High Speed Rudder issue....

Thanks for that redreuben! That mentions making the angle of the fence parallel to the waterline, which makes sense when placed that far down on the rudder blade. Further up the rudder blade, or in my case at the base of the cassette, where the flow streams off the back of the boat at speed, I believe there is a negative angle to the flow. That bears out in the video. My weight on the stern of the boat filming MAY have distorted the results though.

I think I'll have to mount a camera - sail her hard with my weight forward and see what angle the flow hits that cassette. I don't want to have the angle too high, and have the fence "gulping" air and forcing it down the leading edge of the rudder!

BTW - If you couldn't tell, I love noodling through this stuff. Thanks to all for your contributions thus far.

For documentation purposes, Here is a frame of the video showing the possible stream of ventilation bubbles:

 

this question might be more up OS's alley but, do balanced surfaces work in boat apps?


Barry

 

Update:

An update: I rebuilt the busted cassette somewhat beefier and re mounted it. Phil, I will try your stitching. Right now after the rebuild, the gudgeon has a layer of carbon uni along it's length, another layer of carbon uni wrapped perpendicular and then several layers of 6 oz cloth. The lower gudgeon is similar.

I ran the boat under power up to 7 knots and with the tiller bar disconnected, the "slave rudder" seemed to move properly in response to my movements of the master rudder. When left alone, there was no apparent tendency to overbalance or anything, but again, 7 knots was my max speed.

In addition to running the rudders at half-height, I intend to radius the leading edge more and also to add an anti-ventilation plate to the bottom of the cassette.

Thanks again to all for your insights.

 

Paxfish. Sounds like a plan. Keep us informed.

 

Quote. "do balanced surfaces work in boat apps?" Quote.

They may do, but an overly balanced rudder can cause "rudder snatch", and evenly balanced can lead to "rudder flutter".
Regarding high aspect ratio's, they are great in Air.
A wingsail, for instance is embedded 100.000 feet in a sea of air. Air is approx 800 times lighter than water and the difference in pressure from the top to the bottom of the mast is negligible.
The pressure difference from the top to the bottom in a vertical foil in seawater is 64lbs for every foot of depth.
A high aspect rudder then will act like a big crowbar with leverage on its top, with the inevitable damage to both rudder foil and case, as has been demonstrated.
Also , since long skinny foils stall at lower @, a rudder ,which has to constantly endure big @ movements, is also working at a worse L/D ratio and will thus be more draggy.
The late Edmond Bruce showed by both mathematics and experimental testing that an aspect ratio of 1/1 Is perfectly acceptable.
The AYRS book "Rudder design" says that for sailing craft an aspect ratio of 1 is adequate for a spade rudder, or for a transom hung rudder, less than 2 ,
and a rudder behind a streamlined skeg .5 to 1 is very adequate.

 

The pressure thing

Gday Pat

I was left befuddled after having a talk with the yacht designer Joe Adams. He has designed some great monos like the Helsalls and Adams 10 and 13, the Adams 31 and heaps of cruisers. So I went to ask some questions about his boat when he was in Coffs with us years and years ago.

Along with telling me that bulkheads make a boat weaker (they produce a stress concentration) and that his first boat went to windward better (it was a gaff) than a modern rig he told me that the Adams 13s were much stiffer with their centreboards UP. He explained this as due to the increase in pressure due to increased depth in the water as you state above. The board didn't have much ballast in it anyway. I walked away after our talk bemused that my certainties were not as concrete as I thought.

BUT although the pressure is higher in deeper water - go snorkelling to test this and don't blow your nose - is the DIFFERENCE in pressure between the sides of the foil any different at greater depth?

In my high school physics class I teach about balanced and unbalanced forces. So the water pressure can increase but is it not only the unbalanced force that is the determinant of the lift produced by the foil? When I look at the equations for the lift from a foil I can see the input for density but no corresponding input for pressure. As water can't really be compressed how can the depth be related to lift and/or drag from a foil?

BTW - good on you Pax - nice to know I can learn something from your mishaps.

cheers

Phil

 

Pretty easy really. It all boils down to Newtonian physics. F=MA.
Water has to be literally "Pushed Aside" to produce a force to resist sideways movement. Water is heavy and for our purposes not compressible.
A board or rudder needs an angle of attack, (@), to do this.
A board or rudder, centred on the same line as the boats keel does this by leeway. The most efficient angle, (ie:- best lift to windward, with least drag), is 4 t0 5 degrees. A long thin rudder is not able to consistently do this , but a LAR keel or rudder is more insensitive to fluctuations of @ and thus better for our purpose. A skipper of a boat fitted with a leeway indicator, who helms his boat at all times to achieve a degree of leeway of 4dg, will consistently win races.

