Why are some rudder pivots inclined?

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Avocet

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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?
 
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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the cat among the pigeons

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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!
 
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.
 
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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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 ....

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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.
 
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"!
 
You are absolutely right that when heeled part of the rudder force (assuming you are pulling the tiller 'uphill' as described) is trying to lift the boat, in the same way that the sails are pushing it down. Both these forces would be proportional to the angle of heel.

What you don't make clear is why you think that an inclined rudder pivot would make any difference to this. The foresail (inclined leading edge) will be wasting just the same proportion of its power pushing down, rather than across, as the mainsail (leading edge perpendicular to direction of motion). If you think that the traditional, sloping down and forward, rudder pivot would exaggerate this effect, do you think that a trailing rudder pivot (i.e. sloping down and back) would diminish this effect?
 
1) Having pondered the matter further (I've become fascinated by it!) I think I was mistaken to say that the foresail doesn't give any lift on account of its inclined or raked stay. The force generated is perpendicular to the sail, and when a vessel upright and is on a beam reach, for instance, the force generated will be partially forwards and partially upwards. The relative proportions of these will depend on the degree of rake of the forestay, so that a low aspect ratio rig such as on a traditional boat with a bowsprit and shortish mast will have a relatively greater lifting component, and smaller forward driving component, than a modern tall high aspect ratio rig. The lifting component, in either case, would appear to diminish as you get towards close hauled. I guess that there is also a lifting component when running.

This lifting effect wouldn't seem to apply to the rudder because the water is passing (more or less) horizontally from (more or less) ahead to astern, and not across it. Unlike the foresail above, at no point is the water flowing perpendicular to the axis (of the rudder).

2) Willliam H. mentioned above problems with the rudder design of the Sorcerer. I think the problems do not relate to from any upward/downward force arising the inclined/raked axis of the rudder per se. In that case, as I understand it, the rudder was raked aft, which means that the lower end of the rudder is unsupported. One implication of a raked rudder is that you need greater total rudder length to reach the same depth of water (though on the other hand you get the yaw lever effect of it further aft). With such an extreme rake as 30 degrees that's an awful long length of unsupported rudder to get the necessary depth, and both the rudder itself and, especially, the gudgeons/pintles would have been under very high loads. Sounds like they weren't sufficiently stout to take it. Quite a large number of sporty boats have aft raked rudders, including, I believe, First 18, Foxcub, Hunter 19, Achilles 24, First 25, Hustler 25.5, Sadler 34, (though none of these as extreme as 30 degrees) so I don't think there's a fundamental problem with them.

3) Pye End suggested that raking/inclining the rudder gives you some balance to the rudder, but I don't think that's the case. If you have a forward raked (i.e. traditional) rudder axis, the weight of the rudder tends to pull the rudder/tiller central when stationary, but i wouldn't have thought it significant when under way. The sporty aft raked rudders would presumably flop to one side or another left to themselves, unless the rudder is actually buoyant, in which case it'll tend to centre itself at rest.

4) Whatever the mechanics/hydrodynamics of it, raked rudders can obviously do the job. Many boats renowned for their handling - e.g. Contessa 26, Cutlass, Hustler, Twister - have their rudder axis raked one way or the other. Apparently "Early critics thought the raked rudder [of the Folkboat] would make steering difficult under some circumstances, but experience proved them wrong.".
 
I can certainly vouch for the self-centring effect of the "traditional" rudder out of the water - that's exactly what Avocet's does when released. Obviously, the same effect exists in water but much reduced because of the rudder's bouyancy and the forces exerted by the water whilst the boat is moving. I've never sailed a boat with the rudder axis inclined the other way but was wondering whether it might be something to mitigate the "feel" of weather-helm a little?

I still can't agree about it generating no lift though. I can agree that a rudder with a vertical pivot axis wouldn't generate any lift with the boat upright, but as it heels, I think it wold generate more and more. At extreme angles of heel, I think it might even generate more lift than yaw!

As for the handling, I guess that I'd have to agree with your kind words (Avocet's a Cutlass after all!) but there are also disadvantages (like a lousy turning circle at close quarters). I was wondering whether there are any boats with traditionally inclined rudder pivots that didn't have long keels too? In other words, I was wondering whether the nice handling these boats enjoy is actually nothing to do with the rudder being inclined, but is to do with having a long keel? I certainly also take the point in one of the earlier posts that not inclining the rudder would make for an even worse turning circle!
 
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