Twin rudders, alignment?

[Ian_Edwards]

Does anyone know the correct procedure for correctly aligning twin rudders?
And
Should they be parallel?
After I'd finished antifouling the boat I stood back to see how it looked, as you do. Then I noticed that the two rudders didn't seem to be parallel.
It's easy to adjust the tiebar between the rudders, but how and where do you measure from?
I could set-up a centre line using a string from the keel to the middle of the skeg, then extend it to a post or stepladder to establish a mid ships line.
I could then measure the distance between the lower tips of the of the trailing edges of the rudders to the center line, the rudders are rectangular and the lower corners are approximate square.
But where do I go from here?
Adjust the tie bar so that the centre of the leading edge of the rudders are equidistant from the centre line, and the same distance centre as the trailing edges.
That would seem to make then parallel.
Is that the correct procedure?
A yard had both rudders out to put key ways in the stocks for the quadrants. To stop the quadrants slipping, and I'm not sure that they got them back correctly.

 

[westernman]

On my Hobie FX one catamaran, the user manual said there should be a toe in 3-5mm.

 

[Chiara’s slave]

Your method is sound, but they may not be optimal if parallel. The only way to find out is to ask the manufacturer of your boat.

 

[Ian_Edwards]

Thanks, but the manufacture are no longer in business.
I could try the owners association, but most southery's are single rudders and seldom raced.
I was wondering if there was some rule of thumb for twin rudders monohulls.

 

[jamie N]

With no practical experience of this, it did make me think; how about the procedure being to disconnect 'say', the stbd rudder, steering only with the port. The stbd rudder would be 'flaccid', then to reconnect it at that 'measurement'.
As I say, I've no knowledge of this.
Much easier on a Hobie cat than a Southerly of course......

 

[dunedin]

Thanks, but the manufacture are no longer in business.
I could try the owners association, but most southery's are single rudders and seldom raced.
I was wondering if there was some rule of thumb for twin rudders monohulls.

Try the designers office?

 

[Ian_Edwards]

In the past I've tried disconnecting the starboard rudders. The port helm is the master where the autopilot is connected.
Then trying to sail a straight course with the starboard rudder, free to line up in the flow.
It proved to be very difficult.
I tried using both the autopilot on a fixed heading and locking the port rudder to steer dead ahead. Neither produced a stable course, the autopilot kept putting minor adjustment in. It doesn’t know if it's off course until it's off course. And the locked rudders always resulted in a slight drift one way or the other.
When you add the slight friction in the rudder bearing, which causes nothing much to happen until the hydrodynamics produced enough lift to change the angle of the free rudder, resulting in a discontinues movement of the free rudder.
And
The difficulty of adjusting and reconnecting the tie bar without upsetting everything.
I gave up with a frustrated sigh and "that's the best I can do".

 

[Neeves]

If all else fails.

I really find it difficult to belieive that Southerly conducted tests and developed a twin rudder configuration more sophisticated than other yachts/designers.

Look at other yachts, more than one design/manufacturer with twin rudder (check owners association) develop a simple spreadsheet (if there are differences) and try the most common.

Start a new thread, or resurrect this thread, and posts the results.

I suspect you will find its not that complicated but understand your desire to find the optimum.

Do the port and starboard rudders have a different profile?

Jonathan

 

[Chiara’s slave]

I’m sure they’ll work if they’re just parallel, but also is there any Ackerman effect? Our friend’s Hobie FX will have cranked tillers. Dunno if that is any kind of issue with a cruising boat, but Southerlys aren’t slow. My mooring neighbour has one, I’ll look at it’s steering with a whole new interest.

 

[Boathook]

The rudders on my cat aren't parallel. Only just noticed this winter after many years ! Possibly only 1 degree or less out. The bar linking the rudders has no adjustment.

I suspect that for my boat it won't make a bit of difference unlike sail trim !

 

[MikeBz]

We had a Sun Odyssey 32 with twin rudders, they were supposed to be set up with toe-in but I can’t remember how much. I guess on a monohull having toe-in helps mitigate to a degree the increased weather helm you get as the boat heels since the leeward rudder will be doing more of the work as heel increases.

