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      A Few Simple Rules   05/22/2017

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Andrew P

49 er C board section

130 posts in this topic

I believe the opposite to be preferable.

 

Go from say 10% at the root (or what ever depending on boats speed, etc)

and fine it down to 6% (no point in going any flatter) at the tip.

Before any one asks, flat plate drag implication sis why you don't go flatter than 6%.

Seem to be a working limit.

 

JB

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I saw that at the show, Pres Ed, but confess I didn't understand what I was looking at. I understand that the board has a trim tab, but what's with the notch and string etc?

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AIUI, actuator mechanism. The string is rigged to a block that the jib sheets run through. When the sheet is loaded, it pulls the tab to leeward. I would guess that the slot in the top means that all string lengths are correct at all levels of centreboard rake.The U shaped bar flips over the top to lock off the tab offwind.

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Well, many things were talked about but nowhere is there any mention of the profile shape of the boards, the first part of question #1.

ie rectangular foil compared to triangular foil, with an elliptical foil somewhere in between., and foil rake.

 

Here are some hypotheses, which you may be able to shed more light on:

 

1/ An elliptical foil is supposed to be the most efficient shape due to minimising induced drag, but it is not optimised for overturning moment.

 

 

careful here. Elliptical shape is not the most efficient. Elliptical loading is theoretically the most efficient.

 

- An elliptical shape will have an elliptical loading if the twist and section are the same top-bottom, fair assumptions I think.

- Also Rectangular or triangular profiles will tend towards elliptical loading, ie the tip of a rectangular foil is 'lazy' wheras the tip of a triangular foil will do more than its share.

- Elliptical loading is not necessarily the most efficient, depending on its effect on overturning moment.

 

is 1. a fair assumption? Tapering to any great degree means a thicker (% of chord) section is needed at the tip to avoid going too slim.

 

FFS 'the other duncan' I put down a simple set of hypotheses which I was interested in opinions on.

 

As you're so expert, can you list for us all the other variables we need to ignore in order to make a reasonable design descision?

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The aim of this programme is to lower the drag coefficent but maintain the width of the bucket on alternate tacks without requiring the yaw slip to generate the lift. Achievable with the flapped board in theory.

 

Late and a bit OT, but there we go. Only photos I took at the dinghy show. National 12 flapped board.

 

post-419-0-74525600-1363332641_thumb.jpgpost-419-0-94079200-1363332644_thumb.jpgpost-419-0-56782500-1363332647_thumb.jpgpost-419-0-29461200-1363332650_thumb.jpgpost-419-0-31596900-1363332653_thumb.jpg

 

Seems like the flap is a v large % of the total chord. Is this the case?

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Well, many things were talked about but nowhere is there any mention of the profile shape of the boards, the first part of question #1.

ie rectangular foil compared to triangular foil, with an elliptical foil somewhere in between., and foil rake.

 

Here are some hypotheses, which you may be able to shed more light on:

 

1/ An elliptical foil is supposed to be the most efficient shape due to minimising induced drag, but it is not optimised for overturning moment.

 

 

careful here. Elliptical shape is not the most efficient. Elliptical loading is theoretically the most efficient.

 

- An elliptical shape will have an elliptical loading if the twist and section are the same top-bottom, fair assumptions I think.

- Also Rectangular or triangular profiles will tend towards elliptical loading, ie the tip of a rectangular foil is 'lazy' wheras the tip of a triangular foil will do more than its share.

- Elliptical loading is not necessarily the most efficient, depending on its effect on overturning moment.

 

is 1. a fair assumption? Tapering to any great degree means a thicker (% of chord) section is needed at the tip to avoid going too slim.

 

FFS 'the other duncan' I put down a simple set of hypotheses which I was interested in opinions on.

 

As you're so expert, can you list for us all the other variables we need to ignore in order to make a reasonable design descision?

 

I'm no expert, but the point of loading vs. shape is often misunderstood - a simple trapezoidal shape is close enough to elliptical to do the job, as you yourself pointed out.

 

Simple napkin analysis tells you that trying to taper down chord while keeping profile constant quickly leads to a difficult to manufacture tip, unless you're starting with a big chord (unlikely, we're talking dinghies), or using CNC moulds or similar.

 

You asked for an opinion. you got it. It is worth what you paid for it.

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Thought it was very neatly done.

Interesting stuff. Jo Richards article was written 4 years ago. Any idea how it ( the board) worked out. Subsequent articles showed the boat to be very fast down wind in light air but not much info on how the board worked as to up wind performance. Maybe easier to work out on a dagger design than a CB.

Same question for the boat at the show. How much improvement going to weather? Tabs would work on Rectangles, trapazoids or elipticals.

I worked on a airplane project many moons ago that proved squaring the tips on a eliptical wing section improved overall performance with out measurably reducing lift. Easier to measure on an aircraft than a dinghy though.

