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Catalina/Capri 25/250 Sailor's Forums

Catalina 250 Specific Forum

Asymmetrical fins

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Al
Captain

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USA
269 Posts

Initially Posted - 05/04/2006 : 22:30:53

<i>...a pair of asymmetrical fins for aft on the 250. Their function would be simple... they would provide increased lateral resistance aft and as the boat heeled, the windward fin would rise out of the water leaving the remaining immersed fin's lifting effort uncanceled. That effort would counter weather helm. [Arlyn]</i>

Arlyn, wouldn't that cause excessive drag when both fins are in the water? Even though the forces cancel, nothing, including lift, is free. Seems like the same as driving a car with excessive toe-in. You'd drive in a straight line, but at the cost of more power needed to "slip" the tires. I know the Hobie Cats we both had had asymmetrical hulls, but wasn't that a cost-saving design to eliminate dagger boards, which were on the 18s and were more efficient?

I'd been thinking of trying to accomplish the same thing by adding a wing on the bottom of the rudder. It would have zero lift when the boat is tracking straight, but as the boat started to heel and the stern to "slide", the wing would "hook up". And the more the boat heeled, the more top surface of the wing would be presented to the water to provide increasing resistance, so it would be self regulating (sortof...). You could even design in a little positive lift in the straight ahead position to level the boat from its usual "stern heavy" attitude.

Al Maniccia
SeaWolf C250WK #698
Marina Del Rey, CA

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 05/05/2006 : 06:53:30

Al, no question about increased drag but the rudder size could be reduced to help compensate.

Also, my original scheme was to locate them just above the boot stripe so that they didn't add drag while the boat is flat, though I'm not sure this is workable.

I'm thinking a long fin with minimal depth and of course an asymmetrical foil so don't think I'd call it a dagger board or leeboard.

A few reasons I'm interested to test this is that I've retained the original 1st generation rudder, which might be about right. The first generation is considerably smaller and thinner with far less drag than the huge rudders we are using now. It was adequate to helm under fifteen degrees of heel. If the fins provided enough lift and resistance to the increasing weather helm that occurs beyond fifteen degrees the rudder of course doesn't have to be that agent.

I've discovered that a foam apendage that has a good bit of contact surface and doesn't hang down far can simply applied with spray adhesive and stay tenaciously.

Edited by - Arlyn Stewart on 05/05/2006 07:00:06

bill bosworth
Navigator

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USA
172 Posts

Response Posted - 05/05/2006 : 08:23:40

Al;
I put a wing on my 3rd gen rudder, it's piece of 1/4" Lexan about 9" x 14" and it has helped a great deal. I find no ill effects, and the rudder is still ballanced and has control equal or better than my 2nd gen rudder. If you'r interested I can give the design for the shape of the blade and details about attachment.
Bill C250wb #134 Serendipity

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Al
Captain

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USA
269 Posts

Response Posted - 05/06/2006 : 13:03:19

Bill,

Yes, do you have some pix of the wing that you can post? I'd love to see it and how it's attached.

I like Arlyn's idea of the wings above the waterline, but I agree they might not be practical. I don't know how long they'd last with my present docking skills...

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zeil
Master Marine Consultant

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Canada
1314 Posts

Response Posted - 05/06/2006 : 20:37:45

Absolutely love the idea of adding/trying either or both suggestions.

Would it be reasonable to assume that the gain would outweigh the drag, would handle a wider range of conditions and would be easier to sail.

Arlyn what kind of foam board would you suggest and what type of glue would be strong enough to hold it in place.
Do you need to glue several sheets together to obtain the right thickness? Would you have to spine reinforce the foam?
Is there a formula to design the fins?
In case you wish to remove the fins, would a solvent remove the glue without leaving traces on the hull.
How do you suggest to shape the foam?
Would the fin, skeg (anti-weather helm device) protrude more than 6 to 10 inches by approx. 2 to 3 ft long?
Can the force be calculated or will it be trial and error.

