Gybing. Board, yes or no

TeamFugu

Super Anarchist
5,049
33
SLC, UT
When I had my 505, I sure noticed a difference. I tried to make one for my Swift but couldn't get the gasket to stick and gave up. The only drawback I saw was when going down wind, the board would gybe when I didn't want it to and the boat would jump out from under me. I've heard of I14's with them and they have a lever that locks the board in place when the kite is raised. I just didn't want to spend time in the shop and opted for more time on the water.

 

Steam Flyer

Sophisticated Yet Humble
44,269
9,611
Eastern NC
I've got an older 505 and a few old classic Moths.  Both allow gybing boards, but do they really work?
Of course they really work. A better question is, do they make the boat faster over a wide enough range of conditions that they are worth the extra trouble.... and that is a question not so easily answered.

The basic idea behind the gybing board is to construct the board/trunk so that it can assume a slight angle to windward of centerline; after all it is a symmetrical foil and they need an angle of attack to generate lift.

If the foil is aligned with the centerline of the hull, the angle of attack becomes the angle of the boat's leeway and the hull is sliding sideways at that same angle...... hulls are generally better at going straight forward thru the water, or at least, that's the assumption.

But what should that angle be, and will you lose more to drag in conditions where the angle is not precisely optimal? reaching under asym, for example, the rig is generating less side force and going a lot faster, it needs much less angle of attack (still, more than zero).

At one point (long ago), the America's Cup racers had trim tabs on their keels for the same reason; and a good deal of effort was expended on adjusting them to the fastest / most efficient angle. They don't do that any more........ the conclusions seems to be that it's not worth the bother.

FB- Doug

 

Daniel Holman

Anarchist
565
130
They work some of the time i.e. upwind in sub fully powered conditions, IF and only IF:

  1. The system is well executed - with a gybeing dagger there is a shit load going on - I just started to built my 3rd cassette. Understand CB gybeing is a little eaiser. Still not trivial. Retfofit of gybeing CB is doable but hard, retrofit of a gybeing dagger case and cassette would write most boats off.
  2. It can be locked downwind or AoA otherwise adjusted;
  3. The gybe angle is set correctly by user according to condition - i.e. boatspeed and RM ergo leeway;
  4. The user sails the boat accordingly - a bit freer on sheets when gyber is at or near max gybe.

You have to be pretty good to notice the difference IF you are sailig well and using it in the conditions it works best in. Rest of the time it can be detrimental.

Dan

 

Locus

locus
776
99
Seattle, WA
We really notice the difference between our two boards. Both are gybing, but one gybes less. Lose a bout 10d of point in lighter wind (its also smaller so that contributes) with the smaller board. 

We have gybe stoppers that engage downwind. Some have it connected to the spin halyard so full hoist engages the stopper. I still pull a rope. 
IMG_2189.JPG
 

 

CaptainAhab

Anarchist
846
230
South Australia
They are good until they are not. Its one more variable that takes time & experience to get it right. Then you have the maintenance with the gasket. I think the biggest is mental. Do you want one more variable to play around? Instead of focusing on steering and keeping your head out of the boat, you are focused on whether or not you have the new gybing board setup correctly. Some people like complicated. We are always trying to simplify. There is definitely an investment. You also have to ask yourself are all of the top boats using them.

 

fastyacht

Super Anarchist
12,928
2,596
I don't think a gybing board on a 505 is any kind of problem. The old simple way of stopping it was to pull up the board a bit so that the flat is engaging the trunk.
My old boat has one.  4000 series. I'll go take some photos.

 

Mozzy Sails

Super Anarchist
1,197
1,036
United Kingdom
The basic idea behind the gybing board is to construct the board/trunk so that it can assume a slight angle to windward of centerline; after all it is a symmetrical foil and they need an angle of attack to generate lift.

If the foil is aligned with the centerline of the hull, the angle of attack becomes the angle of the boat's leeway and the hull is sliding sideways at that same angle...... hulls are generally better at going straight forward thru the water, or at least, that's the assumption.
What I do not understand is that it seems there are two benefits:

1) Opens sail plan, so you can harvest energy from a larger volume of air. 

