Keel aerofoil shape

[SimonRobinson]

I bought a 26ft cruiser / racer last year (20.5 ft waterline lenght), which had the keel designed by the previous owner (unknown to me), who also turns out was not in any way qualified to do so. I have had to take the keel off, and am about to make a plug to get a new one cast.
Although there were only perhaps 20 boats of this type with properly designed fin keels, I have recently taken offsets from an original boat (plans for keel no longer exist). I was intending to copy the shape exactly. I still intend to copy it to a large extent, but would like to improve a little on the aerofoil if I can.
Original hull and keel measurements:
Displacement meant to be 2.3 tons (actually nearer 2.7)
keel 1 ton
Root length 1665 mm
tip length 915 mm
leading edge rakes back to 51 degrees from hull
trailing edge just aft of vertical
Mast 35 ft with 150% overlapping genoa
Max speed about 6.5 knots

The keel I measured had a very fine leading edge, about radius 6 mm
Max thickness near 40% from leading edge
Chord as % 12% near tip and in middle, 11% at root
Nearest NACA section I could find was NACA 65-012, but the keel I measured had a finer entry, and was fuller aft of the max thickness, particularly 60 - 70 % aft of the LE.

I am a little worried about the very fine leading edge and first part of keel, and the fat aft part to the keel. I am no naval architect, so do not want to try to work out lateral resistance etc. I would though like to get the most sensible keel section.

Assuming I leave the profile more or less the same, and kept the weight pretty well the same, would you suggest increasing the leading edge radius and making a shape more like 63-012 for example?

Thanks in advance

 

[Raggi_Thor]

I think the fine entry and the maximum thickness further aft than in naca 00xx would be called a "laminar profile" by the optimistic designer or a salesman. Theoretically you can have laminar flow in a larger area, but this has been impossible to obtain in real sailing (as far as I know), on the contrary, I think.
Why not use a simple naca 0012, it has very good overall performance and it's not as sensitive to roughness (bottom paints?) ad imperfections as other profiles.

See also
http://www.boatdesign.net/forums/showthread.php?p=120843
http://www.boatdesign.net/forums/showthread.php?t=9136&highlight=keel+profiles+naca
http://www.boatdesign.net/forums/showthread.php?t=8743&highlight=keel+profiles+naca

 

[SimonRobinson]

Keel area

Thanks for suggestion. I had read the threads you suggested, but it seemed there was some debate as to what was the best option from 00xx series to 63-0xx to 65-0xx or some more customised design using xfoil. The 0012 and the 63-012 (slightly modified to make the trailing edge 2mm thick) were my preferred options so far.

I certainly feel the keel on most of my class of yacht (Seawolf 26) have optomistically fine leading edges, but they seem to work OK from what I've been told.

On a linked subject, is there a quick rule of thumb for working out area of a section (so I can get moderately accurately the weight of the keel)? I think it might be in the region of 66% of the rectangular area of that section. I have used the more time consuming method of counting squares to work it out.

Are all the NACA sections using the same area, just distributing it in different places?

 

[Raggi_Thor]

The section area is different, but often close to 70%.
I shouldn't be so categoric, laminar profiles can work OK if the keel area is so large that the leeway is small enough, but the keel may lose the grip when you need it most, in a starting situation or if you need to "pinch a mark" (if you say so).

 

[Tim B]

Be careful about what salesmen say, 99.9% of the time it's not right when it comes to section shape.

I would personally avoid a NACA 63,64,65 series section. The apparrent gains to be had from the drag bucket are unlikely to actually materialise, except on a high-end racer with a big keel... that's kept perfectly clean.

66% of the area of the bounded rectangle is a good enough number to use for a quick sum on a 0012.

I think if I were you, I'd load your offsets into a CAD package (Rhino is good and has a free demo), then load a few sets of aerofoil points (for NACA series etc.) and see which fits best by eye. Once you have narrowed it down to a few sections, then ask yourself whether the boat was optimised for upwind or downwind sailing. We can then determine typical leeway angles, and you can take an educated approach to the foil selection.

I would suggest something similar to a NACA 0012 to good general sailing.

Tim B.

 

[Raggi_Thor]

According to Mr Vacanti, naca 0010 has an area coefficient of 68%.
I think that would be the same for all 00xx.

 

[SimonRobinson]

Keel bolts

Thanks guys. Last question, as I've now decided on keel airfoil.

There are 10 bolts, weight 1 metric ton more or less. What thickness of bolt should I use? Currently 3/4", but the original design said 1". And should they be high tensile steel or SS (A4)? I have read threads on material, and there seems to be 3 view points. SS, or high tensile steel or mild steel? Any opinions on size or material?

Thanks again.

Simon

 

[Raggi_Thor]

You can calulate the moment from the keel in a 90 degrees situation.
Say your keel is 1000kg approx 10kN,
and the distance from the keel base to it's center of gravity is 1m,
you have 10kNm.
This is just an example of course.
The keel bolts must make a simmilar moment,
if the distance between a pair of keel bolts is 0.1m
and the moment per pair is 2000Nm, that is 20kN per bolt.

A 3/4'' bolt has an area of approx 275mm2.
210N/mm2 x 275mm2 = 58kN (before yeld of mild steel)
So in THIS example galvanized mild steel would probably do,
with a safety factor close to 3.

See Eric Sponberg's site for example,
http://www.sponbergyachtdesign.com/ArticlesEngineering.htm

With a short fin, the loadings from a grounding is brobably the worst case.

I hope someone can give som pros and cons regarding SS.

From
http://www.boatdesign.net/forums/showthread.php?t=6275
Tensile:
SF = 2 (safety factor)
Rx = tensile stress per bolt. (N/mm^2)
N = number of bolts.
A = area of keel bolt. (mm^2)
F = weight of complete keel. (Newtons)
R(0.2) = 0.2 % tensile stess or proof stress of the material of the bolts.(N/mm^2)
Rx = F/N*A, SF*Rx <= R(0.2)
(Dutch Peter)

 

[Tim B]

Look in the ABS Guide for offshore racing yachts. It is freely downloadable, and has a short section outlining the minimum keel structure. Calculation of standard bolted structures (as Raggi has outlined above) should be done anyway.

Personally, I'd use High Tensile steel bolts, not mild steel. HT allows you a much greater factor of safety for the same weight as MS. I'm not sure you'd gain anything by using stainless, you can always encapsulate the bolts in epoxy (or paint) if you're worried about corrosion.

Cheers,

Tim B.

 

[SimonRobinson]

Keel

Thanks