| Rudder Geometry #65449 01/25/06 07:43 PM 01/25/06 07:43 PM |
Joined: Oct 2005 Posts: 951 Brisbane, Queensland, Australi... ncik OP
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Posts: 951 Brisbane, Queensland, Australi... | I am building a new rudder system (blades, boxes, tillers, tie-bar) and was wondering what geometry other mosquitoes found to be the best. In particular, I am looking for the angle the tillers need to be angled in at to give ackermann like steering.
If a boat were a car I would angle them towards the centreline of the boat perpendicular to the centreboards. But leeway angle stuffs this up and tends to move this point forward. I have read that the best point to aim the tillers at is perpendicular to the bows at the centreline, but this seems a long way forward when compared to ackermann theory.
The reasons for this change include: - Broken blade (old timber blades didn't have any laminate on them!) - Heavy steering (not weather or lee helm, it gets heavy when manoeuvring) - Dislike kick-up/rotating system when sailing in Moreton Bay with lots of jelly-fish
Anyway, any advice on this would be greatly appreciated.
Nick. | | | Re: Rudder Geometry
[Re: ncik]
#65450 01/27/06 04:02 AM 01/27/06 04:02 AM |
Joined: Jun 2004 Posts: 502 Port Noarlunga, SA, Australia Darryn
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Posts: 502 Port Noarlunga, SA, Australia | I investigated ackermann angles on Mozzy number two, it made no differance. Point the rudders straight down the centreline of the hulls after making sure they are square, line them up with the centreboards too.
To test it I angled the tiller part of the rudder stock inwards so the blade angles would change, relative to each other, as the crossbar was moved. It's a simple formula I found on the internet somewhere, it made no difference, I believe, because, 1, There is plenty of slip (water) between the rudders unlike car wheels which usually have good traction. 2, Mozzy's aren't very wide, might be noticeable on a Tornado. 3, Often when tacking is initiated, where the ackermann angle might have an effect, only one hull is in the water further reducing the benefit.
| | | Re: Rudder Geometry
[Re: ncik]
#65451 01/30/06 06:37 AM 01/30/06 06:37 AM |
Joined: Jan 2006 Posts: 556 Somers, Westernport Bay, Victo... Peter_Foulsum
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Posts: 556 Somers, Westernport Bay, Victo... | Nick,
I've never heard of ackerman. Would you explain for those like me who are in search of any knowledge that could help make the Mozzie go faster.
I did read that the tiller arms are best pointed toward the centre of lateral resitance which is close enough to the mast step. Something about providing rudder hydrodynamic assistance I think. Can't quite figure that one out unless I see a model or something.
Mosquito 1814 Macka Mozzie
| | | Re: Rudder Geometry
[Re: Peter_Foulsum]
#65452 01/30/06 10:29 PM 01/30/06 10:29 PM |
Joined: Oct 2005 Posts: 951 Brisbane, Queensland, Australi... ncik OP
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Posts: 951 Brisbane, Queensland, Australi... | I'll draw a diagram when I get a chance. In the meantime, follow these links to quench your thirst for knowledge... Ackermann Steering 1 Wikipedia - Ackermann Steering 2 Use this information carefully, the principle is correct but the geometry is slightly different. My initial estimations based on basic ship manoeuvring indicates that the point to aim your tillers at (not your rudder blades, these should still be straight) should be just forward of the centreboards at the centreline due to the effect of leeway angle (which cars don't have). regards, Nick. | | | Re: Rudder Geometry
[Re: ncik]
#65453 01/31/06 07:13 AM 01/31/06 07:13 AM |
Joined: Jan 2006 Posts: 556 Somers, Westernport Bay, Victo... Peter_Foulsum
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Posts: 556 Somers, Westernport Bay, Victo... | Thanks Nick.
The info and links you provided helped me understand why the tiller arms are bent where they are. I suppose you would have to map the minimum turning path to determine the exact difference between the inner and outer hulls. This would determine exactly where to point the tiller arms.
I guess that different speeds would give different turning radiuses so there would have to be a certain setting that would suit one speed with a tolerance each side of this. I also assume that in light winds the adverse effects of the chosen setting would be minimised through low flow around the rudders and minimised drag. Converserly at high speeds the number of skipper errors and conservation of rig, sail body and soul would tend to outweigh having a precise tiller angle setting.
Mosquito 1814 Macka Mozzie
| | | Re: Rudder Geometry
[Re: ncik]
#65455 02/03/06 09:15 AM 02/03/06 09:15 AM |
Joined: May 2003 Posts: 4,451 West coast of Norway Rolf_Nilsen
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Posts: 4,451 West coast of Norway | For the Tornado, Ackerman angle is 7degrees. Geoff Dobbs, former Tornado builder (Sailcraft), posted the following generic information on how to determine correct Ackerman angle: An easy way to check if the Ackerman mechanism is set up correctly is to run a string out sideways from the 1/3 chord point of the centerboards and anchor it a good way out from the boat, set the rudders to a fairly hard over position, run a line exactly square from the nearest rudder to the sideways line and make a mark at the intersection, do the same from the farther rudder. The lines from both rudders should intersect the sideways line at the same point. If they do not,disconnect the tiller connector from one tiller and adjust one rudder until both lines do intersect at the same point. Measure the distance from the connector hole is from the pin. Take half this dimension and add it to each tiller to bring the pin to the correct location (it is a good idea to make sure you get it in the right direction). A triangular plate picking up on the existing pin hole plus an extra hole on the tiller to prevent rotation would do this better than bending the tillers but make sure that the tiller pins remain parallel to the pintles in both planes. | | | Re: Rudder Geometry
[Re: Rolf_Nilsen]
#65456 02/04/06 09:28 AM 02/04/06 09:28 AM |
Joined: Apr 2004 Posts: 17 Dotan
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Posts: 17 | Optimal tiller toe-in angle is determined from the boat circulation radius on the turn. Many factors influence this circulation radius - hull shape, catamaran drift, with or without centreboards, distance between the hulls, catamaran length, skipper technics of the turn etc..
Very simple example considering only some points: Circulation radius = 5m, catamaran hull length = 5,5m ; width 2,5m tiller toe-in angle would be 5.4 degrees. If circulation radius = 6m, then tiller toe-in angle would be 4,2 degrees. So it is recommended to make calculations for every catamaran and sailor. | | |
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