2) Align hull with leeway to reduce drag.

If point 1 is true it's only good if you're looking for more power, but it does assume that the slot works by forcing air through and speeding it up, so if you open the mouth (forestay to mast) to get more air through then flow will be even faster through the slot?  But, that's not really how the slot works. Air actually slows through the slot with the jib slightly reducing drag on the main and decreasing lift on the main, but the major benefit is the main increases lift on the jib. 

Rotating the sail plan away from the wind also moves the leech of the jib further back to leeward of the main luff, which from genoa's isn't a terribly efficient place to have sail area. I would imagine it's then very easy to over sheet. 

Point 2 is fine, but it ignores the lift you get from the hull. Don't most waterlines somewhat resemble a foil, especially on traditional hull shapes? Aligning the hull with leeway will reduce hull drag, but also hull lift. 

So with no experience of gybing boards my gut feeling is that point 1 would be good when you're looking for power. And maybe point 2 would be good when hull drag isn't worth the lift it provides due to hull shape. 

 
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Daniel Holman

Anarchist
565
130
Point 1 is a bit poorly worded. 

In physics terms, your sails are sheeted wrt to the track of the boat. 

If you sheet the main and jib to same angle wrt cl and suddenly you switch gyber on and your heading remains roughly the same you are actually sheeting your sails 4 deg more with respect t track, so possibly oversheeting a little. 

You are right re overpower at least in fast boats. If you are doing 4 deg leeway once fully hiked at say 5 kts, then with more wind and twisting sails and going say, 7kts for same sf then the cl and Aoa will reduce a fair bit to say 2 deg, so the benefits of removing 2 deg of leeway from hull are about a quarter of the benefit of removing 4 deg of leeway. 

Worse, if you leave it gybeing at 4 deg when doing 7kts you will be making positive leeway so adding induced drag back on, and likely making a mess of helm balance. 

Dan

 

Steam Flyer

Sophisticated Yet Humble
44,269
9,611
Eastern NC
Point 1 is a bit poorly worded. 

In physics terms, your sails are sheeted wrt to the track of the boat. 

If you sheet the main and jib to same angle wrt cl and suddenly you switch gyber on and your heading remains roughly the same you are actually sheeting your sails 4 deg more with respect t track, so possibly oversheeting a little. 

You are right re overpower at least in fast boats. If you are doing 4 deg leeway once fully hiked at say 5 kts, then with more wind and twisting sails and going say, 7kts for same sf then the cl and Aoa will reduce a fair bit to say 2 deg, so the benefits of removing 2 deg of leeway from hull are about a quarter of the benefit of removing 4 deg of leeway. 

Worse, if you leave it gybeing at 4 deg when doing 7kts you will be making positive leeway so adding induced drag back on, and likely making a mess of helm balance. 

Dan
Right, I was going to use the phrase "net angle" but that sounds like something to do with fishing. Basically, a gybing board does not change the angle between the apparent wind and the boat's track.

I've only played around with gybing boards in slower classes, and found the main benefit to be in holding a lane at the start. 4 degrees sounds like a lot though, I think I was working toward 2 1/2 degrees and sweating whether to get it symmetrical or 1 degree more on starboard.

...   ...

So with no experience of gybing boards my gut feeling is that point 1 would be good when you're looking for power. And maybe point 2 would be good when hull drag isn't worth the lift it provides due to hull shape. 
Hulls are not good at generating lift. This is why a horizontal foil to lift them out of the water can go so much faster. Some hulls don't mind a small skew angle, like scows, but others really hate it..... or rather, the water going by them does

FB- Doug

 

Locus

locus
776
99
Seattle, WA
I don't think a gybing board on a 505 is any kind of problem. The old simple way of stopping it was to pull up the board a bit so that the flat is engaging the trunk.
My old boat has one.  4000 series. I'll go take some photos.
Mine stops when raised part way as well. We tend not to raise the board anymore when wire running. Hence gybe stopper. 

BTW 505s raise the boards as the wind strength increases and we rake the rig. So in heaver breeze the board is up 1-4" so it stops gybing as well. For most crew weights 12kn is where the board starts coming up, on my boat down, parallel and 1" up will still gybe, 1 1/2" up stops the board. 

