New Posts New Posts RSS Feed: V Twin
  FAQ FAQ  Forum Search   Register Register  Login Login

V Twin

 Post Reply Post Reply Page  <1 1011121314 142>
Author
Menace View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 16 Oct 10
Online Status: Offline
Posts: 296
Post Options Post Options   Quote Menace Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 5:52pm
Originally posted by Phil eltringham

I have a number of questions, mostly to try and understand what you expect the waveform it will generate to be like as I can see it having a number of strange effects on the performance of the boat.  please indulge me...
 
Have you got as far as trying to model (numerically if not in CFD) the eddy, spray, added mass, and wave drag that will appear as the tunnel opens up under the hull? 
 
Phil, I've done this for another fast cat project and the indication was the tunnel effect is a myth. As you decrease the beam width, you increase your wave pattern drag due to wave pattern interference. We trialled numerous beam widths and the general consensous was that wider meant less resistance. In the cases with the narrower cats, you would have been better with a monohull.
 
Another few things that I've thought about is that volumous bow is just going to dig in when the rig accelerates, similar to the old cruising cats of the 60s to 80s, where everyone thought more volume was key to keeping the bow up. All that really happens is that the rig accelerates even more quicker than the hull due to the added hull resistance and the bow will go down more if you have a powerful enough rig to be doing it in the first place.
 
That gap between the base of the boat and the water surface where the centreboard enters the water is going to play havoc with induced drag too.
 
I hope I'm wrong....
Back to Top
G.R.F. View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 10 Aug 08
Location: United Kingdom/Hythe
Online Status: Offline
Posts: 4027
Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 7:40pm
The thing you have to consider, most cat hulls are wave piercing displacement, this design is more akin to twin racing long windsurf boards joined, given more volume to support a larger rig.

What would be interesting is any data on Cats (if any) that were built with twin planing hulls. 

It was one of the main reasons I went for flat rather than deep V.

Each hull surface I deliberately set up at .67 mtrs wide, for the very reason that that width will generally support 6.5-7.0 sq mtr rig, double it should give me 13-14 sq mtrs.

Now there have been examples of planing Windsurf style Cats, but they were inevitably under canvassed, didn't perform and they were not really designed with racing in mind, more a simple beach/leisure toy.

The bit about where the centreboard enters the water is the bit that worries me, originally I had intended that the leading edge of the CB would lodge into the front of the cutout, but the way the boys programme has defined what has to be where in order to function, has defined that the plate be further back. 

They're convinced it's not an issue. Legend in my own mind I may be, but at the end of the day these guys at Synthesize are professional boat designers with a lot of experience, so I have to be guided by them.


Edited by G.R.F. - 27 Jan 11 at 7:43pm
Back to Top
Rupert View Drop Down
Really should get out more
Really should get out more


Joined: 11 Aug 04
Location: Whitefriars sc
Online Status: Offline
Posts: 8795
Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 7:48pm
Reg White designed a planing cat if I remember rightly. I'm sure someone will have a pic or 2.
Firefly 2324, Lightning 130, Puffin 229, Minisail 3446
Back to Top
AlexM View Drop Down
Really should get out more
Really should get out more


Joined: 10 Jan 06
Online Status: Offline
Posts: 857
Post Options Post Options   Quote AlexM Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 8:00pm
Back to Top
laser193713 View Drop Down
Really should get out more
Really should get out more


Joined: 13 May 09
Location: United Kingdom
Online Status: Offline
Posts: 884
Post Options Post Options   Quote laser193713 Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 8:11pm
Something we are made aware of very early on is whats called the mirror effect when talking about foils meeting a hull.  Basically saying that if the foil is butted up against the hull where the water meets it this doubles the effective lift of the foil, reducing the angle of attack needed (leeway) and thus drag. I'm sure the guys know what they are doing but have you consided twin boards and perhaps rudders? This would give much better performance from the foils in my eyes.