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i made a new front board (ruder) for a canter, rectangular, same area and section as the old eliptical longer board, the brains trust on the boat and the score card say its an improvement. a rectangular board would have a shallower c of e than a tapered board of the same area and less frontal area.shorter footprint. (it wasnt my idea just doing what i was told)

 

same same arnold.

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This is what I'm talkin' about.

 

Elliptical planform (Prandtl's first theory), Span and lift constrained: Constant downwash

Prandtl (second theory) / Jones, Lift and bending moment constrained: Downwash decreases linearly with span

Klien & viswanathan, Lift, moment, and shear constrainsts: Downwash decreases parabolocally

 

As explained by someone else:

 

""Prandtl asked the question: If I take an elliptical span load wing, and I try to BUILD that wing, I need a certain size wing spar to carry that load. If I constrain that load (called the wing root bending moment), and I need to develop the same lift as the elliptical, and I don't constrain the span, IS THERE A SPAN LOAD THAT WILL GIVE LESS DRAG? got that? Same moment, same lift, unlimited span, MINIMIZE DRAG. Prandtl found the answer was, yes there is. The new optimum is 22% MORE span with a bell shaped span load (a young kid named Reimar Horten was the first to call it "bell" shaped), gives 11% less induced drag for the SAME lift, and the SAME wing root bending moment. And Prandtl published it in a paper the title of which translates as "On the Minimum Induced Drag of Wings." Two pages, two figures, two tables, eleven equations. Classic, simple, elegant, and fantastic."

 

"Another short aside: there has been only one development to the Prandtl/Jones bell shaped span load since, and that was done in 1975 by Klein and Viswanathan where they also added the shear load as a constraint to the spar as well. the end results SHOULD give the minimum structure for a given lift, and given wing root bending moment, with minimum drag. Think about that optimization.

 

 

http://digital.library.unt.edu/ark:/67531/metadc55624/m1/16/

 

http://www.engbrasil.eng.br/index_arquivos/art60.pdf

 

 

 

 

 

post-62166-0-04762100-1369194608_thumb.jpg

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Yep, when you start getting deeply into it the number of variables get too much for my simple brain...

 

You have to start figuring out which are the real limiting factors...

 

Peak loads for instance - are they the sailing loads, or the loads exerted when righting the boat... Can't remember the last time I had a board which I'd trust with my fat a*** sitting or standing on the end for instance.

 

Then IME the major limiting factor on board performance is at low speed coming out of a tack and the like, not when sailing a leg. Can't remeber when I ever had a centreboard stall when sailing along.

 

So the optimum foil needs to balance minimum drag and heeling moment at speed with structural strength both at speed from sailing loads and capsize loads, which are typically exerted in different parts of the foil, meaning layup is as much as a factor as section, plus minimum weight, plus low speed performance coming out of a tack... Damned if I know how I'd ever solve an equation with that many variables...

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Then IME the major limiting factor on board performance is at low speed coming out of a tack and the like, not when sailing a leg. Can't remeber when I ever had a centreboard stall when sailing along...

And that's what Julian was talking about above. Basically if all you are doing is running in a straight line at speed, you can reduce your foil by quite a bit. But most race boats have to tack upwind. Which means you need to balance the total amount of distance lost in the tack (including how much resistance to the turn there is, to how long before flow attaches, to how much leeway you make until you are back in the foil's operating range) against the amount of excess drag you are carrying once you are back up to speed.

 

And as he pointed out, the self-tacking jib did a huge amount to increase the "bottom" of the speed bucket and hence allow for a reduction in the amount of lift the foil has to generate.

 

But its not sure that the "B" foil described above, performs well in that low speed reattaching flow mode.

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At the moment I'm using a foil that's quite pointy (like the 'B' foil). Not sure if it's a benefit or not. The weak tip isn't a problem, as its too narrow to balance on anyway!

The two things I'm uncertain about are

1/ as everyone says, what happens during the exit of a tack? What's the quickest way to get max. lift at low speed? I think, in fact, that a short -chord foil develops lift more quickly (I think it takes 2 chord lengths to develop 90% max lift for a given speed)

2/ Assuming the foil is lightly loaded during normal sailing, skin friction becomes more important and the foil, again, becomes less pointy and closer to an ellipse.

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At the moment I'm using a foil that's quite pointy (like the 'B' foil). Not sure if it's a benefit or not. The weak tip isn't a problem, as its too narrow to balance on anyway!

The two things I'm uncertain about are

1/ as everyone says, what happens during the exit of a tack? What's the quickest way to get max. lift at low speed? I think, in fact, that a short -chord foil develops lift more quickly (I think it takes 2 chord lengths to develop 90% max lift for a given speed)

2/ Assuming the foil is lightly loaded during normal sailing, skin friction becomes more important and the foil, again, becomes less pointy and closer to an ellipse.

