Say guys...I have't kept up with the latest developments in cat sailing and have seen recent references to "gybing boards".Can someone explain??? thanks

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

A broader discussion between some well known profiles:
http://boatdesign.net/forums/showthread.php?t=5783


I think the jury is still out when it comes to their advantages over fixed symmetrical foils.

After discussing the issue with people who do understand how foils work (mostly with Tom Speer), the conclusion was that asymetrical foils or symetrical foils with an angle of attack actually make a boat faster.

Contrary to common belief, they do not make the boat point better - they improve speed by means of reducing drag.

A summary:
- Asymetrical or angled foils allow a boat to head lower while still obtaining the same lift (less drift).
- The boat points closer to its path over water (heads lower).
- The hull's contribution to lift is reduced and a greater % of lift is provided by the foils.
- Since the foils provide lift more efficiently than the hull, the total drag is reduced and speed improves.

Not so complicated, after all.

Luiz,

the theorethical advantages of gybing boards are good, and the reasoning you and Tom have worked out is the same as the one described in some of Marchajs works. Asymmetrical foils looks even better. But they are too cumbersome in use for practical racing on most boats.
The NACRA Infusion F-18 have gybing boards, but as far as I have seen they have not dominated events (yet). If we can agree that talent (sic!) is fairly evenly spread on local events, the Infusions should be generally faster. Perhaps the F-18 is too heavy to showcase the advantage of gybing boards? Anyway, I agree with your reasoning, but still think the jury is out on wether they really make a boat faster.

The Nacra Infusion boards gybe while sailing downwind and are fixed upwind...put that feather in your hat (I still can't figure out the significance of it).

Jake,

do you know why they are fixed upwind? Is this something the crew do actively?

The speed advantage is easy to explain (as I did) and may be mathematically proved (by someone else <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />), but it is a small advantage in theory and even smaller in practice.

Drift angles are in the range of 2 to 3 degrees and the asymetrical or angled foil can not eliminate all the hull's contribution to lift. Drag INCREASES if the foil's angle is too big (or the section too cambered) for a given speed, so it is safer to use a small angle of incidence.

Ideally, the angle of incidence should vary continuously from close to 0 to more than 2 degrees as speed increases (also depending on the section's camber).

In practice this means that a compromise is necessary and that "ideal lateral lift" - obtained exclusively from the foils and not from the hull - is achieved only at one definite boat speed.

I'd expect a boat with angled foils to excell at that specific speed, have a marginal advantage at higher speeds and a disadvantage at lower speeds. This probably explains the situation you observed.

The upper 18" or more (45cm) of the trailing edge of the board is quite thick. It might actually be the same as the height of the daggerboard well in the hull (not sure). Below that thickened edge, the board has a standard sharp trailing edge. When raised normally for downwind sailing, the board is allowed to pivot in the well. When fully lowered for upwind sailing, the board fits tightly in the well and is not allowed to pivot.

The next question you are going to ask me is if they point downhill or uphill when raised...I confess that although I spent a full weekend on one last weekend I neglected to notice - but I have it on high authority that they point downhill. While reaching if raised, they pivot back and forth with the wave action but I doubt they have any intent for reaching.

What about high wind upwind sailing? Will the boards shift in the wells if you raise them up about a foot?

no - as long as the top of the board is captured by the thick trailing edge, it will not rotate. The trailing edge is definitely longer than a foot.

hi jake, did you sail a weekend on the infusion? i'm very interested to know what you think of nacra's new boat and especially how it compares to the "old" f18.

Jake

You are right, the top of the board is thick, but this thick part does not reach the bottom of the slot. The board is loose at the bottom of the slot and this allows it to TWIST. The board does not actually gybe but twist upwind.
I am trying to demonstrate this with the boat on its side in the boat test that was mentioned earlier in this thread; www.nacraeurope.com/download.php?fileid=10
When the board is raised so that the thick bit is above the deck, it just floats evenly in the trunk and so does not 'gybe' for downwind.

Paul GBR7

Just had a look at the picture again - it doesn't demonstrate anything does it? Its just a bloke looking at a daggerboard isn't it!
Trust me, you can twist it with your hands

Paul

I haven't been on (or even seen) an infusion, but generally gybing boards look like this:



When the board is down. The wide back is the only area that occupies the whole case, so the fwd edge is free to "gybe" from port to stbd.

When you pull the board up a bit the max thickness section of the foil enters the case, so that the foil is now locked off on cl.

Hopefully that and my 30 second sketches make sense

AAAAaaaaa...now that makes sense.

Yeah - I crewed on an Infusion this weekend and we had everything from 5knots to 15. I do like how the rigging and many of the nicities are evolving. I still don't care for the appearance of the lines of the hull to the eye but I liked the way it sailed enough that I could look past it. The trampoline to hull joint and the shape of the outer deck makes it a very crew friendly boat.