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  #31  
Old 01-23-2017, 10:25 AM
valery gaulin valery gaulin is offline
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I think people are just affraid in unorthodoxe solution. Of curse there is always some draw back to any design option. But the idea of a blunt bow on catamaran might not be as bad as people think it could be. I need time to be able to try it!!!

@BJN : Where do you live? I am in the Montréal area we seam to be on the same thinking, of you dont live to far we could probably test a scale model?
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  #32  
Old 01-23-2017, 10:29 AM
upchurchmr upchurchmr is offline
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Quit talking about water and illustrating with air.

They are a little different.

Great idea about a model, so long as you compare to something traditional, with some way to measure the difference, at multiple different speeds.
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  #33  
Old 01-23-2017, 10:32 AM
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daiquiri daiquiri is offline
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Boats are not cars - whether it's about aero/hydrodynamics, powering, structure, materials or else.
They are completely different animals, having completely different operational, constructive, structural and maintenance requirements - and hence their shapes cannot be compared in such simplistic way.
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  #34  
Old 01-23-2017, 10:39 AM
bjn bjn is offline
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There are similarities though. The turbulence aft of the stern for example. When racing monohulls we try to distribute the crew to make the stern float just on the surface. Too low, we get a turbulent wake, similar to the illustration with the car. Too high the waterline length becomes shorter.

Maybe there are similar lessons to learn about the front part of the vessel as well?

valery gaulin
I'm on the wrong continent!
When I find the time I will see what a Michlet (free software) simulation will show.
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  #35  
Old 01-23-2017, 10:52 AM
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daiquiri daiquiri is offline
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Quote:
Originally Posted by bjn View Post
There are similarities though. The turbulence aft of the stern for example.
Of course there are similarities, but not to the point where you want to swap bow for stern in order to make the boat resemble an airfoil. It doesn't work that way.

For example:
Quote:
Originally Posted by bjn View Post
When racing monohulls we try to distribute the crew to make the stern float just on the surface. Too low, we get a turbulent wake, similar to the picture with the car. Too high the waterline length becomes shorter.
"The waterline length becomes shorter."
- that one. That's one important point of divergence between boats and cars. Waves, which do not exist in land-vehicle's aerodynamics, are of paramount importance for determining the shape of submerged parts of water-based vehicles.

IMO, the only really valid reasons for a blunt bow on a catamaran would be a gain in the amount of internal volume of the vessel (if the overall length is limited), and to get a smaller draught.
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  #36  
Old 01-23-2017, 03:34 PM
Emerson White Emerson White is offline
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A boat exists at the interface between two fluids. Modeling it in a volume of air will not give you the full picture. Also, there is a reason the front end of a fish that swims fast in open ocean tends to come to a point.
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  #37  
Old 01-23-2017, 07:10 PM
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DennisRB DennisRB is offline
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Quote:
Originally Posted by valery gaulin View Post
This video show exactly that I am not crazy! Round blunt bow can be a good option for a catamaran. The video shows that yes I does push water, that create lost of energy but I think that in a pointy bow we don't see the underwater flow seperation from a sharp poiny bow. What you don't see you don't know!!!

Anyway the conclusion is that the blunt bow is not that bad of an idea and should be tried amd tested on a scale model to see the kind of behavior it could give to a catamaran.

When I am done with my other project I will try to make a model to see the behavior.

On the other hand if someone as a finite element analysis software the simulate flow around a blunt round bow hull shape let me know I would like to try to run an analysis. Or maybe someone knows of a free software?
You have continued to ignore the fact a boat exists between the medium of high density water and low density air, and that wavemaking occurs because of this, and wavemaking is large proportion of hull resistance. You need to think about that for a while so you can understand your graphical representations of shapes moving in a single medium are not applicable.

Also its a leap of faith to say it will work as well on a cat as the racing mono. The waterplane is very lightly loaded on this carbon racing boat. Its more like a surfboard sitting on top of the water. You will struggle to get your cruising cat to lightly float with little hull below the waters surface, instead you will end up with deeply immersed waterbrake bows which act nothing like a foil immersed in a single medium as explained in my first paragraph.

