Foiler Design

[MalSmith]

For those interested in numbers, I have been doing a bit more analysis regarding the feasibility of putting foils on a laser. Attached is a graph showing calculated resisitance values for a Laser and a Moth, both with and without foils.

Looking at the results, not surprisingly, the resistance values for the Laser are significantly worse that those for the moth. The laser needs about 40% higher windspeed to get up on the foils, taking into account sail area differences, but ignoring righting moment issues. So if a Moth can get up on foils in say 7 to 8 knots of wind, the Laser will need 10 to 11 knots of wind. One intersting result is that once the laser is up on foils, the improvement in performance is proportionally greater than that of the Moth.

Also attached is a graph showing my calculated values for the Moth compared to published full scale resistance values (Beaver/Zseleczky). I used this comparison to gauge the reliability of my calculated values, which I estimate to be accurate to somewhere in the order of 10%. However, the important thing is not the actual values, so much as the comparitve values of Laser vs Moth.

[Doug Lord]

Mal, very interesting! What mainfoil area were you using for the Laser?

[MalSmith]

Doug,

For the main foil I used the following:

Section lift coefficient (Cl)- 1.2
Section drag coefficient (Cd) - 0.022
Section angle of attack (AoA) - 10 degrees
Dihedral angle - 40 degrees
Effective Cl due to dihedral - 0.919
Projected span - 1.4m
True span - 1.83m
Chord - 0.12m
Foil Area - 0.172m^2

I have assumed that all lift is provided by the main foil only.

Next task is to investigate whether the foil is structurally feasible.

[Doug Lord]

Originally Posted by MalSmith

Doug,

For the main foil I used the following:

Section lift coefficient (Cl)- 1.2
Section drag coefficient (Cd) - 0.022
Section angle of attack (AoA) - 10 degrees
Dihedral angle - 40 degrees
Effective Cl due to dihedral - 0.919
Projected span - 1.4m
True span - 1.83m
Chord - 0.12m
Foil Area - 0.172m^2

I have assumed that all lift is provided by the main foil only.

Next task is to investigate whether the foil is structurally feasible.

=============
Thanks ,Mal. 10 degrees sounds real high-is that just at takeoff? What speed
do those figures represent?* What section? Bradfield believes an 80% mainfoil loading is best with a rudder foil at 50% the area of the mainfoil. Says it improves pitch characteristics. So light rudder foil loading. Beavers paper mentions 75% mainfoil.
What are you considering for your rudder foil?
Sorry, for all the questions-I appreciate your answers.

* Found the speed above-could you correlate that with section angle of attack w/o too much trouble?

[MalSmith]

Doug,

The 10 degrees is at take off speed, however, because it is a surface piercing foil, it could be left at that and you get the drag reduction from the wetted area reduction. As mentioned, I would like to make it manually variable to experiment with it for optimisation in different conditions.

I forget which section I picked out of the book, except to say that was a 12% thickness NACA section. I should have noted it down, but as I will probably just shape up a section by eye, it's not really critical. I was only looking for typical figures, and I'm being fairly conservative in my estimates, I think.

I have based the design on 100% mainfoil loading, again to be conservative. Any lift derived from the rudder T-foil will be a bonus. I want it to be quite pitch stable, so I thinking lightly loaded symmetrical section rudder foil of generous area to provide plenty of reserve.

Remember that the aim of this project is basically cheap and dirty foiling for dummies, not ultimate foiling performance.

[Doug Lord]

Mal, it is an extremely good idea you have to make the angle of incidence of the mainfoil adjustable on the water-very good thinking. Good Luck!


PS do you have the Icarus book showing a 470 sailing on two surface piercing main foils? It appears to be one foil each side and the whole foil unit appears to be molded in one piece and would seem to be easily removable. No extra RM for the boat....

Here is a Flying Dutchmen doing the same(from Hanno Smitts site) :

[daiquiri]

Hello, have you analyzed the pressure distribution around the NACA 0012 foil at 10° AoA, water speed of 8 kts (circa 15 m/s **) and, say, 25 °C temperature (summer)?
At first sight it appears to me that you are risking a cavitation.

