Bieker B6

[BalticBandit]

From a "normal boat" next to you, you would be "nose down" on them but tracking the same CMG - and you would have a touch more speed. And in the Lulls you would maintain point when they have to "nose down".

And that is EXACTLY what you see when you sail a non-gyber next to a gyber.

I cannot see extra speed (with my emphasis) at all. If you make the assumption that hull drag doesn't vary much with attitude and (its only a medium sized assumption since we know that induced drag must be tiny in proportion to total induced, wetted surface drag is identical and wave drag must be very very similar) then if the foils are at the optimum angle to the wind in both boats, and the sails are at the optimum angle to the wind in both boats then all the dynamics are essentially the same. So how can the behaviour in the lulls be significantly different? In both boats the power available will decrease by the same amount, the heeling force will change by the same amount etc etc etc. I agree that in the gybing board boat the angle of the centre line is always nose down by the gybe angle of the board, but what I don't see is why that should make a blind bit of difference to anything other than the jib/main relationship. Which goes right back to my original proposition that the jib/main relationship, which we all know to be crucial to the upwind tuning, is the only factor that appears to change enough to have a significant effect.

Well as the IC testimony here indicates, hull drag does vary with attitude (and I'd tend to agree)

and yes, once in equilibrium the net lateral forces are zero. But the reason there is leeway is that the initial lateral forces are greater to leeward.

But I really don't understand this

Disregarding gybers, for the boat to move to weather, the foil would have to be at a negative AOA, hence generating negative lift, so where on earth does the weather force come from?

Huh? Again, I am so confused by the terms that I cannot sort this. Normally since the goal is to go to weather, "negative" would be "away from" going to weather. so it sees completely contrary to every notion. And what is "weather force"?

 

[aem27]

I've been following this thread for a while with some amusement, and now feel the need to throw my own opinion into the ring, just to confuse things further, for which I apologise in advance.

The benefit of a gybing centreboard is singular. It reduces hull drag by allowing the heading of the hull to be aligned with the direction of travel. That is all it does. The extra loading on the gybing board (due to loss of any contribution to leeway resistance) is small enough that any change in angle of attack on the centreboard is negligable (at least, for flat bottomed wide planing hulls). There will, or should be a small change in the width of the jib slot, but whether that is a good or bad thing is indeterminate. There is no magic changing of force couples.

In order for there to be a benefit from the gybing centreboard:

(a) the reduction in hull drag must be greater than the added drag due to the gybing board system at the hull exit.

( B) assuming (a) is true, the added time gain must be greater than any time loss due to fiddling with, or tripping over the gybing centreboard mecahanism.

Thats all there is too it. No magic. Sorry.

Interestingly, wide flat bottomed planing hulls which normally contibute little to leeway resistance will also benefit least from a gybing centreboard. The added resistance due to leeway is mainly induced drag due to the lift produced by the hull. A gybing centreboard is probably of more benefit on a deep narrow hull, and this is only because a high aspect ratio centreboard is more efficient than a low aspect ratio hull at resisting leeway. If a gybing centreboard is used on a deep, narrow hull that does normally contribute significantly to leeway resisiance, the area of the centreboard may need to increased to compensate for the extra loading. As stated above, I doubt if this is the case with the i14.

I was going to post something similar, but I think you've banged the nail on the head.

My feeling is that you can feel just a single strand of weed on the dagger board and it noticeably slows you down. Until someone dreams up a completely smooth interface between gybing board and hull, then the increased drag from the cut-out will never be overcome by the reduction in drag from the hull lee-way, which I suspect is minimal. Similarly most gybing cases are carrying around a few liters of water as well!

My experience in 14s is that the more time spent on the water, the better you do. This was true in Japan, with George, and later Sydney with Dan. With that in mind, on my next boat:

- I plan to have no fittings under the foredeck, except pole launcher. (They all turn a 10 minute job into a 40 minute job.)

- A fixed forestay. Just less rope and complication.

- Fixed lowers. (well turn buckles)

- Not a gyber. It adds complication for little perceivable gain. Just use a 7 foot board in light winds.

- Annoyingly no rotating tillers for the rudder. I like the idea, but have yet to see a perfect implementation that doesn't go wrong.

Unfortunately, this dream boat won't happen till after Vancouver :-(.

Archie

 

[Amati]

Just out of curiosity, has anyone tried a centerboard that steers like a rudder?

At the very least, everybody would have a tiller!

:lol:

edit- how did I do this?

 

[JimC]

But the reason there is leeway is that the initial lateral forces are greater to leeward.

This is where I think using the term leeway confuses the issue. Leeway is simply and only the angle of attack on the centreboard foil, plus/minus any difference between the alignment of hull and foil... As a thought experiment imagine a una rig catamaran on hydrofoils with highly variable gybing foils. If the foils are rotated 15 degrees one way you get a bit over 15 degrees of leeway, if they are rotated 15 degrees the other way you get a bit under 15 degrees of "negative leeway" but the track doesn't change.