I sense that a skeg type anti weather helm device or whatever it will be called could be very beneficial and a great addition to an already wonderful trailerable boat

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bill bosworth
Navigator

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USA
172 Posts

Response Posted - 05/06/2006 : 21:53:05

Al:
The fin pictured is the first one that I tried. I made it of 1/8" aluminum plate (forgot what copper bottom paint would do to aluminum). My new one is made from 1/4" lexan measuring 9" x 14". The leading edges are cut back at a 45degree angle leaving a 1 1/4 flat at the center of the leading edge. I cut the plate out with a bandsaw, then routed the edges with an 1/8" R. round over bit to form a bullsnose on all edges. I sanded the center portion where the blade comes in contact with the fin with 120 grit sandpaper. The fin is glued to the blade using 3M 5200 type adhesive (actually I used the quickset type). I built up a good bead of adhesive between the base of the rudder and the blade. I proped the rudder up against my workbence making sure that the fin was at right angles to the axis of the pintails and the faces of the rudder.
I have found no ill effects from using the fin and find that the fin controls the boat in much higher winds and angles of heal. In fact the boat has only rounded out in very high winds and only a couple of times over the past 3 years.
I will confess that the rudder has developed a crack between the pintails on two occassions. Catalina has repaired the cracks both times. I don't feel that the addition of the fin caused the cracking but that it was due to a manufacturing problem.
Hope that this helps;
Bill c250wb #134 Serendipity

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 05/07/2006 : 09:43:20

Henk, the thought of adding asymmetrical fins hasn't gotten farther than thought.

Several years ago, a single center line fin was added as one of the efforts to refine the 250 handling and I've [url="http://www.stewartfam.net/arlyn/skeg.html"]written[/url] about that.

When testing the 3rd rudder, a small portion of the fin needed removed and the residue of glue easily removed in seconds with a dampened cloth of acetone. The fin however has tenaciously held in place. Keep in mind that my boat is trailered and has a clean bottom, an unprotected foam fin would be too soft for a bottom that needed pressure cleaning or scraping.

Foam however may be covered with epoxy resin and cloth to form a hard shell apendage, which was my original intent after testing. My problem is that I never verified the individual contribution of the fin.

I could do that by reinserting the rivets in the rudder but then decided perhaps it would be simpler to make a comparison when the fin fell off... it never did (looks as good as when it was installed).

The point, at least some amount of fin can simply be glued on and will hold well and can be easily removed and the glue (3M spray adhesive) easily removed. Foam is easy to shape by cutting and sanding and can be covered easily by epoxy and cloth for a permanent apendage.

Designing asymmetrical apendages to counter weather helm when heeling would be likely a trial and verify excercise. And... there are certainly more than a few questions about how to design them. What size and foil? What if any angle of attack to mount? Can they be mounted at the boot stripe so that they don't protruce any drag when sailing flat yet when heeling 20 deg find effectiveness? Will they actually reduce the rudder requirements?

A few comments on Bills rudder fin. I've always thought it was a great idea. The fin does at least two significant things... it stops laminar flow losses off the end of the rudder and it replaces the loss of vertical rudder surface as the boat heels. Without question, it increases the performance of a rudder. I've a 3rd rudder but want to stay with a beaching but if the beaching fails, will without question add Bill's mod to the 3rd as it is inadequate for aggressive sailing without the fin.

My druthers however, would be to go back to the original 1st gen rudder, which has far less drag but couldn't control the weather helm beyond twenty degrees of heel, but if asymmetrical fins could do that...

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zeil
Master Marine Consultant

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Canada
1314 Posts

Response Posted - 05/09/2006 : 10:36:59

Thank you for the explanation Arlyn! Before doing much else I'll experiment with raking the centre board aft. Since "Someday Lady" has anti fouling on her bottom and is pressure washed when hauling her out of salt water, gluing anything will present problems. This subject interests me very much...

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