An elliptical un-swept foil, if the taper ratio is correct should stall across its span at the same time. ie. neither the root, nor the tip, as the relative angle of attack is same across the span, and consequently no drag penalty.

If you just swept the same elliptical foil back, and to make matters worse you made it low aspect, you would need to change the taper otherwise the angle of attack will be greater at the tips and you would get a drag penalty. Though it would stall at a greater average angle of attack as bit of bonus. It makes sense to look for optimum curve or taper for a swept foil.

A wing is swept back to address problems with compressibility, as designs have gotten better less sweep is required i.e. compare the sweep on a 777 versus the 747. You have added rake the fin because of the weed, so I doubt you have much choice on the rake...

I have quite a few aerodynamic references on the topic, as wanted to understand why windsurf sails have a twist in them and other phenomena.

This one is probably a good starting point
www.flightlab.net/Flightlab.net/Download_Course_Notes_files/3_Three-DimensionalA%232BA154.pdf

Hopefully some of this makes sense

Here is the paper on cavitation

www.marin.nl/upload_mm/8/3/e/1806737901_1999999096_120_e_8.pdf

It shows that cavitation doesn't have much effect on the stalling angle of attack, maybe even delaying it slightly... Though it does reduce the overall lift and increase drag. See fig 4. This would suggest an increased tendency to spin out, when your 'under finned' at speeds where cavitation is a consideration. i.e. cavitation would have an effect when spin out is least likely to occur.

The video you posted on cavitation, where it covers partial pressure and the interaction between ventilation and cavitation, is intriguing. Still thinking about that one...

racerX said..

A wing is swept back to address problems with compressibility, as designs have gotten better less sweep is required i.e. compare the sweep on a 777 versus the 747. You have added rake the fin because of the weed, so I doubt you have much choice on the rake...

Wing sweep is used to minimize compressibility drag rise but introduces 'Dutch Roll' instability problems. The sweep angle is a function of speed. The maximum speed of a fighter can be estimated simply by measuring the sweep angle on a plan view photograph.

The 777 has less sweep because it has a lower design cruise speed than a 747. The 747 was flight tested to Mach 0.99 in a power dive during development.

Yes that's a good paper, and the last line is the kicker alright! I just wish he had done it with symmetrical foils. foils.org is a fab site but a terrible time thief - I always get sucked in by all the cool looking boats/ hydrofoils and end up reading about all sorts of foil stuff that isn't relevant to what I am after but interesting all the same

yoyo said..
Windsurfing is different. Why? See if you can work it out....

Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.

racerX said..

yoyo said..
Windsurfing is different. Why? See if you can work it out....

Lack of scientific method? No real scientific testing? easier to make stuff up that sounds good. Just a way to have a great time with a few bits of plastic?

Maybe so.

No, no, no and no.
Think first principles. What causes cavitation in first place?

yoyo said..

No, no, no and no.
Think first principles. What causes cavitation in first place?

Sufficiently low pressure, factors that effect the value of this low pressure are: density of the fluid (e.g. salt water/versus fresh), other nuclei in the fluid (e.g. a solid surface, air bubbles big and small, salt), the partial pressure of other gases present.

What is also interesting is that many other the foil shapes/ideas designed to reduce the effects of compressibility in aerodynamics also appear to be applicable to cavitation. e.g. minimise high pressure gradients across the chord, i.e. spread the pressure changes along the chord.

In any case as far as windsurfing is concerned I reckon we are all just guessing...

fantastic fangman be interesting to see decrepits version

Thanks for the pic and I am lovin' the vice idea Capn. Starting to look a little like a cut away there - what's going on there?

At the very start of this thread, I said that none of the ideas in the FangyFin MkII were original. Here is proof. My team-mate Barry made this board after an argument with Noah about the best way to spend 40 days sailing.(Noah wanted to take all his pets too which didn't help) .
Please note; fillet and cut-away married to a thickish foil shape!



The board is 1.5mm Mahogany marine ply skin built over frames. Weighs 9kgs incl.fin and footstraps.
To my eyes it a beautiful thing :-)

yoyo said..
Windsurfing is different. Why? See if you can work it out....

well for a start the quote from Tom Speer is about hydrofoils:

"The other conclusion is that for a practical design having foil/strut junctions, etc., any speed above 18 - 20 kt will have some degree of cavitation present."


I don't remember seeing any foil/strut junctions on a windsurfing fin. I believe that at speeds less than about 45 knots the enemy is ventilation, not cavitation, and even above those speeds I have trouble believing that a fin would capitate before it ventilated.

I don't remember seeing any foil/strut junctions on a windsurfing fin. I believe that at speeds less than about 45 knots the enemy is ventilation, not cavitation, and even above those speeds I have trouble believing that a fin would capitate before it ventilated.

My thought was to consider the board and fin as one hydrodynamic component and so there is a strut / foil interface. I imagined the board to be the strut, and the foil the fin, rotated through 90 degrees. Using the findings on the video, the result is that the system is operating in the worst possible scenario where the foil is very close to the surface, so it will cavitate sooner and the plume from the pressure interference at the junction will be more pronounced at sooner/lower speeds. Does it really matter whether it is cavitation or ventilation? The end result is the same - there is disruption to the flow with the loss of lift. The fillet is there to try and minimise the pressure interference between two surfaces moving at right angles through the water and as a result to try and delay/minimise the formation of the plume. Theorectically, the trade off between increased surface drag and minimising the plume occurs when the radius of the fillet is between 6 -10% of the chord of the fin.

keef said..

I don't think these fins are designed for top speed rather than early lift for the maximum sail area and shallow water , if I could get 34knts max in moderate wind with an 18cm weed fin I would be happy
I've been trying to stay away from the resin but Ill do an east coast version as I like the concept, I think the collar needs to be a tad further up the base without being too fat, maybe a few do's and don'ts from fangy ;)

Exactly Keef, you nailed it :-) and I am my own worst enemy. I get a new toy, and always want to see how fast it can go
(BTW the MkII 24 =>36knots in small chop on my old Missy Screamer II, Koncept 7.5 is the current limit)
PS If you do end up playing with the resin again, I would be really keen to hear what effect varying the size of the fillet has on performance.

Pacey said.


I agree, reminds me of the work that Oracle did on their rudder/foil junction in the 2013 Americas Cup: www.sailingworld.com/racing/biekers-cavitation-fix/

Nice find Pacey Below is a photo from the article:
Of interest to me: 1) the photos extent of the fillet toward the rear, 2) the radius varies from front to back 3) the drag reduction it afforded is so significant ( this boat was one of the massive foiling cats that hammered around SanFrancisco Bay)

Nice find mate! The challenge now is to find Pink anodising for my aluminium.