Swing Keel Foil Pattern?

justsomeguy · Nov 29, 2014

I found a photo of a NACA 0012 at http://commons.m.wikimedia.org/wiki/File:NACA_0012.svg

If you know the chord to be 16-18 inches, you should be able to interpret the units in the span.

Yep, it's math.

$tingy Sailor · Nov 29, 2014

The template that comes with the CD DVD has NACA 13% printed right on it. It's 15.75" long and 1.5" at the widest (half profile). Down and dirty enough for ya?

HillbillyWanderer · Nov 30, 2014

I have read a few threads on people sealing their keel but this is first time I have seen anyone talking about the foil curve. How critical is it that I get the shape exactly right? My intent is mainly for racing. Is there a template for cruising and racing?

Jackdaw · Nov 30, 2014

$tingy Sailor said:
The template that comes with the CD DVD has NACA 13% printed right on it. It's 15.75" long and 1.5" at the widest (half profile). Down and dirty enough for ya?

An NACA foil is ALWAYS widest at 30% down the chord. The taper back to the tail is very important, it is designed to allow the flow to stay attached to the foil. If it tapers too quicky, the flow detaches (stalls) and huge drag results.

$tingy Sailor · Dec 1, 2014

That should work. You won't be able to get the keel perfect by hand anyway. As long as it's close and you fill in the major defects and get a consistent shape along the entire exposed surface with the keel lowered, you probably wouldn't notice the difference.

Jackdaw · Dec 1, 2014

CloudDiver said:
I'm actually more confused than when I started, but I think this makes the most sense.

I have a large .jpg of a NACA 13 foil on a grid. If I print it out and blow it up to 15.75 long (I have 14 x 17 paper in the printer), and I verify 1.5 at its widest point then I think that will be good to go right?

An NACA 0013 foil will have a thickness of 13% of the chord. So 15.75x0.13 = 2.04 inch

justsomeguy · Dec 1, 2014

Jackdaw said:
An NACA 0013 foil will have a thickness of 13% of the chord. So 15.75x0.13 = 2.04 inch

Yah, that's about what I get using the pattern in my previous post.

Give a man a fish.

$tingy Sailor · Dec 1, 2014

Methinks CD messed up and their template is actually a 10% curve (~1.5" thickness), which would result in a lower effective foil percentage in the water (angled 45 degrees). That matches up better with the ~3" max thickness of an actual bare keel, mine anyway. Otherwise, we'd have to add 1/2" to both sides.

It also states on the template:
"The actual foil percentage if the (keel hanging) angle was 45 degrees, the water length would be 22.62", which would come out to 13.2% This is about perfect for a small boat. Generally, a 10% is preferred for higher speeds, but the cross-section is usually too weak. The 13% has better stall characteristics at slower speeds." Al Gunther

That would suggest that, to achieve an effective 13% foil in the water, the cross-section should be more than 13%. If the goal is effectively 10% in the water as he stated, and to achieve that the cross-section needs to be 13%, then the template is too narrow by 1/2".

Or am I missing something?

$tingy Sailor · Dec 9, 2014

Did some more math. The template shape is correct but the caption on it is either wrong or incomplete depending on how you interpret it.

The caption states that it is 13%. It's shape (perpendicular cross-section) if applied perpendicular to the keel centerline RESULTS in a 13% foil curve in the water length, which is 22.62". That's what we want. However, the template length of 15.75 makes it an actual 19% foil curve. Explaining this in the booklet would have helped.

15.75 * .19 = 2.9925

2.9925/2 = 1.496 = the approx widest point of the half section template

By the way, out of curiosity, I called CD Tech Support to ask them for clarification. They forfeited, claiming none of the hundreds of DVD customers have ever asked, it wasn't calculated by them, and it would take them a day of research to figure it out. Hmm, sounds like an answer a salesman would give. I expected better but I work in software R&D, so I'm biased.

The morale of the story is, don't measure the template and expect to come up with 13% and don't call CD Tech Support expecting an explanation. Just use it and git 'er done.

Jackdaw · Dec 9, 2014

$tingy Sailor said:
That would suggest that, to achieve an effective 13% foil in the water, the cross-section should be more than 13%. If the goal is effectively 10% in the water as he stated, and to achieve that the cross-section needs to be 13%, then the template is too narrow by 1/2".

Or am I missing something?

Is the trailing end squared off? I have not seen the template in question, but true NACA foils end at a point. Because this is not practical for keels, it is OK to square them off. That makes them shorter. But the actual foil length is calculated with the phantom 'tail' being there.

$tingy Sailor · Dec 9, 2014

Jackdaw,
Yes, about 1/2" shorter.

CloudDiver,
You're probably close enough for volunteer work

Got any progress pics?

CaptDon01 · Dec 9, 2014

All this math is making my head hurt! :cussing:

Anyway, is all this sifering consistent with the fact that the keel doesn't hang straight down, but is angled? Wouldn't this actually lengthen the actual distance the water is flowing over the keel? It doesn't flow 90 degrees off the leading edge, but probably closer to 45 degrees?

But what the heck do I care, I'm a cruiser with a wing keel, so talk amongst yourselves....

Don

(oh, I forgot, I got a swing keel C-22 also)

$tingy Sailor · Dec 9, 2014

Hehe, you crack me up, Don!

Don't you work in terminal radar approach for the southern half of California guiding ginormous jets hurtling over the heads of hundreds of thousands of helpless people in the dark of the night? I can just imagine...

Whisky Tango Five Seven, you're probably within 5 degrees of the runway, over.

SCT Control, we're flying blind up here with all this fog, can you sharpen the pencil a little?

Papa Xray Two One, fall in behind Whiskey Tango five seven and follow, over.

SCT Control, excuse me?

Whisky tango five seven, what the heck do I care? Talk amongst yourselves, my head hurts.

SCT Control? SCT Control???

Just kidding, buddy. I couldn't do your job!

Jackdaw · Dec 10, 2014

CaptDon01 said:
All this math is making my head hurt! :cussing:

Anyway, is all this sifering consistent with the fact that the keel doesn't hang straight down, but is angled? Wouldn't this actually lengthen the actual distance the water is flowing over the keel? It doesn't flow 90 degrees off the leading edge, but probably closer to 45 degrees?

But what the heck do I care, I'm a cruiser with a wing keel, so talk amongst yourselves....

Don

(oh, I forgot, I got a swing keel C-22 also)

Don,

A quality thought!

But templates for keels are aligned on then at their swept angle, to match water flow. In the case of the C22, 30 degrees.

justsomeguy · Dec 10, 2014

CaptDon01 is absolutely correct.

The OP may want to re-measure the chord at the angle of attack that it presents to the water.

Swing Keel Foil Pattern?

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