The original rudders were 900 deep and 500 chord. The got bent a couple of times. They have since been replaced with much stronger and heavier rudders 600 deep and 500 long. These are strong enough to support the weight of the lw hull. They have also been fitted with a top end plate to prevent vortex shedding over the top edge when the rudder is off centre. You may have noticed that if you studied the previous photo:
https://www.dropbox.com/s/sox099g9dn6mbfo/100_2166.JPG?dl=0
The dagger board was installed to improve windward ability with the lower aspect rudders. The board is almost 3m long and 500 chord. It is immersed a little over 2m being pivoted under the deck and is braced laterally to the longitudinal beam under the middle of the deck.
The board is cambered enabling it to operate around zero angle of attack. That means it can have small radius edges to avoid detectable vortex shedding. It balances lateral sail forces between 50 and 55 degrees apparent wind so going to windward there is no leeway unless pinching higher than 50 degrees. Leeway is noticeably negative if the board is left down when sailing lower than 55 degrees.
Once reaching, lifting the board is like taking a hand brake off. Boat speed increases about 15% as soon as the board is lifted.
The cambered bi-directional board with small robust rudders works quite well. Draft is about 1m with board up. Windward ability is quite good for a boat having such a large cabin.
On 08/09/2015, at 1:27 AM, "robriley@rocketmail.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
what was the original purpose of the dagger board ?
and what was the aspect ratio? roundabouts if u dont recall
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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