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Old 27-04-2014, 10:27   #91
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Re: Placement of windvane steering system.

There is a way to balance your rudder without moving your pivot points aft. It's not really ethical to extend the rudder below the keel but I've seen it on twin keel boats w/o I'll effect.

I'm not a marine engineer so take this as you like.
If you were to extend the rudder down another foot and add a foil forward, which extends forward of the pivot points and under the skeg, this would balance out the hung rudder.

It's a bit out of the box and it would take some good math to get it right but it sure would ease off on the strain on the tiller. Trial and error would work too but that would probably be more expensive then an engineers math time.

Or like I stated earlier if you extended the pivot points aft by adding A-frames to the transom and an extension to the skeg a portion of the rudder could be forward of the pivot point. Or you could shorten the skeg and add the foil, a bit similar to this one below.


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Old 27-04-2014, 10:52   #92
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Re: Placement of Windvane Steering System.

Imagine the lines and nets and crab pot collection you might catch on that.
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Old 27-04-2014, 15:43   #93
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Re: Placement of windvane steering system.

Quote:
Originally Posted by tedsherrin View Post
And just found what the vessel originally had in a windvane.
I have absolutely no probs with the idea of mounting a trim tab on that rudder.... semi balanced or not... look at my aux rudder.. it is barely 'semi balanced' and it works a treat. The only issues I would have with the one shown in the drawing is that the vane looks a bit small. You can see the approx ratio that mine has in the bottom linkage.... maybe yours would need more 'oomf'... ie a shorter arm on the head of the trim tab
I also wonder why you would build it just to hook an autopilot to it.. why not just set it up as a windvane?
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Old 27-04-2014, 16:28   #94
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Re: Placement of windvane steering system.

Quote:
Originally Posted by El Pinguino View Post
I have absolutely no probs with the idea of mounting a trim tab on that rudder.... semi balanced or not... look at my aux rudder.. it is barely 'semi balanced' and it works a treat. The only issues I would have with the one shown in the drawing is that the vane looks a bit small. You can see the approx ratio that mine has in the bottom linkage.... maybe yours would need more 'oomf'... ie a shorter arm on the head of the trim tab
I also wonder why you would build it just to hook an autopilot to it.. why not just set it up as a windvane?
Pure cost. I just want something to get me out if trouble for next summer which building, or rather having my boiler maker make me up a trim tab will be easy. But adding a windvane as well will cost much more. I'm looking at an autohelm upper unit as well which would be $2500.
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Old 27-04-2014, 16:54   #95
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Re: Placement of Windvane Steering System.

Actually, once you have the trim tab mounted and controlled, the rest of the wind vane mechanism could be fabricated affordably. But you will be well on your way with just the electrically operated trim tab.
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Old 27-04-2014, 20:02   #96
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Re: Placement of Windvane Steering System.

I must be seeing the opposite of what this thread is taking about. Our rudder is a semi balanced spade, (but more balance than Demarrey's odd setup) and the complaint I have in following seas it is too easily thrown about. It's an effort to keep the tiller from flaying around, the advice I got was to build a skeg, which means making the rudder unbalanced (as all skeg hung are unbalanced). Supposedly the skeg steadies the rudder.

We have a trim tab on our spade, but the wind vane steering can only manage it if the boat is slowed down. 6 knots & over - no way, reef to keep speeds under 5 knots and sails are balanced too, then the vane/trim tab can manage.

My feeling is a trim tab on Ted's rudder will work, probably better than on ours. Just the tab's area (or chord) needs to be at least 1/4 of the rudder's.
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Old 28-04-2014, 18:34   #97
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Re: Placement of Windvane Steering System.

Skeg hung rudders can be balanced.
Maybe I misunderstood your meaning?
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Old 29-04-2014, 02:37   #98
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Re: Placement of Windvane Steering System.

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Originally Posted by Capt Rottnest View Post
I must be seeing the opposite of what this thread is taking about. Our rudder is a semi balanced spade, (but more balance than Demarrey's odd setup) and the complaint I have in following seas it is too easily thrown about. It's an effort to keep the tiller from flaying around, the advice I got was to build a skeg, which means making the rudder unbalanced (as all skeg hung are unbalanced). Supposedly the skeg steadies the rudder.

We have a trim tab on our spade, but the wind vane steering can only manage it if the boat is slowed down. 6 knots & over - no way, reef to keep speeds under 5 knots and sails are balanced too, then the vane/trim tab can manage.

