Foil Design 1970's Keel Boat

[daiquiri]

Originally Posted by alex_sailor

MY new question is should I go for the "fat" profile to match the boat, or go slimmer? Eric Sponberg's response was the foil no. was not as important as the planform.

If your keel (as I suspect) is not a high-aspect ratio blade like those seen in modern racers, then Eric Sponberg is absolutely right.

Low aspect-ratio keels' characteristics (approx. less than 3-4), are dominated by vortex-induced lift, which in turn is determined by the planform shape. To better understand thais, please see this post: Centre Keel Shape
Keel's aspect-ratio is calculated as

AR = 4*(T^2) / S

where T is keel draft, S is the keel area.

High aspect ratios (more than 6, approx.) are influenced by the choice of airfoil. In between both of them combinations of both planform shape and airfoil type are more or less equally important).

As about laminar foils (I understand that you were considering using them) and their drag bucket, please note that it will exist and will give you benefits in terms of drag reduction only for high-aspect-ratio keels and only as long as you make sure that the keel surface is always clean and smooth. In that way, there will be a laminar flow zone over the forward 30-50% of the airfoil surface, which is the reason for the drag bucket.

If the surface is not smooth or if it becomes contaminated by a marine growth, the laminar flow area (and the relative drag bucket) will disappear and you'll be left with a conventional (but more complex to manufacture) foil, with an ordinary, higher drag curve.

In practice, it means that (for high aspect-ratio keels):

• if the boat will stay in water for long periods, there will be no advantages of using airfoils with a drag bucket, because of marine growth and fouling. Then you could as well opt for a simpler foil with a thicker trailing edge, like the said NACA 00XX series.
• If you intend to haul the boat out of water and clean the hull after every (or so) use, then you will see an advantage of laminar foils' reduced drag.

Cheers

[alex_sailor]

If the attachment made it through, you can actually see the exisitng keel and rudder from a 3D LIDAR scan.

The question of what I plan to do, there are a few options
1) simply fair the keel to a best fit NACA 0018 and leave it as is. This gets me in the water soonest.
2) Try to loft a similar keel and manufacture around the existing SS post and welded plates. This option makes sense if I want to examine the condition of the existing post with its places and if I feel the existing planform and profile are as close to the optimum for this boat as we can get.
3) Build either male or female mould, buy a carbon or carbon/ss post and build a brand new keel around that If this is the option I select, I would then want the most optimum shape for the boat.

[alex_sailor]

If the attachment made it through, you can actually see the exisitng keel and rudder from a 3D LIDAR scan.

The question of what I plan to do, there are a few options
1) simply fair the keel to a best fit NACA 0018 and leave it as is. This gets me in the water soonest.
2) Try to loft a similar keel and manufacture around the existing SS post and welded plates. This option makes sense if I want to examine the condition of the existing post with its places and if I feel the existing planform and profile are as close to the optimum for this boat as we can get.
3) Build either male or female mould, buy a carbon or carbon/ss post and build a brand new keel around that If this is the option I select, I would then want the most optimum shape for the boat.
Attached Thumbnails
If the attachment made it through, you can actually see the exisitng keel and rudder from a 3D LIDAR scan.

The question of what I plan to do, there are a few options
1) simply fair the keel to a best fit NACA 0018 and leave it as is. This gets me in the water soonest.
2) Try to loft a similar keel and manufacture around the existing SS post and welded plates. This option makes sense if I want to examine the condition of the existing post with its places and if I feel the existing planform and profile are as close to the optimum for this boat as we can get.
3) Build either male or female mould, buy a carbon or carbon/ss post and build a brand new keel around that If this is the option I select, I would then want the most optimum shape for the boat.
Attached Thumbnails

[DCockey]

I started a another thread about "The Myth of Aspect Ratio" since the topic: http://www.boatdesign.net/forums/sai...tio-36836.html Aspect Ratio should not be a consideration per se in the design of you rudder. In particular don't decrease the chord in the belief that induced drag will decrease.

