SloopJonB
Super Anarchist
The keel on my Kirby QT was like that - thickness tapered down about 2/3 of the depth then the sides went vertical.
Supposedly it was a helicopter blade profile.
Best way to Measure keel foil section?
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Supposedly it was a helicopter blade profile.
Interesting.. i wonder how the Reynolds numbers compared with the heli blade spinning in air vs the keel in water.
Heli blades are a very peculiar case in aerodynamic: the tip operates at very high speeds while the root almost nothing, and the forward going blade operates at much higher speed than the retreating blade (the root in fact, may have negative airspeed). They are often reflexed to reduce Cm enough to keep from twisting off.
Maybe not so lazy, if the flats are parallel on either side it's possibly a parallel mid body NACA 00xx with the PMB inserted at the widest point.
silent bob
Super Anarchist
Starboard!!
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12 metre
Super Anarchist
It will help if you tell us what type of boat the keel is attached to.
If it is an older design with a longish keel, it is possible a foil section was not used at all. Some boats like a Cal 20 just have a bulb at the end of a flat plate.
Even a flat plate will produced lift - just not an overly efficient section. And when you get below about a 6% t/c ratio, the shape has little or no bearing and a flat plate works jus fine.
The thinnest 4 digit I could find on online was a 0006. I did find a 64A0004 (4% t/c) which I've attached below - however there is such little curvature that a flat plate with a rounded nose and a tapered back as PaulK suggests below would probably work equally well
The Q
Super Anarchist
Having spent hours reading every article I could find on keel Design, before designing my own. I found several top designers recommended a flat centre section in certain circumstances.
It allows you to have a higher number shape say a NACA 15 instead of a NACA 9, which gives a more forgiving sailing characteristic. It's less likely to have a sudden loss of grip at high angles of attack.
But using a flat in the middle means the keel is not as thick as if the NACA shape were scaled for the full length of keel , so being easier through the water.
So you just pull the NACA shape apart at its widest point, putting a flat between, which unless you are sailing at AC level, its almost as good as a full aerofoil.
This is what I did at the top of my keel, it keeps the same NACA shape down to the point it meets the lead "torpedo" by which time the flat in the middle has disappeared and the bottom of the keel strut is pure aerofoil.
Note, due to my boat having to be shallow draft, the "torpedo" itself is an aerofoil, which is a thicker NACA aerofoil shape to get the lead in..
It allows you to have a higher number shape say a NACA 15 instead of a NACA 9, which gives a more forgiving sailing characteristic. It's less likely to have a sudden loss of grip at high angles of attack.
But using a flat in the middle means the keel is not as thick as if the NACA shape were scaled for the full length of keel , so being easier through the water.
So you just pull the NACA shape apart at its widest point, putting a flat between, which unless you are sailing at AC level, its almost as good as a full aerofoil.
This is what I did at the top of my keel, it keeps the same NACA shape down to the point it meets the lead "torpedo" by which time the flat in the middle has disappeared and the bottom of the keel strut is pure aerofoil.
Note, due to my boat having to be shallow draft, the "torpedo" itself is an aerofoil, which is a thicker NACA aerofoil shape to get the lead in..
12 metre
Super Anarchist
I can see how that might work. You may get the upwind lift and stall characteristics of a 15% section with the offwind drag characteristics of a 9%. Seems too simple, but could be a case where Ockham's Razor applies (which it often does).
I haven’t checked closely, but my Salona also seems to have a flat section in the middle. It’s a modern design but I have also wondered whether this was intentional, or something that should be fixed with fairing.
It’s in the water right now but here’s a picture from this summer, it’s wet so maybe difficult to see the lines:
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Could also have been an attempt to make it locally wide to keep the keel bolts further apart (if they are in pairs)
Then if you want to optimize for VCG of the bulb you squash it vertically in a separate transformation.
Then you add a dillet where the fin attaches to reduce the junction drag due to the sectional area change.
The best bulbs (and by that simply lowest drag) are not bodies of revolution of a NACA foil. They scale the vertical coordinates of the foil section to make it fatter (and not just use a fatter foil section). Can't recall off hand the ratio.+
Then if you want to optimize for VCG of the bulb you squash it vertically in a separate transformation.
