Simple, strong and safe rudders. Tunnel-rudders.
Hi everyone!
I have been reading about surfboards and their fins, since i believe proas have more in common with surfboards than normal monohulls (speed among other things).
I stumbled upon Roy Stuarts site about tunnelfins and got very interested.
Tunnel-fins seems to have a lot of good qualitys. They are by nature very efficient for a given size and very directionally stable. The are resistant to stalling and very simple to make out of steel, aluminium, carbon, glass or wood/plywood. I like that they are very shallow (not deep down in the water.
My idea is that one uses two tunnel-fins that go through the bottom of the hull ( in slots). They are couple to an axle going through the bottom so that they can be turned. I don’t know if one want to turn both at the same time or only one at a time.
Under the tunnel-fins-rudders there should be some kind of skeg that will protect the rudders and function as a bearing for the axle.
I will build these kind of rudders on my green sharpieproa. I have made very crude tests by draging the plastic tunnel-fins in the picture through some water, and they have a lot of “bite”. Its hard to notice any drag when playing with them in the sink, so i have no idea about possible low/high drag. I will have to test it on my model as usual…
One of the very nice properties of tunnel fins is that they will have some lift of the hull. If they have the correct angle of attack they will create a stable pitch and counter the nose up - aft down tendency of the deeply rockered hull. I think this will enable them to surf and or semi-plane at speed.
Johannes.
Just one picture of the tunnel-fin-rudder turning. There should be two arc-shaped slots where the tunnel-fins rests on the bottom of the proa.
I will build them today ( i hope) and show some more pictures soon.
Cheers
Johannes
A picture of a possible cross-section of a V-shaped multichine hull with a tunnel-rudder.
I believe it should be a little wider compare to the hull. As it is now there is probably not enough area in the tunnel-rudder for a even easy flow of water.
Johannes
Thanks for the links, some very interesting stuff!
The hydro equivalent of Mal’s Arcsail perhaps?
For turning the thing you might look at having a shallow circular recess in the hull and mount the arc on a turntable set in the recess, at least for flat bottom areas.
Cheers,
Skip
The hydro equivalent of Mal’s Arcsail perhaps?
Yes!
I have been thinking on Mals arcsail many times. I like the concept a lot, but i would build it very differently. I thinks its a great idea, and his experiments are a great source of inspiration.
I have been building and testing flying rings today. Not my video but very similar to my tests:
There is a lot of potential in the arc/circular wings/air and hydro foils.
They are extremly stable compared to “open” foils. I like that stability, and i think it has a lot of potential in high speed sailing, where the foils has to work in very turbulent and dynamic conditions.
The theoretical “low drag” of normal “open” high aspect foils is not always the best option in the confused state of flow that is prevalent in hard sailing.
Cheers
Johannes
Today i cut the slots on the bottom of my model proa. Its scary to start cutting into something that is almost finished!
I need to sand the slots to shape and smooth the edges.
The tunnelfoils are made out of 10x1 mm aluminium bar. I will sand them to a better shape once im done with the slots and a way to fasten them and turn them.
This is a very crude simple model. I’m having a hard time adding any finesse to the rudders. The foils are probably bigger than they need to be, but that is because of my lack of tools and knowledge to make anything in this small scale.
Johannes
More than 300 views in 24 hours, but nobody except Skip writes anything at all?!?
Is there a large elephant in the room i have completly missed or am I not explaining it good enough?
I’m getting nervous about this… Is there anything wrong with my idea soo please say soo….
Cheers
Johannes
Aloha, Johannes,
interesting way of thinking .............. it has way less draft when sailing.
The question will be ; do you have enough lateral area
- at no speed ;manoeuvring in tight space ,such as harbours you have to have big lateral area , otherwise the proa is blown all over the place. specifically flat and round shapes.
- at a bit speed , while shunting , you need a large foil to get “bite” ,otherwise you run the risk to get “caught in irons”.
-at full speed . you need barely any foil , so for that i see a benefit.
two Questions.
-how do you turn the foil in the opposite direction after shunting, or you don’t as I see your slots.
so one of the two is [shape-wise] pointed in the wrong direction? or do you have the idea to have a symmetric foil shape.?
- how do you get rid of the foils when beaching?
cheers Sven
interesting way of thinking .............. it has way less draft when sailing.
Thank you! The shallow draft is very important to me, soo that is in part why develop this kind of rudder.
The question will be ; do you have enough lateral area
According to the different studies i have seen online a tunnel-foil is comparable to a straight foil 3,14*diameter/2 (its only half a circle). So a 30 cm diameter tunnelfoil should have the same lift as a 47,2 cm deep straight foil with the same chord.
