Bridgedeck centreboard why don't they work???

Yet many fast multihulls have surface piercing rudders which are expected to work at both higher angles of attack AND speed compared to a centerboard?

 

Questions;
Assuming one chooses to use a central board and it is set up to balance the boat at say 5 degrees forward angle to minimise ventilation.

What might be the angle forward need to be to get the boat to reliably lie head to wind ?
Would that angle be the same aft to help the boat track off the wind ?
Would the one centre mounted board have a similar aspect ratio to hull installed daggers or would a different aspect ratio work better ?

 

they have much higher-power rigs and so a drop in efficiency is not a problem?

 

A few opinions.

I have no idea on the angle needed to lie to a headwind. But presuming the boat will be drifting backwards slowly the loads would be fairly low, so some extra ability to over rotate it forward should be no issue, comparatively speaking. Also assuming someone who would want such a board would also have retractable rudders means this should work quite well?

I think any angle rotated aft will "help" track downwind. How much I don't know will be a big improvement. But the side loads would be lower downwind so that would offset the tendency for increased ventilation. Any ability to move it back has to be an advantage. How much is anyone's guess.

A higher aspect board would have less surface drag and I imagine the ventilation issue would be less. But loads are harder to answer in this config so I guess it would be an engineering compromise getting worse as the boat gets bigger.

 

If its not a significant issue for them as a racing boat, the slower cruiser with a slower use requirement of a CB (upwind only), plus the fact it operates at smaller angles of attack seem to point toward it being less of an issue for that application?

 

not sure that cruisers have smaller angles of attack as our speeds are much lower. This emphasizes the lateral component of the vector

 

That's true, but rudders certainly have higher angles of attack as they are for steering. The boards angle of attack is the leeway angle. Its still probably unlikely that the angles of attack a racing boat rudder has to deal with will be less than a cruiser CB? The speeds are obviously much lower.

 

yes, but rudders are designed to alter the angle of the boat and then centered. Not designed to be held at large angles continuously as it too draggy. If weather helm or lee helm is excessive then sails should be retrimmed.

 

The design needs to work at the highest angles of attack likely to be experienced in normal use. Just because the average angle of attack is lower than what is used periodically does not mean it should be compared to the average angle of attack. Imagine having a boat where the rudders stalled and ventilated during their intended use to steer the boat, but then it was justified as satisfactory because when they are held in near the middle there is no issue.

The CB simply can not be steered. Its angle of attack is always the leeway. If the rudder does not work at much higher angles off attack than this it is a failure. I have read a few times on this forum the foil shapes/sections of a rudder should be able to withstand higher angles of attack than a keel/board. This makes obvious sense to me.

 

we are getting a little cross threaded here.
for rudders you are talking of stalling, the hydrodynamic lift is completely lost and the rudder becomes useless.
We were originally talking about the free surface effects of the centerboard where ventilation is a concern in contrast to the daggerboard with its lack of ventilation. Ventilation can extend far down the foil as shown in the video and will degrade the lift component -- how much loss? Would it cause stalling? Video's don't show that (wasn't tested) only ventilation from free surface.

 

Its relevant. Many fast multihulls have surface piercing rudders and they seem to work fine even though they are expected to work at higher angles of attack and they are expected to work at far higher speeds, both situations that increase ventilation/drag compared to a slower moving CB. Therefore its not immediately obvious that a surface piercing CB which is only to be used at much slower speeds upwind on a cruising boat will be much worse, particularly when you consider the drag benefit downwind without the board slots.

 

lets stick with comparing centerboards with dagger boards.
same speeds, same design goal -- free surface piercing vs. non-surface piercing.
We can then add in the auxiliary design factors of case drag etc etc.
but you think that class A cat designers haven't gone through this process already?
For efficiency, dagger boards.
For Mulkari, the rocks demanded a kickup centerboard.
For me, nervousness about underwater hull openings is a key concern.

 

Why limit ourselves to comparing only daggers when valid comparisons exist elsewhere? You did that yourself when you posted videos of foils which were not centerboards or daggerboards.

This argument has been addressed and shown to possibly not be as relevant as you might think.

http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-14.html#post797754

 

DennisRB. Please explain.:?:

 

It was just an opinion which I have no proof of other than the following logical explanation. It seems fairly intuitive, but not everything is. For the sake of the argument imagine the following example.

One board is 1m wide and 2m deep from the waterline. 2m2. 2:1

The other is 0.75m wide and 2.7m deep. 2m2. 3.6:1

Since the objections are relating to the surface piecing aspect of these designs causing drag etc, the 2:1 board has a larger chord and thickness piercing the water. Therefore it will have more surface piercing related drag since there is simply more board operating in this drag causing state. Bigger wave etc. Secondly this draggy low lift area is larger as a percentage when looking at the whole board areas on the 2:1 board, since there is more board near the surface and less underwater in comparison with the 3.6:1 board. Thirdly any ventilation needs to travel further down the 3.6:1 board to create equal loss of lift because its deeper, and the surface piercing area to trigger the ventilation is smaller which makes the issue less on this board to begin with.

I am up for hearing criticism but it seems pretty solid?