Hi, this might be of interest;

http://www.hmtmarine.com/keel/overview/

Hi, this might be of interest;

Thanks for the link.

If you are THE Roy Stuart, i want to thank you for a great blog/site about surfboards. Its a treasure of information, wise thoughts and a deep understanding of the dynamics at play in the medium we all love!!
I love your tunnel-foils, and your beautiful surfboards.

Cheers,
Johannes

Cheers Johannes, yes I’m the Roy who’s making the tunnel finned wooden boards.

A few years ago we spent quite a bit of time and thought figuring out how tunnels might be applied as rudders and keel winglets for america’s cup boats, that’s when we came cross the loop keel info.

It will be interesting to see how your tunnel rudders work. I usually use a slightly more than half a circle to increase lateral resistance, but not too much as we like to keep the centre of rotation of the tunnel close to that of the surfboard. Anyway if you don’t get enough lateral resistance you could extend the half circle on straight upstand sections, these could enable the rudders to be raised and lowered for greater or less area.

By the way our inspiration for the tunnels came from those flying hoop toys… we found two on separate occasions on the beach, and had been making hydrofoil fins with large horizontal planar wings, these were nice for swell gliding but not on steeper waves. the first tunnel was an instant success, with vice free handling everywhere including inside the tube, plus lots of speed and drive. ( used behind a single fin).

I have some sailing experience, I used to race dinghies and was a foredeck hand on a half tonner, did some catamaran sailing too on a Tiger shark, and paper Tiger but no proas, yet.

This is pretty interesting also: http://www.esotec.org/hbird/index.html

Here’s a tunnel equipped surfboard in action.  https://vimeo.com/62646967

Anyway if you don’t get enough lateral resistance you could extend the half circle on straight upstand sections, these could enable the rudders to be raised and lowered for greater or less area.

That is a brilliant idea!!!’
Thanks!

I will make the radius of the tunnel-rudder slightly smaller (maybe 15 cm or soo for my 24 foot proa) and than lower them for a greater lateral area when needed.
When fully raised, only the last 15 cm of the tunnel-rudders will protrude under the bottom. That is less than the rocker of the bottom. They still need to be strong and/or have some kind of protective skeg. The skeg only need to be in front of the rudders, more like a small fin in front of them, that will take any impact. If I shape the slots so I pull the rudders a couple of cm higher, they become even more strong and damage resistant.
They can be in vertical cylinders so that they are easy to turn…. Suddenly we are back to cylindrical daggerboard cases, but with less deep tunnel-rudders/foils. We still have a much stronger and stiffer structure, thinner foil-section with more resistance to stalling and no tip-flutter/oscillations and no tip-vortices.

I like this!!!
This is a great forum! I learn new things and get new ideas every day….
Thanks for great idea Roy!

Cheers,
Johannes

Instead of skegs and strong steel tunnel-rudder blades one can use sacrificial thin carbon blades. Its easy to keep a couple of spare ones. The turnable disk bottom part should be possible to lift up out of the cylindrical daggerboard case to easily change the blades.

This keeps getting better… Now i will have to fill my slots with epoxy and make a new rudder-arangement on my green proamodel….......

I am still a little confused by these tunnel foils. I came across the links below which for a simple pipe seam quite powerful

http://blog.floatingislands.com/2013/05/first-sea-trials.html

http://blog.floatingislands.com/2013/04/big-pipe-paravane.html

TINK

Thanks for the links Tink!

There is a lot of “power” in tunnel-foils. Please test a short piece of a tube in your bath tub or kitchen sink. It is very impressive. I have been thinking a great deal on these tunnel-rudders. I think there is a lot of potential in these idea.

Cheers,
Johannes

I think this is really interesting as well!

I personally don’t really protective skeg in front of the rudders at all though. If it’s permanent to allow beaching, then it’s got to be some sort of rail on which the whole thing can sit, kind of like the hooves on a sled perhaps, which would be loads of wetted surface area with an terrible shape where the fluid dynamics are concerned…

For a multichine hull you could have two of these arc rudders on either side of the V at the bottom, instead of one just below the V, so that the bottom of the V takes all the weight when beaching, and the arc-foils, if placed high enough and if they are small enough, wouldn’t be loaded at all by beaching. You could do that for a purely V-keeled hull as well, but because of the steep angle you’d probably be doing more to the trim of the boat than steering unfortunately…

From another perspective though: why not just mount a tube rudder on a symmetric foil at the back of the boat, just like any normal kickup rudder, but shorter and with a tube stuck on the bottom? Then you could get it out of the water when beaching and it would be safe from collision damage while underway. Then it’s basically like any normal kickup rudder, albeit with less lateral resistance than say a rudderboard would give you, but also with a lot less draft.

From another perspective though: why not just mount a tube rudder on a symmetric foil at the back of the boat, just like any normal kickup rudder, but shorter and with a tube stuck on the bottom?

There is much higher loads in every direction from tunnel-foils. They do not only resist leeway, but every other non-forward motion as well. This would put very high loads on the attachement points of the rudder. If the rudder tilted up/aft at speed the tunnel-foil would try to drag the aft end of the boat down under the water. This is potentially very dangerus.

