daggerboard shape: suggestions?

[peterchech]

I have just finished the foil shape on a daggerboard, naca 0012 I believe, 4 feet long by about 14 inches chord and 1 3/4 inch thickness. It is for an outrigger canoe, and I expect to sustain speeds around 10 knots.

I have read that the bottom of the board (the end) should not be left flat. Right now it would be a 1 3/4 inch wide and 14 inch long vacuum in the water. But how should I shape it? The thinner it is at the end the better I believe. This gives less area to create a vacuum. I have also read that a curved profile, basically a circular tip (think a spitfire wing but backwards), will bring the point of vacuum even further back, to the tip, limiting further the area available for vacuum. Is this right?

Any suggestions for the outline of my relatively low aspect ratio daggerboard?

[HakimKlunker]

How is your daggerboard mounted?
If it runs in a case, and if you intend to sail also with dagger/s up, then any curved bottom shape will leave a gap at the slot in the hull and there you need to deal with respective drag/turbulance.

Not sure if you have this book:
"Principles of Yacht Design" (Larsson/Eliasson)
There is a lot of basic information there in the Chapter 'Keel and Rudder Design'.

The Spitfire wing is elliptic, not circular.
By the way: It's match was the Me109 with trapezoid wings (although a little curved too at the tips)
The Mustang wings were similar to Messerchmitt's (no wonder. heh heh heh: Einstein, von Braun, ...)
Wing tips are 'shaped' to control or influence the vortex.

However, the last Spitfire was made in Feb. 1948.
The Me's use after 1945 was somewhat restricted
We are now in the age of Airbus and similar. On the other hand they move faster, too - unlike the boats.
Meanwhile there are additional approaches: Tip caps, sweep angles etc.

"...This gives less area to create a vacuum..."
This is perhaps only an unlucky choice of words. But it is the profile shape that creates vacuum (or you meant: transition from laminar to turbulent flow? Or you meant: lift?)

As you ask for a shape advise: Let us please know a little more about the factors all around.

[Gary Baigent]

HakimKlunker, if we're going to be pedantic, it was Messerschmidt Bf109, not your spelling nor Me109 - and your point of lifting an elliptical tip shaped board right up ... why not leave just the tip exposed, that small length of board tip immersion creates minimal drag - certainly better than lifting the board right up into the case. Agreed, the latter is definitely a brake.
It would interesting to compare the difference on the C Class cats where they have a compromise, ugly, not sophisticated, cut off tip but are able to lift them so they are flush with hull bottom - to that of a true aero or hydrodynamics tip profile, like those fitted to the AC hard wing tips of Stars and Stripes cat ... or the long proven and still excellent Spitfire elliptical tip, (which by the way, allowed the Spitfire to out turn the Bf109 whose clipped wings stalled so they fell away) or to the angled aft and tapering to a trailing edge point of some modern foil shapes - to see where the greater gain/loss occurs. Obviously the C Class guys have made their decision ... but that could change with next swing of fashion.

[Tim B]

There are two common tip-shapes, which are both reasonable:

Leave it as a rectangular cut-off. Simple, but not the most efficient. Or, use a parabolic, or possibly kuchaman planform with the "corners" radiused so that the resulting tip is smooth.

Basically, the smoother the corners, the better it will work. The trailing edge geometry should be kept sharp though for minimum drag (subject to operator safety!).

There is plenty of info about wing-tips on the web, so have a search. Don't confuse tip-shape with planform decisions due to loading along the wing. Tips control vortex roll-up, not induced drag directly.

Tim B.

[daiquiri]

Was it necessary to create two threads for the same question?

daggerboard shape: suggestions?

[HakimKlunker]

Originally Posted by daiquiri

Was it necessary to create two threads for the same question?

daggerboard shape: suggestions?

If it is possible, then it is somewhat necessary

[HakimKlunker]

Originally Posted by Gary Baigent

..........

I love the Messerschmidt (sic!) Bf109 but am not allowed to talk here

So I will (mostly) stay with the topic:
With an elliptic shape I thought of a 'curved' trailing edge all along. But that would mean that any dagger position except fully down will already produce the said gap at the case.
As we do not chase Spitfires with a boat (heh heh heh) it will not matter so much if we fall away in a close turn... as long as it will not end in a - - - BARREL ROLL (heh heh heeeeeeh)

Maybe we should sub-divide:
- Larger cruising cats i.e. are usually not used as upwind-warriors.
- 'Smaller' cats are either just for fun or
- for competetive racing
- The original question mentions an outrigger canoe.

My quick search found no good results: Is there a picture at hand about the C-class daggers?

