Thread: GIS Resurrected

Howdy, interesting and surprising news Clay!

So... use the templates to do the leading and trailing edges.

Then finish the bottom end off by putting a quarter inch wide bevel along each edge and rounding over. Where the different shapings start to meet up in the corners of the board, get rid of any hard edges (except for those two sharp trailing edges). The corners will transform into two specific curves that are quite different.

MIK

MIK

Thanks Michael, that all sounds easy enough. Somewhere in the dark recesses of my mind I recall reading that the bottom of the blade should be elliptical. Is that necessary or is rounding enough? I was about to make a template using an isometric circle template.

I'd like to build the hollow mast so I'll buy a new set of plans for it and any other revisions you might have made in the intervening years (too many of them, the years that is ).

cheers,
clay

Hey, Clay,

I think a half built Goat is evidence that you have the plans already (actually I know Clayton very well, he goes back to my bicycle shop incarnation)!

So I can send you a PDF of the current plan if you like. There is not a lot of difference, more pics maybe, a built up foil building section.

Also the centreboard outline has changed slightly ... for no greater reason than it seemed a good idea at the time and adds no extra labour or materials.

The square ended board does lose a little over a perfect eliptic foil. But you ever tried to shape a perfect elliptically ended foil? Just not possible even with 10 templates.


So, I would suggest not worrying about it - the boat sails like the clappers with the square ended centreboard.

Best wishes
Michael

MIK

I know Clayton very well, he goes back to my bicycle shop incarnation

Hee, hee, hee, that would would mean going back to my incarnation as a famous bicycle frame builder (well not quite famous)

Thanks Michael, that is very generous of you. However, I would like to pay something. What say I pay the difference between what I paid for the plans and the current price. It would at least make me feel better about wasting so much of your time over the years .

cheers,
clay

Originally Posted by seajak

We've started reading 'Swallows and Amazons' so he's already learning about lug sails .

Clay

Have you thought of joining The Arthur Ransome Society - although it is based in England, there is an Aussie section (AusTARS) which is quite active. When my kids are a bit older I think we will be going on the sailing and camping activities. The AusTARS website is:
http://home.vicnet.net.au/~ransome/index.html

G'day Jeremy,

The bloke I bought my Topper from is a member of the Arthur Ransome Society. He reminded me about 'Swallows and Amazons' and I came straight home and ordered a copy for the little bloke for his birthday. I'll check out the website, their activities sound very interesting.

cheers,
clay

Interesting you and your elliptical ended foils. I used these shapes for years, you know the RAF Spitfire thing and all.

Then I noticed that no subsequent piston driven aircraft, of exceptional ability employed this tip shape. In fact, the best performing aircraft of the era has distinctly square tips (P-51, and later the Sky Raider). So I did some studying.

What I and apparently other foil designers discovered, is you do get more tip generated vortices, but because of the much smaller amount of area in the elliptical tip, less lift in this location on the plan form. In direct comparison the square tip does generate more induced drag, but because it has more area at the end of the foil, it generates more lift, over coming the drag loses from additional turbulence.

What does this mean? On low speed craft (S/L's under 1.6) use the elliptical shapes to decrease drag as you're not getting much lift anyway. On higher speed craft, use the square tip to gain the lift advantage.

I have found that elliptical tips work much better on rudders then center or dagger boards. This is because the rudder offers some lift, but not as much as the main appendage, because of it's angle of incidence. Since you aren't needing or generating as much lift as possible with a rudder, you can ease the tip vortices with an elliptical plan form, which keeps the blade "engaged" longer, over a wider range of incidence angle, before stalling.

Howdy,

The theory is based around an elliptical "pressure distribution" not necessarily an elliptical planform.

So the theory starts breaking down a bit.

So with sails you have twist changing the pressure distribution around with vang tension or snotter tension or downhaul downhaul tension or some combination.

With foils you can get a very very close approximation to an elliptical pressure distribution using a parallel sided root section, near the hull then at simple straight sided taper tip. This has generally been the route of high performance gliders too - another area where a tenth of a percent can prove important in competition. They are sprouting winglets now (which don't appear to work very well in sailing craft - notice the fad for wing keels has disappeared).

But my personal theory is that accurate sections shape using templates will overwhelm anything you do with the tip.

The reason I think this is that one company here has been making laminar flow section foils for certain classes when the Reynold's numbers that come from the boat's speed and the chord of the centreboard are far too low to be in the Laminar flow regime of the chosen foil.

So the section was wrong, but the foils still performed well. They certainly performed better than the usual inconsistently shaped foils that were around at that time ... so the only variables not accounted for were surface finish and detail finishing.

As the surface finish was quite good on some of the none templated foils (ie inconsistent shape) but they still didn't perform .. this leads me to something like this priority list. Priority out of 100 (I was going to do a simple numbered list but it didn't really show where things belonged.

100 Enough Area - it is amazing how many boats compromise their performance from the start.
90 Reasonable depth (aspect ratio) for the area
90 Templating or some other method to get consistent shape
90 Detail finishing - no lumps or bumps - good surface.
50 Correct section shape chosen - as long as it isn't something stupid.
30 Square or flat trailing edge unless you are smart enough to make it knife edge without it being vulnerable.
20 Elliptical end

The ranking is somewhat speculative on my part ... but tries to pin down my experience with racing boats where this was carefully followed on a theoretical and week by week racing arenas simultaneously.

Best wishes
MIK

Worth noting that from about Mk.V onwards, the Spit's wings were square ended too.
Improved the roll rate significantly, but at cost of noticeable reduction in lift according
to pilots of the era (eg. Clostermann Not likely to be an issue to a sail boat -
rapid roll rate is probably not a desirable trait. IIRC, later variants increased the
wing area to compensate. Funny how that matches MIK's foil priorities.

