Ljungström Arch Bow hull - Proa.
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.
I cant understand what you mean with “tilting symmetrical frames”. I cant picture it in my head. Could you draw a picture?
Regards.
Johannes.
These pictures is very relevant in this thread too, i think.
Its a very simple explanation to how and why the Arch bow hull is so fast and weatherly.
There is very few and small discontinuities in the flow around the hull as it moves through the water.
Any sudden change of the flow will create a shockwave and create resistans. A smaller radius will create larger preassure differences and larger wake for a given speed.
Johannes.
Any sudden change of the flow will create a shockwave and create resistans. A smaller radius will create larger preassure differences and larger wake for a given speed.
So how about a parabolic curve (or even a NACA section) from midships to bow with the tight end, the small radius, above the waterline? Parabolic curves are used to maintain attachment of flow in sails and wings. Is this the same thing as avoiding the discontinuities and pressure build ups that you are talking about, Johannes?
Regarding tilted bulkheads, I’ll get pen and paper (and divider!) out tomorrow and see if what is in my mind translates onto paper and get back to you 😊
So how about a parabolic curve (or even a NACA section) from midships to bow with the tight end, the small radius, above the waterline? Parabolic curves are used to maintain attachment of flow in sails and wings
There is probably many ways one can get good performance from a hull. I’m not out to prove anything with this. I just want to give my view on it all. As i have said before. I’m no naval engineer, or engineer of any kind. I’m just very enthusiastic about proas, and some simple ways to build semi-performance hulls. I can gladly throw away some speed to gain simplicity and speed of construction.
Regarding tilted bulkheads, I’ll get pen and paper (and divider!) out tomorrow and see if what is in my mind translates onto paper and get back to you 😊
I’m looking forward to that!
Regards.
Johannes.
I was thinking a catenary curve might be a smooth way to flatten the center while keeping turbulence down. this the curve a power line takes between the poles.
Tom
I was thinking a catenary curve might be a smooth way to flatten the center
I thought that would be a great idea, until i tested it. I bought a short piece of chain to compare with a simple radius-curve. See the picture below. Because of the weight of the chain pulling most on the part of the chain farthest out from the center of the chain, the ends of the catenary curve is most straight, and the deepest part is in the center. The catanary curve has the wrong distribution of rocker, being flatter out in the ends and deeper in the center.
As i have said before. I learn something new almost every day from this forum.
I have been playing with a 6 feet piece of chain all day, and finaly something dawned on me.
The catenary curve is very usefull when drawing the rocker in both the bottom and sides on a Matt Layden Paradox or similar chinerunner sharpie.
I will take some pictures and explain if anybody is interested.
Regards.
Johannes.
Johannes,
it might be interesting if you took a piece of chain say 5 m long and tensionednitnso the sag was proportional to the intended depth of you rocker. I think you’d see a much flatter midsection than your short piece is showing.
the reason I think this curve would be appropriate is to achieve a desirable Cp , room in the middle of the hull for accomodation, but still achieve a constant rate of curvature change without any “kinks”.
of course I’m no hydrostatics expert, so I might be completely off in left field this. Thank you for the link!
Tom
There is not much difference between a radius-curve and a catenary curve but i can see some flattening towards the 1/3 point on each side. Its a very crude example, but its all i can do at the moment. I dont have any longer chain at home at the moment.
Johannes.
Two links to some more information and the potential performance of Ljungströms Arch Bow Hulls. These are both quite extreme, but i think there is a lot of potential in the arch bow concept.
I would call these hulls “Variable radius arch bow hull”, as the radius is not constant in all the frames.
I have been sketching on a Proalized Arch bow hull. I hope i can post something soon. I have to test these idea too… I need a huge hangar with a complete set of heavy duty powertools, CNC-milling machine, vacuum pumps, prepreg carbon, a huge autoclave and large tanks of epoxy….. Why is everything that is fun, either expensive, forbidden, dangerous or all the above at the same time?
Cheers
Johannes
One hour of work with my router (?) gave me 20 frames to build an arch bow hull on.
I want to try Fredrik Ljungströms kind of hull. I’m tired of my ASP and barges. I want beautiful round smooth shapes. I think i will make it of styrofoam strips with one layer of fiberglass and epoxy.
Cheers,
Johannes
Today there has been some more progress on my Fredrik Ljungström inspired “Cirkelbågsskrov”-proa.
I cut some foam strips with a sharp knife. That was messy and extremly boring.
I have drawn the keel-radius and made some lines at 90 degrees to the keel-line for the frames.
I hope i can attach all the frame before I run out of steam today.
Cheers,
Johannes
I hope I made the right choice when scaling this design so that I can compare it to my ASP and barges.
I made the waterline length and width and the calculated weight the same as the red barge-proa. The arch bow hull is going to be 170 cm long and 15 cm wide (LOA and BOA) but only 125 x 12 cm on the waterline.
I hope the weight ends up somewhere around 4 kg.
Cheers,
Johannes
I have finally made some progress on this project…
In this speed I will probably test this hull somewhere around July 2021 or soo…..
I found an interesting picture from a study Fredrik Ljungström made around 1940.
Link to study (swedish)
The picture shows the different areas of pressure that will effect the motion of the hull when it heaves in waves.
The hull with big flat areas will react very slow and heavy because of all the water that will “be attached” to the hull. It will move like a much heavier hull, and the structural loads on the plating will be much higher.
Cheers,
Johannes