Go Back   Starboard Forums > Free Forum

Reply
 
Thread Tools Display Modes
Old 8th December 2007, 07:07 PM   #41
C guy
Guest
 
Posts: n/a
Default

High aspect planing surfaces are DEFINITELY more efficient at planing. This has been well recognised for decades due to tank tests. This effect was even known to old dinghy designers like the great Uffa Fox, but in dinghies the losses in going for a high aspect ratio are greater than the gains.

Daniel Savitsky of the Stevens Institute is the biggest name in the field and his 1964 "Hydrodynamic Design of Planing Hulls" is apparently the basic paper. I'm no expert, but I've gone through this with a bunch of guys like naval architects, academics and Boeing aerodynamacists for a project.

The reason a high aspect planing surface works well is fairly simple. What is "planing"? It is when a hull is lifted above its static centre of gravity (flotation state) by the dynamic lifting forces created by its movement. What creates dynamic lift? The acceleration of water particles downwards. When a board pushes water down, an equal and opposite lifting force is created. Thanks Mr Newton.

A wide surface hits more water and pushes more water down than a longer surface of the same area. The front edge (at the "spray root", where spray splashes forward) is the point where most of the water is pushed down, and therefore where most of the lift is created.

Further back, the water is already diverted by the leading edge; it is no longer being pushed pretty much straight down by the board, therefore it creates less lifting force - but it still has drag. At the tail, the water is pretty much just flowing along the board, so it is no longer being accelerated down much. Since the water isn't being shoved down much, it's not creating much lift back up to lift the board on the plane.

A foil does NOT have its centre of lift at the geometric centre of the planing surface, it's much closer to the front, near the spray root - Savitsky's graphs showing this are available in most basic naval architecture books.

So adding length just adds surface friction drag, pretty much. What we want is width, because that means more water is shoved down along the leading edge.

Of course, since nothing comes for free a high aspect planing surface comes complete with added form drag (rising by the square, pretty much), wave impact drag, wetted surface drag at low speeds, etc. Powerboats can use high aspect but at their speeds wave impact is a massive problem I think. Sailing skiff and dinghy designers are sometimes aware of it, but they would lose to much in form drag, wetted surface, wave impact etc, and they can't really adjust their planing length like a board can.

Some designers at the trials that chose the RSX pointed out that across the whole range of conditions, a Lechner is still faster because it's got so little wetted surface (basic geometry of surface of solids), such a long waterline (basic wavelength/speed theory), such a skinny shape (low form drag) etc etc etc. Of course, it also has a fairly limited top speed.

The high aspect planing surface effect is definetely not marketing BS, this is a very well studied and very well known effect of basic physics.

Floyd, a D2/Lechner type well and truly planes when sailed properly! And some scientists do compare planing hulls to foils.
  Reply With Quote
Old 8th December 2007, 07:11 PM   #42
C Guy
Guest
 
Posts: n/a
Default

PS I let some very non-technical terms get through in that last post, it's late and the last guests just left so I'm finishing the red wine. Apologies.

By the way, a Lechner Olympic medallist and IMCO world champ reckons the round-bottomed board is faster thn the IMCO 'till 14/16 knots of wind. In a good planing breeze, surely even with a big sail neither will come close to matching a FW board in steady winds.
  Reply With Quote
Old 8th December 2007, 07:12 PM   #43
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Question

By that definition a square would have an Aspect Ratio of 1.Which is probably most efficient shape for a board ???? Planing Aspect ratio of 1 is perhaps best ???
Mathematically a circle has largest area for minimum perimeter; what would its AR be ???
Floyd is offline   Reply With Quote
Old 8th December 2007, 07:37 PM   #44
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

C Guy
Never said a Lechner didnt pane; it planes at a higher speed than flat board
of simiar width.(Lifting forces to aid planing have to be perpendicular to board.Round hull increases SA without increasing lift.(It increases grip on water to aid displacement upwind performance)But thats not what we were talking about.Didnt want to say Div 2 would beat FW in such winds but I do agree)

