Rudder Center of Effort Migration with Speed

[Erwan]

Hi Everybody,

Sailing a pretty new A-Cat in 15 to 20 knts wind speed, with true wind at 90°. I observed that at high speed, if you pull or push the rudders a little bit, some hydrodynamic phenomenon seems to increase the initial motion just like if the center of effort has moved in front of the rudder axis.

Whether you push of pull the tiller extension, you have to hold it firmly, otherwise it will escape from your hands as soon as ou have moved one degree.

Should I consider possible solution like:
-Putting little flat horizontal device on the rudder blade at the waterline level
-Or sandpapering the leading edge of the rudder blade on a lenght related to the hull boundary layer at the transom level ?

Thanks in advance and best regards

Erwan

[daiquiri]

Since a rudder has a symmetrical airfoil section, the center of pressure does not move very much with angle of attack.
If it was a ventilation problem, your rudder would lose lift during the turn and would tend to get back to the neutral position. So I think the horizontal plate at the waterline level will not resolve your problem.
My first guess - the rudder stock is too far aft along the airfoil chord.
My second guess - a vortex might be created somewhere in front of the rudder during the turn, modifying the pressure distribution, either increasing or inverting locally (along the span) the rudder lift force and/or moving the center of pressure too much forward.
Do you have a pic of that rudder and of the hull in front of it? Or a drawing, possibly with main dimensions?

[Erwan]

The boat is at 500 km and I will go back in 2 weeks. I will take o "blue print" of the rudder blade.
With regards to your remark, I thought that the center of pressure uses to move forward with increasing angle of incidence ?

[Doug Lord]

Could the rudder be angled forward? That could put too much area forward of the axis of pivot.
Are the two rudders parallel to each other or towed in or out?

[daiquiri]

Originally Posted by Erwan

With regards to your remark, I thought that the center of pressure uses to move forward with increasing angle of incidence ?

True for cambered airfoils, not for symmetric ones.

Symmetric airfoils have the center of pressure placed at 1/4 of the chord from the leading edge and it is independent of angle of attack. It is one important result of the Thin Airfoil Theory which has been experimentally verified for a wide range of airfoils, and is valid for angles of attack sufficiently far from the stall - which means in the linear region of the lift curve.

Just to complete the technical info, it also means that the center of pressure and the aerodynamic center coincide on a symmetric airfoil, which is generally not true in case of cambered airfoils.

[DaveJ]

On most beach cat's (Hobie's, Nacra's, etc....) that have kick up rudders have the ability to adjust the rotation of the rudder for or aft. What the does is adjust the pressure on the helm. This biases up the helm to the sailing rig you have so it feels neatural with no push or pull on the tiller when sailing straight.

Now you can adjust it to lee or windward, i have my boat set to windward, so if i fall off it will automatically turn into the wind and stall, but i have a constant pull on the tiller that when the wind picks up can require alot of strenght to hold steady.

High performance cats like an A-Class will have it more to lee, which makes if very twitchy and responsive, but you don't have the constant pull that i get when sailing straight, but if you fall off, the helm will vere off the wind and you can say bye bye to the boat as it will sail away without you.

So to fix your problem, when the rudders are down and locked, adjust them so they don't swing so far forward. This will make it sail less to lee and more to wind.

Dave

[Erwan]

Thank You for taking time to answer, and I think there is probably a combination of many elements. I made some research on my side:
-These new pierce-wave hull shapes use use to sail with the nose "above the water" most of the time, as a result, the boat is very sensitive to rudder change, it is a deliberate choice for speed tacking or gibbing.
So rudder axis is probably perfect for average speeds, and sub-optimal for higher ones.
Some small leading edge " imperfections" could be neutral at moderate speeds and significant at higher ones.
In addition, at higher speed, the hull may generate some hydrodynamic phenomenon which trigger early turbulent separation or something like that. To illustrate this point, I would like to mention Yves Loday interview about his new F18 design: " With increasing power available on F18, we should have increased rudder area, but it would have been a hudge speed penalty, that is why, with water tank tests, we found that some " steps" at the aft part of the hull, untill the transom, increase the rudder efficiency".
May be the solution is to adapt other blades?
Regards
Erwan

[DaveJ]

Having the bows out of the water really helps with tacking in high winds. Needing to get the bow through the eye as soon as possible without loosing boat speed is a real requirement and will make or break you whilst racing.

I have to agee with Yves Loday's statement, never having the chance to sail on a F18/Tiger but have raced against them on my dodgy old Hobie 18. Listening to them talk and debate the best rigging for the condictions of the day, i understand why Yves would make that comment.

Most people rake the mast back as far as possible, so when in the closed haul the main is block to block (which is how i have mine) which is suppose to help with pointing into the wind. Doing this moves the CP of the sails back and away from the design possition in regard to the centre board/rudder relationship.

Increasing the rudder size will help with the change in CP. As a tempory fix, they roatate the rudders forward. Just like Doug Lords and my previous replys.

In regards to your porblem, a possible reason why is that the owner before you may have raked the mast back as far as possible, set the rudders to suit that style of sailing, then you have brought it and rig it with less rake.