Keel section question

Hi experts!

I am looking to optimise the keel of my boat, it is a nationally recognised class similar to the Intl Dragon.
The Class Rules stipulate only maximum and minimum measures at two sections "VL11", at 100mm above the bottom of the keel and "VL9" at 300mm above the bottom, blue and red respectively in the picture:



The section of the rudder is free.

One problem is that the Class Rules stipulate a very wide entry. Below is a comparison between the max and min, in blue, of the Class Rules and some NACA profiles when the rudder is included.

VL11 is 1200 mm long and the rudder is 390 mm at that level.



VL9 is 1600mm long and the rudder is 600mm at that level.



So my questions are:
1. should the rudder and the keel together be faired as a NACA section for minimum drag? or should the keel itself have the right section?

2. Can I use one NACA section at the front and then change? (a necessity because of Class Rules) which is the better? or should I just forgetaboutit?

3. Should the upper sections of the keel, which are unregulated by class rules, be faired using something similar to the section at VL9 or should i use a "true" NACA section e.g 63-010? there must be no concavities larger than 5mm anywhere.

Best Regards
Niklas

 

There is something wrong about your figures. For VL11 I get a t/c of 11.6% so a xx15 section should be wider than class allows, not narrower. The same for VL9.

I'd stick with a standard NACA 00XX section, especially if you want to include the rudder as it will make the gap fairing easier. I would include the rudder in the keel, but you will need to pay attention to the hinge gap (Get a copy of Heorner Fluid dynamic lift for a full discussion of hinge design). The low drag sections can be sensitive to AOA due to their finer entrys and exits, which is not what you want in a typical sail boat.

Yes, you can use different t/c ratios for the fore and aft bodies. Pick where you want the maximum section and then work up seperate fore and aft t/c ratios to get what you want.

 

That wouldn't be a Udell 22, by any chance? The Square Meter rules aren't that strict with regard to the keel.

I'd consider the keel and rudder together.

There's nothing magical about a NACA section. What you really want is a section designed specifically for your application. For example, it has to meet the thickness requirement, you probably need to have the same volume so you don't lose ballast, and it should be designed to work with the rudder deflection.

Again, I'd go with something designed specifically for those regions. Does the concavity requirement only apply to the keel itself, or the keel and rudder? You might want to consider how much weather helm you typically carry upwind, and design the section so it has a smooth contour on the windward side when the rudder is deflected at its normal angle when going to windward. When centered, this could cause a concave profile across the joint.

If you'll post the tabular data for the class curves you've plotted, and the coordinates of your current keel if you have them, I'll take a stab at a section that might fit.

 

more information

Thank You!

The boat is called Neptunkryssare, it was originally designed in 1938. But Udell 22 was an extremely good guess!!! This one is slightly smaller, 1150kg and 9.00m. It is being built out of plastic today and is used for recreation and one design racing two man keelboat, in Sweden only I guess.




Here are the offset tables for the min and max for sections, eh -in swedish, but hopefully you get the important stuff.



The concavity restriction applies to the keel only, so concavities across the joint with a centered rudder is allowed!

Best regards
Niklas

 

Here are what your keel sections look like, with the rudder chord included, compared to NACA 4-digit sections of the same thickness. Two of the profiles have leading edges that are very close fits to the NACA sections. All the sections are more hollow in the rear, due to the rudder. The coordinates need to be smoothed if you were to use them directly.

I think you would want aim toward the maximum thickness so you can place as much ballast as possible as low as possible. To some extent it depends on how accurately you think you can shape the keel - you might want to allow some room for error and design a little inside the maximum profile. At 300 mm, there isn't a lot of choice. The specification calls for a thickness between 6% and 7%.

If one were to deflect the rudder so the windward side was a fair curve, that might be a good basis for designing a section that fits within the limits. It would help to avoid hollows on the keel portion, too, since there is a hollow formed by the rudder at zero deflection.

Continuing....

 

I think these sections meet your requirements. They are close to the maximum dimensions at the leading edge because I was trying to avoid an excessive pressure spike there at moderate lift coefficients. The windward side is faired when the rudder is deflected 3 degrees, so there's no pressure peak there, either, that could lead to premature separation over the rudder.

As you can see from the polar diagram there's no drag penalty for modest deflection of the rudder. A little bit of weather helm will actually improve the maximum section lift/drag ratio when going to weather. I've arbitrarily chosen a Reynolds number of 500,000 for the plot, but other speeds should be similar in nature. Like the NACA 4-digit sections, these are front-loaded and fairly robust to differences in Reynolds number or surface finish.

The minimum drag is predicted to be virtually indistinguishable from a a NACA 4-digit section of equal thickness, and there is less drag at moderate to high lift coefficients with the rudder deflected.

If there is any way to seal the gap between rudder and keel, that will reduce drag. I expect you keep this boat in the water, so coming up wth a seal that isn't subject to fouling will be difficult. You might try a strip of polyethelene folded in a "Z", placed in the gap, and fastened to the rudder leading edge and keel so it rolls with the rudder leading edge as it's deflected.