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  #16  
Old 01-23-2017, 07:41 PM
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DennisRB DennisRB is offline
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Here is one you are really going to struggle to argue against. I didn't troll through dozens of pics either until I found one that agreed with me. That was the first one that came up after a search for catamaran design waterline shape.



Quote:
Underwater cross section shown in red. Waterplane in blue.
http://carbonix.com.au/a-class-catam...he-art-part-2/

Are you really going to ask which end is the front?
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  #17  
Old 01-23-2017, 07:51 PM
upchurchmr upchurchmr is offline
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Random doesn't count.

I can make up all kinds of strange waterlines - none of which match reality.

Still, which end was supposed to be front?
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  #18  
Old 01-23-2017, 07:57 PM
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Quote:
Originally Posted by upchurchmr View Post
Random doesn't count.

I can make up all kinds of strange waterlines - none of which match reality.

Still, which end was supposed to be front?
Only I didn't make it up. Someone else did from Carbonic Boats. Leaders in A cat design. There must be a conspiracy to fool idiots like me into thinking some cat waterplanes have a keen resemblance to backward foil shapes. I mean long pointy bows with more volume aft than forward is a completely strange idea that does not at all match reality.

I can't tell which end is the front either! Perhaps we consult a NA to clear it up for us?
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  #19  
Old 01-23-2017, 08:39 PM
upchurchmr upchurchmr is offline
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Dennis,

I try to take what you say as trustworthy. I didn't see post 16 before I sent 17. Why that happened I don' know.
I was talking about post 13 where you said - That is just a random waterplane I found.

Re-reading the posts from 13, I'm now not sure what's being said.

I went to the Carbonix website and still am not sure what they are showing.
One thing is sure - their current A class is nothing like an old Tornado, so I'll drop that.
It is interesting that with lifting foils, the overal shape of the hull has nothing to do with the waterline plane, since the waterline in their pictures probably starts around 30% of total length from the front.

From the Carbonix article, it is clear to me that their current shape cannot be divorced from the fact foils are being used.
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  #20  
Old 01-23-2017, 09:02 PM
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DennisRB DennisRB is offline
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Ok, I was irritated as it seemed you were trolling because my example was obvious. But it makes sense if you missed it, probably as it was the first post on page 2. No worries.

I don't see the world in back and white. There is a lot of grey in between. I mentioned that a cats waterplane can look a lot like a backward foil section and I think this is still true. Just because every waterplane does not look exactly like a NACA0010 does not mean there is no resemblance to foil shapes in many cases as I have shown. I think most would carry the area a bit further forward.

Here is a laminar flow 63 series which does that.



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  #21  
Old 01-24-2017, 03:30 AM
semelis semelis is offline
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As I pointed in anothe thread about pointy ends, it's not the same IN a fluid that AT the surface between 2 different fluids.
If you add the necessity for buoyance, the 2 problems are really so different that it surprises me than someone pretends to use the solutions of one to solve the other.
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  #22  
Old 01-24-2017, 08:50 AM
bjn bjn is offline
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Quote:
Originally Posted by semelis View Post
As I pointed in anothe thread about pointy ends, it's not the same IN a fluid that AT the surface between 2 different fluids.
If you add the necessity for buoyance, the 2 problems are really so different that it surprises me than someone pretends to use the solutions of one to solve the other.
It's fun to do unconventional thinking!
And maybe I can learn something about boats in the process!
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  #23  
Old 01-24-2017, 09:58 AM
bjn bjn is offline
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I ran Michlet simulations to compare two different 10m hulls. They have both 2 tonnes displacement, 1m beam, and about 0.5m draft (0.47m). The waterline profile (i.e. seen from above) is a NACA 0010 in one case, and a NACA 0010 going backwards in the second case. They have both semi circular cross sections. The center of the semi circles making up the hull are at the height of the waterline. (This means that the underwater profile (i.e. seen from beam on) is the lower half of the NACA profile.) All semi circles are slightly oval since the draft is not exactly half of the beam. I could have adjusted the displacement to get it perfect, but I thought 10m/2tonnes were nice even numbers.

Attached screenshots shows the result.

According to Michlet, the resistance/drag curves for the hull going forwards and the one going backwards are the same(!). But the wave patterns are very different.

Not sure if this means that Michlets resistance calculation algorithms are too simple, and cannot match reality? Or if these two hulls really would have the same resistance in reality?

In another thread, someone posted a link to a paper (http://eprints.soton.ac.uk/46442/1/071.pdf) and a picture (http://www.boatdesign.net/forums/att...resistance.jpg) which showed that hull shape is not critical on a slender hull, but length/displacement is (at least something along the lines of that).
That paper is based on real tank testing, and not Michlet simulations. I lean towards believing the results of these Michlet simulations, since they are in line with the results of that paper/real tank testing. My experience from testing (proa) hull shapes in Michlet in the past align with this. Most shapes gives very similar resistance results if the length and displacement is the same. For the most part, the resistance only changes from a change in wetted area. But in this case, both have the same wetted area.

I'm still sceptic though, since the wave patterns are so different. But I don't know much about these things. I am mostly just able to play with simulation software.

What are your thoughts/comments?
Attached Thumbnails
Why does multihulls have pointy bows?-resistance-10m-1m-2ton-naca-0010.png  Why does multihulls have pointy bows?-resistance-10m-1m-2ton-naca-0010-backwards.png  Why does multihulls have pointy bows?-section-area-10m-1m-2ton-naca-0010.png  

Why does multihulls have pointy bows?-section-area-10m-1m-2ton-naca-0010-backwards.png  Why does multihulls have pointy bows?-waterline-10m-1m-2ton-naca-0010.png  Why does multihulls have pointy bows?-waterline-10m-1m-2ton-naca-0010-backwards.png  

Why does multihulls have pointy bows?-wave-amplitudes-9knots-10m-1m-2ton-naca-0010.png  Why does multihulls have pointy bows?-wave-amplitudes-9knots-10m-1m-2ton-naca-0010-backwards.png  Why does multihulls have pointy bows?-wave-resistance-10m-1m-2ton-naca-0010.png  

Why does multihulls have pointy bows?-wave-resistance-10m-1m-2ton-naca-0010-backwards.png  
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  #24  
Old 01-24-2017, 11:58 PM
semelis semelis is offline
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Quote:
Originally Posted by bjn View Post
It's fun to do unconventional thinking!
And maybe I can learn something about boats in the process!
Or why stones bounce out of water
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  #25  
Old 01-25-2017, 02:13 AM
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DennisRB DennisRB is offline
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It was me that linked those papers. Interesting hey? DLR is the main factor for resistance and hull shape is secondary, well at higher speeds. At lower speeds WSA matters.

The resistance curves of all those real hull shapes were close, but the crude hull stood out as worse. I wonder if Michlet would have also shown the crude to have been as bad as it was on the real test? I'm not sure how Michlet handles shapes that create a lot of turbulence?

I hope Leo finds this thread and comments.

One thing is for sure, Michlet wont tell you anything about the sea keeping of a given hull. It won't tell you what happens when a wave smashes against a blunt bow.

Also, I showed that the waterplane of some cats looks very much like a backward foil shape, but the underwater sections at the stern would have a shallow release angle unlike the semi circular projection of the blunt end of a NACA section. I imagine this to create as much transom drag as it would create bow wave drag when sailed the other direction, hence a similar result.

Also, you should try it in catamaran format. I imagine the wave interference would be different between the 2 shapes. Also can you plot the resistance curves on the same graph so its possible to see the difference?
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  #26  
Old 01-25-2017, 07:14 AM
bjn bjn is offline
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Yes! interesting paper, thanks! Haven't had time to read it in detail though. Was the picture from a second paper?

Today I used Michlet to simulate two other shapes. A symmetric proa hull with semi circular (oval) cross sections, 1m beam. And I also tried a symmetric hull with blunt round bows, similar to what Valery showed in his thread. These results were not identical with each other, so now we have something to compare yesterdays results with! =)

I put the results in graphs of Rw, Rv and Rt. See attached pictures.

Looks like the NACA10s and the Blunt one has a lot more wave resistance than the symmetric proa...

If there is an "ideal" shape (better than this symmetric proa shape) in terms of wave and total resistance, I could add that one as well. Suggestions are welcome.

I will look into simulating in catamaran format also.

EDIT: I made a mistake on the labeling in graph Rt. The red curve is the NACA10 forwards, not backwards as it says.
Attached Thumbnails
Why does multihulls have pointy bows?-compare-rw.png  Why does multihulls have pointy bows?-compare-rv.png  Why does multihulls have pointy bows?-compare-rt.png  

Why does multihulls have pointy bows?-waterline-10m-1m-2ton-sym-proa.png  Why does multihulls have pointy bows?-waterline-10m-1m-2ton-blunt.png  
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  #27  
Old 01-25-2017, 06:55 PM
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DennisRB DennisRB is offline
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Assuming hullspeed is about 4ms going by the curve, and assuming your heavy cruising cat will probably not exceed this limit often as heavy cats often struggle to do. It seems the blunt hull is by far the worst in this range. It gets better at high speeds because its cp is higher (smaller max beam etc). But the proa hull is miles ahead in the boats realistic operating range and only slightly behind at speeds that are probably not going to be reached often or at all.

Just remember when comparing hull shapes the displacement must stay the same as DLR is the overriding factor. Your whole point to this is to get more space in the boat. But by moving volume to the ends you need to remove volume from the middle to keep displacement the same.

As pro members such as Adhoc explain ad nauseam, isolating one variable is good as an exercise to learn trends in naval architecture, but unless you are applying it to the SOR of your vessel its totally pointless.

Have you actually experimented to see if having less volume in the middle of your hull and more towards the end is actually better for real world accommodation layout? Perhaps it means you can sleep people in narrow bunks at the bows? Or perhaps the reduced volume in the middle of the hull will make that area much less useful leading to an overall detraction in usable space?

Doing the sims in cat format will likely change the results a fair bit. This program is designed precisely to model the wave interactions that would occur.
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  #28  
Old 01-26-2017, 06:43 AM
valery gaulin valery gaulin is offline
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First, I have to see thank you to compare those hull shape in Michlet, Very interesting result.

Quote:
Looks like the NACA10s and the Blunt one has a lot more wave resistance than the symmetric proa...
I am not sure if I understand why you say this because from your Rw graph it seams that the blunt nose has a lower coefficient. Is it not better to have a lower coefficient?

Also I can't find the definition for Rv in the Michlet manual. Can you please tell me what it is.

If we look at Rt( total resistance) there seams to not have a big difference from one shape to another.

Does Michlet take in consideration the wetted surface area. A fuller hull will have less wetted surface area than a skinny pointed bow type of hull for the same displacement.

I am actually suprise to not see more of a penalty of a round blunt bow. I was expecting a greater penalty from this shape, even if it was my proposition for a cruising catamaran.

Maybe I am misinterpreting the graphique that you attach from Michlet???
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  #29  
Old 01-26-2017, 06:56 AM
valery gaulin valery gaulin is offline
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Thank you Dennis RB I understand now! Cruising speed of 4 ms is probably the maximum speed for cruising. I though the speed was in knots!

So if I understand as the speed gets above 4 ms the blunt nose as a lower curve( smaller coefficient) than the other shape. I know that it is unlikely that a cruising blunt bow catamaran gets up to 6 ms, but is it what we can conclude from the graph?

Does Michlet can tell you at when it is in planning mode instead of displacement mode? Is it the reason why the blunt bow hull (fuller hull) seams to be better at higher speed?
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  #30  
Old 01-26-2017, 07:09 AM
bjn bjn is offline
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I'm not sure what the v in Rv stands for. But Rv is the same as Rf in these simulations. The skin friction. Your design has about 12sqm wetted area while the others have around 11sqm, thats why yours is a little higher.

But as DennisRB has pointed out, at low speed, Rw, and Rt, total resistance, is a lot higher.

The scale on the vertical axis is in m/s. Multiply by about 2 to get it in knots. So the plot is from 2 to 30 knots. I think I will simulate at a lower speed if I make more simulations.

I made a new graph with Rt as a percentage of the best hull. Really shows how much more resistance the hull has at low speed.

I don't think the hull is planning in the simulation, because Michlet doesn't model planning as I understand it. And we can confirm that by looking at the Rv curve. If the hull was planning, it would have less area in the water, so would transition to a lower Rv.

But maybe the stern wave is "helping" it or something like that. Just a guess.
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Why does multihulls have pointy bows?-compare-rt-percent.png  
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