A steering question

[Guest]

Hello All

On a small yacht (as in photo below) would you expect that a rudder under the hull fully imersed would give better steering than the one in the photo?

Peter

 

PS. not my boat just an example

 

 

 

 

[Guest]

On a boat like that yes. You can get away with a smaller foil and get better steering

 

Check out the config of the whiting 26 1/4 tonners

[Guest]

Thanks Royale 780,

Anything to do with a Ross.

I would have thought the question would have started a bit of a debate, everybody has a opinion on what makes a good sailing boat but it seems I am wrong yet again.

Bugger of a winter here sofar won't stop raining.

Peter

[B00B00 310]

I think with a 4KTSB like that the quarter wake at any speed would make the rudder pretty well 'Immersed' anyway....

[Marshy 30]

And with Quarter Tonners you get 2 in the stern and it sinks it down anyhow...

[Norwegian Blue 10]

Unless it is us, in which case the rudder seems to spend too much time clear of the water (well downhill anyway)

[Island Time 1,234]

The primary difference is that an immersed rudder is less likely to suffer from ventilation (air being sucked down the lee side of the rudder causing loss of steering efficiency). Rudders that penetrate the suface are therefore less efficient when at the limits of their envelope.

 

There can be advantages though, like everything in a boat it is a compromise!..

[Guest]

Interesting question, good for hours of shite dribbling at a seedy bar somewhere.

There are many different things that contribute to the efficiency of the rudder.

Very basically:

Where: Lift = driving yacht forward, drag = slowing yacht down.

All other things being equal the lift of a rudder increases with area, so a larger rudder gives more lift. Area is basically rudder span (length of rudder) x rudder chord. Also, drag is proportional to lift, so more lift = more drag. Also, more area = more drag

Now, the rudder is used to control the angle of attack of the lifting surfaces of the vessel (lifting surfaces is the keel and rig). The lifting surfaces will typically pivot somewhere around the keel. Thus the rudder acts like a lever on the yachts angle of attack. As you know, a short lever requires more force than a longer lever to get the same result. So the closer the rudder is to the keel the bigger it needs to be: bigger rudder = more drag.

 

Except that is not the whole story. The rudder is a lifting foil that has a whole bunch of considerations... The most important here is its aspect ratio (span / chord). Aspect ratio is important because lifting surfaces have 'tip losses'. which basically means the ends of the lifting surface don't do much good work but they do contribute drag. So, a rudder hung under the hull has only one end to suffer tip losses, the other end is up against the hull and will not suffer tip losses to the same magnitude.

 

Whereas a transom hung rudder basically has two 'free' ends. The bottom tip losses are the same as for the inboard rudder,but the losses at the top are slightly different.

At the top there is the free surface of the sea, and so the pressure variations due to the lift/drag of the rudder will cause pressure waves to form off the rudder leading to increase in wave drag (not much though). More importantly there is no end plate on the rudder to stop it ventilating from the free surface when it is heavily loaded (as mentioned in a post above). That is to say when you are trucking downhill and suddenly you are trying to avoid a massive broach, the rudder is really loaded up, one side of it will have very low pressure - it will suck air down along the rudder blade and voila -rudder lets go and all hell breaks loose. In the same situation where the rudder is underneath the hull, it is harder for it to suck air so it will hang on longer.

 

Having said all that, you would probably not notice much difference on most older boats. I think you would generally be better off with a bigger rudder with more drag, than one that stalls and lets go. Mini650's went through a phase of smaller high performance rudders to reduce drag. I notice that they are going back to big rudder and to hell with the drag (but I suppose they have not problem with lack of power..).

 

On the boat in the pic - there would be a lot of work to move the rudder and put it inboard, I dont know how much it would improve performance (negligible?). I suppose you could make it slightly smaller if inboard, but that means building a new rudder. Do you have a problem with it letting go/stalling?

 

TL;DR -and just re-read your question - For the same size a rudder further aft will give more powerful steering in most conditions. However, under extreme loading the inboard rudder might be less likely to stall.