[daiquiri]

Correction: 8 kts = 4.1 m/s - my fault, sorry.
It's looks ok now, you are far from cavitation onset.

[MalSmith]

Phew, you had me worried there for a minute! I wasn't expecting cavitation to be an issue at 8 knots.

[Doug Lord]

I am starting a series of posts that will reveal important technical details of some successfull foilers in order to help people interested in foiler design.
I'm starting with the largest bi-foiler in history-Mirabaud- thanks to the generous help I received from Thomas Jundt, designer and owner of the boat.
This will be followed by more boats including the Rave, Moth and ,hopefully, the R Class, Hobie Trifoiler and Doug Halsey's remarkable surface piercer.
==================
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 take-off(8.5 knots) loading is 50% main foil; 50% rudder foil
b. at 23 knots all load on main foil
--
Altitude Control System-Mirabaud 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.ft-big rig)=2.54
SA/total foil area(both sides-not struts-big rig)= 23.54

[Doug Lord]

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.ft-big rig)=2.84
SA/total foil area(both sides-not struts-big rig)= 18.36

[Doug Lord]

The Moth was the first sailing bi-foiler 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 bow-but 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 18-5, page 219. This book should be the bible for anyone interested in small hydrofoil design-power or sail. It is without a doubt the best new book on small hydrofoils in the last 40-50 years!
==================
LOA 11' + rudder gantry-appox. 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 140-200lb)
c. all up sailing weight= 241lb.
--
Foils- two fully submerged foils, one on daggerboard ,one on rudder-area and loading varies between boats-see 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= 0-1.5 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 System-As 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 sides-not struts)= 22.28
-----------------
pix from Moth Worlds,ThMartinez.com(twin midship wands), Dan Kessler(Raptor Sails-twin bow wand) and from Mach 2 website

[bistros]

Doug:

Rather than regurgitating your no-doubt spectacular compendium of foiling trivia here in Boatdesign.net like a puppy that has overeaten, perhaps you would consider a website dedicated to this information.

This would save folks here from watching your endless foiling information bulimia nervosa all over their screens.

In all seriousness you certainly could save yourself 90% of your Internet time if you did not feel the need to repeat both specifications and photographs. That way, when new tidbits come your way, you could just update the website! With RSS feeds enabled, the interested would be automatically updated if you changed a critical detail like wand placement specs or if Mirabaud changed a critical control line diameter.

Like the bible, you could index the site by designer:chapter:verse so when you are discussing items you could refer to Ilett:3:274 rather than retyping the foil area of your aeroSKIFF mainfoil. Scholars could study your compiled works for credit, just like an on-line Bible, Koran or Talmud.

Imagine how compact your writings would be if you just posted:

Bradfield:1:67-95

instead of reporting the complete Rave specs every six months. I bet the overall Internet speed would go back up, your Internet bills would drop in half and you'd get some time back to work in the shop on your new boat!

Win-win!

Something to think about ......

--
Bill

[PI Design]

Hi folks, long time no post.
Does anone know of any papers that look at the effect of running foils close to the surface of the water? This must have an effect on the lift and drag - how far below the surface do you have to go to be able to assumer their is no impact?

[Doug Lord]

Originally Posted by PI Design

Hi folks, long time no post.
Does anone know of any papers that look at the effect of running foils close to the surface of the water? This must have an effect on the lift and drag - how far below the surface do you have to go to be able to assumer their is no impact?

-------------------
I was told minimum depth of a hydrofoil is 2.5 times the foil chord. You could pm Tom Speer or search this thread if you haven't already. Some foilers have used surface proximity effect as a form of altitude control...

Heres the abstarct from the paper below(pdf for sale,unfortunately):
"Results of an experimental study of the effect of surface proximity on hydrofoil lift are presented. The biplane image theory, a horseshoe vortex model and momentum theory are described in relation to the effect of surface proximity on hydrofoil lift and drag. The biplane image theory and the horseshoe vortex model are shown to predict the same effect on lift, and are seen to be in good agreement with the experimental data. The Payne momentum theory is seen to differ significantly from the measured results. The data indicate a significant reduction in lift at depths less than two chords with very little effect at greater depth."

http://www.sciencedirect.com/science...da95e476179004