Judy B. is right, because the rudder will work very differently under sail than under power with an inboard. The water flowing past the rudder is going to try and get “behind” the rudder with the minimum amount of energy expended to get there. Under sail, the flow is laminar (smooth), and will also stay “attached” and flow cleanly around the rudder, providing sideforce the way a wing on an airplane provides lift. The center of effort is typically at about 25% of the chord length (width of the rudder in the direction of flow) aft of the leading edge. This will be behind the shaft centerline, which is the reaction of the sideforce, so the rudder wants to go straight, relative to the local flow. As the boat heels, the center of effort of the sail is not over the keel, but offset some distance to lee. The sail is pushing forward (thrust), the keel is holding back (drag), so the boat wants to turn to weather. This gets reacted by the forces on the rudder, which manifests itself as weather helm. Under power from an inboard, the flow over the rudder is usually at a much higher velocity, and because of that plus the proximity of the prop to the rudder, is very turbulent (small scale phenomena) plus not very organized as per direction (due to swirl, capitation,etc.). The average flow is also running “downhill”, due to the driveshaft angle. If the rudder is turned far enough that it “stalls”, ie. the flow is no longer attached, the rudder doesn’t work like a wing anymore, it works like a door, and tries to blow open. Because of the combination of the stalled flow, the rudder shaft angle, and the driveshaft angle, the rudder’s lowest energy (most open) position is hard over. Reverse is just a mess. Hydrodynamically, nothing works right in this direction. Try to back up as infrequently as possible.
Keith Sneddon From: [email protected] [mailto:[email protected]] On Behalf Of Judith Blumhorst, DC Sent: Tuesday, March 30, 2010 12:31 AM To: [email protected] Subject: Re: [IC27A] a rudder with a mind of it's own More descriptive details please..... How does it behave under sail power? How does it handle under power? is the boat an inboard or outboard? Take a look at this page: http://www.blumhorst.com/catalina27/catalina-27-rudders.htm The C27 is a nicely mannered boat when the rig is properly tuned and good sails that are properly trimmed. I can sail my C27 with two fingers on the tiller just a couple of inches to windward of the centerline, in winds from 10 knots to 35 knots and higher gusts. (That's true before and after I replaced the OEM barn door rudder) if I take the time to set the right sail plan. If the the tiller violently gets away from you under fully powered-up but not overpowered sailplan, you either have an inadequately tuned rig or an unbalanced sail plan. If you're consistently heeling more than 30 degrees, the boat is carrying too much sail area or the the sails are over-trimmed, and you' can expect to feel the boat try to round up hard. Less likely but still possible, your boat is not sitting on her lines fore and aft. Or, possibly your rudder is grossly asymetrical from one side to the other, a defect that is emphasized when the maximum foil is too far aft, like the OEM rudders. Under power, it's a different story. A little balance area on a rudder is generally a good thing under sail, decreasing tug & load on the tiller, but too much balance can cause problems when powering in forward with an outboard. Balance is not your friend when the prop is pushing water at the rudder. The prop wash from the inboard tends to push her off course when I'm trying to go straight ahead under inboard power. With the old OEM rudder, my C27 yawed strongly to port, and pulled very, very strongly. That's because of two factors - the balance area on the rudder and an asymetry of the rudder foil. Backing is always a problem if you let the rudder get too far from centerline. The longer the chord of the foil the longer the lever arm and the harder it is to keep on course. Awaiting more info from you.... maybe it's something I've forgotten to mention, Judy B 1977 C27 TR #3459 inboard Sailing San Francisco Bay area ________________________________ From: RobAdkins <[email protected]> To: [email protected] Sent: Mon, March 29, 2010 11:57:55 AM Subject: [IC27A] a rudder with a mind of it's own hi all, upon first sailing my c27 i noticed the tiller can get away from me mighty quickly if i'm not careful. this was not the case with my old columbia which had a keel mounted rudder. i thought a bit, yes it did hurt, and after drawing pictures in my head i've come to the conclusion that a small percentage of the rudder, on my c27, is foreword of the rudder post. this would explain it's violent tendencies to leap from my hand when executing a sharp turn and it's equally violent resistance to coming back to center after said turn is complete. i'm wondering if anyone has any clue as to why this design "feature" exists. it does make the helm more responsive/sensitiv e under sail but while under motor it is a definite hazard. any info would be appreciated. thanks-rob ________________________________ This e-mail and any files transmitted with it may be proprietary and are intended solely for the use of the individual or entity to whom they are addressed. If you have received this e-mail in error please notify the sender. Please note that any views or opinions presented in this e-mail are solely those of the author and do not necessarily represent those of ITT Corporation. The recipient should check this e-mail and any attachments for the presence of viruses. ITT accepts no liability for any damage caused by any virus transmitted by this e-mail.
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