Bridgedeck centreboard why don't they work???

riddle me this then, boards should be designed by calculating sail forces and then designing foil profile and board area to match OR is the rough metric of 2~4% of sail area good enough?

 

Yeah, I think surface pearcing CB should be compared to LAR keel not high end racing daggerboad. For my boat I choosed CB over LAR keel because I wanted maximum possible shallow draft capability and safety in case of hitting submerged rock or something. Obviously I get performance benefit downwind because there are no unnecessary stuff dragging along when not required. Upwind performance is comparable to a cruising monohull of similar size despite my crappy sails. Downwind I have hit speeds around 15 knots, I guess around 20 knots would be a limit when windward hull starts to fly.

 

I think the fancy movable fences would be most helpful for surface piercing hydrofoils. It takes less energy to move a fence than change the attitude on the whole foil. In fact you could probably work out a self adjusting attitude control.

For cruising with the central board the forward rake for the basic setting is probably the ticket. I agree that the main advantage of a centerboard or daggerboard arrangement is the ability to adjust draft for shallow running. A dagger could be set up with fences but retraction becomes more complicated and it is going to take up deckspace when up.

I'll read the whole thread, did anyone mention the Jim Brown CC44 charter cat Anna Kay from back in the 80s? It had a central centerboard as described. Be interesting to hear how that held up.

 

Ventilation research

I decided to see if I could ask the author of this video some more particulars about this video as it appears to be a portion of his very significant studies on this ventilation/caviation subject matter.

This was his recent and very informative reply. (I've got a lot of reading to do)....
https://www.youtube.com/watch?v=zJttVDFbYJY&lc=z12jcllbukb2sjn4e04chx4rhlbrcnjadjs.1486600185508882

 

Note from quote above....

That is a speed of about 8 knots, very viable for our cruising catamaran. But note the BIG angle of attack...12 degrees, and that it is a symmetrical foil (more prone to ventilation than an asymmetric one,...from my reading.

 

I'm not sure what this refers to? I would have thought the chord of the foil would be relevant?

Strut is yawed at 12 deg? That would seem to indicate a high angle of attack?

The foil has a square blunt end? That would seem to create a wide "transom hollow" style ventilation which would surely increase the tendency to ventilate compared with a sharp trailing edge?

 

Yes indeed, the image below was taken from a co-authored paper of 2014.
The square blunt end shown is rather a killer in terms of ventilation. As noted lots more reading to do.
As to the Froude number, I take that to mean that the geometry of the strut is not important IF the speed is less than 1.6 * sqrt(gravity*depth)
so for a foil depth of 1.5 m, speeds under 3.8 m/s are safe? Edit: of course, ventilation would be possible at shallower depths than 1.5 meters.

 

So here's that little sketch I promised,...just roughed out quickly while I was on a trip. Hope its understandably,...my printing has NOT improved with age...ha...ha


The board on the opposite side of the sails are set on will be utilized (down). The sail forces are pushing the vessel to that side they are set on,…so the centerboard needs to counter that force,.... the board needs to provide lift in the opposite direction. The board with its flat side on the same side as the sails are set, ie the board opposite the side the sails are set on.

This board’s bottom tip will be forced to the outside beam of the vessel, and thus its top tip will be forced inwardly against the nacelle plate. For this reason it will wise to have a healthy amount of board area above the pivot bearing location.

A snug fitting, large dia bearing, and a good size plate area to bear against, should preclude having to have a full size retaining plates on the outer surfaces of the boards. Flat surface frictions will be minimized by very slim sheets of teflon or delrin material.

The somewhat 'free floating' bushing type bearing will sandwiched together with the centerboards boards and the nacelle plate,..and restrained from side movement by the two very taunt side support cables/wires.

 

...from another subject thread...

So I've chosen a small amount of forward rake (if its found to be desirable), pulse a few fences combined with scallops at that upper front edge that appears to be so critical in starting the ventilation problem.

 

http://www.caseyharwood.com/pubs/umichthesis/

Found the reason they chose that 'blunt trailing edge TE' for his testing work

 

also of interest
http://people.eng.unimelb.edu.au/imarusic/proceedings/18/170 - Binns.pdf
a series of photographs looks like they were taken from youtube video I posted earlier
see reference #10

also from http://www.boatdesign.net/forums/sailboats/best-shape-surface-piercing-foil-3991.html#post23840
" But we decided to abandon the concept, and switched to inverted-T surfaces which worked very well. The sections at the waterline were NACA 0010, which worked very well with no tendency to ventilate, even when going close to 20 knots." no word on angle of attack.

 

Would it be of any benefit to use a different section at the top of the board than further down in order to inhibit the downward movement of the air ?
I think this might be in effect what the "tubicles" do

 

At least I could read this reference without getting totally lost in a lot of theory.....ha...ha. I tried to wade thru a few of those other PDF's, and they were just above my 'pay-scale'...ha...ha

I did notice this quote from that one you referenced...

 

When I was a kid my older neighbor had an Arrow cat , it went pretty well with the central board http://arrowarafura.com/gallery/your-photos/
I sailed on an Ospray/Osprey cruising cat with central bridge dagger a couple of times, once was on a survey/sailing assessment & didn't rate it to well upwind the next time was assisting delivery downwind with board up & made good time from Port Hacking to Pittwater.
Was much like this cat with the board just ahead of the mast, I have a recollection of it being raised & having to tie across to stop from banging... https://www.youtube.com/watch?v=JSrQwWv0pVk
Jeff.

 

Just a small note - the yaw of a strut is exactly what AoA is for a centreboard. So their observation corresponds to saying that the probability of ventilation is directly related to the hydrodynamic load on a centreboard - as by my previous post.

It certainly would help, but where is exactly the top of the board? Think of all the possible positions of sea surface along the board, even with a moderate swell.

Cheers