Balanced Rudder Behaviour.

[DJE]

Had the boat 2 years now but still getting used to a wheel and a balanced spade rudder. When hard pressed and heeling too much the helm stays light and responsive until the wheel is about half way to full lock - which is something like 15 degrees of rudder angle. At this point the load in the wheel increases enormously and very suddenly. But the boat continues to answer the helm. What could be causing this?
If the blade is stalling I would expect to lose control and the wheel load to reduce as the lift on the blade disappears. I think it is associated with friction in the bearings as it was much worse last year before I cleaned and greased them.
I suppose if the blade stalls the drag will increase rapidly as the lift falls and thus will add to the friction in the bearings. Any other ideas?

 

[dom]

It could be that you have to wind on a good deal more rudder at a point of heel where the boat's hydrodynamics require exponentially more turning effort. If as you intimate the strain is more friction related, then it will tend to work more more symmetrically than with extra turning moment and/or stall conditions encountered, i.e. winding off the rudder will also be sticky.

As a matter of interest are your bearings glide, or roller and are they self-aligning? I'm guessing glide, but if they're not self-aligning, what can happen is that your rudder stock bends as the rudder loads up and consequently presents at an incorrect angle at each of the bearings. If this is the problem, then unfortunately they may need to be replaced; glide ones are thankfully cheap ...but as for spindle :ambivalence:

When the boat comes out of the water, why not ask a young rugby player to put his shoulder progressively harder to the rudder and test the steering as he does so. If it's the bearings, you can then easily drop and sort?

 

[anoccasionalyachtsman]

Think in terms of a typical aircraft as it approaches the stall and thereafter. The centre of lift moves a little forward, and the nose rises. This would equate to your helm lightening. After the stall, the centre of lift moves quickly and a long way aft and the nose drops sharply. Moving the centre aft along your rudder will increase the leverage you're having to oppose.

 

[jwilson]

The stall theory may be right.

Also, if you sight along the pivot/rudderstock line, has the lower section less area percentage forward of the pivot-stock line? This might also increase loading as the boat heels and the upper part lifts partly out of the water.... (as it will do on many modern AWBs).

 

[lpdsn]

I think it is associated with friction in the bearings as it was much worse last year before I cleaned and greased them.

I guess they're not the typical nylon water-lubricated bearings then.

It doesn't really sound like friction if the loads increase suddenly. What I've taken to doing this year is to take the rudder angle from the autopilot and display it to the helm. The main reasons were to help teach crew not to oversteer and to provide back up to the turk's heads when manouvering, but it is interesting information so I sometimes leave it on there. I can use up to 15 to 18 degrees of helm if hit by a gust off the hills but the rudder doesn't stall at that. The loads don't kick in suddenly but 15 degrees of rudder is a lot harder than 10 degrees. To get the helm much beyond 20 degrees when travelling at speed requires an lot of force, perhaps too much for an average sized bloke.

One idea: maybe there is sideways force on the bearings that suddenly kicks in. Is there any play in the rudder if you try to move it from side to side when out of the water? If the rudder is held tightly in place I wouldn't expect the sideways force to cause friction to increase suddenly but perhaps it's moving.

 

[Daydream believer]

One point-The rudder is not "balanced" it just has a small part ( I guess up to20%) in front of the pivot point.
My understanding is:-
Once past a certain angle it no longer allows the water to flow as an aerofoil might. A very small amount of water may even be passing around the leading edge( forcing water forward) as opposed to the trailing edge due to the curved ( or raking back) leading edge on the rudder. Instead you are effectively dragging a flat board sideways through the water & because it is not balanced you feel the greater load. As you move the rudder to a greater angle the tip moves further off line as the rudder post usually rakes aft which makes the tip stick out more to the side when at an angle, adding to the load. As well as trailing edge stalling the rudder will also develop stalling at the tip ( forcing water down) & top ( forcing water up) which may come in at different points in the angle or all at the same time. If it all happens all at once, that is when a boat will loose control suddenly & round up which a designer would( presumably) try to avoid
I do not think that it is anything related to bearings but the way the rudder is reacting to the water flow & the way it is stalling.

I would not expect to be greasing bearings. Or at least my Jefra bearings are nylon rollers are meant to be water lubricated. As for wear-I lost my rudder 2 years ago after 11 years from new & the bearings were replaced as a matter of course, but would have done another 2-3 years & i do 2000 miles Per year

Another point-Does it rub against the hull at the angle where you get the greater resistance? My first one might have at that angle. There was some play for it to ride up 10 mm & it did float in water. Not noticed on the hard unless one forced it up on purpose

 

[DJE]

Thanks all for the comments. the rudder bearings are simply a solid stainless steel bar turning in greased bronze bushes. All pretty bomb proof, no discernable play and all in good condition since we dropped the rudder to clean and grease it all last winter. I would expect the friction to increase as the total side force increases but there seems to be a step change at one particular rudder angle which suggests to me that there are some hydrodynamic changes at that point.

I can't recall whether it was easier to take helm off than to apply more. The conditions when this happened on a fast reach on Saturday were somewhat lively and I was more concerned about whether or not the boom end was going to hit the water than sophisticated analysis of wheel forces. Before we serviced the bearings we noticed the same effect when close hauled and in need of a reef. Then the wheel would virtually lock up at the same point where the step change occurs. But the bearings were very dry - when the boat came ashore last winter I disconnected the wheel and autopilot from the rudder stock and tried to turn the blade by hand from below. I could barely move it. Since then the bearings have been cleaned and greased the rudder turns easily to the stops on both sides when there is little or no load on it.

 

[rudolph_hart]

Thanks all for the comments. the rudder bearings are simply a solid stainless steel bar turning in greased bronze bushes. All pretty bomb proof, no discernable play and all in good condition since we dropped the rudder to clean and grease it all last winter. I would expect the friction to increase as the total side force increases but there seems to be a step change at one particular rudder angle which suggests to me that there are some hydrodynamic changes at that point.

I can't recall whether it was easier to take helm off than to apply more. The conditions when this happened on a fast reach on Saturday were somewhat lively and I was more concerned about whether or not the boom end was going to hit the water than sophisticated analysis of wheel forces. Before we serviced the bearings we noticed the same effect when close hauled and in need of a reef. Then the wheel would virtually lock up at the same point where the step change occurs. But the bearings were very dry - when the boat came ashore last winter I disconnected the wheel and autopilot from the rudder stock and tried to turn the blade by hand from below. I could barely move it. Since then the bearings have been cleaned and greased the rudder turns easily to the stops on both sides when there is little or no load on it.

Agree with Dom ref the stock bending. I re-arranged a 32ft IOR racer's aft-swept unbalanced rudder to balanced & unswept by re-profiling the old rudder. This delayed stall out considerably, but progressively heavier helm through stock bend was much more noticeable. This made a wind-over-tide spinnaker run in a blow rather 'interesting'

Another point: is your steering rod-actuated? I know of a wheel steered Dehler 34 which had the system lock up when 'over-wound' beyond full lock, overpowering the bump-stop, until the pushrod & arm on the stock became 'in line'. Also the wheel suddenly spun freely, as it had run off the end of the quadrant. Of course, this happened in a blow near a nav hazard. Scary.