Rudder 10cm up

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Musto999
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Rudder 10cm up

Postby Musto999 » Sun Sep 03, 2017 9:44 pm

I noticed on Dave Poston's video from UK nationals his rudder was about 10cm proud of the cassette i.e. not fully down?
Is this to minimise drag?
A mistake (unlikely)
or other reason?

Would be interested to know.

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Rick
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Re: Rudder 10cm up

Postby Rick » Sun Sep 03, 2017 9:57 pm

Seems to have become vogue amongst some but not all ...

Bruce Keen does not do it.
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Re: Rudder 10cm up

Postby paul manning » Mon Sep 04, 2017 12:41 pm

My understanding of the thinking here is by raising the rudder and daggerboard in stronger breeze you reduce lift on the foils and allow the boat to be sailed slightly more bow down upwind. It's a balance between lift and speed to achieve optimum VMG.
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bigdave
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Re: Rudder 10cm up

Postby bigdave » Wed Sep 06, 2017 5:46 pm

I can assure you it was accidental! I need to re-pack my rudder stock. However Andy Gould showed me a neat trick, you can just put a bit of elastic around the tiller and the knot on the rope on the rudder, to stop the rudder coming up if it's a bit loose.

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Re: Rudder 10cm up

Postby sandyday » Thu Sep 21, 2017 1:35 pm

Interesting topic - I've seen a few people sailing with rudder and/or CB up a bit in strong breeze. Ian Renilson suggested to me to try it in stronger wind.

From a technical perspective however it's probably an over simplification to assume that this works directly as a result of reducing the lift.

Quite a few things will change when you raise the foils... but on any given point of sailing the total lift on the CB and rudder (plus the hull) must be equal to the sideforce generated by the sail(s) when the boat is sailing in a straight line. Hence the total lift on the foils is essentially controlled by sail settings and won't be affected by rudder or CB position.

The effects of lifting the CB and rudder include reducing the friction (and some other components of) drag of the foils via reduced wetted area / submerged volume and also reducing the heeling moment a bit, by raising the centre of pressure on the foils (CLR). Both good.

On the other hand you will reduce the aspect ratio of the foils giving you a little more induced drag for the same lift, and you will also reduce the lift slope of the foils, giving you a little bit more leeway for the same lift. Both bad.

So the trade-off is quite subtle. However if the trade-off works in your favour, the combined effect could well be to make it more effective in VMG terms to sail a course which is a bit more bow down in windier conditions. If you're sailing a bit further off the wind, that change of course may in turn reduce the sideforce from the sails and then the lift from the foils...

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Re: Rudder 10cm up

Postby paul manning » Thu Sep 21, 2017 7:27 pm

Thank you Sandy for such a detailed explanation, much appreciated!
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Re: Rudder 10cm up

Postby Bruce » Fri Sep 22, 2017 2:34 pm

Nice explanation, Sandy.

I have done very little testing on lifting the rudder, but so far I haven't found any noticeable speed improvement (at least for me). Unless I know I can get a speed improvement I prefer to keep it down so I have full manoeuvrability on the start line. The ones who do lift a lot of board tend to lift the rudder as well, which makes sense to keep the relative size of the daggerboard/rudder similar.

Most of the Aussies sail with plenty of rudder up (25cm?) in breeze and they are convinced it is quicker.
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