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  #1  
Old 10-27-2012, 06:06 AM
Erwan Erwan is offline
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Added Resistance & Power Increase in Waves

Hi Everybody,

Thanks Mr Lazauskas for posting 26th ITTC report. I was very interested by Chapter 2.4 regarding the added resistancce in waves.

Unfortunatly, the application is more Ferry boat, Ro-Ro ship or big cargo.

as a result the Froude numbers investigated in such studies are quite low.

That is why I have a question for the specialists:

Is it possible to find some studies on this topic, but for higher Froude Numbers appropriate with 18 feet long beach cats speed (F18, A-Cat) ?

The goal is just to have a rought idea, in order to check was I have been told a few years ago, by a Phd student working on bassin model/ water tank

His opinion was a F18 or A-Cat will have a 50% increase in drag , if the waves are as high as the freeboard of the hull= at the limit of the wet-deck/ gunwhale?

Happy week-end to all

Best regards

EK
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  #2  
Old 10-27-2012, 06:18 AM
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Leo Lazauskas Leo Lazauskas is offline
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Quote:
Originally Posted by Erwan View Post
Hi Everybody,

Thanks Mr Lazauskas for posting 26th ITTC report. I was very interested by Chapter 2.4 regarding the added resistancce in waves.

Unfortunatly, the application is more Ferry boat, Ro-Ro ship or big cargo.

as a result the Froude numbers investigated in such studies are quite low.

That is why I have a question for the specialists:

Is it possible to find some studies on this topic, but for higher Froude Numbers appropriate with 18 feet long beach cats speed (F18, A-Cat) ?
It's not my area of expertise, but here is one very recent paper that looks at the effect of hull shape on the added resistance of catamarans.
https://www.navalengineers.org/Proce...lane_Daire.pdf

There are many more. Use google to search for:
"added resistance of catamarans".

Good luck!
Leo.
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  #3  
Old 10-27-2012, 07:44 AM
tunnels tunnels is offline
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size matters !!

To all those that sit in high places and do big things i would say the majority of people that visit and cruise these pages on Boat design are in the smaller boat section rather than ships !! although it is of interest only what happens to ships and most things apply to smaller boats but in a much smaller way .
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Old 10-27-2012, 09:39 AM
Erwan Erwan is offline
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Thank you very much Mr Lazauskas,

That is a great starting point, just have a quick look, and realize I need to study it seriously, the "non-dimensionalization" issue is not that clear in my mind yet.

It's great there are both wave-piercer & semi-swath hull shape.

Tunnels: There is just more money at stake with cargo fuel consumption, than with an half knots speed improvement for a beach cat.

But for instance, the famous serie 64 of DTMB use a 6 m lenght models with L/B consistent with our beach cat, so you can interpret the result candidly no scale-effect to address. At least it's what I do.

Thanks again and happy week-end

EK
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  #5  
Old 10-27-2012, 10:36 AM
tunnels tunnels is offline
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Quote:
Originally Posted by Erwan View Post
Thank you very much Mr Lazauskas,

That is a great starting point, just have a quick look, and realize I need to study it seriously, the "non-dimensionalization" issue is not that clear in my mind yet.

It's great there are both wave-piercer & semi-swath hull shape.

Tunnels: There is just more money at stake with cargo fuel consumption, than with an half knots speed improvement for a beach cat.

But for instance, the famous serie 64 of DTMB use a 6 m lenght models with L/B consistent with our beach cat, so you can interpret the result candidly no scale-effect to address. At least it's what I do.

Thanks again and happy week-end

EK
There is just more money at stake with cargo fuel consumption, than with an half knots speed improvement for a beach cat.
yes i completely agree with what you say . there should be more pushing in the design of boats , making them more efficent and easier to moving along and throught water as well as air !! . surely with all the people world wide some one some where has had a break through and away from the traditional shapes of most of the boats we all see every where !
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  #6  
Old 10-27-2012, 10:45 AM
BMcF BMcF is offline
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A usefull paper; thanks for posting that Leo.

The authors conclude correctly that model testing is the only reliable way to get it close to correct for a specific design (and that's the only way we've ever done it) but it can certainly be usefull to have at least a rough idea what the reduction of speed in seas might be for a design, well before any tank testing is carried out.