So I'm afraid you reached your conclusion using confirmation bias, by ignoring all arguments against it, and finding arguments for it that may not even be applicable as proof its a good idea.

However, if you made a racing cat with this bow shape that was extremely light it might be fast in flat water by planing. There were some planing cats that were very fast in some conditions. But as a cruiser I'm not so sure.
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  #38  
Old 01-23-2017, 08:05 PM
Mr Efficiency Mr Efficiency is offline
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Where's Ad Hoc ? He insists bow shape in a catamaran hull is not that critical, resistance wise, in the scheme of things, but length/displacement and slimness ratio are.
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  #39  
Old 01-23-2017, 08:21 PM
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DennisRB DennisRB is offline
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Originally Posted by Mr Efficiency View Post
Where's Ad Hoc ? He insists bow shape in a catamaran hull is not that critical, resistance wise, in the scheme of things, but length/displacement and slimness ratio are.
I have followed those threads with great interest. I don't even think he said BT is a much of a factor. I seem to remember him arguing the opposite, that BT (beam to depth) means nothing compared to DLR. That is what this paper seems to show too. I agree the DLR is the overriding factor. I have the read this paper and understand that DLR is the king, and hull shapes should always be compared to hulls of the same DLR as this factor will override any other.

http://eprints.soton.ac.uk/46442/1/071.pdf

Therefore any arguments about bow shape should be made to hulls which have the same DLR. A hull with the same DLR that has waterbrakes attached is going to have a lot more resistance. Therefore its only fair to compare hulls actually designed to be real vessel hulls with low resistance in the SOR. Anyone could design a very unfair hull on purpose to have a poor resistance even at the same DLR.

Look below, here is a series of actual hull shapes used on real vessels that compares resistance. You will see all the real hull shapes have quite similar resistance. There is one hull shape which sticks out as much worse. The crude hull. I don't know exactly what this shape is but I believe it was a pointed rectangular block.

I'm afraid this scow cat hull will likely not be much better than the crude hull in this test if its bow is deeply immersed.

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  #40  
Old 01-23-2017, 08:33 PM
Mr Efficiency Mr Efficiency is offline
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We'd need to know exactly what the "crude" shape, was. If it was a rectangular block, the submerged stern drag would be significant.
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  #41  
Old 01-23-2017, 08:36 PM
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DennisRB DennisRB is offline
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Quote:
Originally Posted by Mr Efficiency View Post
We'd need to know exactly what the "crude" shape, was. If it was a rectangular block, the submerged stern drag would be significant.
The point is the DLR was constant, so shape does play a part if the hull shape is totally neglected for performance. But even then the difference is not gigantic.

Here is the post:

Two hulls, which has least drag?

Quote:
It is a hull made from a square block of wood, with just the bow, cut at an angle to be ‘pointy, just like a child would make with a saw, that’s it.
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  #42  
Old 01-23-2017, 08:40 PM
Mr Efficiency Mr Efficiency is offline
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It is a matter of degree, and what speeds are unacceptably disadvantaged, if the opening poster is prepared to trade off for what he sees as the advantage of a shorter boat. But the words "shorter" and "advantage" are uneasy companions in a displacement cat, performance wise !
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  #43  
Old 01-23-2017, 08:44 PM
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DennisRB DennisRB is offline
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As seen with the real hull shapes, some had lower resistance and low speeds and high resistance at high speeds, and others were the opposite. Only the crude hull was considerably worse at all speeds! What I get from this is that you need to keep away from crude hullshapes, and any regular hull shape is going to be decent compared to it.

There was no cucumber hull modelled here so we can only be left guessing
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  #44  
Old 01-23-2017, 08:50 PM
Mr Efficiency Mr Efficiency is offline
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A flat fronted hull isn't the same as his semi-circle, maybe he needs to make some models and test them against one another. What is certain is he will have a nice fountain feature with a plumb round bow.
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  #45  
Old 01-23-2017, 09:42 PM
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DennisRB DennisRB is offline
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If he has a semi circle section it will at least beat the "crude hull" in WSA. But the pointed block crude hull probably created a better entry!
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