Avocet
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Reged: 03/06/2001
Posts: 1227
Loc: Cumbria
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This is probably an embarrassingly basic question, but can anyone explain in layman's terms why some (usually older) boats have the rudder pivot axis inclined to the vertical?
Avocet is a good example - a long-keeler with sternpost angled at about 45 degrees to the vertical and the rudder hung on the sternpost.
Is it something to do with the angle the rudder presents to the water flow when the boat is heeled?
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Carvel
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Reged: 12/11/2007
Posts: 88
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Others may have more informed/technical explanation, but i would have thought that it has the advantage of having a keel hung (i.e protected) rudder, without having to have the keel brought right back to the stern (i.e. giving greater manoeuvrability and keeping wetted area down), and having the simplicity of a transom hanging, rather than the complications of holes in the hull, bearings, etc associated with a through-hull rudder.
Of course though the rudder pivot is at, say, 45 degrees to the vertical, the water stills flows (more or less!) horizontally across it (unless you push tiller over too far and rudder stalls). As far as i can see (and my rudder is of this description) there is a lot of nonsense talked about imagined effects of downforce, etc. (No one expects the thrust on a vertically hung rudder to be up and down!)
There. That should put the cat among the pigeons!
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VicS
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Reged: 13/07/2002
Posts: 8974
Loc: Home: Kent. Boat: Chichester
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Quote:
the cat among the pigeons
................. 
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Old Chemists never die, they just fail to react
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Engage
regular
Reged: 28/10/2001
Posts: 26
Loc: Preston, Lancashire, UK
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It just so happens that I am trying to extract my rudder assembley and it too is inclined by about 10 degrees from the vertical. I have come to realise that such design features are so in order to make life as difficult as possible. The only way I can drop the whole thing is either to hoist the bow 10 foot into the air or float the boat and let it drop out. I need a plan C!
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At last - Never ending debt, worry, sickness, hard work and sticky mess - Yes I now have a boat.
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oldsaltoz
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Reged: 04/07/2001
Posts: 2570
Loc: Australia, East coast.
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Most people dig a hole.
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Growing old is unavoidable. However, growing up is still optional.
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dovekie
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Reged: 08/06/2003
Posts: 62
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One of the many things "Avocet" is a good example of!
I agree that it must be because of the wish to have the strength of a keel hung rudder, without the keel coming right aft. I can't think of a skeg hung, or free hung rudder that isn't vertical - presumably because vertical is best if there are no constraints. The plane of sweep of the tiller is also affected, but I can't see that being important enough to dictate design.
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William_H
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Reged: 28/07/2003
Posts: 3513
Loc: West Australia
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Just as a contrast one 22ft boat that was popular here called a Sorcerer had an inclined transom top forward ie longer at the water line. This was perhaps for style or to make the water line longer without making the whole boat bigger. Anyway they were supplied with a transom mounted rudder where the pintles were paralell to the transom.
The rudder was soon found to be very inefficient. Brackets were soon fitted to them all to move the top pintle out and make the pintle line vertical.
With the pintle line near 30 degrees of vertical when you ask the rudder to provide lift for turning the boat the lift is inddeed partly trying to lift the stern of the boat so requiring more rudder input and increasing drag.
If the boat is sailed in strong winds then there is an inevitable weather helm caused by heeling the boat. So you are constantly askinng the rudder to turn the boat so drag becomes huge. And stalling of the rudder more likely.
Perhaps with long keel boats ie with bottom rudder pintle on the keel there is not so much weather helm when going hard so rudder does not have to work so hard. Hence rudder away from vertical does not matter. Or maybe people who own these boats don't care. (uncritical love)
Note of course that the rudder when not vertical due to heel also tends to lift in the wirng (vertical ) direction which also adds to the inefficiencies. Hence some boats have twin inclined rudders. Only the leeward rudder being then vertical is used.
Just a few thoughts olewill
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Avocet
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Reged: 03/06/2001
Posts: 1227
Loc: Cumbria
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Quote:
One of the many things "Avocet" is a good example of!
...eh????
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Carvel
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Reged: 12/11/2007
Posts: 88
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Quote:
had an inclined transom top forward ie longer at the water line..... rudder was soon found to be very inefficient... With the pintle line near 30 degrees of vertical when you ask the rudder to provide lift for turning the boat the lift is inddeed partly TRYING TO LIFT THE STERN OF THE BOAT ....
I'd suggest that an inclined pivoted rudder is no more trying to lift (or sink) the stern of a boat than your foresail mounted on an inclined forestay is trying to lift it's bow into the sky! (Though I've heard that myth, too!) Nor is the rearward sloping forward edge of your keel trying to sink the whole boat. The force is generated by the FLOW of water (or air), and is (as I understand it) perpendicular to it. Provided that flow is horizontal and the boat upright, the lift is lateral in all three cases.
Imagine an 'individual bit of water' (as it were!) flowing under your boat and along the side of your keel. When it gets to the front edge of the rudder it is deflected to one side, but not (for our intents and purposes) up or down. It does not 'know' that the rudder starts ahead or behind just above and below it. it just continues it's course at its depth of whatever.
Many aircraft have swept back wings. They lift the aircraft up. They do not get torn off trying to fly in a direction perpendicular to their leading edges! This is because the flow of air across them is (like an inclined rudder) not perpendicular to their leading edge, but parallel to the ground and fuselage (or in our case the centre line of the boat and water water surface).
There are, no doubt, effects such as that arising when when a boat is severely pitching, but they are marginal to, and not to be confused with, the essential flows and forces involved.
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Avocet
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Reged: 03/06/2001
Posts: 1227
Loc: Cumbria
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I stand to be corrected here, but I would agree with you when the boat is upright AND the rudder pivot is vertical. However, when the boat is heeled, I would have thought the rudder would try to lift (or sink) the back of the boat a little depending on which way it is turned?
Imagine being heeled over (for argument's sake) to starboard. I guess that most boats would be applying a little bit of starboard rudder (not sure of that's the correct term but I mean the helmsman would be moving the tiller / wheel so as to turn slightly to starboard to counteract the natural tendency of the boat to round up into the wind). Under these circumstances, surely the rudder would be partly steering the boat and partly acting as a "trim tab" trying to lift the stern slightly?
As far as I can think, angling the rudder axis aft at the waterline and forwards at the bottom (i.e. the "traditional" old fashioned style) would exaggerate that effect...
...but I'm not sure if that's true, and if it is, I'm not sure whther it's "good", "bad" or "irrelevant"!
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