 

[Elise]

... With that in mind, on my next boat:

- I plan to have no fittings under the foredeck, except pole launcher. (They all turn a 10 minute job into a 40 minute job.)

- A fixed forestay. Just less rope and complication.

- Fixed lowers. (well turn buckles)

- Not a gyber. It adds complication for little perceivable gain. Just use a 7 foot board in light winds.

- Annoyingly no rotating tillers for the rudder. I like the idea, but have yet to see a perfect implementation that doesn't go wrong.

Unfortunately, this dream boat won't happen till after Vancouver :-(.

Archie

Sorry to go off-topic a bit...but why fixed lowers?

Thanks,

Elise

 

[Mal Smith]

.... Besides, I'm begining to conclude that it only works for those who believe in it. For instance: I am convinced that I can get better height with the gyber on, and yet I don't see any evidence in the previous posts to support that possibility....

If you can reduce hull drag by installing a gybing centreboard, then you can choose to have either more speed, or more height, or a bit of both.

 

[mark1234]

I cannot see extra speed (with my emphasis) at all. If you make the assumption that hull drag doesn't vary much with attitude and (its only a medium sized assumption since we know that induced drag must be tiny in proportion to total induced, wetted surface drag is identical and wave drag must be very very similar) then if the foils are at the optimum angle to the wind in both boats, and the sails are at the optimum angle to the wind in both boats then all the dynamics are essentially the same. So how can the behaviour in the lulls be significantly different? In both boats the power available will decrease by the same amount, the heeling force will change by the same amount etc etc etc. I agree that in the gybing board boat the angle of the centre line is always nose down by the gybe angle of the board, but what I don't see is why that should make a blind bit of difference to anything other than the jib/main relationship. Which goes right back to my original proposition that the jib/main relationship, which we all know to be crucial to the upwind tuning, is the only factor that appears to change enough to have a significant effect.

Well as the IC testimony here indicates, hull drag does vary with attitude (and I'd tend to agree)

and yes, once in equilibrium the net lateral forces are zero. But the reason there is leeway is that the initial lateral forces are greater to leeward.

But I really don't understand this

Disregarding gybers, for the boat to move to weather, the foil would have to be at a negative AOA, hence generating negative lift, so where on earth does the weather force come from?

Huh? Again, I am so confused by the terms that I cannot sort this. Normally since the goal is to go to weather, "negative" would be "away from" going to weather. so it sees completely contrary to every notion. And what is "weather force"?

That's precisely the point. A symmetric foil pointed down the middle of the boat requires leeway in order to generate lift. It cannot move to windward like a moth (sorry - move towards the side opposite the sails, such that the boat has a vector perpendicular to the longitudinal axis, and upwind, rather than leeway). If it did that, the foil would be generating lift perpendicular, and towards the leeward side of the boat. There is no way it could move to weather (in the opposite direction to leeway) - unless there was some external force.

As for the drag stuff - wetted surface drag and induced drag are not the whole picture. Consider also form drag. A square box of the same wetted surface will have more drag than a pointed shape such as a hull, lift or no - somewhat intuitively, that will change dependant on the angle which you present the shape, and not necessarily proportional to lift (e.g. a stalled foil produces less lift, and more drag than an unstalled one.

 

[BalticBandit]

But the reason there is leeway is that the initial lateral forces are greater to leeward.

This is where I think using the term leeway confuses the issue. Leeway is simply and only the angle of attack on the centreboard foil, plus/minus any difference between the alignment of hull and foil... As a thought experiment imagine a una rig catamaran on hydrofoils with highly variable gybing foils. If the foils are rotated 15 degrees one way you get a bit over 15 degrees of leeway, if they are rotated 15 degrees the other way you get a bit under 15 degrees of "negative leeway" but the track doesn't change.

Fair enough - I now understand your reference frame. And I agree that you need an angle of attack. Leeway is normally considered WRT the idealized vector along the fore-aft through line, but your point about how this changes with a gyber is valid.

And yes, then "lifting to weather" essentially reduces your angle of attack

 

[Andrew P]

Can I move the discussion onto another part of the article by Dave Hollom in Seahorse.

Mr Hollom argues that the Cb looses effective span with increasing speed because the Lift Induced Wave Drag (LIWD) changes the effective span of the centreboard.

He states that it goes from twice the nominal length down to the actual length at some higher speed where the viscous forces dominate the gravitational forces on the water flowing around the centre board. He then goes on to argue that because of its lower Froude number the hull is more effecient at providing a portion of the lift required to resist the righting moment than the CB at speeds where the LIWD become apparent. I can see this with the rudder where there is a free surface, however IMO this is not the case with the CB.

