Hydrofoil Profile Question

I am looking for a suitable profile for an ama centerboard.

An asymmetrical profile (flat one side) such as NACA4312 would be suited for moving through water in one direction. Is there a profile series that is suited to movement in either direction? It would presumably be symmetrical about the mid-chord position. Would lift/drag be much degraded?

 

Does your craft tack or shunt? That would influence what kind of symmetry you seek - right/left or fore/aft. The right/left symmetric version of the NACA 4312 is the NACA 0012, and it's a good choice for centerboards.

For fore/aft symmetry, you have two choices: sharp-edged, like ogival sections, or rounded edged, like these. CFD says the round-edged sections can be as efficient as a conventional section, but I have my doubts as to whether that will work out in practice. The sharp-edged sections suffer from leading edge separation for all but a narrow range of angles of attack, so both approaches have their problems.

 

I am looking for a shunter type profile, that is, with fore-aft symmetry, which I cannot get with the NACA formula. I am thinking of using a section with a circular arc on one surface and flat on the other, about 12% thickness, with about 2% radius on the edges. Does that sound about right?

Who or what is CFD?

 

What you've described is an ogival section. You can find data on them in Hoerner's "Fluid Dynamic Drag".

In Abott & VonDoenhoff's "Theory of Wing Sections" (Dover Books), you'll find the NACA m=1 camber line. That might be another suitable shape for a sharp-edged section, either with a flat surface on the leeward side, or scaling the m=1 camber line for both the leeward and windward sides, so as to have some droop for both the leading and trailing edges.

CFD stands for computational fluid dynamics. Numerically calculating the flow to predict the pressures, boundary layer development, lift and drag. Unfortunately, numerical approximations do not necessarily capture all the detailed flow physics and can sometimes be misleading.

I've recently gained access to a more sophisticated program than the one used to analyze the Proa 3 series sections. It might do a better job of calculating the flow at the trailing edge, which is critical to the performance of these sections.

 

Thanks. I have taken the liberty of copying the P-30208 data for off-line study, as that seems closest to my needs; hope that is OK with you. It will take me a little time to come sufficiently up to speed on CFD to understand what you have provided. Assuming the P-30208 is a good choice it can be made in a fabricated foil since the upper surface appears to be circular between 10% to 90% of the chord and the lower surface appears parabolic. I can easily get those shapes by bending sheet material, with carved LE/TE.

 

I'll reiterate my original caution: the computer says it performs well, but that doesn't mean it will work well in practice. It's quite possible that the flow will separate from the trailing edge in a way that will decrease the lift and increase the drag.

Based on what you've described about your methods of building the foil, I think a sharp-edged section would have a better chance of performing the way you want.

There is no part of the section that is either circular or parabolic. It's very important that the foil be built as accurately as possible to the coordinates as given. And that you put a turbulator, such as strip of zig-zag tape or something like a length of fishing line glued to the surface, down the mid point on the windward side. This section was designed by defining the hydrodynamic characteristics and then computing the shape that would have those characteristics.

What you are describing is essentially the same as the Proa 1 series. That series did have circular arcs with radiused edges. And it was predicted to perform poorly.

 

AK
I started to look at the performance of this foil before you settled on the TS P-series.

For what it is worth I have attached the comparative data for three foils. The data is for an aspect of 4 and Re#200,000.

I cannot comment on the limitations of JavaFoil in producing this data. For foils with thin trailing edges it gives very good results compared with measured data.

Rick W.

 

Tom: If the profile is too sensitive to errors I may not be able to get it close enough to get the predicted performance.

Rick: I need to study your data, haven't had time yet. I am rethinking the fore/aft symmetry. Perhaps I should go for the NACA 4 digit series; I used those way back in my model aircraft days and they did not seem too sensitive to errors. The old Clark-Y was an old standby, flat one side, easy to build, not sure how it behaves in an incompressible fluid though.

 

The attached is the Clark-Y data on a comparable basis to the ones above. It is probably much thicker than you need as it has a wide working range. It does not perform as well as the other foils from a drag perspective. They have L/D as high as 17 whereas this one only gets 12.

Rick W

 

I think the old Clark Y was around 12% which is around what I planned to use. It looks similar to the profile I get for NACA5312 but the NACA profile is (more or less) flat over a greater portion of the chord.

I didn't understand the results in your table: Cl does not become negative and Cd doesn't pass through a minimum, at least for the range of alphas shown.

 

AK
You are correct. I loaded the coordinates straight from a data file I picked up on the web. It drew the section correctly but JavaFoil did not like analysing in reverse. The attached is the correct data and it is now much closer to the others. The foil is very close to 12% thickness.

Rick W.

 

We may have a winner, but I want to study all the numbers a bit more. Thanks for all your help!

There is very little difference between the Clark Y and the 4512 despite what looks like a radical difference in profile. That suggests that minor build errors would not have drastic results.

It also reflects my model airplane building experience, which was that hand-drawn "looks OK" profiles worked about as well as carefully plotted ones, at least outside of the laboratory. However, that experience may also be due to the inability to accurately control the shape of a fabric covered wing between ribs.

The profiles that have fore/aft symmetry might be more picky.

 

AK
I am not certain about the detail of TS's P foil but I believe it aims to achieve a drag "bucket" and this is likely where the sensitivity to shape may be.

Generally results do not differ a huge amount if the profile is smooth without unintended bumps, the trailing edge is thin and the leading edge is nicely curved.

The numbers I have given you are for an aspect of 4 at Re# of 200,000.

If you get into more detail design then you can use numbers closer to where you would like to operate. You may do a little better with a NACA 16-series designed for the required lift but it will likely be harder to make.

The attached image is the Clark Y rotated 2 degrees to give the horizontal base. I have made templates from these images by scaling them to suit in powerpoint and cutting them out from the printout. Works for any section smaller than your printer.

Rick W

 

http://www.basiliscus.com/ProaSections/ProaIndex.html

Try this for more info.

Myself, i've made a number of sculling oars. These must fulfill the same symmetry requirements as you are after.

In the case of a sculling oar the camber and thickness vary along the length, but i would make the edges parabolic and pretty "sharp". These foils would exhibit remarkably high efficiencies, no separation problems or anything.

In the case of your daggerboard, why are you aiming for 12% thickness? It seems needlessly thick. Myself, i'd favour 10 or even 9 per cent. Camber shouldn't be too high either. I'd say around 2 % is good for optimum L/D ratios. Then figure out at what speed you need best performance and calculate area accordingly, so as to operate at design coefficient of lift.

 

<<<<<<<I am rethinking the fore/aft symmetry. Perhaps I should go for the NACA 4 digit series; I used those way back in my model aircraft days and they did not seem too sensitive to errors. The old Clark-Y was an old standby, flat one side, easy to build, not sure how it behaves in an incompressible fluid though.>>>>>>

The clark Y foil is good general purpose and easy to build foil for aircraft wing, but it is far from impressive for performance. Could do better, in other words.

Air, as long as you are travelling significantly slower than mach1, is for all intents and purposes incompressible. I know, i studied incompressible fluid flows. So take any notions of compressible flow and throw it out the window, no sailboat ever needs be concerned about compressibility.

The fabric sag you mention between ribs-didn't you use sheathing? In my model gliders i would always sheath from the spar to the leading edge, upper and lower side, and quite often from the spar to the trailing edge but only on upper side, the lower side there is so near flat it's not worth it. This would completely eliminate sag.