If the situation is as you described (ie lift concentrated towards front edge of planing surface) the planing centre of force would be towards the front; which I dont believe it is. According to what I have read planing centre of force of "any" planing surface is its geometric centre.And paper I read inferrred that lift is a function of difference in height between leading and trailng edge along the entire surface.Ie the wing anology again is not a good one because with a difference in height between leading and trailing edge the entire surface IS encountering previously undisturbed "water".Again suggesting CoE of planing surface would be its geometric centre.Is centre of Force towards front edge on planing surfaces ???
BTW
Uffa Fox`s greatest design was in Catamarans. Which dont plane ! (Except very modern ones. (Spitfire has square planing pads on its stern)
I`m not making out to be an expert but am questioning what we often see as "accepted" statements used to prove a certain board is faster than an other .
I`ve yet to find any evidence/graphs showing lift/drag ratios for different Aspect Ratio planing surfaces. From my investigation it still seems a square/flat surface gives the best lift/drag ratio.
PS
If you were stood on a square flat board being towed behind a boat whare would you stand ?? I would stand in the middle ?? If leading edge produces most lift presumably you would stand toward front ??? Wonder what would happen ???
Floyd is offline   Reply With Quote
Old 8th December 2007, 08:21 PM   #45
sailquik
Speed Demon
 
sailquik's Avatar
 
Join Date: Aug 2006
Posts: 37
Default

Quote:
Originally Posted by Unregistered View Post
But if wider is more efficient surely it would have been even wider;bearing in mind its carrying a ton and half ; and was still only 1.2 m wide.(Not much wider than a Formula ???) To my mind thats pretty narrow !!!
And if High Aspect Planing surfaces are more efficient how come speed boards are narrow (relatively)
PS
True meaning of Aspect ratio is Wing span Squared /divided by area.
And High Aspect wings have much higher parasitic drag. (From longer leading edge)

From a geometric point of view best planing area would be a square; ie max area for minimum leading edge ??? (I think anyway!)
(Planing pads on speed boats (back of V) are square !!!
Bet it was square on YPE ??? (1.2 m by 1.2 m ????)
Anybody know ???

I have actually experimented with 6.5m sails on a Formula in light winds (10-11 knots) and I can tell you that the Formula board planes much easier and downwind much deeper than any other narrower board with the same size sail and in the same wind. There is a feeling the the board skims the surface and the wake is very flat. It is like the board makes a very small 'hole' in the water. It is amazing how little pressure one feels from the sail and yet the board is still planing downwind at quite a deep angle. Of course, with a much bigger sail one also gains more speed and a deeper angle. In other words, the hull is very efficient at planing with very little power. That seems to bear out the advantage of the 'higher aspect' planing surface to me.

Waterskis are narrow because they go very, very fast (at least racing ones do - 100mph anyone?) and need to reduce area to stay in control and provide superior directional stability, that is they need to be narrow so they don't plane too high and fly too much and still go straight. They also have the luxury of almost unlimited power so low drag is less of a factor than the former things.

Speed boards are somewhere in between. They need to be relatively narrow to provide stability and control in chop (even the flattest speed courses are certainly not dead flat and it might surprise you how rough 15cm 'corrugated chop' feels at 45 knots) and not produce too much aero lift that makes them fly away, and they go almost twice the speed of Formula boards so they produce a lot of hydrodynamic lift. This means that at top speed (which is the prime design aim) they actually have quite a small, and short contact patch on the water. Most modern speed boards are markedly wider in the tail than '80's and '90's speed boards and actually have a higher aspect planing contact patch in comparison. They also get to 40 knots a lot easier and in less wind than their ancestors. Some of this can be down to more aerodynamic rigs and better fins, but some is most certainly down to the higher aspect planing surface.

YPE's hulls and MI's for that matter are not square, the flat surface is actually wider than it is long although the 'bow' section makes the overall hull close to square. There is a 'step' at the rear of the hulls so at top speeds the contact patch is very high aspect.

photos:





Last edited by sailquik; 9th December 2007 at 04:46 AM.
sailquik is offline   Reply With Quote
Old 8th December 2007, 08:45 PM   #46
Unregistered
Guest
 
Posts: n/a
Cool

Its probably higher aspect because its more stable.
Floyds argument re centre of planing makes sense but something just seems to say Higher Aspect is more efficient. Personally cant see any reason why .
With a wing/foil its easy to see (even feel) leading edge is providing most lift. (Harness lines dont go in geometric centre of sail)
A gliders wing has its centre of lift around a third of its way back from leading edge.
In these cases its easy to see why higher aspect will give more lift; simply because they have more of whats doing the lifting.
But with a planing surface ???
Where is centre of dynamic lift on a board ???
And I`ve just examined my Formula 160; its planing surface approximates to a square. (At lower speeds anyway) (Actually its still longer than wide I`d guess up to 12 knots thinking which part is in water whilst sailing)
So it sort of fits both arguments ??? !!!
Its square (ish) and becomes higher aspect with speed ???
At speeds Sailquick is talking of its probably the square bit giving early planing ????