[alex_sailor]

I have decided on the following plan and want to hear if this makes sense.
Based on the fact that the rudder was/is in basically sound shape and on an article I read years ago to explain why C&C, after some tank tests, went from their scimitar rudder on the 35-1 to this spade design, to simply re-fair this one, grind down some of the thickness and make templates for a 0017 foil which I will extend an inch beyond the present chord, then chop off the trailing edge to make it square.
I am in the process of lofting the cross sections, as the existing rudder tapers 2" over the 50" length and has that rounded tip. I believe I should simply keep the same 0017 foils all the way down to the tip progressively getting thinner as they get closer. The only things I am not sure about, are 1) should the tip be squared off (flat shoe), or rounded off as it currently appears to be? 2) Because as you can see from the photo, the leading edge of the rudder is not square to the waterline, should I make the foil cross sections parallel to the water line, or right angles to the leading edge, if it makes any difference at all?

[alex_sailor]

By way of further clarification, this boat had 2 iterations, the "offshore" variety of which 20-30 were built with a slightly (4") shorter keel and shorter rig which was double stayed and the "Great Lakes" version which had the longer keel and a mast about 2' taller. MIne is the shorter variety and because she generally handles well and can even be balanced so I can take my hands off the wheel for up to 15 minutes in up to 1 metre seas, I think does well. Just the previous owner seems to have done a repair which was rough, assysmetrical and not a real NACA foil at all. So, I thought, while I have it out of the boat, this is my chance to do a re-design and re-build but having read all the posts, and understanding I know virtually nothing of how to calculate the bending moment of the existing or new keel posts and nothing about what happens if I change the rudder A/R without changing the keel, leave sleeping dogs lie, fe-fair according to a real foil.

Thanks for all the help and useful information I got on this site!

[alex_sailor]

One last thing, I have been using Mr. Ginsberg's NAC4GEN software for DOS and tried to upgrade to the version which allowed one to generate a DXF file but couldn't. Is there any shareware out there which permits me to generate either a spreadsheet, or DXF of a specific foil if all I input are the foil no. and the chord?
Thanks in advance

[Perm Stress]

Originally Posted by alex_sailor

... to simply re-fair this one, grind down some of the thickness and make templates for a 0017 foil ...
I believe I should simply keep the same 0017 foils all the way down to the tip progressively getting thinner as they get closer. The only things I am not sure about, are 1) should the tip be squared off (flat shoe), or rounded off as it currently appears to be? 2) Because as you can see from the photo, the leading edge of the rudder is not square to the waterline, should I make the foil cross sections parallel to the water line, or right angles to the leading edge, if it makes any difference at all?

I would be careful with grinding, not to take too much off and compromise integrity of blade...

"1) should the tip be squared off (flat shoe), or rounded off as it currently appears to be?" As you do not make new rudder anyway, just perfecting existing shape would be much simpler to do....

"2) Because as you can see from the photo, the leading edge of the rudder is not square to the waterline, should I make the foil cross sections parallel to the water line, or right angles to the leading edge, if it makes any difference at all?" The flow close to hull is more parallel to hull bottom, lower down -more horizontal, when in seaway the flow direction is changing constantly... when ....
You get the idea

[daiquiri]

Originally Posted by alex_sailor

Is there any shareware out there which permits me to generate either a spreadsheet, or DXF of a specific foil if all I input are the foil no. and the chord?

You can use this little program to generate a text file with NACA airfoil coordinates:
http://tracfoil.free.fr/airfoils/downloads/nacalte.exe
I have tested it on my PC, no viruses found.
Cheers!

[alex_sailor]

Daiquiri,

Thanks for this, it appears to work just fine. Not being a big AUTOCAD user, I was wondering if it has a routine so I can import the x,y, coordinates into the software and have it create the shape.

I have a work around using 3rd party software, but was asking just the same.

Thanks