Ljungström Arch Bow hull - Proa.
This is another one of my ideas.
A couple of years ago i got really interested in Fredrik Ljungströms ( http://en.wikipedia.org/wiki/Fredrik_Ljungström ) invention, the Arch bow hull (i think one would call it), in swedish its called “cirkelbågsskrov”.
Its a cleverly designed hull, drawn with only a ruler and a divider.
Seen in profile the hull is made up with parts of a circle. It has a totally round keel-line, as every “line” is from a part of a arch. Every “line is drawn with a divider. People i have talked to (who sail this kind of boat), says that this kind of hull sails very good. They are very fast and moves very smoothly and calmly in waves. One did 11,7 knots with a waterline-lenght of only 5 meters.
This is actually where i got Phil Bolgers Advanced Sharpie from. An Advanced Sharpie is the logical result of trying to build an arc bow hull with plywood.
One does not have to use the resulting very large overhangs, as seen in the picture of the arch bow hull. I would use some type of scow bows instead. I will draw a vaka along this lines and post it here soon.
Just want to get this thread started.
Johannes.
Some comments about the pictures below.
First i need some essentials. A large cup of tea and a laptop.
Then i draw a box as large as the largest points of the hull. This is an example only, so i drew a box 10x10 cm.
Then i draw a arch from the mid-point (where the keel-line will be).
Then i draw a new radius to get the right position to be able to draw the hull side.
Its a simple use of a divider. One draws one frame, and use the same one all the way through the hull. The only differ in height, as the follow a much larger arch that is the keel-line. Its very simple to build one of these hulls. I would strip-build it. There is only one width of strips. The frames are made with a router, much the same way i draw them with a divider.
I draw the keel-line the same way as the hull-sides.
Finally i have a almost complete drawing of the hull. One needs a router and a staplegun. A couple of sheets of plywood or particleboard, and some thin wooden strips. Some epoxy and glass ofcourse….
Johannes.
I started building a small model of it a while ago, but i never completed it…
My girlfriend did not like all the dust in the kitchen. I cant understand why she got angry! 😉
Johannes.
Stupid as i was, i used masonite for the last part of the hull. I never got the epoxy to bind the thin strips of masonite together, so i thrashed this model, before it even got off the frames.
I have been thinking of building a new model, but i cant really get the time to do it. I like the simple and very strong hull this way of designing and building gives.
Any questions, comments and technical discussion is most welcome!
With best regards
Johannes.
My girlfriend did not like all the dust in the kitchen. I cant understand why she got angry!
I don’t know, either, Johannes. But to paraphrase a famous US president, Some people look into a kitchen and say “table”. But I look into a kitchen and say, “workbench”
The Arc Bow Hull seems like a cross between Gothic architecture (that’s exactly how you draw a Gothic Arch) and the yet to be invented Constant Camber design of Jim Brown.
My immediate question is, does it hobby horse? and if not, why not?
My immediate question is, does it hobby horse? and if not, why not?
That was one of my first questions too…
The sailors i have spooken too about this says it doesnt hobbyhorse noticeably, but rides very smooth and nice over oncoming waves. I have never sailed a Ljungström-boat, so i cant say anything based on my own experience.
I think the very low angle the bow has where it meets the water will damp hobbyhorsing and pitching, but im a bit worried about that potential problem. Im also worried about excessive piching will throw a lot of water forward and steal a lot of potential speed when sailing. But reading about them, they seem to very fast and weatherly sailboats, that hardly throws any spray.
Regards
Johannes.
I’m attracted to this kind of hulls, because of their strength and simplicity. The arch is a very rational and efficient geometric figure. The least amount of surface for a given amount of displacement. Highest stiffness of any geometrical form. Waves are some form av circular motion, and somehow the circular shape seems very well adapted to that kind of environment. I know that sounds a bit strange and fuzzy, but there is something in all this that appeals to me. The arch hull deflects water very evenly along its underwater-body.There is no sudden change of angle or shape. Its very smooth and gradual, and i think thats a big part of why his boats where very fast for their day. A more boxy high-PC hull will have more transitions from over-pressure to under-pressure along its length. A faster build-up of pressure in the front were the hull pushes the water apart, followed by a lowered pressure as the hulls expansion of width decreases. The same process repeats itself in the aft part. Compared to this the arch hull only has three different zones of differing pressure. It was the process of “translating” the arch hull to sheet-material that gave me the idea of Phil Bolgers Advanced Sharpie. The advanced Sharpie is an arch hull seen from the side or from underneath. Its the same rational shape, and rational use of plywood.
