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Gybing Centreboards

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Dougaldog View Drop Down
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Post Options Post Options   Quote Dougaldog Quote  Post ReplyReply Direct Link To This Post Topic: Gybing Centreboards
    Posted: 04 Oct 17 at 12:17pm
All,

This is nearly associated with 'that' other thread running elsewhere on the forum - but isn't quite.

So...gybing centreboards. How far back to they go and when did they first come into practical use (as against just the theory behind their operation)

D
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davidyacht View Drop Down
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Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 12:34pm
I look forward to iGRF's theories on this one with interest.

We used one on an N12, always felt that the benefit was the ability to foot more while acheiving the same height as those around you.  The dificult bit is to ensure that the board is not wobbly when it is not "locked" in position, usually acheived by pressure.

Malcolm Goodwin's Hornet "Revolution" was probably the boat that raised awareness for the gybing board.  Built in 1973.  Also renowned for its asymmetry (the boat not the board).  
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 12:43pm
I put one on a Div2 board in 1981, used it at Rutland, 1st race, rocketed out from the line sailing higher and faster and won by miles sailing in a force, 2, won both races day 1 and thought woa this is it. I'd seen them on 505's a lot of the dinghy crowd I hung out with back then from Racing Sailboats before LDC acquired it, spoke with mixed views about their performance gains, but that day I was convinced.

Then day two it upped to 3-4, then 4-5 and I couldn't hold it down, the whole board kept railing up (think heeling in a dinghy but with the rig remaining vertical) then stalling, try as I may even sheeting out as soon as the speed built the lift from the plate overwhelmed my ability to keep the board on track.

So dropped the idea, but can see the virtues with the additional weight of a boat, as to how early I only go back as far as 1977 and I first heard about them in 79 from the 505 community.

Edited by iGRF - 04 Oct 17 at 12:44pm
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 2:06pm
Gybing dagger boards change the aspect of the hull and rig to the wind. The foils still travel through water at the same leeway.

Board tend to have a leeway of 3 degrees or so, so gybing the board will keep this leeway, but align the hull with it. The boat will now be pointing in the direction it is going (the foils will still be slipping sideways at the same 3 degrees).

So the question is, what is the advantage of lining the hull up with the direction it travels? Well, first people will say less drag and you are no longer forcing the hull through the water at a 4 degree angle. 

The second reason is it twists the sail plan off the wind, effectively sheeting in 3 degrees more. Kind of like bringing the traveller up to windward and or barber hauling the jib. But crucially, it won't close the slot. That might be nice in conditions when you're trying to build power. 

But there are drawbacks. Firstly, hulls themselves act like a foil in that they resist side slip. So angling the hull with the line of travel means you'll be faster in straight line but your tacking angle will be greater. The trade off here I would very much imagine depends on how the hull shape works and resists side slip versus it's drag. 

The other disadvantage is normal foils side slipping are offset, so the rudder is going through clean water. Gybing the centreboard brings the foil in to line and now the rudder is in turbulence of the CB and maybe produce less lift.

I think they would feel nice, in that it would always feel like the boat was crabbing up to windward relative to the angle the bow was pointing in. I thin the main gain is being able to sheet in further whilst not closing the slot... but that would be specific to certain boats and conditions. 


Edited by mozzy - 04 Oct 17 at 2:10pm
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Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 3:00pm
The experience in the 12 was that the boats with gybing boards tended to foot better when fully powered up in a drag race to windward.  As Graeme describes, there are conditions where the gybing board was very sweet, but in some conditions (I would suggest light) there was no discernable difference.  

Never really noticed problems with interferance with the rudder.  In our case we had a very small rudder on the basis that the centreboard is the primary lifting device and benefits from the endplate that is the hull.

For further reading I consider this to be the seminal work  https://drive.google.com/file/d/0B3Zekt2OTzO4VHp0dk5PckhtOFE/view


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Post Options Post Options   Quote MikeBz Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 4:00pm
Originally posted by mozzy

Gybing dagger boards change the aspect of the hull and rig to the wind. The foils still travel through water at the same leeway.

Or to put it another way - the boat's heading (where the bow is pointing, or what your compass reads) changes but its course (the bearing of the groove you carve through the water) doesn't.  Except that... IF changing the angle that the hull drags through the water does yield an efficiency gain then you will travel a bit faster - the marginal speed increase will generate more lift in the hull/foil combo and hence reduce the leeway marginally (thus improving your course over the ground marginally), and you could of course trade that marginal speed increase for a bit of extra height and hence improve your course over the ground a tiny bit more.

If you overdo the gybe angle of the board then you will drag the hull through the water the 'wrong' way (bow to leeward of direction of travel, stern to windward of direction of travel) - it's hard to see how that would be anything but bad.

In the early 1970s Malcolm Goodwin built a Hornet with a gybing board (1782, Revolution) which absolutely cleaned up for many many years.  However he also greatly optimised the shape, so how much of its pace was down to the gybing board is debatable - faulty intuition lead many to believe that the gybing board made the boat 'move up to weather'.
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 5:38pm
The thing it did for the sailboard was to point high without the normal effort you had to do on a board to outpoint the mean. (Body angle, rig over sheeted hard across the centerline and beyond) the problem came when in order to engage the 'knock' that turned it off, (a block at the top rear of the foil) you had to rake it aft, this then depowered the foil by obviously altering the profile it presented and the board wouldn't point that comfortably at all, so it became all or bugger all, it also could develop a nasty 'rattle' on a dead run or deep reach.

I didn't just bin it out of hand, at the time I was working with Parkers who were in those days the go to 5 oh supplier and we were shipping hundreds of these boards I'd designed, so it was worth working on and we had a couple of goes, Milanese foils, so not cheap and a potential break through like this would have been worth thousands, but sadly it wasn't to be, even heavier riders couldn't make it go above the top end of a three early four, it also co-incided with the other break through we'd built in which was the fully retracting CB which was helping sales anyway, so two things to cope with was felt to be one too many.
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Post Options Post Options   Quote Wiclif Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 7:57pm
To reply to Dougal's first post, the guy I first started work with, a chap called Billy Morton, made a gybing board for his 505.

He had stopped sailing 505's by the time I started work in 1970, so some time before that
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Post Options Post Options   Quote drifter Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 8:35pm
Interesting topic. I sailed in a Goodwin  Hornet in the 70's which had a gybing board. Always wondered why, but lovely boat anyway
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 10:16pm
It's one of those things that seems like a 'no brainer' when you first encounter it..... Seems like it is not quite so simple.
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