3/ won't fill the slot as nicely when pulled up.

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Then IME the major limiting factor on board performance is at low speed coming out of a tack and the like, not when sailing a leg. Can't remeber when I ever had a centreboard stall when sailing along...

And that's what Julian was talking about above. Basically if all you are doing is running in a straight line at speed, you can reduce your foil by quite a bit. But most race boats have to tack upwind. Which means you need to balance the total amount of distance lost in the tack (including how much resistance to the turn there is, to how long before flow attaches, to how much leeway you make until you are back in the foil's operating range) against the amount of excess drag you are carrying once you are back up to speed.

 

And as he pointed out, the self-tacking jib did a huge amount to increase the "bottom" of the speed bucket and hence allow for a reduction in the amount of lift the foil has to generate.

 

But its not sure that the "B" foil described above, performs well in that low speed reattaching flow mode.

We've been doing some GPS tracking looking at tacking speeds.

 

A great tack - 4.2 knts minium speed recorded, an average tack is 2.3- 2.5 knts and a bad tack is barely moving. Can happen in a sloppy lumpy seaway when you go into the tack off balance, (not flat)

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At the moment I'm using a foil that's quite pointy (like the 'B' foil). Not sure if it's a benefit or not. The weak tip isn't a problem, as its too narrow to balance on anyway!The two things I'm uncertain about are1/ as everyone says, what happens during the exit of a tack? What's the quickest way to get max. lift at low speed? I think, in fact, that a short -chord foil develops lift more quickly (I think it takes 2 chord lengths to develop 90% max lift for a given speed)2/ Assuming the foil is lightly loaded during normal sailing, skin friction becomes more important and the foil, again, becomes less pointy and closer to an ellipse.

3/ won't fill the slot as nicely when pulled up.
True, I'd add a 3" long parallel-ish section as it leaves the hull

 

I haven't tried the freeship software, might have a look sometime-

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if you sail a skiff with a dagger board as you lift it you can feel the boat slide into that easy speed band width (reducing area and heeling moment)

 

if you sail a boat with a swing board as you raise it to easy speed zone (which seems to be a fairly critical setting) the area does not reduce so much, the profile is now up the creek, but the heeling moment is reduced probably more significantly than a dagger board, and the boat comes to life.

 

seems to indicate that heeling moment has a far greater impact than profile loading considerations.

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And the designed foil twist in a daggerboard contributes how?

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Fact, having sailed both 5ohs, 14s, 49ers and Mustos, And Lasers and 420s, the swing board also has the benefit of moving CLR aft quite dramatically. Seems to me this has much more effect that its heeling moment reduction because as you move CLR aft, weather helm reduces. Less weather helm less rudder attack angle. Less rudder attack angle both less drag but also a lot less heeling moment

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And the designed foil twist in a daggerboard contributes how?

 

well hopefully you can charge more for them.

I have made some boards with laminates stacked aft, the idea being they will emulate a gybing board, theres probably some other subtle effects there that im not bothered with.

these boards certainly weren't any slower, were they an improvement I doubt it.

 

I think people get caught up in the detail and miss the larger picture.

its this simple, you need to setup your lift engines (sails,boat,foils) to operate at there optimal aot relative to each other, the platform should promote the stability of the setup.

sure you can fine tune the bits, but it has to be done in the context of the larger picture.

 

why twist the board when you can just sail with a bit more helm. use the force luke.

i know a guy that got to the ramp only to find he left his board at home, so he use a reject board that had been stripped from the mould green,it was bent and twisted badly, he went out and won a heat in the nats.

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Hey mr fact, you've hit the nail on the head.

What you've described below is what got me interested in the pointy board profile. In essence there is less area near the tip, so it doesn't trip up as much - there is more area up near the hull. The pointy-ness helps keep the board efficient by maintaining a high aspect ratio.

How to know whether this is any more than a theoretical benefit in certain conditions..

 

 

if you sail a skiff with a dagger board as you lift it you can feel the boat slide into that easy speed band width (reducing area and heeling moment)

 

if you sail a boat with a swing board as you raise it to easy speed zone (which seems to be a fairly critical setting) the area does not reduce so much, the profile is now up the creek, but the heeling moment is reduced probably more significantly than a dagger board, and the boat comes to life.

 

seems to indicate that heeling moment has a far greater impact than profile loading considerations.

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BTW, there is a vast array of empirical data on this going back many years. (early 70's)

 

If your sailing a displacement of Corinthian type of boat you opt for a wash-in or gybing type of twist arrangement.

If your sailing a skiff, or a Contemporary type of boat you opt for a wash-out or feathering array.

 

Again 5o5 is the defining boat, same reason as have been spoken about before so I wont labor on the point.

 

Has to do with the trade off between Yawl drag and tip losses.

Its as simple as that.

 

JB

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