Argument could be settled if someone could catogorically say where centre of lift is on a planing hull ? Doesnt feel to be towards front to me? Is it in geometric centre or nearer leading edge ???
  Reply With Quote
Old 9th December 2007, 02:34 AM   #47
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Cool

Hi Sailquick/ C Guy
Theory of optimum shapes in free-surface flows. Part 1. Optimum profile of sprayless planing surface

--------------------------------------------------------------------------------
T. Yao-Tsu Wu a1 and Arthur K. Whitney a1p1
a1 California Institute of Technology, Pasadena, California

Article author query
wu ty [Google Scholar]
whitney ak [Google Scholar]



Abstract

This paper attempts to determine the optimum profile of a two-dimensional plate that produces the maximum hydrodynamic lift while planing on a water surface, under the condition of no spray formation and no gravitational effect, the latter assumption serving as a good approximation for operations at large Froude numbers. The lift of the sprayless planing surface is maximized under the isoperimetric constraints of fixed chord length and fixed wetted arc-length of the plate. Consideration of the extremization yields, as the Euler equation, a pair of coupled nonlinear singular integral equations of the Cauchy type. These equations are subsequently linearized to facilitate further analysis. The analytical solution of the linearized problem has a branch-type singularity, in both pressure and flow angle, at the two ends of plate. In a special limit, this singularity changes its type, emerging into a logarithmic one, which is the weakest type possible. Guided by this analytic solution of the linearized problem, approximate solutions have been calculated for the nonlinear problem using the Rayleigh-Ritz method and the numerical results compared with the linearized theory.


(Published Online March 29 2006)

Yet us windsurfers know High Aspect Planing surfaces are OBVIOUSLY more efficient !!!

(This is a genuine quote from a paper; written quite a while after Uffa Fox designed boats)
Floyd is offline   Reply With Quote
Old 9th December 2007, 02:48 AM   #48
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Cool

And Another to just complicate it a bit more !

Abstract : The steady motion of a planing surface of moderate aspect ratio at small angles of attack is considered. Linearized theory is used with a square-root type of pressure singularity representing the flow near the leading edge. As asymptotic solution for the pressure distribution on the planing surface at large Froude number (or small beta, the inverse of the Froude number) is sought. The lowest order term of the pressure distribution, obtained by setting beta equal to zero, is found to be the same as the pressure distribution on the lower side of the corresponding thin wing. Higher order terms in beta are obtained by an iteration process. Explicit solutions are obtained to order beta squared for rectangular planforms. Numerical results are calculated for rectangular flat plate planing surfaces of aspect ratios from 0.5 to 2.0. It is found that for large aspect ratios the lift coefficient is reduced by the gravity effect and for small aspect ratios it is increased, the dividing aspect ratio being about 1.5. The results compare reasonably well with experimental data. (Author)

Note
"it is found for for large aspect ratios the lift coefficient is reduced"

I think it says the optimum AR is 1.5 !!!

But I`m not sure.

We cant simply say
"High Aspect is more efficient"
Floyd is offline   Reply With Quote
Old 9th December 2007, 04:40 AM   #49
sailquik
Speed Demon
 
sailquik's Avatar
 
Join Date: Aug 2006
Posts: 37
Unhappy

Sorry Floyd, I didn't follow any of that. It does seem to suggest a lot of 'qualifications' though. I would like to know what the 'experimental data' showed.

Are we assuming a 'large' aspect ratio is a 'high' aspect ratio?
sailquik is offline   Reply With Quote
Old 9th December 2007, 06:37 AM   #50
C Guy
Guest
 
Posts: n/a
Default

Floyd, the area of the highest dynamic lift is very well known in aerofoils and planing hulls. Illustration 2 of Savitsky's classic 1964 paper (I think you can find a pdf at Boat Design Forum somewhere) shows the "spike" near the stagnation point/spray root area. I have a pfd but I can't find the web address I got it from, sorry.

I have been through this with America's Cup designers, Boeing aerodynamacists, Jim Drake, naval architects, mathematics academics specialising in performance prediction programmes and tank tests, and small craft designers. Some of them have checked my project, where I refer to this. It's a very well known effect. Apart from Jim, none of them use the high aspect effect in their craft so it's not marketing that makes them speak of it.

If the centre of lift was at the centre of the planing surface, you wouldn't be able to hang ten on a surfboard - but you can.

"Uffa Fox`s greatest design was in Catamarans. Which dont plane ! (Except very modern ones. (Spitfire has square planing pads on its stern)"

Catamarans? I have just about every book Uffa ever wrote, and his biography, and I think the unspectacular Bell Cat was his only catamaran design.
  Reply With Quote
Reply

Bookmarks

Tags
None

Thread Tools
Display Modes

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +7. The time now is 01:51 AM.