Twin rudders, alignment?

[flaming]

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.

Depends what you call noticeable... Certainly I know about it if it's wrong....

Thinking about it some more from a cruising perspective, what I think I'd do is to tune for about 10-15 degrees of heel. So go out on a nice day with flatish water and a nice F3-4. Sail the boat on a reach with circa 15 degrees of heel, then tune the toe in so that the windward rudder has even flow on both sides.

This will give the best performance on reaching and lightish wind upwind sailing. As the wind builds upwind you should find the windward rudder moslty out of the water, so no big deal. This should mean your only point of really less than optimal would be downwind flat boat sailing. Which is probably the trade off I'd take for cruising as on a reach you can find the helm loading up if pressed, which this should help with. But downwind you will only really have a slight increase in drag to contend with, not any adverse handling characteristics.

 

[Ingwe]

As has been hinted this is complicated. As flaming has said the top race boats with twin rudders now adjust them depending on angle of heel and speed.

If you are going to leave them fixed then how much toe in angle will be optimal overall will vary from boat to boat depending on the top speed of the boat and how far apart the two rudders are. The faster the boat and the further apart the rudders are the closer you get to setting them parallel, so the last 2 race boats I have had have been set with about one degree toe in, wheras a slower cruiser that doesn't lift the windward rudder out of the water upwind would proably want about 2-3 degrees toe in.

The other issue is obviously measuring this to set it up, for race boats the designers often give an optimal distance between the rudder tips as this is the easiest thing to measure, so if you can find a setting that suits you using flaming's method above the next time the boat is out the water measure this distance so that you can easily set the rudders back up the same way again in future.

 

[Neeves]

I'm surprised that on a cruising yacht the rudder angles are enabled to be varied.

A racing yacht is largely set up for a. fixed crew, minimum extra on board - and the weight is effectively fixed. The rudders can then be varied for the conditions. A cruising yacht has so many variables, call it weight

I would have thought the angle might vary with the loading of the yacht, large/small crew, a weekend sail vs a trip across the Atlantic.

Jonathan

 

[Ian_Edwards]

Lots of interesting comments, thanks.
The Southerly 46 hull was designed by Jason Ker. However, it's my understanding that there was parting of the ways part through the prototype build.
I have corresponded with Jason Ker's design office once many years ago, I got quick reply to my first enquiry, subsequent emails weren't answered, so I moved on.
As in my post #7, I've tried adjusting on the move, by allowing one rudder to self align. But found it really difficult to do, technically on the boat and finding the right conditions, a suitable wind speed, minimum waves plenty of space, no boats around, pot boys, or in my part of the world nuclear submarines.
I think the best bet to get a good a start, is to try Jason Ker's office, and hope that they have some info' on a 20 year old design, where only 4 were built. And set up say a 12mm toe in for each rudder, using the bit of string method that I outlined in post #1.
It'll be hard to tell how much of a change it makes, I have no really yard stick to reference to. I don't race, and when I'm sailing the outer Isle, I might see 2 or 3 yachts, in the distance in a day.

 

[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.

I have had a look and page 22 of the S38 manual (nor anywhere else in it) doesn't have this detail, sorry.