I’m afraid of any tendencies to hobbyhorse, so i will probably have to build a model and try it out. I think its important to keep the keel-radius as large as possible, and that where the blunt bows fits in. The arch hull does not have the hard chines to rely on to damp out unwanted motions like pitching.
Johannes.
My immediate question is, does it hobby horse? and if not, why not?
I have been thinking about this one since you asked in your last post.
Hobbyhorsing is essentially a resonance, where the hull has too little damping and the waves excite the resonant frequency. A too high Q factor.
There is many ways to lower a Q-factor of any resonant system.
One way is to lower (or increase) the resonant frequency to a point where it doesn’t get excited, ( little rocker is an example of that) or the energy exciting the frequency is not big enough to sustain a resonance. Another way is to make the shape damping by adding resistance like i have done with the sharp chines in my AS-proa. The chines will create powerful eddies and transfer a lot of energy from the hulls motion to the water. If i would round my chines to a radius of a couple of inches it would pitch and hobbyhorse much more. The chines are really important in an advanced sharpie.
I think that making the keel-radius as large as possible will lower the resonant-frequency and the very low angle the hull meets the water will push a lot of water around - when pitching, effectively creating a lot of resistance to pitching and hobbyhorsing. It will transfer a lot of energy to the water and thereby greatly damp any unwanted motion.
This needs to be carefully studied and i thank you for pointing this out. That made me think, i and hope i have solved the problem. Forums is a wonderful thing. A simple question will create a lot of interesting thoughts and discussion.
All this is probably every day stuff to a lot of people, but since im no engineer, i have to study everything, and try to learn as i go.
With best regards.
Johannes.
Johannes,
Thinking more about the lack of hobby horsing, I think it might be because the angle of the flare of the topsides increases towards the bow instead of decreasing which is more usual. Because the bulkheads are the same profile and because the angle to the vertical increases towards the keel line and the fact that the bulkheads are raised progressively in respect to the waterline, then the angle of the topside at the waterline actually increases towards the bow. So although this may send spray everywhere and thus dissipating forward energy, it will have a profound damping effect and probably keep the sails filled and driving.
That’s how I’m visualising it anyway. I could, of course, be entirely wrong. It has happened 😊
Thinking more about the lack of hobby horsing, I think it might be because the angle of the flare of the topsides increases towards the bow instead of decreasing which is more usual.——— So although this may send spray everywhere and thus dissipating forward energy, it will have a profound damping effect and probably keep the sails filled and driving.
That’s how I’m visualising it anyway. I could, of course, be entirely wrong. It has happened 😊
That seems like a really good explanation!
I have been studying David Raisons Team Evolution and its scow bow. It throws a lot of spray in a video on his site, but as you mention above - “keep the sails filled and driving” is probably more important than some energy-losses because of the spray. I know that the Arch Bow hulls can be very fast and nice behaving. I’m very tempted to build a model and try it. I think it would sail really good. To much interesting stuff, and way to little time.
Johannes.
With the arch bow hull its very simple to create asymmetrical cross-sections.
Just use different radius for the two sides, and place the keel-line wherever you think its best.
I drew a slightly asymmetrical cross-section, just to show what i mean.
I see that i am very tired. I mix the scale on the last drawing. 40, 50 and 90 cm should read 4,0 5,0 and 9,0 cm.
I made another one. Much more pronounced asymmetry .
Johannes.
Would tilting symmetrical frames off centre effectively give you an asymmetric proa hull? It would be a simple way to make a more traditional proa hull, for sure. Though it would bear weight much better towards the bows which would not necessarily be a good thing as far as steering through weight transfer is concerned.
In building construction and in engineering, the use of centimetres is avoided. Only metres and millimetres are used to help avoid mix ups.