I had assumed the programme was rhino actually, at uni we work in maxsurf which can be exported to rhino easily.  The uni doesnt own a copy of rhino but i have worked with it before and it is definitely more accurate when it comes to trimming and mating surfaces.
Back to Top
Peaky View Drop Down
Really should get out more
Really should get out more


Joined: 15 Nov 08
Location: United Kingdom
Online Status: Online
Posts: 2776
Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 8:11pm
My word, that's it! 
Bootiful was a planing cat, as is this:
 
 
Back to Top
Phil eltringham View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 16 Mar 04
Location: England/Hitchin
Online Status: Offline
Posts: 1105
Post Options Post Options   Quote Phil eltringham Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 10:00pm
I ran some numbers on a planing cat while i was at uni (I liked the idea on paper, If i find the files i'll send them), I even did an iteration where it should plane while flying a hull (asymetric hulls), the net result was that you ended up either going back to something more like modern a-class or F18s or you go back to a mono that looks like Vlad Murnikov's SpeedDream.  It was interesting chattting to the C-Class guys at the boat show this year, their current hulls were designed to plane, but they have found it doesn't work and the next ones will aim to minimise wetted surface area instead. 
FLAT IS FAST!
Shifts Happen
Back to Top
I luv Wight View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 28 Jan 05
Location: United Kingdom
Online Status: Offline
Posts: 628
Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 11 at 10:41pm
Originally posted by Rupert

Reg White designed a planing cat if I remember rightly. I'm sure someone will have a pic or 2.

Hurricane 500 had flat bottom hulls, but the platform was very twisty, it had feeble skegs instead of daggerboards.
This looks fun:


http://www.bloodaxeboats.co.uk
Andy P
foiling Int Moth GBR3467
Freedom 21 Codling
Back to Top
Menace View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 16 Oct 10
Online Status: Offline
Posts: 296
Post Options Post Options   Quote Menace Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 11 at 8:03am

Originally posted by laser193713

Something we are made aware of very early on is whats called the mirror effect when talking about foils meeting a hull.  Basically saying that if the foil is butted up against the hull where the water meets it this doubles the effective lift of the foil, reducing the angle of attack needed (leeway) and thus drag. I'm sure the guys know what they are doing but have you consided twin boards and perhaps rudders? This would give much better performance from the foils in my eyes.

I had assumed the programme was rhino actually, at uni we work in maxsurf which can be exported to rhino easily.  The uni doesnt own a copy of rhino but i have worked with it before and it is definitely more accurate when it comes to trimming and mating surfaces.

 

If the Uni has got Solidworks, try to use it instead. I found Solidworks a lot easier to pick up and from my experience, the general trend nowadays is towards higher end modeling packages such as UG, Solidworks, CATIA, etc. A Soildworks rendering can make a rhino one look pretty basic in standard.

 

The more I think about the V Twin thing, and the more cycles I go round in my head, the more I think a cat would be a better solution, just because the going wide thing will benefit from 4 perspectives.

 

1) Originally stated, reduces wave pattern drag by decreasing wave pattern interference.

 

2) The wider you go, the more righting moment you get.

 

3) Increase width means you can lift a hull at a lower heel angle respectively. The heel angle thing for me is a bit of an over-riding concern as due to heel your sail plan loses a lot more efficiency than most imagine. It has similar effects to pinching way too high; your angle of attack on the sail seriously decreases along with your lift coefficient. I've seen people optimise a mono-hull so they can sail at an optimum angle of heel dropping a few percent in bare hull resistance, only to find that the heel angle drops firstly the efficiency of their foils, then more importantly, when the start looking at the drive force, the sail efficiency has dropped way off and overall, they are taking a big hit and the decreases in performance are seriously outweighing any gains made by designing the boat to operate at an optimum heel angle.

 

4) Putting beams across the boat would seriously drop your weight compared to forming a solid moulded mid surface.

 

I think what Phil was indicating about the planing cat hulls actually not performing as well as modern mainstream cat hulls has a lot of validity, most cat hulls are very efficient and trying to get them to plane will just increase resistance. The C-Cats toyed around with T-Foils, think from what I can remember, they have left them off the boat for a similar reason, lifting the hull out of the water via the foils was less efficient than the hulls acting in displacement mode.

 

There is a relatively quick and easy way to test some of your theories out Graeme without throwing a few grand at the project. Without wanting to sound too Doug Lordish, get the Synthesize boys to knock out a scale radio controlled model. Not everything will be scalable, but you may learn a lot from just general behavior as Maxsurf and design spreadsheets only tell you so much. If you get it wrong at model scale, it's not the end of the world, just adjust the model appropriately. If you leave these changes until the boat is in production, it will cost you a hell of a lot more.



Edited by Menace - 28 Jan 11 at 8:05am
Back to Top
aardvark_issues View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 24 Jul 05
Location: Weston-Super-Mare
Online Status: Offline
Posts: 504
Post Options Post Options   Quote aardvark_issues Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jan 11 at 8:28am
Planing cats can work... Set some records if I remember rightly.


Back to Top
 Post Reply Post Reply Page  <1 1011121314 142>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy