Sailing Foiler Design: Foil Assist and Full Flying

Dynamic Stability Systems(DSS)

This is probably the newest "foil assist" system for monohulls-keelboats so far- and not only lifts the boat but aids in pitch control and, most importantly, adds righting moment. The foil slides side to side.
Here are some relevant threads:

-- http://www.boatdesign.net/forums/sailboats/design-dss-foil-assist-keelboats-41729.html
-- http://www.boatdesign.net/forums/sailboats/quant-28-foil-assist-keelboat-dss-38421.html

 

Sailing Foiler Design

One of the most exciting developments in this field ever is the adoption of lifting foils on the new Americas Cup boats. The first to fly was Team New Zealand on a main foil that appears to be a surface piercing "V" foil which would allow virtually automatic altitude control. They also appear to fly level with both t-foil rudders immersed most of the time.
Oracle appears to have adopted manual altitude control with "L" foils, but they have not yet flown as well as TNZ and the recent pitchpole has set their program back a bit.
I think that the altitude control system the teams adopt may shed some brilliant thinking on an important subject for foilers. And both teams(and Luna Rosa) are breaking new ground that may some day filter down to the rest of us.

 

Sail Rocket

Sail Rockets extraordinary performance has raised the speed sailing bar incredibly high and in so doing prompted a debate on various forums along these lines: "Is Sail Rocket a foiler?"
On boatdesign, the "debate" is in the SailRocket thread; on Sailing Anarchy the debate is sort of tucked away in the "Future Tech in Dinghy Design" thread in "Dinghy Anarchy".
In considering the debate it is often brought up that a hydrofoil "lifts up" and
"supports" a boat. It should also be borne in mind that Bernard Smith in his classic "The 40 Knot Sailboat"(which inspierd SailRocket) called the Sailrocket type an "Aero-Hydrofoil".
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Here are a few ideas on the subject provided seriously but in good humor:

--New "hydrofoil" definition proposal: 1)" A generally wing shaped foil used to create an upwards force or a downwards force with a vector of those forces acting vertically at 90 degrees to the surface of the water." 2) "A craft that uses such a foil or foils." Oh well, its a new world.....

--Proposed Name of the SailRocket* type: " Balanced Forces Hybrid Planing Hydrofoil"
* Paul Larsen doesn't think "Aero-Hydrofoil" realy gets it right.
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It seems to me to limit the definition of hydrofoil to one that describes lifting up only as qualifying as a hydrofoil is dated to a period where the concept of a hydrofoil pulling down(like Sail Rockets foil does) was largely unknown. And for years foilers have been designed where the windward main foil would pull down when required and would switch automatically between pulling down and lifting up due to the action of dual independent main foil altitude control systems.
Sail Rockets main foil does what any other surface piercing foil does:
1) develops lateral resistance,
2) controls flight altitude.
And it does it in a more or less opposite way than a "normal" surface piercing
foil does and accomplishes the same exact thing!
Sail Rockets Main foil has a force vector pulling down and another pulling parallel to a line drawn between the center of lift of the foil and the center of lift of the wing(sail). So it develops lateral resistance and downforce which acts to control flight altitude.
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Comments earnestly solicited........

 

Sailrocket can in no way be considered a foiler if by "foliler" one means a craft that is supported by hydrofoils. Sailrocket's main foil does not support the boat, instead it produces a downward force. Its purpose is to oppose the sideways force of the wing. It is really no different than a leeboard, except for being mounted on the opposite side of the boat.

Sailrocket is a planing boat. The portion of its weight that is not supported by aerodynamic lift on the wing and beam is supported by the planing surfaces. In that regard it is quite similar to Yellow Pages Endeavor and Macquarie Innovation. All three boats have widely spaced, short planing hulls. It is just like a sailboard with regard to having vertical lift from an inclined sail rig and the rest of the lift provided by a planing hull.

Sailrocket is a wind-driven planing trimaran, and I don't think there's any new nomenclature needed to identify it.

(As for whether Sailrocket's hulls are actually planing or operating in displacement mode, I think it's safe to say they are planing when at speed even if the aerodynamic lift causes the craft to trim bow down!)

 

===================
Tom, there is more to SailRockets foil than just lateral resistance, isn't there? As I understand it, the foil develops considerable downforce and controls the flight altitude of the boat-thats according to the "Facts" section of the Sail Rocket site. Seems to me the foil does exactly what a "normal" surface piercing foil does-lateral resistance plus altitude control? (But a little differently-while developing downforce?)

 

Er.. no.

I get the following estimates the four largest SR forces involved at 65kt speed and 25kt crosswind (70kt relative wind):

Main sail lift 2500 lb (assumes CL=0.8 on 18 m^2)
Foil lift 2500 lb
Weight 800lb
Outer sail lift 200 lb
Planing lift ~600lb (assumes Sail and Foil forces aligned)

The weight and planing lift are minor compared to the huge Sail/Foil 2500lb force couple. So there isn't "much more to SR" if we look at the force magnitudes --- the Sail/Foil forces completely dominate the overall force vector diagram. Replacing the planing lift by tilting the foil lift a little bit would hardly matter it seems.

PS
The fact that the wing lift greatly exceeds the weight was proven by the SR1, which was easily tossed into the air by the sail, and that was at "only" 50 kt. And that was even though the sail force was way ahead of the CG, so much of the energy went into rotation rather than vertical translation.

 

Sail Rocket

So, in trying to understand how the foil works, we ignore that according to Larsen, part of its function is altitude control?
If we consider lateral resistance and altitude control it seems to me that the foil is doing what a "normal" surface piercing foil does-except that it creates downforce?
I appreciate the comments-and I don't mind being wrong because I'm trying to understand exactly whats going on- but this doesn't cut it :

Especially in the face of Larsens explanation from "Facts", under "The Boat" on the SailRocket site: ( http://www.sailrocket.com/node/292 )

•The curve of the main foil determines how high the boat rides.

 

I was discussing only force balance, which is not the same thing as stability and control.

Assuming the SR2 3-view is precise, the outer part of the foil is "over-angled" relative to the foil/sail axis, so the outer part of the foil tries so suck the whole foil down. This is balanced by an upward lift on the inner part of the foil which pierces the surface. The upward force increases strongly with submergence, while the downward force is nearly constant. Hence, the foil strongly seeks and holds some stable depth while providing a nearly constant force component along the foil/sail axis. It's a very clever setup.

But regardless of its stability characteristics, the fact remains that the total force of the foil is comparable to and opposite to that of the sail. It must be so, because there isn't much else left to balance.

 

Sail Rocket

--------------------------
I said earlier:
"Sail Rockets main foil does what any other surface piercing foil does:
1) develops lateral resistance,
2) controls flight altitude.
And it does it in a more or less opposite way than a "normal" surface piercing
foil does and accomplishes the same exact thing!"
>Can you agree with that?
--
From the Facts page:

•The back of the boat will lift onto the curve of the foil at around 25 knots. The leeward float will begin to fly clear of the water over 50 knots. Only the main foil, the rudder and the ‘step’ of the forward float will be in the water at high speed.

Picture: front float levitation @ 65 kts:
click-

 

Thanks for a very interesting thread.

Your lift formula and Ray's correction are a little confusing for those of us who prefer the metric system.

Lift = Coefficient Lift x (half density x velocity squared) x Surface Area
L = Cl.1/2pV2.S
L=Lift= Newtons =N
Cl=Coefficient of Lift = dimensionless
p=Density= kilograms per metre cubed= kg/m3
V=Velocity= metres per second =m/s
S=Surface Area= square metres =m2

Standard Density Atmosphere = 1.225kg/m3
Density Water = 1000kg/m3

I would like to recommend "Mechanics of Flight" by A.C. Kermode (ISBN 0-582-23740-8) as an excellent book on aerodynamics. It is regarded as a bible by many pilots and engineers. It begins with quite basic school boy physics and ends somewhere around degree level.

 

Lift formula expressed in the metric system

Dear Number 4

Earlier in post number 7 of this thread I answered a similar question from Olav about conversion from American units to Metric units. It is shown in part as follows:

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=V^2 X S X Cl. " Note on units used: pounds, ft/ sec, and square feet.