For a multichine hull you could have two of these arc rudders on either side of the V at the bottom, instead of one just below the V, so that the bottom of the V takes all the weight when beaching, and the arc-foils, if placed high enough and if they are small enough, wouldn’t be loaded at all by beaching. You could do that for a purely V-keeled hull as well, but because of the steep angle you’d probably be doing more to the trim of the boat than steering unfortunately…

This is a very interesting idea. This combined with the “fad” some years ago among Wharram-owners to put a Anti-Vortex plate on their hulls, http://www.ikarus342000.com/Antivortexp.pdf gave me an idea. Why not have a fixed Anti-Vortex plate as the bottom part of a triangular tunnel-rudder. Only the vertical part should move like a normal rudder. You loose some effectivness by the larger wet surface compared to a true round tunnel-foil, but you gain the effect of an enclosed area of water being forced through the tunnel-part with relating properties of stall-resistans, lower drag through use of a much thinner foil section, less or no end-flutter, vibrations and a stiffr foil.

The rudders should be on the high-pressure lee-side to have maximum lift. I suppose the triangular tunnel-rudder would damp pitching motions quite well, and resist squatting.

johannes - 25 May 2013 02:34 AM

Why not have a fixed Anti-Vortex plate as the bottom part of a triangular tunnel-rudder. Only the vertical part should move like a normal rudder.

I think that would be a funnel more than a tunnel.  The difference is that all walls of the tunnel move when you turn the rudder.  Your funnel rudder moves only one of the walls, it will no longer be parallel with the other walls, and I think you’d get more drag than lateral force.

it will no longer be parallel with the other walls, and I think you’d get more drag than lateral force.

This might be a problem, but with a tunnel-rudder going round the bottom of a deep V or deep multichine hull you would get the same problem. The hull will not be parallel with the rudder when turning. I hope that with these thin and short chord rudder-blades this will not be much of a problem. Because there is two rudders working on opposite ends of the hull, they will have a lot of turning power even if they are small. Rudders always have a lot of drag (up to about 30% of the total drag of the boat) when turning. By making the rudder a part of a closed funnel, it can be much smaller and much higher aspect without stalling prematurely or other problems assosiated with very thin section foils.

I have not tested this though.It is only a thought I have. I want to try it.

Cheers,
Johannes

How about this:

Take a pair of dagger-board “cassettes” and have them pass through the hull and deck at a diagonal angle. Let’s say that they penetrate the windward side of the hull right at or slightly above the waterline. Your tunnel-rudder can be a full-circle shape mounted vertically to the bottom of this foil so that it rotates only horizontally. The angle of the board will be determined by the minimal clearance needed for the rudder to turn when it is deployed to a depth equal to the bottom of the hull. Of course, when not running in shallow waters the boards can be pushed deeper.

The downside to this is a more complicated steering system, as the dagger-board will have to have a pair of steering cables (dyneema dux would seem ideal) run inside it to operate the rudder. I can’t come up with any solution that uses a simple rotating shaft due to the rudder not being mounted along the same axis.

Johannes, I’m new to the forum, and been browsing some threads of yours. Very nice ideas, concepts and solutions!

On the subject at hand, reading thru this thread an idea came to mind.

I don’t know much about this foils at all, but from the very informative posts here and a quick read thru some links, It seems to me this can work… Might be a catch somewhere thou, I didn’t really give it much though.

If you treat the tunnel ruder as a normal board you can deal with it in the same way, as long as you can transmit the necessary energy without snapping anything.
I can’t really think how to describe the form I have in my mind, so I attach a drawing with it.
I think such form can be made strong enough for the job.

I think the easiest and less complex way would be to hinge it from the side of the hull somehow, maybe with a system of lowering and raising it, or even kick up.

I hope it makes sense!

I find this very interesting.  I have a couple naive questions.  I do not know much about hydrodynamics.

1) What problem are you trying to solve here?  Leeway? Stalling? More vertical lift?
2) When you turn a tunnel rudder a few degrees don’t you get greatly increased drag?  Regular rudders act as brakes at speed.  It would seem like you are increasing this phenomenon with a structure that has greater lift.
3) Depending on what characteristics you want to change or improve, it seems like you might want to keep the foil fixed so that all that force is aligned with the primary direction of the hull? This would be more like the surfboard.

If stalling is your issue, there are “Tubercles” .

http://perryboat.sail2live.com/yacht_design_according_to_perry/2011/10/rudders-lets-start-here.html

I saw these on the leading edge of some of the surfboard tubes and on one of the leading skegs.

Thanks,
-jak

I find this very interesting.  I have a couple naive questions.  I do not know much about hydrodynamics.

No questions are naive. Every question is a “wish” to understand more, and as such is the very core of a thriving forum and progressive development.

I don’t know much about hydrodynamics either, but I’m trying hard to learn.

1) What problem are you trying to solve here?  Leeway? Stalling? More vertical lift?
2) When you turn a tunnel rudder a few degrees don’t you get greatly increased drag?  Regular rudders act as brakes at speed.  It would seem like you are increasing this phenomenon with a structure that has greater lift.

The tunnelfoils has a couple of very interesting advantages over straight “open” foils. The cylindrical shape has a much higher structural strength and stiffness than a straight foil for a given thickness and cord. Since it can use a much thinner foil-section it can have greatly reduced drag for a given amount of lift = higher efficiency.

Because of the encircled “pillar” of water it will be much resistant to stalling.

Every kind of rudder will always increase drag when not aligned with the flow. The tunnelfoil is somewhere between a hydrofoil and a rudder, as it will stabilize the boat, reduce pitching and keeping a clean even airflow through the sails - powering the proa. Two small low drag rudders acting on each end of the vaka will (hopefully) control the proa better with less drag and power-draining turbulens than a normal deep straight rudder.

Cheers,
Johannes