We developed, quite some years ago, some fairly crude empirically-derived "predictive tools" to estimate the speed reduction a seaway for SES' (air-cushion catamrans) and the results were accurate enough for "first cut" design works and vessel propulsion sizing. That methodology was far too coarse to have ever predicted any difference between one cat hull or another though.
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Old 10-27-2012, 10:49 AM
BMcF BMcF is offline
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[Bsome one some where has had a break through and away from the traditional shapes of most of the boats we all see every where !
Ta da!!

http://seawayblog.blogspot.com/2010/...king-boat.html
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  #8  
Old 10-27-2012, 12:29 PM
Erwan Erwan is offline
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Thanks BMcF, That is a breakthrough for sure!!!
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  #9  
Old 10-29-2012, 03:22 AM
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Leo Lazauskas Leo Lazauskas is offline
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A usefull paper; thanks for posting that Leo.
I'll see if I can find more from the authors. They have been collecting similar model data at the Aust. Maritime College Tank for quite a while now.

Quote:
Originally Posted by BMcF View Post
We developed, quite some years ago, some fairly crude empirically-derived "predictive tools" to estimate the speed reduction a seaway for SES' (air-cushion catamrans) and the results were accurate enough for "first cut" design works and vessel propulsion sizing. That methodology was far too coarse to have ever predicted any difference between one cat hull or another though.
I have thought about doing something similar, but I was turned off by the
complicated behaviour of the seals when they plough in.

Good luck with "Sandy" which I believe will keep you indoors for a few days!
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  #10  
Old 10-29-2012, 04:49 AM
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sottorf sottorf is offline
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Dear Erwan,
I have searched for such information myself and there are very few technical papers hat deal with this subject of added resistance for small catamarans. Alik, perhaps you know of something from Russian literature, if you are following this?

The main difference between the small craft and the papers available for fast ferries, is that the tunnel starts slamming much sooner due to its relative low height. Once the tunnel starts slamming the added resistance can be as much as 100% of the calm water resistance.

Tank testing is probably your best option for determining speed loss in waves if you are unable to take full scale measurements.

Attached is another paper on added resistance in waves - again for larger vessels.
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  #11  
Old 10-29-2012, 07:21 AM
Erwan Erwan is offline
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Thanks for all these references, I have some serious homework to do now.

My application is beach catamaran, but these boats are flying the windward hull most of the time, as a result, the issue is more similar to high L/B ratio monohulls, than catamaran configuration involving waves "interferences" between the 2 hulls wave systems.

I think, another big difference between this beach cat issue, and all studies available, is that for Ro-Ro, Ferries, or any transportation boats, the free board of the hulls are most of the time vertical or "wall side", in order to have loading capacities.

Beach cats instead, can have almost any kind of free-board shape, it can be convex ( =flared ??), flat or concave (=cusped?) like wave-piercer, or even have triangular cross-sections, with the triangle base at the water line or below and the tip of the triangle being the deck.

Thanks again and good week everybody

Best regards

EK
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  #12  
Old 12-14-2012, 06:42 PM
johnhazel johnhazel is offline
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I would like to be able to make a similar analysis as shown in the link below:

http://www.youtube.com/watch?v=_j-zTbKIPgc

Where do I start? It seems there should be some one out there that has this in their back pocket already.
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  #13  
Old 12-14-2012, 07:07 PM
DCockey DCockey is offline
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Information about OpenFOAM is here: http://www.openfoam.com/
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  #14  
Old 12-17-2012, 09:43 AM
johnhazel johnhazel is offline
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I've been studying many places found through google searches on openFOAM for the past month or so. I'm trying to find some hand hold ding such as is done in this German language tutorial:
http://www.youtube.com/watch?v=G4tK3auhUu0

What I am specifically wanting to find the steps for fre pitch and sinkage models as shown in the other youtube video I referenced.

BTW It seems that you need 1-2Gb memory for every 1Mb of grid cells in openFOAM.
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  #15  
Old 12-30-2012, 11:38 AM
johnhazel johnhazel is offline
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The main driving force for me to gain access to evaluations of openFOAM is to find optimum hulls by first getting a low drag hull in michlet. Then evaluate the hull using openFOAM with a free to pitch and trim model that wil show the wave profile along the hul. Then modify the shape of the hull until I get an immersed volume and pressure distribution more like the optimum found in michlet. maybe a crazy idea... maybe not.
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