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Obtaining a NACA template


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I'd like to craft a fairing for my trolling motor shaft. Over at boatdesign.net, a fellow used a NACA 0025 template which greatly reduced the load on his trolling motor and improved the efficiency greatly.

I know a lot of you guys have faired your rudders and keels. How can I obtain this template?  I mean, I see the "image" all over Google but not sure how to get the template. I'd like to make a fair 1.5" thick by 11" high.

The bottom line is, I am unsatisfied with the offerings from Torqueedo, Electric Paddle and other electric outboard vendors due to their cost and/or failure modes and warranty support.  I'd rather just fair the shaft on my 40lb. thrust trolling motor and pair it with a battery from Dakota Lithium.

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I do not know where you can get a physical template, to build your foil from, other than going to foil/rudder manufacturers like Phil's foil (still in business? not sure...).

But if what you want are the coordinates of regular foils, you may want to check out  airfoiltools.com  . They have a library of the most common foils ordinates. The X value is the chord of the foil. Everything is normalized. So X goes from 0 to 1. You multiply both X values and Y values by the length of the chord you want, and you get your profile.

You can check below for common 4 digit NACA profiles.

http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm[camber]

The site gives way more details on lift coefficient, drag coefficient, based on Reynolds number and angle of attack, but there is a small window in the page, with a scroll on the right, that gives all coordinates.

 

Hope this helps.

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9 minutes ago, weightless said:

The four digit series was specifically designed to be easy to create. There may be extra style points for using a slide rule but there's no shame in using a spreadsheet :)

https://en.wikipedia.org/wiki/NACA_airfoil

image.thumb.png.a6de1d56b46c34059e8fe74ea8e1279e.png

Sounds like a lot work there, I can send you a template file to your specs as a jpeg file that you can simply print out on paper and glue to some 1/4" plywood and then cut with a jigsaw and some sanding and you are good to go. ORCA3D Foil Assistant in Rhino3D makes it easy!

image.thumb.png.67df26e6ae91e235ec1abd072111a3bc.png

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2 minutes ago, Rasputin22 said:

Sounds like a lot work there, I can send you a template file to your specs as a jpeg file that you can simply print out on paper and glue to some 1/4" plywood and then cut with a jigsaw and some sanding and you are good to go. ORCA3D Foil Assistant in Rhino3D makes it easy!

image.thumb.png.67df26e6ae91e235ec1abd072111a3bc.png

 

I'm just trying to eliminated the shaft of the trolling motor as a blunt object in the water. The foil would be 1.5 inches thick by 11 inches tall and I would cut out the hole for the motor shaft. So, each half of the foil would be .75" thick so that I could clamp the halves around the shaft. See photo:

 

 

IMG_2185.JPG

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3 minutes ago, Ajax said:

Niiice.

Even better you can download a 3d section of foil from the parts library at GrabCad and scale it yourself. How do you plan on fabricating the fairing? Do you have access to a 3d printer? My local library has a 3D FDM printer as part of their MAKERLAB for patrons use. 

https://grabcad.com/library/naca-0025

 

 I downloaded the GrabCAD .sldprt (Solidworks file) and drag and dropped it onto the open Rhino window that I am working on something else and this is a screenshot of the results.

image.thumb.png.f11672b8db0f9dd887930568ed529922.png

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That photo is from a very old thread on Boatdesign.net. It is not my project.  I'll throw a caliper on my trolling motor shaft and get back to you with precise measurements.

I was just going to work with wood. I don't have a 3D printer and I don't think any of our local libraries have them. My co-worker has a small, entry level printer and several of the plastics we looked at were not suitable for salt water so he couldn't print the parts for me. Alternatively, we have a shop called Maritime Plastics here in Annapolis. They may be able to fabricate it if I present them with the proper file.

Honestly, I'm stunned that no one is selling fairings. It's such an inexpensive, obvious improvement. I see tons of motor fairings and propeller cages but nothing to smooth out the shaft. I thought I'd find something on Amazon, but nope.

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4 minutes ago, Ajax said:

That photo is from a very old thread on Boatdesign.net. It is not my project.  I'll throw a caliper on my trolling motor shaft and get back to you with precise measurements.

I was just going to work with wood. I don't have a 3D printer and I don't think any of our local libraries have them. My co-worker has a small, entry level printer and several of the plastics we looked at were not suitable for salt water so he couldn't print the parts for me. Alternatively, we have a shop called Maritime Plastics here in Annapolis. They may be able to fabricate it if I present them with the proper file.

Honestly, I'm stunned that no one is selling fairings. It's such an inexpensive, obvious improvement. I see tons of motor fairings and propeller cages but nothing to smooth out the shaft. I thought I'd find something on Amazon, but nope.

Be sure to get the diameter of the motor pod itself where it meets the vertical shaft. May as well print the cope at the bottom of the new fairing to fit up snug against the motor case.

