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  #46  
Old 01-28-2017, 08:01 PM
valery gaulin valery gaulin is offline
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Yes that would probably be a couple of years at least. But maybe a model would come eventually.
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  #47  
Old 01-28-2017, 08:12 PM
Mr Efficiency Mr Efficiency is offline
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If blunt bows were always a resistance disaster, bulk carriers and oil tankers would not have such full bows, which are obviously a compromise where internal volume has been prioritized. Speed, of course, is not that great, a discussion point that seems to be lacking here. If you want speed, there will be a penalty.
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  #48  
Old 01-28-2017, 08:47 PM
valery gaulin valery gaulin is offline
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Thank you MrEfficiency,
This is exactly the point i am trying to understand myself. Speed is not the main priority, basically what i have in mind is to get the most volume out of a 30ft x 14 ft catamaran.

To get that extra volume I am ready to make a compromise on speed. Of curse not ready to compromise to the point were it would sail slower than a similar size 30ft sailboat.

In full round blunt bow i see these advantages: increase Volume amd displacement , shallow draft, no hobby horsing, might be able to surf in the right condition.

disadvantage : confort in short frequency waves, slower in light wind, unorthodoxe solution.
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  #49  
Old 01-28-2017, 09:25 PM
redreuben redreuben is offline
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Valery;
Explain no hobby horsing ? A full bow is going to pitch a lot more than a wave piecer surely.
In the 6.5 mini video the sea is pretty flat, whats going to happen in a swell ?
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  #50  
Old 01-28-2017, 09:50 PM
Emerson White Emerson White is offline
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"unorthodox solution" isn't a penalty. Surfing can be a disaster though and can be a high penalty, greatly amplifying the damage done and risk to life in a storm. Surfing can be good if you want speed around the buoy, but I wouldn't call it an advantage when viewed in the light of your other statements.
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  #51  
Old 01-29-2017, 12:54 AM
valery gaulin valery gaulin is offline
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Well, no hobby horsing! Because from what I understand when the hull midsection has to much volume compare to the stern and bow it tend to create a pivot point in the mid section most boyant part of the hull with the stern and bow going up and down around that center point. Therefore the fuller round blunt bow as I suggested as a high primatic coefficient, high volume in the stern and bow.

Also weight distribution can also exacerbate hobby horsing.

Now most design try to prevent hobby horsing by creating a more volumious stern than the bow.

On the earliest design of catamaran they also realize that adding some more volume in the bow it helps prevent pitch polling.

Basically the fuller the end of the hull are compare to the midsection it help prevent those two condition, hobby horse and pitch polling. But don't get me wrong I don't believe it is a necessity to go all the way to a full round blunt nose to achieve this goal.

The full round blunt nose that I am suggesting is mainly a solution to increase volume for a maximum size of a cruising catamaran 30ft X 14ft without too much penalty from this choice.

Basically I believe that the pros surpasses the cons.

What everybody said in the reply make sense and I understand their points but they seam to forget that everything is about compromises.

As I shown by video, mini transat, this choice cant be as bad as they make it sound. Now I am not trying to say that this option will win a race on a cruising catamaran even if it did for the Mini transat.
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  #52  
Old 01-29-2017, 03:36 AM
semelis semelis is offline
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Quote:
Originally Posted by Doug Lord View Post
---------------------------
I don't think Parlier's problem had anything to do with the bow-it wasn't blunt like the mini's.His hulls were stepped planing hulls and didn't have less drag than a "normal" cat hull until boat speed was over 20 knots-then drag dropped a lot.

Right, it had to do with the assumptioin that he would be planning more often than it could in reality.
Planning was the thing needed to reduce Parlier's cat drag and is the thing needed to get the blunt bow out of the water as so nicely works in the mini, and it's the thing that is not going to happen often enough, as Parlier tested in a real life 60 feet boat.
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  #53  
Old 01-30-2017, 07:00 AM
bjn bjn is offline
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Quote:
Originally Posted by Mr Efficiency View Post
If blunt bows were always a resistance disaster, bulk carriers and oil tankers would not have such full bows, which are obviously a compromise where internal volume has been prioritized. Speed, of course, is not that great, a discussion point that seems to be lacking here. If you want speed, there will be a penalty.
The results showed the opposite. High drag at low speed, low drag at high speed. So if he wants speed, it might be a good idea. But he can only reach that speed in rare conditions.
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  #54  
Old 02-03-2017, 01:56 AM
bjn bjn is offline
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Originally Posted by DennisRB View Post
I hope Leo finds this thread and comments.
I asked him in a PM about the hulls going forwards and backwards. He says the Michlet results were correct! Backwards and forwards gets the same resistance, but different wave pattern!

Here is the PM, and don't think he minds that I put it here in the thread:

Quote:
Originally Posted by Leo Lazauskas
Quote:
Originally Posted by bjn
Hello Leo,

I want to thank you for providing the software Michlet!

I have a question about the hull resistance calculation part of the software.

I made a comparison of a hull going forwards, and then going backwards. The resistance curves are the same(!)

See post:
Why does multihulls have pointy bows?

Is this correct, or could it be that the resistance calculations are not made to be accurate for this kind of input?

Best Regards
Björn
Yes, that is correct!

In fact, it is also true for multihulls.

There some experimental results from Japan that validate the result. Wave patterns made by an arrow-shaped trimaran are very different when going forwards or backwards, but the wave resistance (which integrates the wave pattern, sort of!) is identical in both cases.

All the best,
Leo.
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  #55  
Old 02-03-2017, 01:59 AM
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DennisRB DennisRB is offline
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If the wave pattern is different, it will effect the total difference in a catamaran form due to different interference. Have you tried it yet as a cat?
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  #56  
Old 02-03-2017, 02:01 AM
bjn bjn is offline
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If you read his pm, he says it's also true for multihulls, which I interpret as meaning that the wave interference drags will also end up being equal.

I haven't tried it yet as a cat. I will see if I can make it work today.
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  #57  
Old 02-03-2017, 02:18 AM
Mr Efficiency Mr Efficiency is offline
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Originally Posted by bjn View Post
The results showed the opposite. High drag at low speed, low drag at high speed. So if he wants speed, it might be a good idea. But he can only reach that speed in rare conditions.
I'm not sure what results you are alluding to, but I can't recall seeing any fast boats/ships with that bow shape, except possibly modern subs, and they are optimized for submerged running.
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  #58  
Old 02-03-2017, 03:14 AM
bjn bjn is offline
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I did a comparison in Michlet with this result. The blunt hull was as good as the best hull (symmetric proa) above 10 knots. But much worse at low speed.



And if the blunt flat bottomed hull would plane at high speed, it would get less resistance.

But this is all simulations and theories, so maybe I should not put so much weight on these results. But it seems plausible that the resistances at high speed doesn't differ a lot, since it aligns with the chart Dennis posted:
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  #59  
Old 02-03-2017, 03:37 AM
Mr Efficiency Mr Efficiency is offline
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With planing hulls, the difference between a very full entry and a more slender one can become very noticeable, as a distinct braking as the bow dips in following seas. This "bog" is mainly a result of shape, not an increase in wetted area, a good hull will slice through with little wash-off of forward speed.
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  #60  
Old 02-03-2017, 06:32 AM
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Kestrel Kestrel is offline
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A sharp bow (pointy and about vertical shaped) entrance angle is one of the factors which reduce resistance in displacement end pre-planing conditions, and wetlenght could be greater a bit too, so as hull displacement critical speed. In planing, sharp bows could be usefull if a wave piercing effect is desired.
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