Starboard!!
Member
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Trying to identify what foil section was used, any ideas on how to best take measurements of the keel?
Best way to Measure keel foil section?
- Thread starter Starboard!!
- Start date
12 metre
Super Anarchist
What I have done is get a couple of boards (I think I used 1x4) about a foot longer than the max chord length and picked up a couple of bolts longer than the keel thickness.
Drill holes near the end of both boards large enough for the bolts. Place one board on top of the other so the holes line up. Insert a bolt through one set of holes and thread a nut down to the approx width of the keel.
Assuming you are doing this on your own - place assembly around keel (it's okay if it is lying on the ground. Insert second bolt through other end and thread nut down to approx width again. Then raise assembly to the desired span width and tighten one of the bolts till you get a friction fit between the boards and keel face. Or just raise the assembly until you get a friction fit regardless of span. Measure the length of exposed bolt at each end and adjust until they are equal (while maintaining light friction fit). If you have a helper it makes it a whole lot easier and less finicky.
Now you have thickness. Measure the chord length at that point and you now have the t/c ratio. Finally, mark of the point where the board is touching the keel. Measure from the mark to the leading edge of the keel and you have maximum thickness as a % of chord.
If the max thickness as a % of chord is 30% and has a fairly round and bluff leading edge it is likely a NACA 4 digit. If the leading edge is more of a sharper parabolic shape or max thickness is at 40%, it is more likely a NACA 6 series say a 63Axxxor 64Axxx.
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Starboard!!
Member
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- 105
- Thread starter
- #4
@12 metre. That’s genius, thatnk you!
The middle of the keel is basically flat (lazy builders/casting) so I don’t expect it to match exactly with any NACA foils, and will require a lot of fairing build-up. Is there Any software or method you recommend for matching the points to the particular NACA foils?
The middle of the keel is basically flat (lazy builders/casting) so I don’t expect it to match exactly with any NACA foils, and will require a lot of fairing build-up. Is there Any software or method you recommend for matching the points to the particular NACA foils?
sailorman44
Member
Some builders were known to cast their keels flat instead of vertical which made the up side of the keel flatter as the metal cooled and shrank.Check both sides of the keel.
What you could do is enter your measurements into a spreadsheet. Do the same with the normalized NACA section coordinates and scale the chordwise coordinates to your chord. Next you probably need to do some interpolation of one of the datasets to have your chordwise coordinates line up. Then you can perform a least squares fit of the thickness coordinates with the thickness of the NACA section as a variable.
Once set up it should be easy to check a few different sections manually. As others said before start with the 4-digit series. You will probably find that your measured section does not fit any airfoil section, but you can use the tool to check which you are closest to and where you need to add material to get a proper section. Plotting the coordinates will help as well!
SloopJonB
Super Anarchist
Or you could contact the designer and ask.
PaulK
Super Anarchist
If the builders/casters were so lazy as to leave the middle of the keel basically flat, why would they have made the rest of it to match any specific foil at all? 'Round the front and taper the back... we're done'.
ryley
Super Anarchist
silent bob
Super Anarchist
Years ago, a shop was doing a refit on a rare Porsche. When it came time to replace the windshield, they ordered the original design blueprint from the factory. They made a windshield, and it didn’t come close to fitting the car. They assumed they made a mistake, and proceeded to make another. It too did not fit. So, they called the factory and spoke to the original workman that made the windshield and explained their dilemma. He replied “Just because they were designed that way doesn’t mean we built them that way!” They made their own design from measurements taken off the car, and nailed it on the first shot!
An old article in Professional Boatbuilder nailed it. It said that a decently built plain section foil was way better than a poorly built sexy section. It basically proved that the Winged Keel on the Cunter Shitbox 37.86 was about as hydrodynamic as it’s anchor.
An old article in Professional Boatbuilder nailed it. It said that a decently built plain section foil was way better than a poorly built sexy section. It basically proved that the Winged Keel on the Cunter Shitbox 37.86 was about as hydrodynamic as it’s anchor.
A few more things to consider.
If the designer was sophisticated (or thought she/he was) then the foil section can vary with depth.
If the casting company was lazy or not too good, the shapes on either side may not match
Many years ago we did an investigation of why a Hunter sailed differently on different tacks. We set up a jig and measured the keel at about 4 or 5 locations. The 2 sides were somewhat symmetric but not exact.
You can use a free program like Profili to get shapes of all typical NACA sections.
If the designer was sophisticated (or thought she/he was) then the foil section can vary with depth.
If the casting company was lazy or not too good, the shapes on either side may not match
Many years ago we did an investigation of why a Hunter sailed differently on different tacks. We set up a jig and measured the keel at about 4 or 5 locations. The 2 sides were somewhat symmetric but not exact.
You can use a free program like Profili to get shapes of all typical NACA sections.
The NACA sections are based on mathematical formulas, so the coordinates can be calculated exactly for any chord position that you may have measured.
Yes, of course. But that does not tell you which thickness fits best to the keel at hand. You can of course eyeball it, but the idea of the approach was to minimize the change required to the existing section. You don't have to use a 10% or 15% thickness section, but you can also use a 13.38% section if that is closest to the existing foil.
