Cal 20 Hull fairing question

[WeeHopeIII]

I would like to gain a modest windward ability in my Cal 20. The existing keel is approx 950 pounds, 7/8" thick, and the length above the bulb is 42 1/2". Whole boat draws 3' 5" so the flat height above the bulb is about 24 to 28". I've looked at some of the hull threads and realize there are tradeoffs, lift vs drag.

From the attached graphs, a profile like 12% Joukowski looks like a good choice if I'm reading them correctly. Another possibility is to just shape the leading edge to something like NACA0004 and fair the rest of the keel smooth.

Your comments are most welcome!

Chris B

[Paul B]

Originally Posted by WeeHopeIII

I would like to gain a modest windward ability in my Cal 20. The existing keel is approx 950 pounds, 7/8" thick, and the length above the bulb is 42 1/2". Whole boat draws 3' 5" so the flat height above the bulb is about 24 to 28". I've looked at some of the hull threads and realize there are tradeoffs, lift vs drag.

From the attached graphs, a profile like 12% Joukowski looks like a good choice if I'm reading them correctly. Another possibility is to just shape the leading edge to something like NACA0004 and fair the rest of the keel smooth.

Your comments are most welcome!

Chris B

Be careful. You won't want to take the boat out of class. That would hurt resale value. It would also prompt local PHRF raters to give you a whack that would probably exceed the speed increase.


From the class rules:

KEEL:
The leading and trailing edges of the mounting flange may be faired into the hull. A .25 inch fillet is allowed between the sides of the flange and the hull.
The vertical fin may not be altered from the original factory profile. It may be faired, but the maximum thickness shall not exceed 1.5 inches. Maximum profile cord lengths:
6 inches down from flange: 49.750 inches
12 inches down from flange: 47.750 inches
18 inches down from flange: 45.625 inches.
The bulb may be faired but not altered in shape.

[WeeHopeIII]

Hi Paul,

I've rescued the boat from the scrap heap and I'm not concerned about re-sale value. I'm also aware of the class rules, but am still wondering if anyone has a comment on attached profiles for a boat with ablative hull paint at speeds in the five to six knot range.

Thank you.

[Paul B]

Originally Posted by WeeHopeIII

Hi Paul,

I've rescued the boat from the scrap heap and I'm not concerned about re-sale value. I'm also aware of the class rules, but am still wondering if anyone has a comment on attached profiles for a boat with ablative hull paint at speeds in the five to six knot range.

Thank you.

I would tend toward the 0010-34. It is a very well proven section for this type of application.

I wonder how you will be able to thin out the existing iron to match with the sections these foils?