Then you add a dillet where the fin attaches to reduce the junction drag due to the sectional area change.
The Q
Super Anarchist
The photos above do illustrate the information I was trying to convey.
The only chance of designing and building a boat of my own was in 1996-2001 in Saudi, the internet was illegal at the time of design. It was for solo sailing and with my back problems, it's sit in like an international 2.4mR. 16ft long, 23ft mast and 3ft 3in draft. The rest of the shape of the boat was came from weight and local UK styles of boat purely calculated by hand.
Our waters mean we have 45ft yachts with 3ft 6in draft holding up a 60ft mast. I wouldn't try to design anything that big or extreme.
Yes 20 years+ after the original design by hand without internet information, there are many things I'd change. Now for this major rebuild, the keel is one of them.
I'm no design engineer, and the more technically correct calculations, in order to shape 3D aerofoil translations are beyond my facilities and abilities for this 2019/20 redesign of the keel.
I followed information, as to general shape, from naval architects articles and some university learned papers. Particularly as to the leading edge, bottom shape and the recommendation that if you're shallow draft you need as much aerofoil as possible, a round torpedo doesn't give you lift .
So once I'd worked out the volume I needed for the quantity of lead,. the CLR needed, and the VCG required, I experimented with various foil shapes it get the best fit. Luckily because we have a lot of weed in the area I sail , the C of G of the torpedo, need moving aft, so no forward projections were needed.
So it's a thicker NACA shape forward, then parallel sides to move the C of G Aft, then the NACA shape back to the aft edge. Some filleting /shaping between torpedo and keel.
Note, I've not gone for laminar flow shapes, the benefits are not enough for the slow speeds we sail with repeated tacking , every 30 seconds to a minute or so up river, and I doubt my abilities in constructing something smooth enough.
Yes a Dillet was fitted, I almost missed it, as there isn't a huge amount on the net about Dillets, on its shape, positioning etc. So I looked at what info I could find, and estimated the shape as best I could.
By chance today I'm putting on the final glass fibre coat on the Dillet, and leading edges. Glassfibering 3D shapes is fun..
Time to drink this muggacoffee and head for the workshop..
The only chance of designing and building a boat of my own was in 1996-2001 in Saudi, the internet was illegal at the time of design. It was for solo sailing and with my back problems, it's sit in like an international 2.4mR. 16ft long, 23ft mast and 3ft 3in draft. The rest of the shape of the boat was came from weight and local UK styles of boat purely calculated by hand.
Our waters mean we have 45ft yachts with 3ft 6in draft holding up a 60ft mast. I wouldn't try to design anything that big or extreme.
Yes 20 years+ after the original design by hand without internet information, there are many things I'd change. Now for this major rebuild, the keel is one of them.
I'm no design engineer, and the more technically correct calculations, in order to shape 3D aerofoil translations are beyond my facilities and abilities for this 2019/20 redesign of the keel.
I followed information, as to general shape, from naval architects articles and some university learned papers. Particularly as to the leading edge, bottom shape and the recommendation that if you're shallow draft you need as much aerofoil as possible, a round torpedo doesn't give you lift .
So once I'd worked out the volume I needed for the quantity of lead,. the CLR needed, and the VCG required, I experimented with various foil shapes it get the best fit. Luckily because we have a lot of weed in the area I sail , the C of G of the torpedo, need moving aft, so no forward projections were needed.
So it's a thicker NACA shape forward, then parallel sides to move the C of G Aft, then the NACA shape back to the aft edge. Some filleting /shaping between torpedo and keel.
Note, I've not gone for laminar flow shapes, the benefits are not enough for the slow speeds we sail with repeated tacking , every 30 seconds to a minute or so up river, and I doubt my abilities in constructing something smooth enough.
Yes a Dillet was fitted, I almost missed it, as there isn't a huge amount on the net about Dillets, on its shape, positioning etc. So I looked at what info I could find, and estimated the shape as best I could.
By chance today I'm putting on the final glass fibre coat on the Dillet, and leading edges. Glassfibering 3D shapes is fun..
Time to drink this muggacoffee and head for the workshop..
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