My question is given the hull/centerboard intersection is in fact a solid boundary, wouldn't the effective span of the CB remain at twice the nominal length and therefore the premise that the hull becomes a more efficient provider of lift when at speed than the centreboard is not necessarily correct. I would think that the solid reflective boundary remains in place and the effective span remains as it is at low speed and the CB froude number doesn't decrease. Or is it the case that the limited area of the hull surface allows the wave drag to manifest itself downstream of the hull surface. Can't say that I've seen it in practice, Unlike the rudder blade where you can see the LIWD effect at speed.

The intersection of the hull and CB still sees an increase in drag with speed due to the presence of the fixed boundary but this is constant across all scenarios.

Comments?

 

[Mal Smith]

Mr Hollom argues that the Cb looses effective span with increasing speed because the Lift Induced Wave Drag (LIWD) changes the effective span of the centreboard.

He states that it goes from twice the nominal length down to the actual length at some higher speed where the viscous forces dominate the gravitational forces on the water flowing around the centre board. He then goes on to argue that because of its lower Froude number the hull is more effecient at providing a portion of the lift required to resist the righting moment than the CB at speeds where the LIWD become apparent. I can see this with the rudder where there is a free surface, however IMO this is not the case with the CB.

Comments?

I agree. As long as the width of the hull in the vicinity of the centreboard is similar to the length of the centreboard, the hull is going to act as a pretty good end plate and I would think the effective span will remain close to twice the length of the centreboard at any speed.

Even if the effective span was halved, the efficiency of the centreboard drops by less than half. So in line with Mr Hollams argument, if the hull was, say (being generous), 10% as effective as the centreboard at very low speeds, it might be as much as 15% as effective at high speeds. Bonus. Not a convincing argument against gybing centreboards in my opinion.

 

[Amati]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Oh hell, I'll look for it again, but the gist was that hulls aren't that great an endplate. Pressure fields under the hull and around centreboard interacting and all....

He could also mean the bunching up of the flow as it approaches the hull/centerboard join.

Or all of the above?

Paul

 

[Andrew P]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Oh hell, I'll look for it again, but the gist was that hulls aren't that great an endplate. Pressure fields under the hull and around centreboard interacting and all....

He could also mean the bunching up of the flow as it approaches the hull/centerboard join.

Or all of the above?

Paul

I can understand that the hull of a keel boat is not a great end plate when it is heeled over or even when it is sittng vertically because there is a easier path to the free surface. On a properly sailed 14 the hull presents a pretty good end plate. Pauls and others have said that the placement of fillets at the intersection is a tricky exercise and has little gain. A two D model of the CB section indicates that the field of influence of the low pressure doesn't extend very far out relative to the width of the hull.

Andrew

 

[Mal Smith]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Paul,

I'm sure I've read a similar article somwhere in the distant past. I think it was to do with why elliptical keels should be narrower at the root with max chord lower down the keel. As Andrew says, I can understand this with keels that are exposed to the free surface due to heeling, but not with a wide hull sailed flat.

Of course at high speeds the i14 is planing and the centreboard may well be exposed more to the free surface. But with the planing hull half out of the water it will be even less effective at providing lateral resistance, so the argument still doesn't stand up.

Mal.

 

[Amati]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Oh hell, I'll look for it again, but the gist was that hulls aren't that great an endplate. Pressure fields under the hull and around centreboard interacting and all....

He could also mean the bunching up of the flow as it approaches the hull/centerboard join.

Or all of the above?

Paul

I can understand that the hull of a keel boat is not a great end plate when it is heeled over or even when it is sittng vertically because there is a easier path to the free surface. On a properly sailed 14 the hull presents a pretty good end plate. Pauls and others have said that the placement of fillets at the intersection is a tricky exercise and has little gain. A two D model of the CB section indicates that the field of influence of the low pressure doesn't extend very far out relative to the width of the hull.

Andrew

(In a transparent attempt to continue this thread)

2D? 2D?

Although I must admit that the T rudder holding the bow down in extremis must have some relevance here. Although since Irrational 14 hasn't chimed in yet, that must mean something, nicht wahr?

Paul

 

[Amati]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Paul,

I'm sure I've read a similar article somwhere in the distant past. I think it was to do with why elliptical keels should be narrower at the root with max chord lower down the keel. As Andrew says, I can understand this with keels that are exposed to the free surface due to heeling, but not with a wide hull sailed flat.

Of course at high speeds the i14 is planing and the centreboard may well be exposed more to the free surface. But with the planing hull half out of the water it will be even less effective at providing lateral resistance, so the argument still doesn't stand up.

Mal.

But it might explain the lack of the mirror centerboard.

(?)

