Rudder angle

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Graham_Wright

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Although I realise that it is impossible to generalise, what is the optimum maximum rudder angle on a sailing boat. I have heard of rudders snapping their posts when the boat was going astern a bit smartish and the helmsman letting go of the wheel.

30°?
 
Sorry, not really an answer but more of an observation - the form of the hull must have some bearing on it. A flat-bottomed boat with a narrow fin would have a tight turning circle and maybe could go that far - or a bit further. Our current long-keeler rudder doesn't look like it will go much past 20 degrees - or that there would be any point if it did.
 
The max rudder angle as said depends on the speed of turning of the boat. My little boat will turn almost in its own length having a fin keel. This means that with the rapid rate of turn even more rudder angle can be applied without stalling the rudder. But a slow to turn boat would tend to stall the rudder especially when applied before the boat begins to turn.

Obviously any more rudder angle than will stall the rudder is bad gives less turning power and as I suspect your concern is, is a greater problem when reversing as the rudder pressures can overpower the helmsman.

We might note here that if the rudder has more balance ie area ahead of the axis then that balance will also reduce rudder pressures when going astern. So you might connsider on lift out if you can add more rudder area forward of axle to improve balance.

good luck olewill
 
A few spade rudders will turn through 180deg in this situation without damage....
A fin keeler can usefully use a lot of rudder angle when the stern is sliding in a turn, so the only answer is to have a very strong rudder, or take care when going astern.
forty five degrees is probably the minimum full lock you would want, and thats enough to generate huge force astern.
Any mechanical stops on the helm need to be very strong.
Perhaps it should be designed to break in a non-catastrophic way in this kind of event? i.e with the hull and rudder stock/shaft intact.
People do get hurt by the tiller going astern, care is always required.
 
The centre of effort is never more than 30% or so aft, so a balanced rudder can only reduce the problem by half or so. Most modern rudders have quite a lot of balancing area already.
Having a high aspect rudder will help as the lever arm is shorter in astern, but this brings its own strength requirements of course.
 
It is impossible to generalise as you say but on a sailboat the answer is about 36 degrees. Above that angle the water "stalls" the rudder and all you get is more turbulance. In fact this "stall point" occurs at a decreased angle the faster you go but for slow speed manouvering 36 degrees is the practical maximum. It's a common fault when manouvering to use too much rudder, especially in reverse.
 
acording to Ian Nicolson's 'Build your own boat'

<span style="color:red"> A device to prevent the rudder going over more than 35 degrees may be built into the steering gear. </span>

This is where I have fitted the stops on my tiller.

This seems much in line with what other posters have sugested.
 
G'day Graham,

Yes 30 degrees is fine for most sailing rigs, just make sure the stops are good and solid.

If adding balance, calculate the area behind the rudder stock and add 12% of that in front of the stock.

Hope this helps.

Avagoodweekend......
 
I think it depends on the boat, a fin keel which turns tightly will benefit from more lock, as the angle of attack is nothing like the rudder angle when the boat is turning through a small radius. Sometimes you need to maintain the turning force against the wind on the bow, despite the fact that the boat is already turning. This has been my experience in a light fin keeled boat.
On a long keel boat, with a keel or skeg hung rudder, 20 degrees may be enough to stall it totally into a brake.
Also I sometimes find full lock is needed when springing off using the propwash on the rudder.
I agree using too much helm is a common fault (and its not intuitive for some people to ease off a stalled rudder in a panic), but sometimes it's good to have the option!
All boats are not the same!
Also letting the rudder crash to even 30 degrees won't be good in astern. There is no substitute for the helmsman understanding this, particularly with a big tiller steered boat.
 
I believe that 35 degrees is generaly considered to be the optimum angle because after that the rudder starts to act more as a brake than a ruddder. Some boats like mine have built in stopps to prevent you going past this point while others like the trapper 500 can rotate through 360.

Yoda
 
My boats have had anything from nearly 90 deg with a transom-hung rudder to my current about 35. Of course there is no advantage having any more than, say, 35 deg in manoeuvring, except in the situation where the boat is stationary and you want to give a burst of forwards power to kick the stern over, when a large angle can be helpful - not with my present saildrive though!
 
As suggested 35 is the normal max, the real problem being the potential forces going astern. Limiting the angle to 35 helps in the system sizing. Some specialised systems do use greater angles to gaim more side thrust but they also usually invole more rudders, I once worked on a system that had three rudders per shaft and there were three shafts.

As pointed out at 'normal' speeds the sideways lift forming flow starts to stall around 35 degrees (it will vary at different speeds and with different profiles) so you get a fall off in turning effort with a sharp increase in drag.
 
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