Best shape for surface piercing foil

tom,
thnx for the confirmation on the NCrit.


andrew,
ventialtion only seems to be a real problem on the higher CL loaded course racing fins.
speed fins when chosen with the right size do not seem to suffer that much ventilation.

i don't think cavitation can be cancelled out at speeds over 40kn...
i working with a couple of guys now that have already done +42kn runs over 500m and had peaks of 45kn. i'll keep you guys posted on anything interesting regarding those developments.

when you made those symmetric root/asymmetric lower fins how did you know which angle of attack they would be running at? to have the base of the fin non lifting symmetrical it must have been twisted away from a center postion.

i have decided against asymmetrical sections as they are just too expensive to make with the double number of CNC machined moulds and number of fins required as speed courses can go either direction. if my calculations are right the angles they run at are so small anyway that asymmetrical sections have a negligible advantage.

instead of fences i think i'll try a bit around with different fairings.
does anyone know of literature regarding fairing of wings/foils into hulls/fuselages?

cheers
boogie

 

Boogie

speed fins when chosen with the right size do not seem to suffer that much ventilation.

Depends very much on the board, but in my experience (3 times Australian speed record holder and 6 times competitor at Weymouth) it is still a real issue. My speedsailing may have been a fair time ago, but fins and physics haven't changed that much.

i don't think cavitation can be cancelled out at speeds over 40kn...

My point about cavitation was not that it won't happen in theory, but that in practice the fins are operating close to a turbulent waterflow with entrained air, and as a result ventilation will always occur first.

when you made those symmetric root/asymmetric lower fins how did you know which angle of attack they would be running at?

Basically guesswork, estimated 3-4 degrees of leeway, chose camber of main section to give minus 3-4 degrees angle of zero lift.

to have the base of the fin non lifting symmetrical it must have been twisted away from a center postion.

No, by cambering a foil differently along its span you introduce aerodynamic twist i.e. the angle of zero lift changes up and down the foil. At a given angle of attack the lift coefficient for different portions of the foil will be different. Physical twist is not required.

regards

Andrew

 

How do you morph sections along the span? If you have a symmetrical on top, and further down there is an asym section, in between there would be a lot of different unspecified sections, how could one guess how they would behave?
Would you take a fair number of them and analyse in Xfoil for instance?

 

The simplest case would be to maintain the same foil profile and vary the camber line smoothly from zero up to the full camber.

Conceptually this is no different from having a sail that is very flat at the bottom and progresses into a fuller shape (i.e. more camber) as it goes up.

The key to this approach is that when the board is running straight through the water, the symmetrical foil at the root is producing zero lift, whereas the asymmetrical foil further down is operating at a positive lift coefficient. As there is little chance of a separation bubble forming at the root of the foil the cambered foil has some protection from air sucking down the fin from the surface.

I built a fin based on the concept and it worked exceptionally well, to the extent that I sailed two speed competitions with it and I never had it spin out.

 

I don't use Xfoil, but Profili 2, which is based on Xfoil. In it, morphing sections and analyze them one by one, would be a breeze.
It could also be done in Dr. Hanleys "Sailing Aerodynamics", but only with linear morphing (and with limitations regarding the shape of the planform).

 

hi andrew,

interesting and good to know that you have actual experience of speed trials on windsurfers. what was your top recorded speed over 500m back then?
i have only started proper speed sailing a few months ago, but as it is winter here now, chasing the right conditions had been a bit difficult and so far i only managed peaks of 36.2kn on the GPS and an average of 34.25kn over 500m.
but spring is just around the corner...

i still can't really picture how you can have a symmetric section in line with the centerline of the board and by just changing the camber down the foil you create proper lift with it.
as far as i know the camberline goes pretty much through the LE and the TE of the section, so no twist means to me that the LE's and TE's of the sections along the span of the foil are in one plane.
but that would mean that the cambered sections are actually working with negative lift.

the only way i can picture a fin with a symmetrical section at the base at zero AoA and lifting sections further down would be by twisting the foil [with cambered sections] so that the camberline at the LE or TE is tangent to the centerline of the board.


cheers
boogie

 

Boogie

I think my best official time was just over 34 knots in the Canaries in 1987, unofficial best timed run was about 38knts, ironically in practice the day before the event. The wind died as soon as the event started and we struggled to get wind speed much over 20 knots for the rest of the time. Note that these were timed runs over 500metres, not GPS estimates or instantaneous measurements.