CaptainAhab
Anarchist
Spiling. This will allow you to quickly take accurate sections. You can use a piece of ply and a stick to takeoff. Redraw it and measure away. There are a few good videos on YouTube
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SloopJonB
Super Anarchist
Simplest and most accurate way to spile is to use a joggle stick.
After I learned about them I "joggled" a new bulkhead, cut it out and it fell into place with zero trimming or fitting.
The key is to ensure many pickup points.
12 metre
Super Anarchist
Many interesting solutions for transcribing keel measurements.
However, in spite of the thread title, what the OP really wants to know is how to identify what foil section was used
So with that in mind: I suspect most if not all production fin keelers will have a NACA foil and the vast majority of those will be a 4 digit. Some faster lightweight designs may have a 6 series (all of my boats have had a 6 series). On a very rare occasion you might stumble across a NASA LS, Eppler, Wortmann, etc. but I suspect the 4 digit and 6 series cover 99% of production boats.
You only need 3 data points: chord length, max thickness, and % location of max thickness. If the % location is around 40% it will be a 64Axxx with the last 2 digits denoting the t/c ratio as a %. For example the section at the keel root on my boat is a 64A008 - meaning it is a 6 series with max thickness at 40% of chord length with an 8% t/c ratio whereas the tip is a 64A012 meaning it is the same section as the root but is relatively thicker. The A designates it has the lightly hollow trailing edge filled in.
if the % location of max thickness is at 30% it is most likely a 4 digit although possibly a 63Axxx. I can tell the difference blindfolded by running my palm along the leading edge - big difference between the two. A 4 digit has a much more blunt leading edge and has the shape of a semi- circle. A 6 series is noticeably finer/sharper and more of a parabolic leading edge.
Anyways to get the 3 required points all you need is to make the jig I described in post #2 (or something similar), a measuring stick or tape, and a pencil or Sharpie.
Now, if you want to make templates, you can make them yourself or contact a place like Phil's Foils. I did the latter on my previous boat and provided them with the three measurements and he immediately said "Sounds like you have a 64010 section". Fortunately it had uniform sections so all he needed after that were the 3 chord lengths I needed the templates for.
So I had them CNC three templates for me (they used G10) for $200 IIRC. Pretty good deal to me to have accurate templates made without the effort of laying out and cutting plywood templates.
However, in spite of the thread title, what the OP really wants to know is how to identify what foil section was used
So with that in mind: I suspect most if not all production fin keelers will have a NACA foil and the vast majority of those will be a 4 digit. Some faster lightweight designs may have a 6 series (all of my boats have had a 6 series). On a very rare occasion you might stumble across a NASA LS, Eppler, Wortmann, etc. but I suspect the 4 digit and 6 series cover 99% of production boats.
You only need 3 data points: chord length, max thickness, and % location of max thickness. If the % location is around 40% it will be a 64Axxx with the last 2 digits denoting the t/c ratio as a %. For example the section at the keel root on my boat is a 64A008 - meaning it is a 6 series with max thickness at 40% of chord length with an 8% t/c ratio whereas the tip is a 64A012 meaning it is the same section as the root but is relatively thicker. The A designates it has the lightly hollow trailing edge filled in.
if the % location of max thickness is at 30% it is most likely a 4 digit although possibly a 63Axxx. I can tell the difference blindfolded by running my palm along the leading edge - big difference between the two. A 4 digit has a much more blunt leading edge and has the shape of a semi- circle. A 6 series is noticeably finer/sharper and more of a parabolic leading edge.
Anyways to get the 3 required points all you need is to make the jig I described in post #2 (or something similar), a measuring stick or tape, and a pencil or Sharpie.
Now, if you want to make templates, you can make them yourself or contact a place like Phil's Foils. I did the latter on my previous boat and provided them with the three measurements and he immediately said "Sounds like you have a 64010 section". Fortunately it had uniform sections so all he needed after that were the 3 chord lengths I needed the templates for.
So I had them CNC three templates for me (they used G10) for $200 IIRC. Pretty good deal to me to have accurate templates made without the effort of laying out and cutting plywood templates.
El Borracho
Barkeeper’s Friend
Sounds like the goal is to fair it to a NACA section. You need to choose if you want the designer’s section or the proper section that is the closest fit to what you have (my choice).
For best fit it seems some posters here could do the math...or an accurate drawing might do.
Some designers did use thicker sections down low to get more lead down there. Would be worthwhile to measure sections at around 25% and 75% deep rather than the middle.
A paper template of the nose might be necessary to differentiate between parabolic and circular. But may not matter if your goal is a new section vs. designed section.
Never underestimate the sloppiness of builders and yards.
For best fit it seems some posters here could do the math...or an accurate drawing might do.
Some designers did use thicker sections down low to get more lead down there. Would be worthwhile to measure sections at around 25% and 75% deep rather than the middle.
A paper template of the nose might be necessary to differentiate between parabolic and circular. But may not matter if your goal is a new section vs. designed section.
Never underestimate the sloppiness of builders and yards.
12 metre
Super Anarchist
Yep - G10.
They had smallish hooks on the leading and trailing edges. Not really long like you see in a lot of photos - maybe 1/4" to 1/2" but enough.
12 metre
Super Anarchist
It was also done because any rolling or pitching of the hull is amplified at the foil tip. You want a more forgiving shape at the tip and a thicker one gives you that.
