NACA airfoil

The Vacanti paper linked earlier shows the area coefficients for 10% thick profiles in Table 3/Fig 8. 0010 has the largest area thus allowing more volume for ballast. 66-010 has almost the same, but the most commonly used 63 about 7 % less. So volume is not the reason for not choosing 00-series despite the fact that many keels are quite thick (12-18%) in order to have enough volume.

 

I don't think any designer worth his fee would need to make a keel 18% to get the correct amount of ballast (say 40% of displacement).

For performance fin keels you might see the range of 8 - 10%. For longer, shallower cruising fin keels you might see something as high as 12%. You have to remember that as the keel is longer and shallower the chord is already much longer, so the chord thickness at a similar percentage is also already much thicker.

 

This depends a lot on the type of keel and era of the boat. Modern keels have quite short chords and if there is no distinct bulb the keel becomes thick. Also a bulb keel may have rather thick fin in order to be strong enough. This 12-18% is actually from my boat (35 ft). It has a fin keel with 2 m draft. In the middle it is about 12%, but both ends are thicker. Close to hull it is 15-16% in order to make the hull keel joint stronger. The low end has sort of a bulb, but the fin just gets thicker. Chord is 1-1.2 m for the keel including the "bulb". Ballast is 33%.

I have nothing to do with the design, but I think it is not bad at all. The boat is fast, especially in light air. Making the foil thinner would have added surface area or required a "real" bulb, if the draft is kept constant. Both would have made the boat slower, at least in light air.

 

I agree a "bulb", or flared false bulb at the bottom of the keel can be thicker. But I don't think it is so good to have the top of the keel getting thicker.

In fact, on the old elliptical keels we would strive to minimize the thickness at the keel/hull interface. There is a lot of turbulence there, and keeping the keel thin is important. So even if the top chord was only 75% of the mid chord, and the thickness at mid chord was 10%, we would make the thickness at the top chord only 8%. Viewed head on, the keel shape would look quite lenticular, but thinnest at the top.

In a longer cruising keel we would keep the chord thickness the same, so the top was longer and correspondingly thicker than the midspan or bottom. I can't recall ever going more than 12% (except when doing a faux bulb sort of squash at the bottom).

 

Just saw a 2012 keel design for a C/R-boat by a very highly respected design office. Rather deep racing keel, no bulb, not even a faux one, about constant chord. Thickness 12-15% increasing downwards.

 

Interesting. Sounds like they are getting weight down low. Is the Ballast/Displ under 40%? You would think the thick tip would be quite draggy.

 

31%. Thick isn't that much draggier. For the same volume it has much less drag than a thin one, which would have a much longer chord.

 

Not necessarily, if you take wave drag of the keel into consideration. To decrease the wave drag, you have to keep submerged volumes far from the water surface, hence the increasing chord length or thickness towards the tips.

There will be some sweet spot, where this gain in wave drag is lost by increased induced and residual drag of the resulting planform shape and thickness/chord distribution.

Cheers

 

The foil profile of the 2012 keel I mentioned seems to be extremely close to 66-series. If one makes a 15% thick and 1.8 m deep (from hull) keel with 1 m chord using 66-series, the volume is 0.183 m3. Thus from lead it would be 2030 kg and from iron 1320 kg.

If one insists on using 12% thickness, for the same volume the chord must be increased to 1.115 m. Thus the 15% profile can have 11.5% higher Cd for the same absolute drag. Looking at the data it seems that 15% profile has about 10% higher Cd than 12%, thus it has less absolute drag. Additionally the thicker profile has a much wider low drag bucket, which may even work on a beat.