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Gybing dagger boards - what is it mean ? #141550
04/30/08 06:52 AM
04/30/08 06:52 AM
Joined: May 2006
Posts: 96
Budapest, Hungary, Europe
CatSailingHu Offline OP
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Could somebody please describe me what is it mean ?

thanks

sanyi


Sanyi
Nacra Infusion
www.catsailing.hu
-- Have You Seen This? --
Re: Gybing dagger boards - what is it mean ? [Re: CatSailingHu] #141551
04/30/08 07:21 AM
04/30/08 07:21 AM
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South Carolina
Jake Offline
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Boards that pivot in their trunks. Initially I used to believe this meant downwind, but they actually pivot sailing upwind pointing slightly to weather. The AHPC Capricorn and Nacra Infusion use gybing daggerboards - basically, they're designed to have slop in the top of the daggerboard when they are all the way down. When raised, the board engages better in the trunk and does not pivot.


Jake Kohl
Re: Gybing dagger boards - what is it mean ? [Re: Jake] #141552
04/30/08 07:44 AM
04/30/08 07:44 AM
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jcasto1 Offline
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So if I have an old NACRA, where the boards may have a little slop, do they automatically act like jibing daggerboards, or do you have to manipulate some feature of the trunk in order to have it operate properly? i.e. make tighter at the back, but allow front to still have some slop?

Maybe my sloppy boards are gybing the wrong way which is why I'm so slow?


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
Re: Gybing dagger boards - what is it mean ? [Re: jcasto1] #141553
04/30/08 08:50 AM
04/30/08 08:50 AM

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It is my understading that gybing dagger boards are a mechanical device that has seperate parts to it that allow it to pivot / rotate...

slop in a single, solid daggerboard would not have the same effect (i dont think).

Re: Gybing dagger boards - what is it mean ? [Re: jcasto1] #141554
04/30/08 08:50 AM
04/30/08 08:50 AM
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South Carolina
Jake Offline
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Quote
So if I have an old NACRA, where the boards may have a little slop, do they automatically act like jibing daggerboards, or do you have to manipulate some feature of the trunk in order to have it operate properly? i.e. make tighter at the back, but allow front to still have some slop?

Maybe my sloppy boards are gybing the wrong way which is why I'm so slow?


The gybing part has to be very specific - sloppy boards may not gybe in the proper direction (like you said). However, speaking from experience, there are a lot of things that can make you slow - ASK QUESTIONS of the sailors around you that are beating you. As they get to know you, they'll start to look at you on the water and provide some critique...this will help you more than anything.


Jake Kohl
Re: Gybing dagger boards - what is it mean ? [Re: Jake] #141555
04/30/08 09:24 AM
04/30/08 09:24 AM
Joined: May 2006
Posts: 96
Budapest, Hungary, Europe
CatSailingHu Offline OP
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thanks, really interesting!!

So if I'm not mistaken
upwind --> boards all way down --> enable to pivot the lower end of the board backward --> the CENTER OF LATERAL RESISTANCE move backward

downwind --> boards halfway up --> no pivoting --> the CENTER OF LATERAL RESISTANCE move a bit forward

All together could effect a neutral helm both upwind and downwind... (what the infusion really has)

??

I will check my dagger boards.

I sailed my infusion only a few times (5-6), and realized that the trailing edge of the boars can be damage really easily even nothing was happened during the sailing, probably because of this pivoting ability.


Sanyi
Nacra Infusion
www.catsailing.hu
Re: Gybing dagger boards - what is it mean ? [Re: CatSailingHu] #141556
04/30/08 09:37 AM
04/30/08 09:37 AM
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John_C Offline
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The CLR doesn't move, the board is not necessarily supposed to move aft for the purposes of gybing. The board being angled with respect to the hull means that lift can be made to weather while the hull is travelling straight forward to make less leeway. Some boats were built with toe in in the trunks of the boards. With that method you have to raise the windward board. As said above nothing fancier than slop built a specific way into the board is needed. Pictures make it easier to understand.

http://www.philsfoils.com/designTips.html#gybinghead

John

Re: Gybing dagger boards - what is it mean ? [Re: CatSailingHu] #141557
04/30/08 12:09 PM
04/30/08 12:09 PM
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South Carolina
Jake Offline
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Quote
thanks, really interesting!!

So if I'm not mistaken
upwind --> boards all way down --> enable to pivot the lower end of the board backward --> the CENTER OF LATERAL RESISTANCE move backward

downwind --> boards halfway up --> no pivoting --> the CENTER OF LATERAL RESISTANCE move a bit forward

All together could effect a neutral helm both upwind and downwind... (what the infusion really has)

??

I will check my dagger boards.

I sailed my infusion only a few times (5-6), and realized that the trailing edge of the boars can be damage really easily even nothing was happened during the sailing, probably because of this pivoting ability.


Yes, the relief on the trailing edge of the top of the daggerboard allows it to actually twist in the trunk and it does put a lot of pressure on the trailing edge of the daggerboards - I've seen quite a bit of minor damage there on these boats.


Jake Kohl
Re: Gybing dagger boards - what is it mean ? [Re: Jake] #141558
04/30/08 12:46 PM
04/30/08 12:46 PM
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Asuncion, Paraguay
Luiz Offline
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Quote

upwind --> boards all way down --> enable to pivot the lower end of the board backward --> the CENTER OF LATERAL RESISTANCE move backward

downwind --> boards halfway up --> no pivoting --> the CENTER OF LATERAL RESISTANCE move a bit forward


This CLR remains in the same (longitudinal) place in both cases, just slightly higher (transversally) when the board is lifted.


Luiz
Re: Gybing dagger boards - what is it mean ? [Re: Luiz] #141559
04/30/08 01:36 PM
04/30/08 01:36 PM
Joined: May 2006
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Budapest, Hungary, Europe
CatSailingHu Offline OP
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Quote
Quote

upwind --> boards all way down --> enable to pivot the lower end of the board backward --> the CENTER OF LATERAL RESISTANCE move backward

downwind --> boards halfway up --> no pivoting --> the CENTER OF LATERAL RESISTANCE move a bit forward


This CLR remains in the same (longitudinal) place in both cases, just slightly higher (transversally) when the board is lifted.


Yes, I see now the pivoting axis is vertical, so the CLR will not move...

thanks to all !


Sanyi
Nacra Infusion
www.catsailing.hu
Re: Gybing dagger boards - what is it mean ? [Re: CatSailingHu] #141560
04/30/08 04:10 PM
04/30/08 04:10 PM
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Yardley PA
DanWard Offline
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Consider also that the angle of attack for a gybing board
is not determined by the gybing angle of the board. The angle of attack is a function of the relationship between CE and CLR. What actually happens is the boat rotates away from the wind so the hull tracks more in line with its course through the water. This rotation also has the added benefit of presenting more sail area to the wind.

Re: Gybing dagger boards - what is it mean ? [Re: Jake] #141561
05/07/08 09:10 PM
05/07/08 09:10 PM
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jcasto1 Offline
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Thanks Andrew & Jake - but I think I didn't explain. Best way is to look at "Phil's foil" page, http://www.philsfoils.com/designTips.html#gybinghead

It seems to me that if trailing edge of daggerboard is fixed in the trunk, but front edge is a bit sloppy, it produces the efect of a gybing board. No extra mods necessary. What am I missing?


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
Re: Gybing dagger boards - what is it mean ? [Re: jcasto1] #141562
05/08/08 12:20 PM
05/08/08 12:20 PM
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Asuncion, Paraguay
Luiz Offline
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Quote
...best way is to look at "Phil's foil" page, http://www.philsfoils.com/designTips.html#gybinghead

It seems to me that if trailing edge of daggerboard is fixed in the trunk, but front edge is a bit sloppy, it produces the efect of a gybing board. No extra mods necessary. What am I missing?


Nothing. This is precisely what the Penguin class has been doing with their centreboards for more than 30 years. I think it is the only solution for centreboards and works well in daggerboards as well.

Phil's Foils design has one big advantage over a sloppy board: it becomes symetrical when lifted (downwind).


Luiz
Re: Gybing dagger boards - what is it mean ? [Re: Luiz] #141563
05/08/08 07:42 PM
05/08/08 07:42 PM
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jcasto1 Offline
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Thanks for the vote, Luiz. I actually pull my boards all the way up when sailing downwind, so that's not an issue.
I think the difference is only the following - if the pivotpoint is at the rear, as mine is, the angle is less than if you did it where the pivot angle was closer to centerline of the board (Phils foil diagram). I suppose I could grind/fill my board somewhat to change the pivot location and increase the angle when boards are down.

But I'll procrastinate some more before I do that.


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
Re: Gybing dagger boards - what is it mean ? [Re: jcasto1] #141564
05/08/08 09:04 PM
05/08/08 09:04 PM
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Asuncion, Paraguay
Luiz Offline
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Quote

I think the difference is only the following - if the pivotpoint is at the rear, as mine is, the angle is less than if you did it where the pivot angle was closer to centerline of the board (Phils foil diagram).


It doesn't really matter how exactly you do it, just make sure the angle of attack does not exceed 1,5 degrees (aprox.).

I believe you sail with the boards partially raised sometimes, at least when reaching. Phil's geometry is best when the foil is partially raised.


Luiz
Re: Gybing dagger boards - what is it mean ? [Re: Luiz] #141565
05/09/08 07:04 AM
05/09/08 07:04 AM
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Kevin Cook Offline
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The idea behind this must be that each foil section has it's own unique lift versus drag cutve. For instance a NACA 008 foil has a specific angle of attack where performance is optimum (highest lift to drag ratio). Due to practical design and arragnment considerations boat foils cannot usually be positioned to operate at this optimum angle. The gybing dagger boards are a way to improve the situation and bring the fiols closer to the best angle of attack.
When looking at these angles, the leeway (yaw) angle of the boat (angle of the boat with respect to true path through the water) needs to be added to the the angle introduced by the gybing action of the boards to come up with the angle of attack seen by the foil. In other words - need to take into account that boats don't sail in a straignt line.

Re: Gybing dagger boards - what is it mean ? [Re: Kevin Cook] #141566
05/09/08 11:30 AM
05/09/08 11:30 AM
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Luiz Offline
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Quote
The idea behind this must be that each foil section has it's own unique lift versus drag cutve. For instance a NACA 008 foil has a specific angle of attack where performance is optimum (highest lift to drag ratio). Due to practical design and arragnment considerations boat foils cannot usually be positioned to operate at this optimum angle. The gybing dagger boards are a way to improve the situation and bring the fiols closer to the best angle of attack.
When looking at these angles, the leeway (yaw) angle of the boat (angle of the boat with respect to true path through the water) needs to be added to the the angle introduced by the gybing action of the boards to come up with the angle of attack seen by the foil. In other words - need to take into account that boats don't sail in a straignt line.


Kevin,

The situation looks different.

The angle of attack is a function of the balance of forces. Those vary with wind pressure and point of sailing, mostly. This angle is authomaticaly adjusted by the boat by means of varying its drift speed. The adjustment mechanism is completely independent on the board's angle of incidence.

In other words:

-When the boat "needs" more lift force, the drift speed increases, also increasing the board's angle of attack and, therefore, the lift force.

- When the boat "needs" less lift force, the drift speed is lowered, also reducing the board's angle of attack and, therefore, the lift force.

A gybing board only positions the board at a given angle of incidence. The effect is a reduction of the drift speed necessary to generate the same lift - but the angle of attack between the board and the water flow remains exactly the same as that of a symetric board.

Like in an airplane, the angle of attack is a function of the balance of forces, not of the angle of incidence.

Instead of thinking that the board was rotated in relation to the boat, think of the boat rotated in relation to the board, wind and flow. Visualize it this way:

1) A symetric section board in a normal boat is generating lift at a fixed angle of attack with the water flow.

2) Now we keep the same conditions (wind, water flow and board), so that the drift force remains the same, and rotate only the hull/boat/sails a couple of degrees in relation to the board.

3) Note that the initial situation is that of a normal board and the second situation is that of a gybing board.

4) The main difference is that now the boat does not need to drift due to the angle of incidence. The wind, flow, etc. remain the same.


[Linked Image]

This is actually a simplified case. The boat's hull was also generating lift in the first case, differently from the second case, where it is aligned with the flow. Also, the rotated boat gives more room to set the sail at a lower angle of attack with the wind flow. Lastly, in practice it is recommended that the angle of incidence remains smaller than the drift angle, because you don't want a negative angle of drift when less drift force is required.

I hope this helped clarify the concept,
All the best,

Attached Files

Luiz
Re: Gybing dagger boards - what is it mean ? [Re: Luiz] #141567
05/09/08 01:46 PM
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Kevin Cook Offline
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Thanks Luiz - good diagrams


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