 

Going to have to agree to disagree here Pat. I can't find much on the web to support the idea that water at a greater depth has a higher lift co-efficient. I get Newton's 2nd law but can't see how that has much to do with increased lift at greater depths. If that were the case then submarines would work far better at certain depths and everyone would be trying to get their propellers as deep as possible. Certainly Arneson wouldn't have done a surface piercing drive and fast ski boats would not be trying to reduce leg length.

I will keep on looking but I think that the planform distribution of lift on an immersed foil is the same along its length apart from surface effects. If that were not the case then hydrofoil boats like the Moth would develop strange habits as they started to fly. Hopefully Tom Speer or a designer of foilers can tell us if depth is an issue.

The foil is breaking probably because of leverage and twisting effects. For plane buffs the Fokker D1 seems like the correct example in this case for why the rudder may go overbalanced under load.

http://www.theaerodrome.com/forum/showthread.php?t=1440

Of course the best explanation for this is in the bible for amateur engineers - Structures by JE Gordon. For those of you who like to understand how your boats hold together this is a fabulous read.

cheers

Phil

 

You can't compare ventilated propellers that are run in that mode to have large face pressures to a submerged foil used for lateral resistance.

 

Sorry Phil if I confused you. I never intended to say that water at a lower depth has a higher lift co-efficient. But a rudder (and indeed any water piercing foil also), simply pushes a lump of water to one side and this produces the F=MA effect, which of course pushes the back of the boat in the opposite direction, thus turning the boat. That lump of heavy water requires energy to move it, and that energy comes from the sails, which in turn deducts the amount of energy available to move the hull through the water. It's called DRAG and that's why it is imperative to move the rudder as little as possible in a light wind race where every 1/10th of a knot counts.

 

What we need to know is the hull designed to lean on the rudder or not
That determines whether you need a balanced rudder or not.
That along with speed determines the naca profile due to it either needing an angle or attack (lift) or not
I am not saying that as a designer but i have sailed plenty of experiments.

 

knowing well the effects of wings and air and it being compressible, reminding me water isn't (Least in this operating range) explained it all to me in a second! haha thanks OS


Barry

 

Some real data and testing

Pax

Have a read through this paper. It is relatively easy to follow and goes through a few concepts about rudders and loading.

http://www.dtic.mil/dtic/tr/fulltext/u2/a542380.pdf

From what I can glean

- pressure distribution along the foil is uniform apart from surface effects (Your rudders should have the same amount of lift all the way down apart from ventilation)
- the rudder will undergo torque along its length altering Angle of Attack (Notice on this model the AOA is reduced by torsion loads. I think that with your foil the foil bends in an opposite manner because the edge is forward of vertical and the centre of pressure is in front of the torsional centre of the foil structure so therefore the AOA may be INCREASED by loading - the Fokker example in my last post)
- leading edge smoothness is important for stall angle.
- high aspect and low aspect rudders work similarly well in a steady sea state but high aspect foils work better at in dynamic loading conditions (so any condition we sail in with waves - ie when you really need a rudder).

cheers

Phil

 

Does that mean that every high-speed multi, dinghy, skiff and yacht designer, windsurf fin maker and surfboard fin maker is wrong when they select high aspect foils?

Isn't the testing done by America's Cup teams, for example, more sophisticated and extensive than that done by Bruce? But don't they go for high aspect foils? So why do they always get it so wrong? Is Dr Martin Fischer, PhD, an expert in foil design for cats, completely wrong when he designs foils like this;



rather than like this;



It would seem strange if the trimaran, cat, skiff, Moth, America's Cup, development class dinghy and offshore racer guys were all wrong, and the US catboat guys did it much better 145 years ago.

 

For me I look at those planforms and think to myself maybe there not a case of right or wrong but right for there purpose? Those look like glider type planforms to me, low drag high lift for specific speed, which maybe what "they" need, but not one everyone could say ideal for every app? An f-16 has best wing design for an f16, but on a glider it'd seem woefully inadequate in lift, and visa versa for an f-16 with glider wings. It wouldn't allow it to get to speed nor lift enough, and would take a state to turn.


Barry