V Twin

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?

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

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.

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. 

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