 

[greeny]

On my ben 323, I removed the rudders to do the bearings. There was definitely toe- in on them. Stupidly I didnt measure it. I was advised by a "boat expert" that the toe-in should be set to align each rudder to the leading edge of the keel. So I did. That season she sailed as though there was a brake on all the time, especially when heeled. I made temporary adjustment to correct it for that year. Then when lifted, they were reset to minimal toe_in which was actually not far from my temporary settings. About half inch difference between leading and trailing edge of rudder as i remember. Moral of the story. Dont listen to "boat experts". Different boats, hull shapes, lengths will all affect the requirement for toe-in. Try to get the designer to advise on settings. Failing that I'd start with them aligned and try it. Then add toe in on the water. While shes out of the water you could make adjustments to see how much toe in for example 5 turns of the adjustment make to the rudder for reference when in the water. Also mark the starting positions so its easy to return to the starting point.

 

[Blueboatman]

The toe in is there so the rudders don’t chatter or vibrate or fight each other as you sail along. So just enough toe in is ….enough. I think

Why not set the boat up on a beat on autopilot then adjust a bit for max speed and least a/p input or correction?
That’s how I would do it, empirically, or ‘ meddling’ until it feels right on the helm and performance.
I’m surprised there isn’t some printed manufacturer/ designer stuff available though to get you started.
There’s an occasional poster here who used to work for Southerly ? e30Mad? He might be able to ‘ point’ you in the right direction for further fact based enquiries

 

[Poecheng]

I looked in my (PDF) manual last night (S38) and page 22 re the rudders is the only one missing ! The drawing for the steering system shows the layout but not any detail about the settings. I will look on the boat at the weekend in the paper manual.
My recollection is that there is a setting, which means they are not parallel.
Re #6, if yours is a Stephen Jones design, I found him most helpful in the past (about Sadler 290) but he doesnt have an internet presence. I was given his phone number and he picked up and had a chat. Even if not his design, he will know the answer !

 

[Chiara’s slave]

It would be fascinating to know the upshot of this. A really interesting question.

 

[marcelherrera]

oddly, we've always had a slightly offset between rudders so that they weren't perfectly parallel, on the JPK 1080.
I think our test was to sail at a heeled angle, and loosen the windward rudder so that it was perfectly inline with the flow. Then stop the boat and measure the distance between leading edge and trailing edge.
Repeat on the other tack, and measure those distances too.
Then take the measurements and ensure that 50% of them is from the centreline of the yacht.

(Note: we had tiller steering, on a wheel I don't think it matters as much)

 

[flaming]

Toe in on twin rudders is a big subject.

In racing circles we adjust the toe in for heel angle. On my boat we have a manual adjustment system, that is essentially a threaded bar that is turned by means of a continuous line through a "grip" block. To be honest I'm really early in the learning curve of exactly how to trim them best, but what I've been told about how you should be adjusting them is that you look at the windward rudder when under way, and you should see that the water level is equal on both sides of the blade. If it isn't move the toe in and see if you made it better or worse.

What we do for most of our racing is look for a setting that is essentially parallel for flat boat downwind sailing, on the assumption that for upwind work on a ww/lw track our windward rudder is mostly out of the water anyway. If we're faced with a long reach we then start "playing with the pink string" and convincing ourselves that it's making a difference. Get it really wrong and you definitely know about it, the drag is noticeable. I have a plan to use a linear potentiometer (sp?) to generate repeatable settings that we can display on the instruments so we can start tuning to repeatable settings for given heel angles rather than leaning over the back looking at a rudder and applying some educated guess work every time....

If I could justify the cash I'd have one of these.... But as I already have a working adjustment mechanism it does feel significant overkill.

E-BAR | FourthCape

 

[dunedin]

....

I really find it difficult to belieive that Southerly conducted tests and developed a twin rudder configuration more sophisticated than other yachts/designers.

..

But the point is that Southerly didn't design the boats. The designers did, and the late models (like i believe this is) had very competent and internationally recognised designers. The design office is therefore where the best advice may come from.

 

[Neeves]

But the point is that Southerly didn't design the boats. The designers did, and the late models (like i believe this is) had very competent and internationally recognised designers. The design office is therefore where the best advice may come from.

I am sure you are correct -

It will be interesting to read what the design house says.

Adjusting toe in angle for heel......

flaming's post seems to suggest its very much an art, suck it and see. It appears you know if you get it wrong but anticipating success in fine tuning looks overkill on a cruising yacht as I find it difficult to believe the OP will want to alter the tuning to suit conditions. I'd also suspect that trim of the yacht might have a bigger impact - once you have fixed for 'average' conditions (see flaming's post) -

How many worry about trim?

Jonathan

 

[Bilgediver]

I doubt if there would be noticeable difference between parallel rudders or toe in / out. Just adjust so rudders parallel after setting the rudder with the feed back to straight ahead and matching the other to it by tie bar (track rod ) adjustment.