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Tip Vortices!

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CLINT View Drop Down
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    Posted: 30 March 2009 at 13:53

I noticed that we don't do all that much technical chat so I thought I would have a go. If this is something a 12 yr old Mirror sailor should know I apologise!

At a DSC work party I was chatting about the new boats with the 'ol man and Eric. I mentioned that someone had said at the champs the people with new boats were struggling to get them to point. I thought that it might be due to the lack of foredeck. I remember a talk saying you want the jib down on the deck and thought it was to do with making sure the wind was forced out the back of the sail. Eric disagreed and said he sailed with his jib off the deck to get more sail area and reduce turbulence.

I bumped into Ian Campbell in the lift today (has worked with Americas Cup Team sail development) and asked him. He said that I was right that you want the jib down on the fordeck but my reason was wrong . It is to do with reducing tip vortices from the bottom of the sail which would increase drag and hurt pointing ability if the jib was raised off the deck.

Just wondered if there is an issue with pointing ability of the new boats and if so whether a mylar foredeck like Smeggy might be worth a try?

As someone with a 30 year old boat I don't want to sound like I'm teling poeple what to do with their new boats but just thought it might be interesting. Anyway I had better go on the RYA forum to get the address of the Maritime Museum to drop the boat off. The 'ol man hit his 70th year over the winter (does anybody know if they would take him too!)

Cheers

Clint

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Jono View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jono Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2009 at 16:54

Hi Clint,

Personally we tend to set our boat up for a little more speed and slightly less pointing, realistically we didn't point too badly in the champs last year but it is something we are looking at early this year  to try and improve on.

We may consider trying this out if nothing else appears to work for us, just not entirely sure if we have really have a major problem with it or not to be honest.

Wizard 2186
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Tom Guy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom Guy Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 12:37

By having a foredeck you can use the endplating effect to remove the tip vortices at the foot of the jib - result = less drag. Presumably if you have more drag, you have to ease your leeches to re-establish flow. Less leech tension = reduced pointing ability 

Tom

Induced Drag

Watch a movie sometime with an airplane flying through a cloud. In that footage, it is easy to see large vortices of fog swirling off the tip of the wing of the airplane.

What is happening is that the low and high pressure areas meet at the tips of the wings. Much of the air and fog "leak" suddenly from the high pressure side (the windward side) to the lower pressure side, creating big swirlies. These swirlies require a lot of energy to form-energy which could be better used in propelling the airplane or boat forward.

figure 7

There have been numerous solutions proposed to slow this effect, such as small vertical winglets on the ends of airplane wings. On a boat, there will be two places this leakage occurs: the head and the foot of the sail. In one of the early 1900's America's Cup races, the American yacht had a 4 foot wide boom, dubbed the "Park Avenue Boom" for its size. The purpose of this was to slow the flow from the windward, high pressure side, to the leeward, low pressure side. This slowed the creation of the induced drag.

There are other, more reasonable solutions to this problem. For instance, on modern windsurfers, the sail is built so that at high speeds, the sail can be leaned back all the way to the board. This closes the gap between the sail and the board, using the board as an "end plate" to stop the flow, much as the fuselage of an airplane does for its wings. Another instance of this end plating is the "deck sweeper" jib, which is cut to come all the way down to the deck of the boat. Deck sweeping jibs are now very common on most modern sloop-rigged boats.

Neither of these solutions addresses the problem of the leakage at the top of the sail. There is no way, so far, to prevent this type of leakage.

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roger View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote roger Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 13:14

Very intersting topic

Thanks

 

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CLINT View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLINT Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 16:35

Hi Tom,

Great explanation. The questions is: Did you do an engineering degree or are you just loads brighter than me?

If you fancy working at the Uni I can pass a copy of your CV to Ian Campbell!

Clint

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Strangler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Strangler Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 18:17

"I remember a talk saying you want the jib down on the deck and thought it was to do with making sure the wind was forced out the back of the sail."

"He said that I was right that you want the jib down on the fordeck but my reason was wrong . It is to do with reducing tip vortices from the bottom of the sail"

I would say "making sure wind was forced out the back of the sail" and "reducing tip vorticies" is basically the same thing. If the wind can leak under the jib as it travels back along the foot of the jib a spiral ie trailing vortex is induced ie drag.

Fat head mainsails are an attempt to reduce the mast head tip vortex, and the Lionheart bendy top mast, and winged keels for the other end. I have a pic of Volvo 60s in fog showing the vortex. I'll dig it out.         &n bsp; And it may be one reason why raking the rig in strong winds works - lowers the boom to increase the end plate effect.

If wind turbines had winglets to reduce the tip vorticies they would not be half as loud.

Foredecks rule ok.

 

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CLINT View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLINT Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 19:20

Cheers Strang

does that mean I'm not thick after all?

Clint

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Strangler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Strangler Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 22:18

Yup.

Now how do you add a pic thats in your documents. Tried the tree, and paste.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Shelton Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:22

I think your theory is wrong. Last season, we tried a new technique, whereby after each tack, I went up on the foredeck and taped the foot of the sail to the deck with gaffer tape. The problem with this is that it forces a one-tack beat, as you have to peel it off every time.

Anyway, it wasn't fast, so I think there is no truth in what you say.

Wind turbines are not noisy Strangler, shame on you. I stood in a windfarm the other day - a beautiful sunny calm day - and they were really really quiet

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Strangler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Strangler Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:39
Sounds like you got your head stuck in the bow chute, John. Bum in the air does not help air flow! On second thoughts....
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