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Old 24-04-2014, 22:51   #76
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Re: Placement of windvane steering system.

Well thanks for the advice. At this time only planning in putting a raymarine auto pilot on the tri tab/auxiliary rudder, so as TN has suggested, I'll give what I've got a go and using an auto pilot. If it works then I've at least done it cheaply. Adding a pendulum windvane system on an oar looks like it's a whole different thing and an ex.pensive exercise.
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Old 25-04-2014, 00:24   #77
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Re: Placement of windvane steering system.

Ted

I'm not qualified to have an opinion on whether a realistically sized trim tab would drive your particular transom-hung rudder, but I never let small impediments like that stop me:

If you can steer for six hours, and are not built like Arnie S, I think there's a good fighting chance. Trim tabs which mount full length of a deep rudder blade like yours are very high aspect ratio, and that makes them very efficient at providing lift for a relatively small area.

An ounce of depth is worth a pound of chord (well, not quite, but it's a much more desirable attribute).

They (unlike you) are tireless, and their force increases in proportion to those on the rudder, as boatspeed increases. And transom hung rudders are not usually balanced or semi-balanced, yet trim tabs are very popular for them, and generally turn out well suited.

Jim's point is sobering however, presuming the three owners he speaks of were acting independently on the basis of observed failure of competent and well proportioned trim tabs, with powerful vane inputs, to steer yachts of the same design. (The hull does look to have very steep run aft, which can make steering difficult in strong conditions)

The polar opposite possibility is that one guy theorised (rather than proved) it would not work, and the others followed his lead.

Setting that aside:

There's no problem for the autopilot: you just need to come up with a linkage which, in the course of providing the steering for the tab, reverses the direction.

You should also position the pivot points in such a way that, if the pilot applies a correction and is then switched off, as the rudder swings away from the centreline, the tab will progressively reduce its angle to the rudder.

The linkage proportions should be such that they're back in a straight line with each other (not the boat) when the rudder has turned through (I dunno) maybe 30 degrees at most. Ideally do a quick mockup in non marine materials to establish what this "negative feedback" or "rudder differential" should be.

The greater that angle is, the more powerful the steering influence, but the more likely you are to cross over into 'oversteering', which is where the boat describes a wavy course even with an autopilot set to minimum sensitivity.
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Old 25-04-2014, 00:45   #78
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Re: Placement of windvane steering system.

While I'm dispensing ill-qualified advice, in case the OP is still pondering his options:

Pillow blocks sound like a good idea in your situation, to me. (Self aligning, as you intend)

I would give consideration to finding ones with aluminium housings. This will help them to be anodic to the bearing raceways and balls. They will need to be replaced at intervals, but in the meantime will help keep the important contents from corroding.

You should try to find a series where the inners are available in stainless. I guess ideally the same size and profile would also be available in conventional bearing steel, because these could be fitted for proof of concept, and (with alu housings) would last a season or more if kept slathered with lanolin. The stainless ones (if you can get them) will be surprisingly spendy, although in recent years the gap has narrowed.

In your case, too, I would privilege length of the blade over chord, (ie high aspect ratio) even if it means going a size bigger on the bearings and rudder post.

Apart from delivering more lift from less area, and being less prone to disturibing variations of balance, there's the advantage that pitching will not affect the available steering effort as much as a shallower, wider chord blade.

I personally would not lose any sleep over the small distance from center. The Vikings had similar hulls, and look how far forward on the stern quarters, and how far out from midships, they carried their steering oars. They were an adaptable and innovative seafaring culture, so I don't think they would have done that heedless of the disadvantages.

Ann's point is worth taking on board, though: you won't have the luxury of knowing that a mirror image of the vane or autopilot setting will deliver the exact same results on the other tack.

You may be able to fudge things though, but simply moving the fiducial mark (the datum line representing a notional head-to-wind attitude of the boat, on the vane turntable) to a point midway between the optimum settings closehauled on each tack, in settled conditions, on flat water.
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Old 25-04-2014, 01:16   #79
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Re: Placement of windvane steering system.

Quote:
Originally Posted by Andrew Troup View Post
While I'm dispensing ill-qualified advice, in case the OP is still pondering his options:

Pillow blocks sound like a good idea in your situation, to me. (Self aligning, as you intend)

I would give consideration to finding ones with aluminium housings. This will help them to be anodic to the bearing raceways and balls. They will need to be replaced at intervals, but in the meantime will help keep the important contents from corroding.