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, F. The variables inserted into the formula will be in kilometers per hour and square meters. The resulting lift will be expressed in kilograms.

To this we may add:

For Americans using mph: Use a conversion factor, F, of 2.16. The variables inserted into the formula must be in miles per hour and square feet. The resulting lift will be expressed in pounds.

For metric users wanting the resulting lift to be in kilograms and the velocity to be kilometers per hour, use a conversion factor of 4.06. The variables inserted into the formula must be in kilometers per hour and square meters

For metric users wanting the resulting lift to be in kilograms and the velocity in meters per second, use a conversion factor of 52.63. The variables inserted into the formula must be in meters per second and square meters.

For metric users wanting the resulting lift to be in newtons, use a conversion factor of 515. The variables inserted into the formula will be in meters per second and square meters.

However, I might add that expressing the result in newtons may be inappropriate. Newtons are measures of mass. Lift equals weight, so it is weight we are concerned about. Kilograms are units of weight.

Mass relates to weight as a function of gravity. Because gravity is the same for you in Europe and Asia as it is for me on the West Coast USA, we can use newtons safely, even if technically incorrect. Just remember to use the appropriate conversion factor as shown above. Of course newtons will come in handy when calculating extraterrestrial hydrofoiling.

About Coefficient of Lift, Cl. Typical numbers range between -.5 and +1.6 (see NACA 63-412 as shown in Figure 13-14 in my book). Extremes for other sections can go as high as +2.8. These Coefficients are dimension-less and may be inserted in both American and Metric formulas without conversion. Unless you own a wind tunnel, to determine Cl and Cd (drag) not shown in my book, you may refer to: Theory of Wing Sections by Abbott and Von Doenhoff.

Incidentally, if you are looking for a copy of my book in Europe or UK, go to Amazon.fr or Amazon.uk instead of Amazon.com. You should save on shipping cost and speed. If you chose not to buy the book, to feel good about your decision, view the video at: http://www.youtube.com/watch?v=Va1bM3hE9yc

Ray Vellinga,
Author, Hydrofoils: Design, Build, Fly

 

Sailing Foiler Design

Thanks for the info, Ray!

 

Hi Ray,
I am afraid to say that you have things the wrong way around with our wonderful metric system.
kilograms=mass
newtons=weight

weight is a force (N) = mass(kg) x acceleration of gravity(g)
The acceleration of gravity is 9.81m/s2. This means that an 1kg mass weighs 9.81 Newtons whether it is in Europe, Asia, or the West Coast of the USA.
But if you were to take it hydrofoiling in space it would weigh 0 Newtons.

http://www.thefreedictionary.com/kilogram
http://www.thefreedictionary.com/newton

The lift formula that I posted was taken straight from a text book.
http://www.aerospaceweb.org/question/aerodynamics/q0015b.shtml

Good luck with your antiquated measurement system and all of your brilliantly confusing conversions!
I think I will just stick to using my simple metric equation all the way.
It makes my tiny brain hurt alot less.

Best Wishes, I look forward to reading your book.
Adam

 

Hi Doug,
I hope you do not mind, but my favourite hydrofoil is motor driven. It keeps popping up as a random picture on the bottom of the screen.
The Carl Boat.
Someone called the Coastguard and said they had seen a flying boat that was having problems at take off.
It has an interesting hydrofoil arrangement. There are several hydrofoils on each leg. Each smaller than the last. It appears that as speed increases, the unnecessary area rises out of the water.
The body looks like it should achieve lift from ground effect.
The air propellors seem a good solution to trying to get drive to the water.
I believe this boat would do very well today with a pair of 2.0 turbo diesels.

Surely this boat could also sail?

 

==============
Adam, my gut is that it couldn't sail but a type somewhat like it could-and already has. The US Navy/Baker mfg pioneering( early 1950's!) sailing foiler used ladder foils which are multiple foils that rise out of the water as the boat lifts off. Monitor also used an innovative system that applied some force from the rig to automatically adjust the angle of incidence of the foils to create righting moment.

click on image: