Gybing Daggerboard?

Doug,

I don't think the easing of the mainsail meant that it was being eased relative to the wind, only relative to the boat centerline. The boat just turns a bit more itoward the direction of travel and the sails are let out to maintain their optimum trim relative to the wind.

 

gybing board

Tom, if the sails are set for going upwind with a conventional board and you suddenly allow the board to gybe -and stabilize- then the sails will be in too far for the new heading which is below the old heading. So it seems to me that if the sails are then slightly eased the boat will go faster. When the sails are eased they then are set more or less the same as they were before the board gybed relative to the wind-just exactly the same as if you turned the boat down a few degrees and eased the sails to match. There are a lot of factors here-I'm just trying to understand them better. A 505 sailor posted this on SA in response to a topic I started on gybing boards:
http://www.int505.org/gybecb.htm
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A 505 sailor on SA has pointed out that most US 505's use gybing centerboards....

 

That makes sense, to me. Modern ultralights have sort of a triangular shape, such that the waterline at the stern is displaced quite a bit laterally when heeled. So if you drew a line from the bow to the center of the heeled stern waterline, it would be rotated to windward and the effective leeway angle of the hull would be exaggerated.

By rotating the keel, they are essentially undoing the rotation of the waterline plane due to heel.

 

Exactly. The board and sails end up with about the same relationship to each other, but it's the hull that gets rotated. This sets up a subtle change in the rig geometry - it "opens up" the foretriangle, you might say.

From the post:
"The boat's leeway angle is determined by the net side force on the sails and the area and effeciency of the foils. The hull has a small effect. An estimate of the average leeway angle is 4 degrees, so the boats in the diagram are actually sailing slightly to the right of straight up on the page.

"This means that for a given set of conditions and equipemnt, if you change from a fixed to a gybing CB, the leeway angle will not change. What will change is that the bow will rotate to leeward about 2.5 degrees. "

I think that's exactly what I've been saying, too.

 

gybing board: performance gain?

Tom, I posted that article because it IS exactly what you said! With one small exception: that particular individual and other 505'ers see a PERFORMANCE GAIN as a result of using a gybing board.
What I'm interested in is your take on the sentence where he says that the boat goes faster as a result of the sheets being eased and since it's going faster also points(loose term-probably means vmg is better) higher( lift varying as the speed squared).
It appears to me that the conclusions you drew above were not anywhere near as positive as this and I'd like to hear what you think.....

 

Lorsail,

I think there is less here than meets the eye. The fact that the boat hull heading may change a bit to windward by jibing the board and easing the sails does not necessarily imply that the course that the boat is sailing is changed.

 

gybing board faster?

Exactly Tom; the boat's course hasn't changed only it's heading(lower with a gybing board). But is it faster with a gybing board as the 505er's say it is? For the reasons they say it is?

 

Interesting thread.

I don't understand why the gybing daggerboard wouldn't go faster. The hull is designed to float and move with minimal drag, and the keel is designed to provide lateral resistance, so why not let each do its job optimally. That means zero angle of attack for the hull, and nonzero for the keel.

Since the boat won't be headed upwind of the true course like it does with a fixed keel, you could initially sheet out a little, but would widening the slot really get you more speed than straightening out the hull?

Here's another question: Would it be feasible to have an adjustable daggerboard with a little tiller on it (lockable) so you have the optimum angle of attack in all conditions? On a larger boat, you might have a little submerged weather vane that tells you the hull's angle of attack.

 

It sounds like the 5o5 class is one where the gybing boards are effective. Other people who've tried them in their boats have not found them to be as effective or the gains not worth the effort of changing the board. He says himself that the rounded hullform of the 5o5 makes it insensitive to the leeway angle, so the gains are probably not coming there.

He does cite the inability to sheet the jib in as much as needed because of the geometry of the deck hardware on the 5o5. Relative to the forestay, the gybing CB would move the jib leads to windward. This seems to be exactly what they need, so it pays off for them. The rougher conditions he cites as being ones where the gybing board is not as effective are the conditions where you might want to open things up and trim for power rather than pointing.

My only point was that the benefit from a gybing board comes from factors that are more related to the sail rig than the board itself. If you had a highly V'd hullform, then maybe leeway angle would affect the hull more and the gybing board would reduce the leeway angle on the hull and produce a benefit there. It's all going to be very dependent on the particular boat.

But the common factor is, gybing the board rotates the hull - it doesn't pull you up to windward. Once you look at it that way, you can start to make sense of what it does for a given design.

 

And I wrote that wrong, of course. The boat heading is more downwind toward the true course with a jibing board.

 

at risk of beating a dead horse

I find it useful to visualize the gybing board issue by use of example. I happen to use a gybing board in a fleet where they are allowed but not all that common.

Picture a fleet of boats driving off of the start line. All have set their jib leads according to the popular tuning sheet. All are on the same heading, jib telltales streaming nicely. If the guy with the gybing board is driving on the same heading as the others (ie: all bows pointing the same way), his board is facing a higher angle of attack than his competitor's and is not in a happy place in the lift/drag curve. Boat speed will suffer as a result, decreasing lift off the board, increasing leeway, and he falls behind and into the lee of his fellow racers.

This is a very easy scenario to fall into - when you're the one guy in the fleet with a gybing board it takes real discipline to let your bow fall off that couple degrees on the start (and set the jib accordingly). It will look like you're sailing down into the guy to leeward, when in fact your actual track through the water will be the same.

Point being, its not automatic that gybing the board rotates the hull. Its the knob on the end of the tiller that rotates the hull.

And its not an either/or situation either. Just as some sailors will pinch and others will foot, there may be a sweet spot that favours the gybing board. Of note with the 5o5s is that with the trend to short chord / high aspect centerboards, they're also using thicker sections (10% and even more)... this may be from mechanical strength requirements, as well as the necessity of having a section that is thick enough to jam the board and stop it gybing once you start to raise it. Just more grist for the mill...

Phil

 

Advantage?

Phil, does the gybing board provide you with a significant advantage? What class are you talking about?
One more question: was it/is it hard to get used to the tiller position with the gybing board; does it give you the "feeling" of excess weather helm w/o any greater force?
Your experience is important to trying to understand this-thanks!

 

I sail Fireball (hard chined dinghy, 16') ... not all that popular in North America these days, but there are pockets of activity.

If it gave me a solid advantage I'm sure everyone would be using them

The guys down in the south-east US swear by them, but not too many in Canada use the gybing board. I've used one for a couple seasons just to try something different.

The sailing angles are different enough that I've had people take note of it out on the course and ask me later if I was using a gybing board (and we're only talking a difference of 2 or 3 degrees). But then, my style has often been to foot for power, so taking that into account the gybing board might be good for me.

Phil

 

I have one more question about keel direction.
When a planing monohull, for instance an Australian racing skiff, is heeled leeward in light air, it rests on its lee bilge.
But that part of the hull is significantly toed in toward the centerline,
so if it's moving even fairly straight, the daggerboard should be pointing leeward!
How does this work, and would it benefit from being able to control the AoA? Thanks much.

 

Once again, think of the daggerboard as fixed relative to the motion of the boat, not the hull. The effect of heeling the kind of hull you describe is to rotate the waterlines to a greater leeway angle. Gybing the board would rotate the bow to leeward to line up the waterlines better with the direction of travel.