I wonder where "90 - no lumps and bumps" fits with PAR's observation on blue whale
fin protruberances enhancement of performance ... ?

cheers
AJ

Show me a whale that holds his fins at about 4 to 7 degrees angle of incidence for a start.

That is usually the deal breaker .. the different speeds and widths (they give you reynolds numbers).

I'll be guessing a whale would love to have hard finished foils of the level of perfection that we make.

But you ever seen a whale move ... much slower than any normal sailboat .. they are about reducing drag by reducing speed. Whales are not meant for higher speeds at all.

Then angles of incidence and the loads being carried.

And that whales ... um ... flap them.

I've just checked articles .... eliminating ones from the windmill guys who already have a design out ...

• The 16 degree stall angle found in the first wind tunnel experiment has been enhanced to produce airfoils that don't stall until they reach an astounding 31 degrees - far above anything previously known.
• Unlike virtually every other airfoil which can stall violently and even damage the machines they‘re employed on, tubercle airfoils always stall gradually. That changes the rules for turbines -- forever.

OK ... stall angle - centreboards are very unlikely to stall - they operate in such a narrow range. Rudders sometimes stall, but carefully shaped ones seldom do.

So low stall is not important on most normal boats ... America's cuppers and other modern boats with their deep, extremely narrow keels can tend to stall if not sailed by experts. If they appear on the next generation of Americas cup boats ... I will be impressed indeed.

Going back to shaping ... until someone works out exactly the shape dimensions and spacing (which Mr Fish may have done) and finds a way to duplicate them... then that is the way. Just adding bumps with some degree of hopefulness is likely to be unsatisfactory.

"It's high risk, high return," says Ben Greenhouse, a manager of business development at OCE. "The business models will depend on how well this works."

It sounds like they are lacking data!

Also just thought ... if it increases efficiency so much in normal use ... why don't all whales and fish have it? I would be guessing it is for specialised conditions that don't correlate with a wide range of uses. This makes it far less likely to correlate with normal foil parameters.

Yap - in a paper it describes how it relates to the whale's feeding pattern. So if your PDRacer has a similar feeding pattern ... it might just work.

This is fun ...

OK the mother lode

THE INFLUENCE OF PASSIVE, LEADING EDGE TUBERCLES ON WING PERFORMANCE
P. Watts and F. E. Fish

We have developed an efficient panel method simulation applicable to
wings that move immersed in a fluid at large Reynolds numbers. The numerical
simulation is used to evaluate the forces acting on the wing. When acting on a control
surface, these forces govern maneuverability. We compare lift and drag forces for a wing
with leading edge tubercles versus the same wing without tubercles at a 10° angle of
attack. We find a 4.8% increase in lift, a 10.9% reduction in induced drag, and a 17.6%
increase in lift to drag ratio. Tubercles enhance wing performance at modest angles of
attack while offering no detrimental effects at zero angle of attack. We evaluate viscous
drag forces acting on the wing and show that tubercles may incur an 11% increase in form
drag at a 10° angle of attack. Tubercles may also extend the operational envelope of a
control surface by delaying the onset and severity of stall. Consequently, the maximum
force of a control surface may be substantially increased.

In general maximum force of a control surface is relevant to rudders but not generally relevant to centreboards or keels. The effect of the bumps has little effect at low angles of attack (rudders when they are not steering) but will have the beneficial effects above when steering at higher angles.

Also applicable to high reynolds numbers (read faster or bigger boats)

So, maybe not that useful for sailing boats.

MIK

MIK

Higher reynolds numbers = bigger or faster boat...

That takes it out of PDR territory into GIS territory.
So then, the question must be asked.... just what is the feeding pattern of The Goat,
and would tubercles benefit it ?

cheers
AJ
(you are right - this is fun!)

Originally Posted by Boatmik

Howdy,
....
100 Enough Area - it is amazing how many boats compromise their performance from the start.
90 Reasonable depth (aspect ratio) for the area
90 Templating or some other method to get consistent shape
90 Detail finishing - no lumps or bumps - good surface.
50 Correct section shape chosen - as long as it isn't something stupid.
30 Square or flat trailing edge unless you are smart enough to make it knife edge without it being vulnerable.
20 Elliptical end

...
Best wishes
MIK

Hi,

most planes with high gliding angle today have fin shaped wingtips for efficiency, not sqares, and propellers with high efficiency as well (e.g. Torqueedo). Induced drag is an issue of some percent, and in racing I would care, but with a recreational sailboat... Tip shape for me is more a question of stability/vulnerability. I hate repairs.
A proper finish, a good profile with enough area, stability, and thats it.

greetings - Jörn

Clay -

Welcome back to the frenzy - if a relatively recent arrival can, in fact, welcome back a veteran who has been around for quite a while... But, don't blame us - you rekindled that flame all on your own. I'm more than happy to raise a cold one in honor of those 10 year old parts; and I'll celebrate the launching of those completed parts when the time comes.

Swallows and Amazons is a great series. I was introduced to those books only a few years ago but I have read the entire series at least twice. My home waters (Sacramento River Delta) put me in mind of (if I remember correctly) the Downs. My favorite book in the series is, We Didn't Mean to Go Out to Sea..

Enjoy the books, enjoy bringing your Goat to life, and most importantly, enjoy that little bloke.

Bob

Thanks Bob,

I've been enjoying reading about your progress and filing away bits of information for future reference. It's really gratifying to see this interest in the GIS, it is such a superb design and Michael deserves to have greater recognition. I spent a lot of time (and money) looking at other designs before I settled in the GIS and 10 years on I still think it was the right decision.

cheers,
clay