA 15'

BEAM max 17'
Between tips of "V" foil 13'9"
 SAIL AREA:
1) 108 ft^2 using the usual rig (main + jib)
2) 143 ft^2 using the larger main + same jib (haven't used since 2007)
 WEIGHT: ~210 lbs for usual setup (smaller rig, 7' amas, etc.) + crew=355lb
20 lbs less when I don't use the amas190lb+crew= 335lb
20 lbs more when I use the larger rig230lb+crew= 375lb
145 lbs = crew weight
 FOILS: Describing surfacepiercing foils is obviously harder than fullysubmerged foils, since
the area is constantly changing, but maybe this will make sense:
Dihedral Angle = 60 Deg.
Chord = 6"
Section Shape = Conventional, uncambered, untwisted ~NACA 0012
Foil Span = 23.3". This is the horizontal projection of the span of 1 foil at takeoff, which I'm
defining as the point where the lowest point of the main hull just touches the water, assuming the boat
is level in both pitch & heel. At higher speeds, the span of the leeward foil reduces to some fraction
of this value, depending on the speed, position of my weight, mainfoil incidence setting, aftfoil
incidence, etc. Span of the windward foil reduces much more, sometimes almost to 0.
Foil Area = 23.3" X 6"= 139.8 In^2 = .97 Ft^2 (Again at takeoffeach side)
MainFoil Incidence : Currently ~8 1/2 Deg. with respect to the design waterline. This has been changed a few times over the years, each change requiring patching & redrilling bolt holes in the foil brackets (attached to the crossbeam) & the vertical stubs (at the top of each foil). Too small an angle gives insufficient righting moment when hullborne. Too large an angle gives too much bowdown pitch angle when flying.
Note:I've designed a new set of main foils, with similar planform, but cambered & twisted sections. Also a new deeper aft foil. It will probably be several more months before I sail on foils again though.
 AFTFOIL INCIDENCE : This angle is adjusted with a screw mechanism & control bar under the tiller. In
past years, I would have to luff up, lean over the aft deck & go thru all sorts of contortions to adjust
it, so it was usually set at some sort of compromise angle. Last year, I improved it so that I can
adjust it (on 1 tack anyway) by twisting the tiller extension. That way I could try to find the
"optimum" setting where the boat flies at just the right height for the existing conditions.
 Construction
Main Hull planked foam, epoxy glassed
Amastich and glue/ 3mm Okume ply
Foils wood,hand shaped and epoxy glasse
More info here(scroll to near bottom): http://www.foils.org/gallery/sail.htm Since that time there have been several improvements & speeds over 25 knots have been recorded on 3 separate occasions.
==========
Pictures below by Terry Curtiss:
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.