Rudder angle & control surface.

[OGITD]

Hi all,

Today someone posed the question of rudder control surface and its angle when going astern.
I know about the relationship of speed and windage, but what I would like to know is at what angle while going astern does the rudder change from control surface to just resistance.
Also once this has been established is it good practice to manufacture and install stops to prevent this from happening?

“These learning curves are a good thing”

& thanks once again.

JM.

 

[oldsaltoz]

Mechanical rudder stops are a must, around 30 degrees each side.

Avagoodweekend......

 

[William_H]

Just to waffle on..... A rudder is an airfoil in water. It provides lift which increase of angle of attack to the point where the rudder starts to stall. The drag increases with angle of attack then increases dramatically when stall occurs.

A have a transom mounted rudder and stall or at least the start of stall is accompanied by turbulence noise and when you look at it you can see a clear hole in the water at the aft low pressure side and a tendency for the flow to try to go both toward the leading edge as well as toward the trailing edge.

Of course stall doesn't occur at any specific angle but depends on speed and also the actual angle of attack (approach) of the water. ie if the boat is turning then the stern is moving sideways reducing the angle of attack.

When going astern the ideal foil shape ie thicker at the front tapering to a fine aft edge is all wrong. The fine exit when a leading edge will promote stalling much more quickly then when going forward.

Rudders always have unbalance in their control when going astern. That is the area aft of the turn axis is very large so any turning commanded will result is a force on the tiller trying to increase the tiller angle. This can really take you by surprise if you are going fast astern then command a little turn. It can be reduced by having more balance of the rudder ie more area forward of the turn axis. But this should be limited to no more than 20% area in front of axis which will balance 20% of the area aaft so reducing tiller loads by 40%.

So the answer to the question is impirical. It varies with speed and actual turn rate. As the tiller angle is increased the drag (force trying to stop the boat) will increase but as it reaches approaching stall the drag will increase dramatically while the lift (turnig force) will start to decrease.
Rudder stops could be usefully fitted but the location will be tricky. If you have limited travel then you may deny yourself some turning ability under some circumstances. ie once the boat is turning you can get more turning with more tiller angle without stalling compared to what would be a normal limit when going straight at lower speeds. olewill

 

[jwilson]

On a traditional long keel hull form, allowing a rudder blade to turn more than about 30% is pointless, unless you WANT a brake (which is occasionally useful!). I believe blades in water actually stall at about 17 degrees angle.

On some modern light hulls with a deep narrow fin and spade rudder, with a flat hull underbody, however, the boat can turn very fast indeed, and while putting on 45% rudder instantly might stall the rudder, you can in the space of a second or two go smoothly from 15 t o 30 to 45 or more degrees rudder without stalling the blade - the turn just keeps getting tighter until you are almost spinning around the keel. The rudder is no longer operating in water coming from straight ahead, but in 'sideswept' water. Most builders put rudder stops at 45 degrees, but this is more a convention than anything else.

Although putting 45 degrees rudder on instantly will stall the blade if you are sailing, 45 degrees when stationary followed by a blast of propwash works well in manouevring. 30 degrees might be fine on a long-keeler that never goes into tight spaces under power, but is not enough for most real-world use on modern boats.

 

[OGITD]

Thanks guys.
I do believe this information will be of use to many other YBW members.
My new issue now is what to manufacture and where to site them.
I think my first port of call (no pun intended) is to have a look at other boats in the club.
It’s just one more job before she goes back in the water.
Thanks once again.
JM.

 

[Mudplugger]

Interesting comments! Having just combined a Whitlock Cobra with a Vetus Hydraulic system,to provide an internal helm position. All the advice received from the knowledgeable, was to ensure that the rudder stops, were a) big and solid! b) not more than 35% either side! all of the hydraulic steering systems seem to be at a max of 30-35%

 

[alan006]

I've been following this post with interest because on Dutch barges I think I've seen rudder stops set much wider than 35 degrees.
They can turn very tightly for boats of their size.These tight turns are when there is very little forward motion.
Have other people noticed this?