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1 hour ago, Rasputin22 said:

Sounds like a lot work there, I can send you a template file to your specs as a jpeg file that you can simply print out on paper

I wonder if getting a .jpg to print to scale would be more work than lofting the template on a bit of paper? A pdf might be easier than a jpg. Then again, plotting the coordinates is easy, too. Maybe the easiest is to figure out the fore and aft length of the foil (cord) and the needed width, plug those into one of the many online calculators to get coordinates and then loft the template by hand? Not that the CAD thing isn't awesome and possibly amusing. I'm just saying it isn't required and, at least for me, it'd take more effort than doing it by hand.

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56 minutes ago, mookiesurfs said:

I hate to rain on this parade, but NACA foils are only efficient (lower drag per unit of pressure differential) at generating lift. They are not the lowest drag fairing.

True, but anything resembling a foil

 shape as opposed to a circular cross section will get you a very long way to cutting the drag.  If you wanna get that last ~2 percent of drag reduction by using a low drag section, then you better make the shape really accurate or it's not worth the effort.  And then one ding from bumping the fairing on something, and there goes your low drag efficiency section.

Ajax, you could just cutout something that by eyeball looks pretty good and foil-like, and you'd be about 90 percent there.  Depends on how meticulous you wanna be about it.

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21 hours ago, Ajax said:

 

I'm just trying to eliminated the shaft of the trolling motor as a blunt object in the water. The foil would be 1.5 inches thick by 11 inches tall and I would cut out the hole for the motor shaft. So, each half of the foil would be .75" thick so that I could clamp the halves around the shaft. See photo:

 

 

IMG_2185.JPG

Many times it cheaper to purchase premolded fiberglass  airfoil sections

 

https://www.uflyit.com/streamline_fairings.htm

 

these profiles are readily available  and are commonly used on  the landing gear on small aircraft

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20 minutes ago, slug zitski said:

Many times it cheaper to purchase premolded fiberglass  airfoil sections

 

https://www.uflyit.com/streamline_fairings.htm

 

these profiles are readily available  and are commonly used on  the landing gear on small aircraft

Ah, nice!  I need to get into the garage today and measure the shaft tubing. I think these all might be too large but you never know!

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21 hours ago, Ajax said:See photo:

IMG_2185.JPG

My guess is that with this large foil your improvement will be zero or less. First it has much greater displacement. Second it had much greater wetted surface. Both are negatives. Scale it down to the thickness of the housing. Sharpening the leading edge could help as well because unlike the NACA foils this is always operating at zero angle of attack. 

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38 minutes ago, Ajax said:

Ah, nice!  I need to get into the garage today and measure the shaft tubing. I think these all might be too large but you never know!

Do some googling 

 

there are many suppliers of stock airfoil profiles 

carbon seems to be the latest fashion , but fiberglass and  extruded plastic profiles are also available 

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I suspect that the drag thing is complicated. There's the interaction with the boat and the air-water interface and so on. If using this as an auxiliary then having something acting as an extra rudder might not be desirable. Maybe minimize the cord length in that case. On the other hand if it using it as the main rudder then a foil that works well as a rudder might be desirable. The four digit foils are pretty good and forgiving rudder shapes. The 6xA foils are fairly easy to make and are decent strut fairings. A fairing more or less centered on the strut, pointy on the ends and mostly symmetrical is probably okay, too.

Not to get too darned cranky but drafting these things by hand is super easy. I get the attraction of going the high tech route. The first time I wrote a program to plot airfoils to make templates was in the early 1980's and I've repeated the exercise a few times since. There are certainly circumstances when they help greatly. I don't think this is that. Anyway, here's a plausible section from Abbott:

image.thumb.png.91ab580e0e90df0abb4ac7aa40c30d9f.png

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On 8/12/2021 at 12:15 PM, Ajax said:

 

I'm just trying to eliminated the shaft of the trolling motor as a blunt object in the water. The foil would be 1.5 inches thick by 11 inches tall and I would cut out the hole for the motor shaft. So, each half of the foil would be .75" thick so that I could clamp the halves around the shaft. See photo:

 

 

IMG_2185.JPG

This looks like a perfect project for a 3D printer.  Let robots do the hard work. 

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4 hours ago, Borracho said:

My guess is that with this large foil your improvement will be zero or less. First it has much greater displacement. Second it had much greater wetted surface. Both are negatives. Scale it down to the thickness of the housing. Sharpening the leading edge could help as well because unlike the NACA foils this is always operating at zero angle of attack. 

Agreed. At a typical trolling motor speed of around one knot, a one inch round shaft is fairly low drag. You also lose the ability to raise or retract the trolling motor if you install a fairing, but that may not be an issue here. If you want to get serious, you’re going to need to incorporate Reynolds numbers when you change fluids, like from air to water.

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