Paducah said..

Sandman1221 said..

A doctoral thesis?, are you kidding me Paducah, that is all you have? How about a paper published in a peer reviewed journal? That I would take the time to read.

Coming from a guy who posted from a one page "Hydrofoil Basics" and has cited it about a dozen times?

Keep digging, dude. You're almost out of the hole.

That basics page was from MIT professors, who likely have dozens, if not more, of published papers to draw from, see main website below, these are the guys that our foils are based on in one way or another! Of course Paducah if you had taken the effort to press on a link in the original document you could have gotten to this page, but I realize that was expecting too much.

go to this website for links to videos and published work on hydrofoils.
lancet.mit.edu/decavitator/

Decavitator Human-Powered Hydrofoil
Check out the videos, brush up with the hydrofoil basics, or go to the table of contents for a complete listing of documentation. Please see the reading list for aero/hydro dynamics references.
On 27 October 1991, Mark Drela pedalled the human-powered hydrofoil, Decavitator, to a world-record speed of 18.5 knots (9.53 meters/second) over a 100 meter race course on the Charles River in Boston, Massachusetts. In the spring of 1993 the Decavitator team was awarded the DuPont prize for the fastest human-powered water craft. This prize was to be awarded to the first team to break 20 knots over a 100 meter course, or to the team with the fastest speed on record as of 31 December 1992.
A Little Background Information...The trek to the world record flight began in 1988 as Daedalus team members Marc Schafer and Bryan Sullivan returned from Greece. At that time they decided to build a human-powered vehicle (HPV) that an armchair athlete could operate. Marc initiated a student-run project (under MIT's Undergraduate Research Opportunities Program - UROP) with funding from the MIT SeaGrant program. When the $25 000 DuPont Prize was announced in 1989, the goals of the project changed and speed became the primary objective. For the rest of the story, see the history page. Where did the name "Decavitator" come from? Traditional boats often encounter problems at high speeds when their propellors cavitate. This boat uses an air propellor, and thus never suffers from cavitation. Also, if you stand too close to the propellor you could lose your head.
What's In This Page...This page contains links to a history of the project, a summary of who was involved, a short time line of the project (including a summary of the versions the boat went through), and the project sponsors. In addition, you will find the specifications for the boat as well as a page that describes some of the methods and materials used to manufacture the boat. See the 3-view drawing of the boat if you are confused about its layout. You can skim through the history text and view the videos and still images in historical context, or go right to the list of video segments or still images to get a visual outline of what happened. Unfortunately, there aren't many sounds unique to this project, so we don't have much of an audio history. If you're interested in still more human-powered hydrofoil stuff, stop by the International Human Powered Vehicle Association for information about all kinds of human-powered vehicles.

Index to all the Decavitator documentation
Introduction to HydrofoilsHistory of the Decavitator ProjectConfiguration HistorySponsorsTeam MembersSpecificationsManufacturing MethodsMaterialsStill Images (about 30 inlined gif thumbnails)Videos of DecavitatorVideos from IHPVA racesVideos of the flying fishBibliography

Copyright 1989-1995
Matthew Wall, Mark Drela, Steve Finberg
Massachusetts Institute of Technology

Sandman1221 said..
That basics page was from MIT professors, who likely have dozens, if not more, of published papers to draw from, see main website below, these are the guys that our foils are based on in one way or another!

It would be great if reading the web site from experts would make you an expert, since we would not have most of this discussion.

Mark Drela is one of the world's most knowledgeable people about foils. To give just one example, he's published XFoil, a program used extensively in the design of windsurf fins.

His Decavitator project is actually a very good example to learn what the difference between cavitation and ventilation is, because it touched both issues over the development. It was drive by muscle power, and older designs used propellors in the water, which encountered cavitation problems. It therefore used a large air propellor to avoid cavitation problems at the propellor (not the foil).

The first version of the foil had a V-wing, but "Tow tests revealed serious ventilation problems with the V-foil design" (anyone who ever used the original MUF Delta fin in chop will understand). Note that these problems were so large for the v-foil that they never managed to even get the boat out of the water - so it happened even at very low speeds. That's a logical consequence of the entry angle.

So the initial version of the Decavitator suffered from ventilation problems at quite low speeds. In Nico's case, the speed was substantially higher, and there was considerable sideway pressure, basically pushing the foil mast into an non-zero angle of attack.

For ventilation to occur, 4 criteria need to be met: (people.eng.unimelb.edu.au/imarusic/proceedings/18/170%20-%20Binns.pdf):
1. a region of sub-atmospheric pressure,
2. a concurrent region of flow separation or cavitation,
3. proximity to a free surface and
4. a path of low impedance to air flow that stretches from the atmosphere to the region of sub-atmospheric pressure

The sub-atmospheric pressure is created by the foil aspect of the mast when it is pushed from the side. The pressure drop will increase directly with the angle of attack (so, in first approximation, with the amount of sideway pressure), and with the square of the foil speed.

Flow separation is the critical issue that sanding addresses. The chance of flow separation also increases with higher speed, and with a higher pressure differential. It's a complex issue, since the effect of surface structure and hydrophobicity on flow separation depends a lot on speed, angle of attack, and shape. There are plenty of theoretical arguments why a certain surface roughness would minimize flow separation. Practical experience from foilers seems to indicate you can reach this by careful wet sanding.

