NACA Sections

This is probably a good question for you Eric (Sponberg)...I have noticed that the conventional NACA 0012 section has its maximum draft (thickness) at about 30% (of total chord length) aft of its leading edge. Some modified sections based on this section appear to have this position moved to more like 25% and the trailing sections are hollowed out.

We have looked at making our 25 foot trailer yacht (Noelex 25) perform better, although it's pretty grey when you look at the rules (Aussies have more explicit rules and I'm treading a foot in each camp path)

My questions are:
1) What will be the effect of changing the draft further for'd on a) the rudder blade and b) the keel? In terms of helm balance

Note that we have already moved the mast further for'd by around 20mm to reduce an inherent 'windward helm'. Even with new sails this is still there but it can be removed to a large extent by good trimming work.

2) If the keel was slighly changed in its fairing to bring the max draft further for'd - will this result in a corresponding change to the CLR (Centre of Lateral Resistance) ..presumably making the situation worse in terms of weather helm??..or will the lift characteristics change and this balance issue not be a problem??

3) In your experience if a foil is properly shaped will much of the drag one feels (even sailing dead downwind with small rudder angles) be removed?? We have taken the step also of cutting a recess out of the front face of our rudder (at the stock) so that the leading edge is now within 8mm of the gudgeon pivot line - as opposed to simply raking the blade radically for'd at the bottom..this seems to be working quite well so far (original blade configuration had the leading edge 40mm aft)

4) When you get the feeling on very odd occasion (usually steady breeze 12 knots / flat seas / no current) that the boat is perfectly balanced, the helm feels light but responsive and everything seems to feel right.(nice angle of heel etc)...are the settings that cause this phenomena worth recording?? In your experience is this what we should be aiming for in the absence of 2 boat testing and / or scientific data collection to maximise boatspeed?? Many thanks

 

Simply moving the maximum thickness forward is not likely to improve the section very much. See the XFOIL polars below.

If the thickness is farther forward and the contours are hollow, chances are it is a completely different section that was not based on the NACA 4-digit sections. It sounds like a section designed for a higher maximum lift.

 

I just happen to fall across this thread and saw my name at the top....

First, I would ask that you be more specific in how you want your Noelex 25 to "perform better"? Does this mean simply be faster on all points of sail, to point higher, to run faster? There are many facets of performance that one could improve upon, and usually, when you improve one facet, others will diminish, so overall you may not be gaining too much.

Second, your questions focus around section shape, and in that regard, I agree with Tom Speer that changes in section shape will have very little effect on performance, by which I mean speed and control.

Generally, I think that way too much importance is placed on section shape, and really, more emphasis should be placed on profile area and planform shape, and the location of the different blades in relation to each other and the rig for adjusting/refining balance.

I looked at the history link on the Noelex 25 class, and it seems to have a long and favorable history among "down unders", which means that it probably already has pretty respectable performance. It would not have survived as a class otherwise.

I sounds like you are wanting to reduce overall foil drag. You can't really do that too much by changing the location of fore/aft maximum thickness. You would have a much bigger effect by reducing the total thickness of the section. If it is 12% now, then try 10% or 8%. You will reduce form drag a bit, at the expense of smaller stall angle. That is, the foils will stall at lower angle of attack, and this could reduce your pointing ability, particularly in moderate air and choppy seas. That's one of those trade-offs that I mentioned above.

I tend to use foil shapes that have the maximum thickness at about 40% chord, because structurally, this gives me a stronger and stiffer blade, which is important particularly for rudder blades and carbon fiber keel blades. I also like fat sections, 15% thickness or greater, because they do have wide drag buckets for running downwind, relatively high lift a slow speeds, and they are very forgiving in stall characteristics--they stall less easily, particularly in waves.

Without seeing a picture of the boat's sailplan and its foils, it is not possible to comment much further. Of course, on the balance between the blades and the sails, some weather helm is always desired, and your description suggests that you have very nice light touch on the tiller. So you may not want to mess with it too much more. Any other changes that you try would require a radical redesign of the rudder or keel, and that would require major building changes. That costs money and time, and it would probably violate the class rules. A change of blades would render the boat something different than a Noelex 25, and that may affect its class standing and resale value. Those are major non-technical aspects that will color your decision to make such changes. If you were to do that, too, you would do best to consult a naval architect near you to advise on or carry out the changes smartly.

I hope that helps.

Eric

 

Thanks Eric

Thanks Eric for a very comprehensive answer. Yes you are right the Noelex has survived the test of time, and has depreciation values similar to the little Laser (i.e. very modest).

They are particularly good above 12 knots of breeze, and it gets better as it gets near the 18-22 knot range.

Our Class Association is having to address these issues presently and I'm involved to a degree. The Australian rules seem to make small modifications possible, and they seem to be more comprehensively written. We seem to have lost our way a bit because the class templates the rules relate to may be lost or even destroyed. The solution will no doubt be find the original moulds or measure up a number of boats, and make a decision on shape and tolerances.

