sheet to tiller - what elastic

dylanwinter

dylanwinter

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my sheet to tiller steering system plans have got to the point where I am after the elastic part of the deal


I am going to start simple - stright from tiller to boom via a block

I am told that medical tubing is the bees knees for the other side

what gauge and where do I buy it

or can I use thin shock cord?

can I use catapult elastic?

http://www.ebay.co.uk/itm/Catapult-...Hunting_ShootingSports_ET&hash=item27cbabfe9b

D
 
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I think it needs to be pretty stout stuff.
You can also vary the mechanical advantage by perhaps attaching the elastic further along the tiller than the sheet.
If you look at a certain patented system, it has a large mechanical disadvantage for the jib sheet, so a small movement of jib sheet is magnified at the tiller, and the force reduced by the same ratio. The force in the elastic balances this, give or take the steering force on the rudder.
There are two variables the way I look at it.
The rudder movement per inch of sheet
The elastic force per inch of sheet or per angle of rudder.

Wind strength is another variable, but you don't have control of that.

I would guess that experimenting a lot is the way forward, being able to easily alter the 'effective bungitude' of the elastic, the preload in it and the sheet tension would be the key.
Being able to change the sheet/rudder movement ratio might help too?

But balancing the tension in a sheet is a lot of force, unless you have some sort of mechanical ratio.
 
Thin shock cord will allow you to double treble etc up on it.

Catapult elastic might be something to consider.

Nick Skeates, in one of the articles I referred you to also mentions bands cut from an old inner tube

I dont know what medical tubing is but ordinary red rubber laboratory tubing comes in a wide range of diameters and wall thicknesses
 
Might be worth using something like shock cord, which is hard to snap, because the outer braid takes the load?
There is possibly a lot of energy stored in elastic at these forces, if something breaks it could be painful or worse.
Watch out for any elastic chafing under load.
We have a system on the dinghy which has about 5ft of 3mm bungee, which packs a punch!
Please take care and report back!
 
It might be worth splashing out on shockcord of a known brand, e.g. Marlow, because then you have repeatable characteristics, and multiple sizes available.
Anything rubbery will age though.
You can get digital spring balances very cheaply on eBay, maybe measuring some of the forces when you get it working would be instructive?
 
bluerm166 said:
Why ignore the progress made by others.
Surgical tubing seems to be recommended.
http://www.jsward.com/steering/
Click to expand...

+1 for following others' research.
That system has much less tension and stored energy in the bungee than the steersman as far as I can see.
Whether more tension means there is more steering effort available in waves etc I'm not sure.

I would think the design of the rudder must make a difference, if the rudder has little are forward of the shaft, it will give plenty of force pushing the tiller to leeward. A very balanced rudder will be more reliant on the system pulling the tiller to leeward.
In waves, some boats need quite a push on the tiller some of the time, other boats its always a pull.
It's the same with autopilots on yachts. A wheel pilot like mine worked up to a point, I guess the motor is 50W? On the same size boat, a racing B&G system intended to hold a course downhill with the kite up would have an erking great hydraulic ram and have a lot of force and power available. The wheel pilot was OK in easy conditions, I did not want to spend thousands on a 'serious' system.
Interesting stuff.
 
Shock cord is not very good for sheet to tiller steering as it has a non-linear response to load - at first it stretches easily but as the outer braid compresses it gets harder. You will find it hard to get good results with shock cord.

Medical rubber as used by physiotherapists is good, bicycle inner tube sections usable too, Catapult elastic might well be OK too.

Don't expect miracles from sheet to tiller, but it can usually be set up to give you a reasonable hands-off course anywhere from close to broad reaching. To windward most boats can self-steer reasonably well with tiller or wheel lashed, slightly oversheet jib and ease main.
 
rubber tube

jwilson said:
Shock cord is not very good for sheet to tiller steering as it has a non-linear response to load - at first it stretches easily but as the outer braid compresses it gets harder. You will find it hard to get good results with shock cord.

Medical rubber as used by physiotherapists is good, bicycle inner tube sections usable too, Catapult elastic might well be OK too.

Don't expect miracles from sheet to tiller, but it can usually be set up to give you a reasonable hands-off course anywhere from close to broad reaching. To windward most boats can self-steer reasonably well with tiller or wheel lashed, slightly oversheet jib and ease main.
Click to expand...

I have had a poke around ebay for rubber tubing

not much on offer

and I have not got a clue what size I would need

I wondered if I increased the length of the std shock cord with a block or two then I ciould extend the period when the stretch is reasonably linear
 
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