You should try to find a series where the inners are available in stainless. I guess ideally the same size and profile would also be available in conventional bearing steel, because these could be fitted for proof of concept, and (with alu housings) would last a season or more if kept slathered with lanolin. The stainless ones (if you can get them) will be surprisingly spendy, although in recent years the gap has narrowed.

In your case, too, I would privilege length of the blade over chord, (ie high aspect ratio) even if it means going a size bigger on the bearings and rudder post.

Apart from delivering more lift from less area, and being less prone to disturibing variations of balance, there's the advantage that pitching will not affect the available steering effort as much as a shallower, wider chord blade.

I personally would not lose any sleep over the small distance from center. The Vikings had similar hulls, and look how far forward on the stern quarters, and how far out from midships, they carried their steering oars. They were an adaptable and innovative seafaring culture, so I don't think they would have done that heedless of the disadvantages.

Ann's point is worth taking on board, though: you won't have the luxury of knowing that a mirror image of the vane or autopilot setting will deliver the exact same results on the other tack.

You may be able to fudge things though, but simply moving the fiducial mark (the datum line representing a notional head-to-wind attitude of the boat, on the vane turntable) to a point midway between the optimum settings closehauled on each tack, in settled conditions, on flat water.
I'm sorry andrew, but that was all gobble gook to me. Can u put this in laymans terms? And what's a 'pillow' thingy?
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Old 25-04-2014, 01:33   #80
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Re: Placement of windvane steering system.

Andrew, you make some good points. Perhaps with his very high aspect rudder and a biggish tab it might work, but I'm pretty doubtful still. Dealing with yaw when sailing downwind in biggish conditions requires quick rudder response. Driving a non-balanced rudder with a tab will, I suspect, have a pretty slow response time, exacerbated in this case by trying to drive the tab with a small autopilot which adds its own delays in response. My guess is that by the time the trim tab gets moved by the a/p, and then the rudder reluctantly responds, the boat will be well off course.

As to the BCC's, they at least have the advantage of a longish keel and its inherent yaw resistance. Ted's boat appears to have a fin with a pretty short chord length and little natural yaw resistance (although the big fin-like skeg may help a lot). I don't know if any of our three friends actually tried the trim tab option, but at least one was in frequent contact with the builders whilst outfitting his boat to do the milk run, and he is, well, frugal is a nice word so had it been a decent option I'd have thought he'd have tried it. As it is, two of the boats fitted Monitors, and I can't remember what the other one had, but it was a similar servo pendulum rig.

And incidentally, my take on Ted's description of steering in a seaway was that it was pretty heavy. You seemed to have taken the opposite view. I wonder which is correct? Ted, could you give us an idea how much force is required to steer manually?

Cheers,

Jim
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Old 25-04-2014, 01:48   #81
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Re: Placement of windvane steering system.

Quote:
Originally Posted by tedsherrin View Post
I'm sorry andrew, but that was all gobble gook to me.
Not surprised, Ted, I was aiming my second post at the OP (Original Poster) who had quite a different set of questions.

Jim:

I understood that Ted was saying the helm was heavy, but my point was that if a person of ordinary strength can manage it for six hours in heavy conditions, that's possibly not too big an ask for a trim tab.

Servo pendulum gears have gained ascendancy because they can steer almost any boat, and because their configuration lends itself to a stand-alone product (which is essential for commercial reasons) but that doesn't mean almost any boat needs them.

Trim tabs used to be the de facto first choice for transom-hung rudders in the days when people generally built their own gears, and they did not develop a reputation for being underpowered, when designed properly.
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Old 25-04-2014, 01:53   #82
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Re: Placement of windvane steering system.

Andrew, my experience has an N of one! But when I built my aux rudder/trim tab steering system, it required that the aux rudder be semi-balanced to get adequate response time and deflection. Getting the degree of balance was tricky, but some ad hoc experimentation using the prop wash as a test rig eventually sorted it out.

Cheers,

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Old 25-04-2014, 08:58   #83
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Re: Placement of windvane steering system.

Quote:
Originally Posted by Andrew Troup View Post
...There's no problem for the autopilot: you just need to come up with a linkage which... reverses the direction...
All tiller autopilots I've seen had a built-in method of selecting direction, so they could be mounted on either port or starboard side.
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Old 26-04-2014, 22:57   #84
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Re: Placement of windvane steering system.

This is fun! I might as well toss in my 2.2 cents.

I am with Jim in thinking that a trim tab on that rudder on TedSherrin's boat will be well short of satisfactory. In fact, I would go so far as to say it would be pointless, but then there's no harm in trying it if you have the skills and equipment to do so.

