Fabric Centerboard

Daggerboards and Centerboards require pockets for storage when not in use, which can take up a lot of space and be hard to design cabins around. Ultimately (if I understand correctly), they are foils in the water, much like the sails are foils in the air. Has anyone made a fabric "sail into the water", to gain the advantages (and disadvantages) we see with sails over rigid wingsails?

 

The loads are comparable between foils in air, and foils in water, or else they couldn't meaningfully balance each other, and sailing wouldn't work. So anything you stick down into the water must be as strong as the mast it balances

 

I suppose you never encountered telescoping keels. Telescopic Lifting Keels - APM Keels https://www.apm-keels.com/project/telescopic-lifting-keels/

As for a "soft sail in the water" you only need to convince the fabric to maintain its shape in all directions with a much higher load per surface area. Right now that fabric is called unobtanium.

 

If you made a centreboard out of any fabric, it would have to be as stiff as a board anyway. A flexible form would bow due to the sideways pressures of the wind pushing to the lee, so your steering would be affected by a curved, and probably bent over centreboard, and that curvature would introduce flow inefficiencies, and probably oscillations in some circumstances, causing many other issues resulting in further inefficiencies. The unobtanium would need to be switchable between flexible for stowage, and rigid like a board, to have any useful effect.

 

It could be an interesting concept to experiment with.I can see the appeal of trying for a flexible surface that would be moved to imitate the effect of a cambered wing section and I can imagine the water pressure doing that part.I can just about imagine an elongated teardrop shape for a rigid leading edge and it might need to be made from something quite exotic.I can even imagine an adjustment system within the part inside the case to behave like a vang and control the amount of twist.I suppose a sleeve luff type of leading edge would be the best thing.What I find difficult is creating something that could support the crew in the event of a capsize,even if a rigid enough spar could be produced for the leading edge.

 

The way I see it, keels do a different job from sails. Yes, they counter the lifting forces that drive a sailboat forward in much the same way below the water, but their job is to resist the sideways forces, not provide driving force.

If righting moment is unnecessary for your underwater foil, you will always need it to control direction, not provide drive. A keel represents, not just lateral resistance, but also directive forward tracking, like a sled runner in snow, or blades on ice.

A fabric underwater foil seems like it would take a curved foil shape that would only result in greater heeling due to increased lateral lift. That would reduce lateral resistance and thus reduce the driving force of the sail by allowing the boat to move leeward more.

Perhapse, a short, but full length fabric keel, something only a hand breadth in depth, but the full keel length, might be effective.

-Will

 

Just calculate the membrane tension needed for a 5% camber, 2 meter deep, 1 meter cord fabric supporting 2.5 tons of side force. What sort of frame is supporting that membrane?

But there is this - it's been around forever - Springfield fan centerboard.

 

I think that in the context of something we are describing as a centreboard,there won't be a need for ballast as the ballasted version would normally be be a drop keel in the minds of most sailors.It is the notion of a cambered surface that would make it more effective on both tacks and which might allow a reduction in surface area as a result.I really don't see it happening any time soon due to the structural challenges mentioned earlier but in time it might be achievable and using a variation of the normal rig controls it might be possible to adjust the shape to achieve the optimum over a range of conditions.The fan centreboard is something you might read about in historical texts but you won't find any racing classes using them as we have since learned about aspect ratio and the importance of a high quality finish.Which means that a good board can be made with less effort and less cost by sticking with the simple concept that we all use.

 

Experiment with a (stern hung) fabric rudder migh be in place before a keel..

 

It would have pressurised air chambers like an inflatable pvc SUP and fold down like a {carbon} dorsal fin..sucking the air {or gas or hydraulic} out would help it retract... back into its rabbit hole,, not for me I have plenty.

 

If it was feasible to use a fabric, the mounting would be the greater part of the problems. The faster you go, the more tonnage force is striking the front edge, or radius or whatever is at the leading edge of the device. That means a lot of leverage over its length to counteract, needing strength, reinforcing, and possibly bracing. Then when under way, immediately behind the leading edge, two flat expanses have to resist the water pressure from both sides, as well as the lee pressure; so internal framework, or foam, or fluid pressure. The clamping of a fabric to a rigid leading edge could be simple or complex, and the strength needed means weight; then what holds the trailing edge from distorting under load. The underlying framework would need to be designed so the fabric doesn't wear excessively along any edges when flexing. I can't imagine anything inflatable being useful beyond a few knots, before creasing and folding up. Try holding a stick steady and vertical in the water at say 5 knots.
I think you would need more structure just to make it work, than what would be needed in a normal solid board shape.
I did imagine an electromagnetic fabric which either locks up under power, or goes limp under power, but electricity and water often mix in a bad and unreliable way.
Fabric soaked in resin would work, like electrical insulation board, but defeats the initial idea; its a board.
Hydraulic rubber and steel wire fabric could be fashioned into a 'fabric' centreboard / keel which is flexible to a degree, and rigid when pressurised, but it would be heavy, expensive, need ancillary equipment, and would be hell to make.
Will's idea of: "a short, but full length fabric keel, something only a hand breadth in depth, but the full keel length, might be effective." seems like the most likely method to not fail immediately. Something like that could be powered and become a low speed propulsion system too, sinusoidal oscillating like squid fins.

 

Inflation could be with seawater, less compressible then air, the elasticity of the fabric would determine the keel's rigidity. Internal fabric baffling would help the keel hold its shape.

 

That's not actually the case though. Both provide driving force.

Going back to the OP's proposition though, no I'm not aware it's been done, although someone somewhere has tried pretty much everything. I don't think there's any reason why one shouldn't have a spar and fabric foil under the boat, but what with the loss of efficiency, the structural problems, all the detail engineering, and all the rest of it, I think the advantages would be worse than minimal. If you consider a dinghy with a 5 foot by one foot board then you still end up with a 5 foot spar and a 1 foot spar to deal with, even if you can remove the fabric. A solid 5 foot by one foot object is a size one can deal with, whereas a (say) matching 25 foot by 5 foot rigid sail is impractical in almost every circumstance.

 

I would be happy to get into this more with you. I am not a naval architect, but I have had physics, so I am both a long way from knowing a lot and capable of learning.

Where, under the water, do the forward vector forces apply? I've heard and read others talking about the keel "sailing" just like the sails do above the water, but I can't picture the keel contributing to any motion forward except as a resistancing sideways force. If I were to put it in terms of 'lift', the lifting forces from a keel are perpendicular to motion.

Would you also suggest that the runners on an ice boat add to the forward forces like the sails do, or do they just keep the ice boat from slipping sideways so the forward vectors from the sails can move the boat? Maybe it's just semantics I'm playing with, but I don't see it would work to trim a keel to the water "current" and successfully tap it for forward energy, the way a sail does. I think that would just bleed energy away from the above water sail, as the boat slipped more leeward.

-Will

 

Driving force yes, but negative.