[Gary Baigent]

Bf109, there are plenty of postings on board and foil shapes. On the elliptical or whatever tip is on your board, it only extends 10-15% of immersed board length - we're not talking of a hydrodynamic tip extending right up to the case exit position on a fully lowered foil - that would be an extremely weird shaped profile board. My point about elliptical tip wings/foils compared to clipped wing/foils is that the latter stalled earlier.

[Tim B]

This is pretty much the definitive reference on the subject:

http://www.scribd.com/doc/35866801/H...amic-Drag-1985

The discussion on planform effects on induced drag starts at section 7-4 (page 116)

Tim B.

[rayaldridge]

Just to contribute a lazy person's perspective on this discussion, Slider's foils are all rectangular in planform. This was done primarily because I could accurately make them using only one template, but upon reflection, I don't think they suffer greatly from their chopped-off tips. They have more area than the elliptical tip board of similar depth, and more lift. Also more drag, but it might largely cancel out. Anyway, I believe there have been fast boats with squared-off foils, so my next boat will have the same.

[HakimKlunker]

Originally Posted by rayaldridge

....I believe there have been fast boats with squared-off foils, ...

I share your view here. From the practical point of view.
I had a quick racing dinghy once with c/board and rudder having no profile at all.
On the other hand, there is someone who already took the trouble to go into profiles, and why not carry it a step further. Just for the 'science'.

[daiquiri]

Since this seems to be the "alive" one of the two duplicate threads, I'm copying here my reply from the other one...

If you want to round the shape of the tips, do it on the leading-edge side and just as much as you think is necessary to let seaweed, plastic bags and ropes slip below it smoothly, and to minimize damage if you hit a ground below. From the hydrodynamic point of view, sharp square tips are better than rounded ones because they tend to increase the distance between the wing-tip vortices. Or, since we talk about a daggerboard, a sharp square tip will push the keel-tip vortex further down than a rounded-tip could do - and hence tend to increase the apparent aspect ratio of the daggerboard.

Cheers

[rayaldridge]

Originally Posted by daiquiri

. Or, since we talk about a daggerboard, a sharp square tip will push the keel-tip vortex further down than a rounded-tip could do - and hence tend to increase the apparent aspect ratio of the daggerboard.

Cheers

That makes me feel better about being so lazy.

[peterchech]

Sorry for posting twice, I thought the thread would get more responses here so I moved it, but then couldn't figure out how to delete the other thread.

I have gone for the squared off tip, mostly because it is quicker to build and I wanna be in the water asap. Thanks for all the advice! I rounded the edges of the tip to about a 1/8 inch radius, and have covered one side with glass and graphite already. Waiting for it to dry and I'll sand and glass/graphite the other side. The leading edge of the board was too sharp initially, and I was worried about low angle stalling, so I rounded it significantly though it still worries me somewhat... hard to compare it to the template accurately in that small area of the very leading edge...

As for the discussion of non-profiled foils, I had a small monohull with just a plank for a centerboard, rounded on its edges to a 1/4 inch radius. The boat probably wouldn't win any races but went upwind well enough. Then on my outrigger canoe (24' modified dierking wa'apa) I used a clamp-on leeboard that was given a profile of sorts, really just sanded/planed to have a sharp leading and trailing edge. This worked fine initially, especially at low speeds. But once I started getting the boat up to speed (5 knots and above) it started ventilating (literally sucking a 8 inch deep hole in the water on the windward side of the board). I also began having control issues, especially since I'm using a steering oar. So... that was my impetus on building a "real" foil this time. And a daggerboard to avoid any future ventilation issues. Simple boards work alright at monohull speeds, but once you start having fun in a multihull and playing with gennakers/etc a simple plank on edge it seems might just be dangerous!

[Tim B]

@Daiquiri: "sharp square tips are better than rounded ones" - please read the section in Hoerner referenced above, as I don't think you're right here. The tip shape in the Y-Z plane (assuming X is fore/aft) if square will cause secondary vortices from the edges which add drag. in the XZ plane (ie. in planform) square tips will hurt performance when sailing with a pitch angle (particularly bow-down), as you get sharp corners and "rearward-facing suction areas".

You will find it hard to beat a tip-shape where in planform, the last few inches (take 15% to 25% chord for the sake of argument) is a smooth curve (parabolic or cubic, it doesn't really matter), stream-wise at the trailing edge. In the Y-Z plane, a smooth parabolic curve from full thickness to the chordline at the tip will serve nicely.

Attached piccy is for a wing, (hence lifting one way only so slightly tailored), but the overall shape is good.