Paul

 

[BalticBandit]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Paul,

I'm sure I've read a similar article somwhere in the distant past. I think it was to do with why elliptical keels should be narrower at the root with max chord lower down the keel. As Andrew says, I can understand this with keels that are exposed to the free surface due to heeling, but not with a wide hull sailed flat.

Of course at high speeds the i14 is planing and the centreboard may well be exposed more to the free surface. But with the planing hull half out of the water it will be even less effective at providing lateral resistance, so the argument still doesn't stand up.

Mal.

Ooh - I remember that paper - damn if I can't remember the title. I vaguely remember it being around circa 1995...

But it was more than just the exposure to the free surface as I recollect. It also had to do with reducing spanwise migration of surface induced aerated water if I recollect...

 

[Liquid]

It also had to do with reducing spanwise migration of surface induced aerated water if I recollect...

This thread hurts my brain.... I wonder if the above had anything to do with why I capsized my 14 so often. Damn if I can't control that spanwise migration of induced aerated water! "spanwise migration of surface induced aerated water in 3...2...1...."

The only thing I would add is being on a starting line and watching the gyber boats climb away from you both in speed and hieght is eye opening! It's rather noticeable, granted I have a B3 and I'm generally looking at the sterns of the B5s....

As you were.

 

[joe.bersch]

Todd- Your observations are accurate. While B3 vs. B5 may explain some of this in your case, we have enough B5/B5, no variables other than gybing/no gybing to confirm unequivocally that gybing boards produce superior upwind vmg in a wide range of conditions. This is not theory. Trevor B and others in the 14 Class have reached these same conclusions. Paul Bieker has just completed the drawings for a plug and play system for the B6 which should be retrofittable to any boat with the most recent Russell Brown built board and able to accommodate a 3" centerboard trunk insert. This will take the fiddling out of the equation for those who have shyed away from their installation and use because of complexity. Stand by for details.

Nobody in their right mind would argue that a gyber is the magic bullet that makes you win the Worlds or any other major regatta. Archie has proven over and over that it can be done without one. As you can attest, Steve and Alan prove it locally as well.

FJB

 

[gui]

It also had to do with reducing spanwise migration of surface induced aerated water if I recollect...

This thread hurts my brain.... I wonder if the above had anything to do with why I capsized my 14 so often. Damn if I can't control that spanwise migration of induced aerated water! "spanwise migration of surface induced aerated water in 3...2...1...."

The only thing I would add is being on a starting line and watching the gyber boats climb away from you both in speed and hieght is eye opening! It's rather noticeable, granted I have a B3 and I'm generally looking at the sterns of the B5s....

As you were.

How long is your board?

I sailed on one of the first B5 for the NZ worlds a few years back, it had a gyber and a long dagger board. We were higher than most (all?), in anything less than 12 kts ... Dunno if it was the long board or the gyber. Like Archie says, could be that you just need a longer fin.

 

[Amati]

When Hollom talks about Viscous forces dominating gravitational forces, you don't suppose that he means that the fin's low pressure reaches up around the hull to the gravity interface? I just spent an hour looking for a hydrodynamics paper on that very thing and of course I can't find it.

Paul,

I'm sure I've read a similar article somwhere in the distant past. I think it was to do with why elliptical keels should be narrower at the root with max chord lower down the keel. As Andrew says, I can understand this with keels that are exposed to the free surface due to heeling, but not with a wide hull sailed flat.

Of course at high speeds the i14 is planing and the centreboard may well be exposed more to the free surface. But with the planing hull half out of the water it will be even less effective at providing lateral resistance, so the argument still doesn't stand up.

Mal.

Ooh - I remember that paper - damn if I can't remember the title. I vaguely remember it being around circa 1995...

But it was more than just the exposure to the free surface as I recollect. It also had to do with reducing spanwise migration of surface induced aerated water if I recollect...

Yup yup that's the one...

My point (yes, I believe I had one :lol: ) is that too much aoa over a certain Reynolds Number regime (or maybe even speed?) excacerbated the effect the article above deals with, which might be why under 12K works for a gyber, but over 12 doesn't induce it? How much leeway resistance to you need off the wind anyway?

Just my imagination, or is the chine crowd happier with gybers than the non chines folk? If you think about it, if that's true, then I had it backwards ( ), and the more the hull resists leeway, the more a gyber is an advantage. So D.H. may have a point. (I love his stuff, but sometimes he riffs like Mr. Greenspan used to.)

Mr. Maas- you have chines, no?

Kind of makes me wonder about no chine hulls and gybers under 12K. Are Int 14 chines above the water enough over 12K so the hull essentially lifts up enough and becomes essentially a roundish no chine hull on plane, and then develop more leeway so the gyber becomes aoa overkill?

It seems to my eye that Paul usually puts his chines on the high side (?)- care to comment, Irrational 14er?

Paul