If I were in the same situation as TedSherrin (and in a way I am, I have a barn door rudder hanging off the back of the skeg) I would fit an auxiliary rudder with trim tab, and make sure that auxiliary rudder was well balanced (approx. 30% area in FRONT of the pivot axis) and I would drive that auxiliary rudder's trim tab with a tiller pilot.

WHY so complicated? Well, I think that rudder would be as heavy as ours to operate due to the total lack of balance, so I would lock the main rudder to balance any weather helm, then use the auxiliary rudder to make minor corrections. And because the auxiliary rudder could be built with good balance, a trim tab would operate it easily. Then, the tiller pilot itself would benefit from the very small effort required to operate the trim vs the greater effort required for even a well balanced auxiliary rudder. (remember if you balance a rudder too much, it will over steer after it passes its critical angle, and that can be even harder to correct.)

But then, I would say all this, as it is exactly what I am planning for my boat.

Matt
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Old 26-04-2014, 23:04   #85
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Re: Placement of windvane steering system.

Quote:
Originally Posted by Andrew Troup View Post
While I'm dispensing ill-qualified advice, in case the OP is still pondering his options:....
Thanks Andrew, I reckon you have pretty well nailed it with the Viking comparison. (Actually, having a canoe stern I like to pretend I have a viking ship from time to time.)

And yes, as long as practical is the aim, for the reasons you've given, namely keeping the darn thing in the water.

I'll also go for stainless inners on the bearings, so far I have been pleasantly surprised at the prices, maybe I have been softened up by the boat bit prices till now, and these are just plain good quality engineering components, and fairly priced at that.

Matt
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Old 27-04-2014, 00:21   #86
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Re: Placement of windvane steering system.

I've been reading up on what u guys are talking about re 'balanced' rudder. I can't see any way I can easily balance it. I'm also amiss to understand why a well known naval architect such as Guy Saillard would have designed and built a boat with no rudder balance of it's such a bad idea. But he did, so that's what I'm stuck with.

I've attached a photo of a plan of the rudder and as you can see it included a trim tab. 110mm wide and as long as the section of the rudder is under water. I can't work out what it was attached to though.

I've also seen there is a lot of comment (and argument) else where on whether a trim tab on an 'unbalanced' rudder will work at all and it seems clearly it will, though with some degree of accuracy in doubt.

Given the cheapest option for me this winter is to make a trim tab and give it a go, connected to an automatic electronic pilot seems the way to go.
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Old 27-04-2014, 00:28   #87
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Re: Placement of windvane steering system.

And just found what the vessel originally had in a windvane.
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Old 27-04-2014, 01:29   #88
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Re: Placement of windvane steering system.

Well Ted, if the designer did his job correctly, and he said that the trim tab would work, I agree that it is worth a try. As drawn, that tab is pretty small, but sheesh, perhaps he did something clever with the shape of the foil and dialed it in. I'll be surprised if it really works, but pleasantly so. It is always good to see a very simple design do its job.

And if it does not, you can always add some balance area to the rudder later on...

Good luck,

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Old 27-04-2014, 02:46   #89
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Re: Placement of windvane steering system.

Quote:
Originally Posted by tedsherrin View Post
I've been reading up on what u guys are talking about re 'balanced' rudder. I can't see any way I can easily balance it. I'm also amiss to understand why a well known naval architect such as Guy Saillard would have designed and built a boat with no rudder balance of it's such a bad idea. But he did, so that's what I'm stuck with.
TedSherrin, I reckon I know why he designed it that way... what I think you have there is a really strong rudder setup, supported at the bottom, all the advantage of a rudder well aft, without some of the risks of it getting knocked clean off.

And I for one am not suggesting it is a "bad" idea to have an unbalanced rudder, just more effort to steer with, that's all. Sometimes I really wish our rudder were balanced, but then I take heart from the fact that it tells me really clearly when I have the sails out of balance.

A while back I posted about steering problems, thinking my steering system needed be geared down, but I subsequently learned (thanks to CF members) that the problem was not the steering at all, but how I had rigged the sails. Without the unbalance rudder, I might never have realised what I was doing wrong.

Good luck with the trim tab idea, I reckon there's no harm in trying it to see if it works, at least you have the choice of doing so, which I sure don't. And if it does work, it will be brilliantly simple to implement.

Matt
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Old 27-04-2014, 08:40   #90
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Re: Placement of windvane steering system.

Ted--you've received a lot of misinformation that trim tabs won't work on non-balanced, transom hung rudders. Ignore it. It is nonsense.

It is marvelous that you have the actual drawings for your boat's rudder trim tab. Good luck with your project. And post, here, if you need some help with it.
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