Wow! Very informative. Thanks. Mark C27 Solution
--- In [email protected], "Sneddon, Keith - ES/IS" <keith.sned...@...> wrote: > > 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 âbehindâ the rudder with the minimum amount of energy > expended to get there. Under sail, the flow is laminar (smooth), and will > also stay âattachedâ 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 âdownhillâ, due to the driveshaft angle. If the rudder is turned > far enough that it âstallsâ, ie. the flow is no longer attached, the > rudder doesnât 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 rudderâs 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 <robadk...@...> > 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. >