This was at a time that the world record was 38.86 knots and just before the contests started using the trench in the south of France. I went from being ranked about 12th in the world at the Canaries contest (Bjorn Dunkerbeck was 14th), to not even being in the top 100 one contest later - the trench was such a radical improvement that basically no times done on open water courses were ever competitive again, see below -

1986 Board Pascal Maka Sotavento, Fuerteventura, Canary Islands, Spain 38.86
1988 Board Erik Beale les Saintes Maries de la Mer, France 40.48
1990 Board Pascal Maka les Saintes Maries de la Mer, France 42.91
1991 Board Thierry Bielak les Saintes Maries de la Mer, France 43.06
1991 Board Thierry Bielak les Saintes Maries de la Mer, France 44.66



Regarding the cambered sections, if you have water flow that is exactly parallel to the line between the LE and TE of the foil, the foil will produce lift. To get a cambered foil to produce zero lift, you have to rotate it negatively by a few degrees. This is the angle of zero lift. If you have a symmetrical foil that morphs into an asymmetrical foil, their angles of zero lift will be different, even though there is no physical twist of the chord lines between LE and TE. This gives an effective twist to the aerodynamic properties of the foil.

Note that commercial airliners use this principle to increase the aerodynamic twist in their wings to reduce the risk of wing tip stall at landing speeds by using flaps at the wing root rather than across the entire wing span. The flaps increase the camber of the wing locally, changing the angle of zero lift of that portion of the wing. This results in an effective aerodynamic twist that has the effect of increasing the lift coefficient at the wing root and decreasing it at the wing tip.

 

D'OH!!!

i just had a look at some sections with XFLR5 [a GUI for XFOIL].
i don't kow why, but i had the angle at where a cambered section produces zero lift all mixed up and inside out....

you are of course absolutely right.
i'm not that familiar with cambered sections, so this is a good reason to look into it a bit deeper.

the times we get of the GPS we carry for the speed trials are amazingly accurate. Martin van Meurs [who has been competing in the "masters of speed" competition where Finian Maynard has done three runs in the 46kn range now] has been carrying a GPS to cross check the speeds and he has been usually within a few tenths of a knot of the certified time [video timed over 500m].

i download my tracks to the computer at the end of the day and analyse the tracks that way. my best average over 500m with a large slalom board [62cm wide] a 6.6m sail and a 34cm fin in about 20-22kn of wind was 34.25kn. i sailed the same combo on a different day over 1nautical mile with an average of 31.4kn.
i am soooo hanging out to get some good wind and direction to see how far i can push it with the small kit....


thnx for showing me the light about the cambered sections
boogie

 

What you say about GPS is pretty impressive, I had no idea it was that accurate for short time periods/short distances.

We always used to be pretty skeptical of people who claimed to have done high speeds measured over a short distance or with radar, they never seemed to be able to reproduce them over a measured 500 metre course, however if Martin says he is getting accurate readings it sounds like your speeds should be pretty reliable.

 

GPS measured speed sailing

hi andrew,

you are right, the speeds recorded over very short distances are not that close to 500m averages.
martin's speeds that were close to the certified ones were not the max-speed reading from the display. those showed speeds up to 45kn, but his best run had an average of just over 42kn.

my peaks are around 2kn faster than the averages too.
GPS has become a lot better since the US military has removed the SA [selective availability] in 2000 that artificially produced a less accurate signal for private users.

the handheld GPS units might only be accurate to within 20m or so, but that seems to be for the absolute position. as long as you stay with the aquired satellites the accuracy from trackpoint to trackpoint [calculated every second] seems to be a lot better. i seem to get spikes in top speed when i loose reception and the unit finds a different sattelite after that.
my best top speed just after i went for a swim when the win dropped was 902kn... but those things are easy to spot on the computer when analysing the tracks.

i don't know if you are aware of this, but MI is using a differential GPS system now [approved by the WSSRC] to measure their speeds. they use two independent Trimble5700 units to verufy the data. it's much better for them not having to stick to a certain course, but being able to pick any 500m out of their data afterwards.

all good stuff
boogie

btw, are you still windsurfing these days?

 

Boogie

Looks like I will have to get a GPS!

I still sail, just not as much as I used to due to the demands of having 2 small children, but wavesailing only. My biggest board is 8'6", biggest sail is 5.0 sq.m., and I don't bother going out if its less than 22knts of wind - one of the benefits of living in Perth.

Andrew

 

the Garmin Foretrex201 is the one to get for windsurfing and sailing.
completly sealed unit with internal lithium batterie and a big display.

but the best thing is the integrated wrist or armstrap so you can wear it like a big watch.

i still use a slightly older Garmin Gekko201 in a watertight celphone/walky-talky case with strap on the arm. [the batterie compartment is not quite that water tight and saltwater + batteries don't mix very well].

they are god fun in the car and for tramping too.

back to work
cheers
boogie

 

FYI, some authorities will now accept GPS measurements for speed records, provided appropriate measures are taken. For example, the North American Landsailing Association has adopted GPS as a primary measurement. Dual differential GPS units with data logging are installed on Bob Dill's Iron Duck, the yacht Bob Schumacher used to set the current Landsailing World Speed Record of 116.7 mph. (BTW, averaging over 10 sec would meet the 500m average requirement)

Here are the NALSA regulations:
"The primary method must have an accuracy of plus or minus 0.5 mph (0.8 km/hr) or less. Accuracy is defined as twice the combined measurement uncertainty of the measurement system (ie at 95% confidence). At present the only methods that are acceptable are high performance GPS, radar and timing traps. Other methods will be considered by the NALSA Board but must be approved by the Board prior to being used for the primary measurement.

A high performance GPS with data logging is the preferred method. It is very accurate and records a great deal of supporting data. Being an onboard system avoids having to sail close to a measuring station at high speed. A suitable GPS system must have a data storage device that will allow it to record speed, time, position, velocity and relevant quality parameters every second during the speed runs (the combined system must be able to output and record NMEA sentences ‘GGA’ and ‘VTG’ at one hertz). At present this will require a sophisticated GPS unit. Differential GPS is preferred but not required. Inexpensive GPSs are not currently designed to output velocity data and do not have the necessary processor speed and accuracy to be used as the primary measurement method. The top speed will be the average over three consecutive seconds. The NALSA board or observers may have the unit inspected by their expert at any time. The measurement plan must address calibration, measurement uncertainty, handling of the GPS equipment (eg: to assure it does not get a ride in a fast car in the middle of a measurement session), and handling of the data to assure that false data can not be substituted. It is preferred that the GPS be used without filtering (smoothing of the raw data before recording it). If any filtering is present it must be thoroughly explained. "

 

Former speed sailor/course director/board designer

I can confirm that Mr Mason competed many times at the Johny Walker Black Label world speed trials in Weymouth. His enthusiasm for going fast and innovative design was matched by his good humour and taste for our sponsors product.

The physics in Weymouth are still identical and we all believed he understood them better than the locals, apart from Nick and me of course.

Warm greetings to andy and his family from Portland.

 

Hey Andy

Great to hear from you, are you still in Weymouth? What about Nick, what is he doing? Have you still got my solid wing in storage or did you burn it long ago??? Drop me an email - AndyM at formsys dot com

cheers

Andrew