 

Doug Halsey

Member
341
100
If the foil is aligned with the centerline of the hull, the angle of attack becomes the angle of the boat's leeway and the hull is sliding sideways at that same angle... hulls are generally better at going straight forward thru the water, or at least, that's the assumption...
Starting from this assumption, you might conclude that the board should be angled at whatever angle makes the hull go straight through the water.

That's not generally correct; the situation is much more complicated.

For one thing, if the board has any lift (i.e. side force) on it, it will induce a curvature in the flow over the hull, so there is no angle that will allow the hull to go straight through the water.

For another, if it were possible to have zero side force on the hull, the loading on the board would be zero at the point it intersected the hull. This would more-or-less eliminate any beneficial end-plating effect that the hull has on the board, so the board's induced drag would be higher.

The situation is analogous to the aircraft design problem of determining the optimum incidence for mounting the wing on the fuselage. This requires CFD calculations or wind-tunnel testing to determine, and invariably finds that it is best for the fuselage to carry some of the loading. 

 
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Steam Flyer

Sophisticated Yet Humble
44,269
9,611
Eastern NC
Starting from this assumption, you might conclude that the board should be angled at whatever angle makes the hull go straight through the water.

That's not generally correct; the situation is much more complicated.

For one thing, if the board has any lift (i.e. side force) on it, it will induce a curvature in the flow over the hull, so there is no angle that will allow the hull to go straight through the water.

For another, if it were possible to have zero side force on the hull, the loading on the board would be zero at the point it intersected the hull. This would more-or-less eliminate any beneficial end-plating effect that the hull has on the board, so the board's induced drag would be higher.

The situation is analogous to the aircraft design problem of determining the optimum incidence for mounting the wing on the fuselage. This requires CFD calculations or wind-tunnel testing to determine, and invariably finds that it is best for the fuselage to carry some of the loading. 
Interesting points, thanks.

Getting to the italized bit.... I don't quite get this. Why would there be zero load, or zero dP, on the gybing board at the hull or root? It's taking an angle to the centerline of the hull, so if the flow is perfectly aligned with the hull, there is still the same angle of attack for the board and the same flow/dP regime.

It is complicated. The times I worked on gybing boards I deliberately undershot the angle rather than over, seemed likely to keep more of the benefits of both worlds to my primitive reptilian brain.

FB- Doug

 

Doug Halsey

Member
341
100
Getting to the italized bit.... I don't quite get this. Why would there be zero load, or zero dP, on the gybing board at the hull or root? It's taking an angle to the centerline of the hull, so if the flow is perfectly aligned with the hull, there is still the same angle of attack for the board and the same flow/dP regime.
Sorry, I was being a bit sloppy with that part because the physical situation is a little unrealistic. (That's why I added the "more-or-less"). Maybe a better way to look at it would be that having the hull carry some of the load gives the board a larger effective span & therefore smaller induced drag.

I think everyone agrees that it's very complicated & requires a lot of trial & error to get it right.

When I was a kid, my sister's Moth had a gybing board, but our Moths were totally different designs, so we never figured out if it was helping or not.

 
I believe I bought your sister's boat, Doug Halsey. 1975? It was either the "Florida" Moth, like  a Mistral, or the scow moth, both bought from your Moth legacy, that had a gybing board. 

There was a wider ridge on the aft edge of the dagger board up high. Woe to one who forgot to raise the board at speed off the wind! Uncontrollable, seemingly random jogs.

Dave Ellis

 
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Doug Halsey

Member
341
100
Hi Dave,

I've lost track of the different owners.

My sister's boat was the scow with the gybing board. I'm not sure, but I think she sold it to Pidge Powell (or maybe Pat Reischmann, or Janice Robertson). Mine was the round one, similar to Florida Moth or Mistral. I sold it to Earl Patterson in 1970.

Here's a photo showing both of them on the beach in about 1967. MothsOnBeach_1967.jpg

 

Doug Halsey

Member
341
100
Here's a photo of the scow with a better view of the hull. (That's a friend sailing it & believe me, he's been told about the sail shape!) GailsMoth_1968.jpg

 

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