Many thanks for the time taken Eric..I appreciate your remarks

regards

Grant

 

I think most good design offices would agree with you on using one of the NACA sections with the max thickness at 40% for keels, but it seems the 00xx is still preferred for rudders.

Do you really use 15%? That's incredibly thick.

 

I have used foil sections for sailboat keels and rudders from 10% to 21%. Rudders often operate at high angles of attack, and you don't want them stalling out at such high angles. Spade rudder designs and some keel designs also need to be very strong and stiff. Thick sections help tremendously in these regards.

In the case of Project Amazon, with her trimtab keel, it was easier to make the trim tab and keel blade fair across the joint in the deflected position with a thick section. This is a really good asymmetrical shape. She certainly did not suffer much in the way of speed loss because of her thick keel blade--she was a very fast boat.

Thick sections don't scare me. Read Marchaj's "Aero-Hydrodynamics of Sailing" and you can get an appreciation for thick sections.

Thinner sections do very well on boats in round-the-buoy's racing in calm water. I am not a round-the-buoys racer--I don't do well at it and I find it rather boring. I much prefer the adventure of offshore cruising.

Eric

 

Hello Eric,
True what you said about the possibility of benefits from the drag bucket (in case of foils which do have that characteristics) when running downwind. But it should be noted that a drag bucket is there thanks to the laminar flow over the forward half of foil's surface. If the foil surface becomes hydrodynamically rough because of marine fouling, or if the flow ahead of the foil is not smooth, then it is very likely that the flow will not be laminar but turbulent. The drag bucket will then disappear (with a big jump in drag coefficient) and the situation will become again a classical: more thickness = more drag.
So, in addition to what you said, I would add that the rudder (and keel) surface needs to be maintained very clean if one wants to exploit benefits of the drag bucket. It is certainly the case of racing boats, which are pulled out and cleaned at the end of the day, probably not so in a case of a boat which is to be kept moored in the water.
All true about the other benefit of a soft, progressive stall, extended at higher angles of attack, for fatter foils.
Cheers!

 

Hmmm...Other, faster boats with trim tabs don't seem to have the need for such a thick foil.

Thinner sections are not only good in calm water.

Think about this as well: When you are designing a keel for offshore cruising compared to inshore racing the keel will generally be shorter and a longer chord. That longer chord will already make the keel thicker for the same foil shape.

 

When one lays out the geometry of a keel section in which the after 30% of the section is the trim tab, it quickly becomes obvious that a thinner foil, say on the order of 10-15%, will have a knuckle at the joint on the lee side of the foil when the tab is fully deflected. At 21% thickness, the knuckle isn't there. There were other practical and engineering reasons to go with a thicker foil on Project Amazon, that would be a discussion for another time. I was happy with the choices that I made with regard to its design and structure.

When sailing in significant waves, the angle of attack to the keel is changing all the time, and on thinner foils, there is an easier tendency to stall the keel or increase drag considerably as the boat pitches and yaws about. On thicker foils, there is less tendency to do so. That's one reason why I like them.

Eric

 

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The lifting hydrofoil sections on the Rave foiler are all 15% t/c ratio. And it has a very high top end speed and takes off in about 8-10k with a single crew.

 

JavaFoil doesn't match

I've been a little concerned that I was getting separation at what seemed like relatively low lift coefficients when I've been playing in JavaFoil, or at least less than I hoped for. Then here in an earlier post, the plot from Tom Speer/Xfoil for a NACA 0012 was better than what I was getting with two element wings. I tried to match Tom's results as closely as I could and the JavaFoil result is much worse.

Reynolds number makes a big difference, but I'm pretty sure I matched that. JavaFoil also lets you select several different transition models and I tried to select the one that claimed to best match Xfoil. That choice didn't really help and the difference between transition models is much less than the Xfoil/JavaFoil difference.

I'm wondering if I'm doing something wrong. It does seem like ncrit should make a difference, but I don't see how to set that in JavaFoil. Separation/stall is maybe the biggest deal when looking at wing design and for other foils too at the low end of the speed range. Separation may also be the hardest thing to model. I guess I'm a little discouraged.

Any ideas?

 

There are many very well mannered cruising designs sailing the world with keels much thinner than you propose. I'll remain in that camp, with the knowledge I am in very good company.

 

Eric: surely keel thickness would be heavily influenced by the aspect ratio, from a structural point of view?

I can see how the thicker section would be desirable in disturbed water.

In aircraft thick sectons are more common in slow craft. Would this not also apply to a boat's keel?

Doug: are the Rave's foil sections asymmetrical?

 

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Terry, they are symetrical sections designed by Dr. Bradfield.

 

Peter
Here are some comparisons of Xfoil and Javafoil against test data using a NACA0012 at higher Re#:
http://www.aerospaceweb.org/question/airfoils/q0259c.shtml

I do not know why the results differ. You can get some difference based on the resolution. Attached is for 140 points.

I note Mark Drela has been visiting the site again. You could ask him if he has any idea for the difference. Martin Hepperle also will respond to emails. If you find out I am interested to know why. I believed that Javafoil overestimated maximum lift but this shows otherwise in these examples.
Rick W