My feeling is a trim tab on Ted's rudder will work, probably better than on ours. Just the tab's area (or chord) needs to be at least 1/4 of the rudder's.
I wonder if your problems are due, not to the semi balance, but to the trim tab. Depending how the linkage is designed, trim tabs can cause funny effects on a semi balanced rudder, including introducing instability and over-balancing. Is there any way to set the trim tab so it simply stays in line with the rudder when steering manually?

If your spade rudder is inherently overbalanced, this will change the balance by moving the centre of effort aft.

If it is STILL overbalanced when the tab is locked to it, the linkage could be reconfigured so the trim tab will CORRECT the overbalance when steering manually, by countersteering.

The usual problem with trim tabs on spade rudders (apart from access) is instability when backing up.

If the trim tab is free-swinging when steering manually, this might also cause stability problems, but backing up would be impossible, so I'm guessing that's not the case here.
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Old 29-04-2014, 03:31   #99
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Re: Placement of Windvane Steering System.

Quote:
Originally Posted by Andrew Troup View Post
I wonder if your problems are due, not to the semi balance, but to the trim tab. Depending how the linkage is designed, trim tabs can cause funny effects on a semi balanced rudder, including introducing instability and over-balancing. Is there any way to set the trim tab so it simply stays in line with the rudder when steering manually?

If your spade rudder is inherently overbalanced, this will change the balance by moving the centre of effort aft.

If it is STILL overbalanced when the tab is locked to it, the linkage could be reconfigured so the trim tab will CORRECT the overbalance when steering manually, by countersteering.

The usual problem with trim tabs on spade rudders (apart from access) is instability when backing up.

If the trim tab is free-swinging when steering manually, this might also cause stability problems, but backing up would be impossible, so I'm guessing that's not the case here.
The rudder has about 5% balance, less than Belcher's ideal of 16 or 17%. We sail with the trim tab free as locking it increases the weather helm. Its totally fine free swinging, it just trails the rudder. Wave action, from 2m following seas while downwind or on a broad reach does knock it (the rudder) around regardless locked or not. (I think its worse locked.) We lock the trim tab for reversing.

Usually for long bouts of manual steering downwind, because I want to make time (6-7knots), I'm rigging bungy cords to the tiller to help stability. Not perfect.

ps, I had a traditional style in my head of a skeg hung rudder, I now know some are balanced.
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Old 29-04-2014, 06:03   #100
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Re: Placement of Windvane Steering System.

Balancing skeg hung rudders without introducing catchpoints for lines and debris is not at all straightforward, and in any case, the mere presence of a skeg lightens the steering load on any rudder close behind, which is significantly deeper than the chord.

"Horn" balance, as proposed by Delmarrey, and as we saw on early aeroplane rudders like the DC3, is (as Terra Nova implies) effectively inviting lines into the foward-facing V 'nip' when the helm is over, only to jam the rudder when it is moved back towards midships.
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Old 29-04-2014, 07:16   #101
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Re: Placement of Windvane Steering System.

There is another way of balancing a skeg hung R. One would have to hollow/notch out the skeg so it would be more like a U-shape or even just an angled support for the bottom of the rudder.

This would deflect any stringy debris that could catch. But we're talking about a structural refit to the skeg. There would have to be a significant gap between the skeg and the front of the rudder for water to round up and create the proper forces for the front of the rudder to redirect the flow. IAW what's remaining of the skeg would need a narrowed trailing edge like the keel or the rudder itself.

If this is too hard to understand I could do up a drawing?
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Old 29-04-2014, 13:26   #102
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Re: Placement of Windvane Steering System.

So, effectively a freestanding rudder with the addition of a bottom bearing, on a strut or bracket?
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Old 29-04-2014, 16:31   #103
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Re: Placement of Windvane Steering System.

FWIW,

Our semi-balanced rudder has a narrow skeg that stops at about 2/3 of the depth of the blade. There is only a very narrow gap between the balance area and the bottom of the skeg. To avoid catching weed or lines in that gap, the builder welded a bit of ~ 1/2" round stock projecting downward from the lower gudgeon, lapping several inches beyond the gap. This blocks entry into the gap, and has worked well for many miles of cruising.

I can't see any real disadvantage to fitting such a device to any semi-balanced skeg rudder.

Cheers,

Jim
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