Trim Tab Self-Steering - NO Windvane

[colo.sail]

http://www.sailsarana.com/selfsteering gear/sarana trim tab.pdf

Who has used/built/seen.... a self steering system such as Sarana uses without a windvane. How effective are these? Advantages or disadvantages?

Looks pretty simple to fabricate

THANKS

[tager]

I am assuming you mean an electronic autopilot linked to a trim tab. If you just have the trim tab, with no force input, it won't be self steering, it will be a trim tab flopping around uselessly in your wake.

I am pretty sure that they use some type of input to their tab, be it wind vane or electric autopilot.

The advantage of linking the autopilot to the trim tab is that the energy required to steer the boat is harnessed from the boats motion relative to the water, rather than from it's battery banks.

The disadvantage is additional complexity, maintenance, and expense.
If I were setting up a self steering system, it would have both vane and electronic inputs. Sarana has a nice style because it does not require lines through the cockpit, holes in the transom, or any of the complexity associated with most commercial vanes.

[FSMike]

Terminology problem I think. I consider a windvane the part that sticks up into the wind. I think sailsarana is using a wind vane without an attached rudder, he is using a windvane attached to the trim tab on the boat's rudder. As sailsarana notes, lots of folk have done this, but he seems to have put a lot of thought into his version.

[colo.sail]

at the attached link.
The tab is simply set by trimming the boat and using the trim tab to hold the rudder in a relative position with a set screw setup. No vane. No autopilot. Just set the tab as you would in a Cessna and leave it.

[hummingway]

They are using an electric autopilot. Self-Steering Gear - Build your own trim tab self-steering gear
That document is part one apparently.

[fmooy]

With respect colo.sail, I think you misunderstand what that pdf is meant to teach. It is only concerned with the design of the trim tab. You still need some input to the trim tab to make it work whether that is a wind vane or electric autopilot. If you look at the pictures of Sarana on the website you can clearly see that the author is using a simple windvane as the input for that trim tab. Take a look at the picture on this page, for example. Having a trim tab that is simply held in a fixed position with a set screw would be no better than just lashing the tiller or wheel.

[dpex]

Ive been agonising over the relative math required to make a trimtab effective in all conditions.

I'd appreciate your opinions on my theories.

It seems to me that I'm dealing with three interconnected levers which then drive a fourth.,..the rudder.

So I started from the view, 'What force is required of a tab to cause the rudder to change its angle of attack.' Answer. 'Don't know.' Then I went on to how far the tab had to move to cause the main rudder to move. Answer. Don't know.

But my experience with aeroplanes suggests it doesn't take that much effort from an aeleron to lift/drop a wing, especially if the lift/drop is minor. Obviously, major inputs ( a significant shift in attitude) require greater forces.

So allow me to address the input forces. The first is the vane. The size of this beast 'must' determine an initial input force. That force, in turn, would be determined by the wind-speed...wouldn't it?

Sure, there are some minor vaguries in this, in as much as a hot wind will have less substance than a cold wind, but how much diff is problematic.

Then there's the arm which is turned by the vane which, in turn, turns the tab. The length of this arm has a force relative to the pressure exerted by the vane.

For example: Let's say the vane applies 10kgs pressure to the arm...The pressure, may I add has to be a quantum of pressure along the length of the vane. But for the moment let's say at a certain wind-speed it delivers 10kgs to the arm.

This arm is, say, 200mm long, whereas the vane is 1000mm long. That says, unless I'm way off the mark, the vane is delivering a force of 5-to-1 (50kgs) to the arm which is connected to the tab.

The tab, as mentioned, is fixed in both length and width...although it can me made less.

And so, is a tab which is 1/10th the size of the main rudder enough to cause th main rudder to turn? Is 1/8th, or 1/6th the requirement?

Dunno.

So here's the theory.

I figure on making a tab of about 1/5th the area of the rudder. I can hack this down as the experiment continues.

However, the main input force (the vane) will be a simple sail, captured between two horizontal arms, and on a simple roller-reefing system. Ergo, I will be able to increase or decrease the presented area of the vane, according to conditions.

Next is the relationship between the lever-arm (attached to the vane) and the levered arm (attached to the tab's axles). I figure that making this a multi-choice arrangement then will enable me to fiddle with settings.

For example, I may find the tab is too large in all conditions and that over-steering is an issue. No probs. I'll take to it with a saw and lop of some area.

But, because I can fiddle with the pressure applied by the vane, via foller reefing, and fiddle with the relative lever arm connected to the tab arm, I should be able, over time, to discover a whole bunch of ideal arrangements.

For example, I may find, in 30 knots on the beam, with my particular boat, that I need 2/3rds vane sail and a 6-to-1 lever. Or half vane and a 4-to-1 lever.

Then I might discover that hard on the wind, in 10 knots, I need full vane and 8-to-1 lever.

The beauty of the design I'm working on is, apart from the size of the tab, I can adjust the two other major parameters in seconds, which is really good for instant gratification when it comes to experimenting.

As asserted. Your opinions would be most welcome.