Dennis
You should take the time to look ALL the way through Todd's clip. It gets more interesting in the shunts.
You should also take a look at rudders designed for higher speed applications. Above 15kts or so cavitation becomes an increasing factor and eventually dominates performance for water foils. You may find this interesting:
You will see that the entries get sharper and the tails get chopped. The tight radius twin nose foil I proposed will performa better than a symmetrical 4-series NACA for speeds where cavitation is a consideration.
Reducing appendage drag by 30% through one single change like use of an asymmetric section will be even more beneficial when it is accounted for in the next loop of the design spiral. 30% drag reduction on the appendages is equivalent to maybe 10% reduction in hull drag or 15% reduction in weight - again these benefits are only the first time through the spiral. The benefit will amplify second time through.
The proa as sailing configuration has not undergone a lot of development. There is a whole lot of unexplored potential. The asymmetric rudders seem to be one such area but good to see some have actually tested them.
The twin rudders would be more aptly termed articulating keels. They are larger than conventional rudders need to be and will have a narrower angle of operation because they have more power. You would design them to operate around the point of maximum L/D, which is closer to a keel or centreboard criteria than conventional rudders. With conventional rudders you have an interest in the maximum lift but for articulating keels it is a different requirement. Under way they will have ample turning power with only small changes in AoA so they do not require a wide working range and the maximum lift is really well above any steering force you would need.
Suggesting that you would only use symmetrical foils on a proa is akin to saying that airplanes should only use symmetrical section wings. Those that are meant to fly well upside down like stunt planes often use symmetrical sections but most use asymmetric wings because there are huge advantages in reducing drag by doing so. Like proas most planes are designed to lift in only one direction. (I wonder if the early aircraft designers had a discussion on merits of asymmetric and symmetric wings. Maybe they looked at what birds used. Like a proa, birds are intended to lift only upwards. )
Rick
On 03/08/2010, at 11:01 AM, Dennis Cox wrote:
Yes, I took the time... the first minute or two. Maybe, I didn't watch enough. It certainly shows packaging and mechanical efficiency. However, I don't recall if tsstproa makes any claims nor testing (yet) that shows it was faster or prevented leeway more efficiently. What happens at higher speeds when vortex shedding and flutter gets involved on a larger and/or faster scale?The rudders on my POS are accidentally assymetric (bad molding) and have rounded trailing edges... and they stall and miss behave. A friend with a similar boat who contoured his to a NACA symmetric foil behaves beautifully... it stalls predictably in both directions and there is no shimmy in the wheel. The before and after were night and day. Back to tsstproa... does it stay stable with a hinge at 50% of the chord? At slow speeds, looks great...If higher speeds were run... I might try playing arm chair quarterback and suggest that he allow it to pivot rearward (whichever rearward applies) to allow the center of effort to "guarantee" being behind the pivot axis. It seems like several degrees is all that it would need.However, for a boat designed to be stretching for top end speed, my I'd think that its going to have more drag than a standard type foil (symmetric or not). So, I guess its a decision of what one wants to optimize... mechanical simplicity (pivot +/- 20) or ultimate speed (with more complexity by pivoting through 270+.On a cruiser, it might be the way to go... on a racer... doubtful. On a racer/cruiser. Hmm... I'll definitely curuous and will look into it more.Dennis
From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Mon, August 2, 2010 8:10:09 PM
Subject: Re: [harryproa] Re: Leeway Prevention
Dennis
Did you take the time to watch the video that Todd linked to:The rudders are bi-directional. What surprises me is that they happily work swinging free with the side pivot. I did not expect this but it is possible with the large radius nose he is using.I have not gone through the moment analysis thoroughly but the stable angle of operation gets smaller as the nose/tail becomes pointier. A nose radius of 0.3% of chord gives a stable working range of 10 degrees at Re# 10E6. Going larger radius increases the stable range but there is a point where the L/D will suffer due to trailing edge turbulence.Irrespective of the stable working range it gets down to whether you want to allow the rudders to float or not. This runs the risk of a rudder suddenly becoming a brake. Rudder stops could be used to limit the range.Rick
On 03/08/2010, at 8:58 AM, Dennis Cox wrote:
I think the biggest issue would be... reversing them. Rotating (as Rob does now) during the shunt gives you the asymmetric on the wrong side going one of the directions. You'd have to have a rudder that is twice as long with half out of the water and rotate the other end into the water going the other way. If I were to use Rocket Science... I'd say something like aero-elastic tayloring (well fluid-elastic tayloring) and have the foil adaptive. Hmmmmm.... don't get me started. It might be painful.Dennis
From: Rick Willoughby <rickwill@bigpond. net.au>
To: harryproa@yahoogrou ps.com.au
Sent: Mon, August 2, 2010 6:42:26 PM
Subject: Re: [harryproa] Re: Leeway Prevention
Ben
Leeway on a hull is added drag and loses VMG. Some keel boats are using flaps or even adjusting the keel or board angle on each tack to overcome leeway. If you have two large rudders rather than a keel/ boards and rudder then you can set rudder angles to avoid leeway.Using asymmetric rudders means you can get the lift more efficiently with a smaller area than symmetrical sections that have to work in both directions. At zero lift (negative AoA) they have less drag than a symmetric section capable of generating the required lift on the wind. So downwind they will have less drag than symmetrical rudders and keels/boards. . You could also lift the leading one if set up to do so.The rudder drag will be a significant portion of the overall drag on a harryproa using a long slender lw hull so anything to reduce it can have a significant impact on performance. You could reasonably expect to reduce the rudder drag by 30% using an asymmetric section compared with a symmetric section for the same lift.I do not know how common asymmetric rudders are on proas but it seems the logical choice and another natural advantage of the breed.Rick
On 03/08/2010, at 12:24 AM, bjarthur123 wrote:
> The unique feature offered with two large rudders on a proa is the potential advantage of using asymetric sections for the rudders.
what a brilliant idea! by "potential", do you mean to say this is NOT customary on modern proas? if so, why not!?
so upwind the more curved surface faces upwind. lift is generated even when the angle of attack is zero. so less leeway and drag. course is more upwind even though heading is the same.
but what about downwind? you maximize VMG by sailing on a broad reach re. true wind, which on a boat that can sail windspeed is a beam reach re. apparent. you don't want to prevent leeway (correct?), so do you flip the asymmetric rudders around so that the more curved surfaces point downwind? this would then generate more leeway, which when trying to go downwind is good. am i getting this right?
new member by the way. just bought a weta trimaran, and am dreaming about retiring on a larger boat. maybe a harryproa!
ben arthur
weta #358, "gray matter"
ithaca, new york
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby
03 9796 2415
0419 104 821
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