Practically, this is primarily an issue to racers, since they reach both the speed and the sideway pressure necessary to get close to flow separation. The average foiler may well be perfectly fine with a non-sanded foil, and sudden catapults are more likely to be due to sailor error or hitting something.

Paducah said..


Since you enjoy invoking Prof. Drela, why don't you read in his own words what they did to inhibit ventilation (his word) on the T-foils of Decavitator: www.boatdesign.net/threads/foiler-design.2447/page-44#post-246398

Incidentally, this gives credence to the speculation of some, including me, that the ventilation bubble on Nico's foil affected the stab rather (or more than) the wing.

You know, I was wondering a while back why you didn't see canard-like designs on foils instead of the typical main wing in front type. This would explain why. A ventilated mast would ventilate the main wing instead of the stab, probably resulting in much more severe crash.

Seems like to eliminate this you'd need a raked mast with the wing and stab both in front of the mast/fuse connection. That poses its own stiffness/design/danger problems. Maybe just adding some mast fences would be a more positive result.

Good point about the fences seeing a varying AoA as well.

Transonic wouldn't apply in this case, as that's for close/through the speed of sound. Nobody is close to that in either water/air on a hydrofoil...

It seems since roughening is needed we need to either have sanding or something akin to a dimpled golf ball surface. The golf ball dimples trip the flow to turbulent and help delay separation, which otherwise is very bad on a perfect sphere. Similar realm of problem, complicated by two fluids of two different densities.

A couple empirical things about this. I am not an expert in hydro flow, but I remember a couple things. The argument was that a "shark skin" surface reduced the thickness of the boundary layer to reduce overall drag.

1. Formula racers used to sand the bottoms of their boards to make them a tad faster. The argument was always which grit fineness to use. Many of them sanded their fins, too.

2. I am retired from Boeing. We had a special experimental 757 with wing riblets which were about 0.2 cm high. The aerodynamic drag was reduced considerably, as measured in flight and in the wind tunnel. The riblet finish never went into production because the manufacturing costs were so high that the airlines would not recoup the higher price for the airplanes.

Nico Prien's follow up video. Let the fight begin (or continue)!

Sandman1221 said..

thedoor said..

aeroegnr said..
Interesting thoughts from Nico Prien on his Defi Wind crash. I think this is the best explanation of a potential problem causing a sudden crash, and a remedy. I don't think that this is what happened to cause my crash at my first post in this thread, but I've had some crashes that seemed to act more like what he shows here.

That is very interesting. The fish foil session where i experienced at least a half a dozen non breaching violent catapults was actually the first day I used a brand new carbon foil (of course not sanded). My only explanation was hitting fish as a felt a soft "impact" with each catapult. Perhaps Nico's ventilation theory is more accurate.

Look no one has shown that lightly sanding the mast of an iQ foil stops it from dropping someone out of flight, that includes Nico and aeroegnr. So why doesn't someone who has been having the problem sand their mast down and find out? Better yet contact Starboard to see if they recommend doing that and if it will invalidate any warranty claim.

And again, there is no video evidence from the videos that the mast is ventilating (air snaking down the mast), which by the way does not normally occur on a vertical mast and would not drop you out of flight suddenly like cavitation does.

Nico is right, there's no mystery to this.

The pressures generally aren't extreme enough for cavitation to occur unless you're pushing towards 50kn.

I have actually fixed this ventilation problem on quite a few masts by sanding the mast to a hydrophilic finish. Ventilation 100% occurs on a hydrofoil mast and will drop you like a rock. This is because your mast spends very little time flying at 0 degrees Aoa so is actually acting as a lifting surface. A combination of loading and something kicking if off, like a scratch, wave, or mast graphic is the most common cause.

The sensitivity of a mast to ventilation can be affected by the Airfoil, dimensions, stiffness, and surface quality. A good design can resist ventilation without sanding, but sanding can fix or massively improve the sensitivity of an ok or bad mast.

This is exactly the same phenomenon as "spin out" with a fin. Usually feels like you dig rail, catch a plastic bag on the foil, or hit something. its a big problem in surf and wing foiling too.

FoilAddict said..
I have actually fixed this ventilation problem on quite a few masts by sanding the mast to a hydrophilic finish.

What grits and sanding direction works well in your experience?

FoilAddict said..
Any mast scratches or wing graphics/lettering you can feel generally have a greater impact than the finish.

When sanding, be careful around the leading and trailing edges as not to change the shape. Use light pressure and clean paper to avoid deeper scratches. If your mast makes noise while riding you probably changed the trailing edge treatment and should follow the guide by Levitaz foils on youtube!

Thanks - appreciate the info. There seems to be a lot of hand waving and anecdotal evidence when discussing the topic of what grits work. It seems very few people want to be accused of not using a high enough grit despite the majority of us barely crossing 20 kts.

Not sure why NP decided to put the graphics/red trim on top of the clearcoat on the HP wings

the video for others interested in this - btw, there's an earlier video('17) where he misspeaks about which side to sand so it can be confusing.