I will use two and steer with both at the same time (linked by wires) so that doubles the foil-size.
- at a bit speed , while shunting , you need a large foil to get “bite” ,otherwise you run the risk to get “caught in irons”.
-at full speed . you need barely any foil , so for that i see a benefit.
Sharpie-hulls with hard chines have a lot of lateral area and are very stable by them selfs. Compare with Matt Laydens Paradox that can beat to windward without any other lateral area than the hull and the chinerunners.
The tunnel-foils are only rudders. They will add to the lateral resistance of the boat, but that are not their primary task. I hope this works. I have to test it with my modell before i decide on using them on anything bigger.
two Questions.
-how do you turn the foil in the opposite direction after shunting, or you don’t as I see your slots.
so one of the two is [shape-wise] pointed in the wrong direction? or do you have the idea to have a symmetric foil shape.?- how do you get rid of the foils when beaching?
The foils should be symmetric foils. They should not turn when shunting.
I hope they can be small enough to not interfere much with beaching. There should be some kind of very strong steel skeg under the tunnel-foils that will take the ground before the rudders.
Thanks for questions, and thanks for all the great info on proas you share on this forum!!
Cheers
Johannes
One GO Science representative told The Register that the ring-wing form can travel up to 8 knots on battery power, and with 33 percent more efficiency than other alternative underwater shapes.
From:
Info - ringwing sub
I found this today. Interesting that they claim 33% more efficiency from something that seems to have more wet surface than a “normal” torpedo tube - shaped sub.
I have been thinking on ringwinged sails, rudders, ama and almost everything i can imagine today. My kids love the flying paper-ring i made.
I lost my patience with trying to make some smart simple way to turn the foils. I filled parts of the hull with polyurethan-foam, and i will cut some slots in that foam once it sets. I’m making it as simple as possible. Rubberbands, cable-ties and crudely shaped pieces of plywood have worked before….
Johannes
More than 300 views in 24 hours, but nobody except Skip writes anything at all?!?
Is there a large elephant in the room i have completly missed or am I not explaining it good enough?
Well, I can tell you that I looked at this and had no idea what to say. This is such a new concept, I was, like, “Hmm, I’ll believe the efficiency claims when I see these things in action and they prove to work in real world conditions.”
But I can say I like that you are putting them to the test on a kind of hull that would probably work for them.
Is there a large elephant in the room?
Are you calling me fat? 😊
I really like this theoretically. Practically, the problems are the slots in the hull, and beaching. You can’t use them as “kick up” either because they turn into giant brakes. I’m trying to think how this could work on my small hull.
-Thomas
How about this, instead of cutting a slot in the hull, you router a recessed circle into the bottom. Mount the foil onto a slightly smaller disc and stick in the recessed hull mount, with a lip to keep in place. Through hull tube in the centre with steering axis. Can even rotate 360 degrees if so desired.
Problems: friction, ...
How about this, instead of cutting a slot in the hull, you router a recessed circle into the bottom. Mount the foil onto a slightly smaller disc and stick in the recessed hull mount, with a lip to keep in place. Through hull tube in the centre with steering axis. Can even rotate 360 degrees if so desired.
Problems: friction, ...
I like this approach. I don’t think friction is the problem. I don’t like a totaly flat spot on the bottom, even though it is probably mostly a cosmetic thing. You would loose a lot of strenght., but it is a much cleaner approach.
The main boost of efficiency comes from the use of a much thinner foil. A 100 cm deep and 15 cm chord foil will have to be at least 2 cm thick to not bend and stall at speed. If you roll it into a half-circle i believe 3 mm thick steel or carbon should be stiff enough not to bend or vibrate.
A much thinner foil has much less drag, aand even more so at higher speed.
Cheers
Johannes
They’d have to be inline with the hull and super robust to be able to beach them?
Another problem I see, is that they will catch things like seaweed etc.
Then I had a crazy idea…...
A kick up version. Large hoops that could swing up past the back of the hull - ie not on the bottom but attached to the sides….. Crazy.
They’d have to be inline with the hull and super robust to be able to beach them?
Another problem I see, is that they will catch things like seaweed etc.
I believe the skeg should solve both those problems. There should be a very strong steel skeg going all the way under the tunnel-rudders. The proa should be able to stand on the skegs. They also hold the bolt that the tunnel-rudders pivot around. This way its easy to adjust angle of attack by moving the top of the rudders forward or backward, and of course turn the rudders.
I will show pictures soon. Today the polyurethan-foam is dry. I have cut slots in it with a sharp screwdriver.
Cheers
Johannes