Steering sheaves / pulleys

[coopec43]

Apparently others have no objection to cast iron sheaves on boats.


https://stainlessdirect.co.uk/index....oducts_id=1203


Clive

[coopec43]

I think I have it worked out.

Some yachts are aft cockpit and have their pedestal virtually over the top of the quadrant so only need about 3 ft of steering cable. We know all cables stretch under load so the stretch could be about ¼ in. over three ft. That ¼ in at the pedestal sprocket translates to 2½ in. at the finger tips.

But my yacht is center cockpit and needs 12 ft of steering cable so the stretch could be about 1 in. which translates to 10 inch at the finger tips.

I believe that is why the local experts said I needed 8 mm cable. And when they saw my sheaves and quadrant could take 10 mm they said 10 mm cable would give a more positive feel? (and I can't remember their exact term).

If I was still going to use ss cable I would use 10 mm and it would only cost $20-30 more

Clive

[kmacdonald]

If you up the cable size you more than likely need to up the sheave size also. As the cable size increases, the CRITICAL bending radius increases also. If the sheaves don't meet the minimum radius the cable will fail under fatigue.

Edson uses aluminum for smaller sheaves and bronze for larger sheaves.

[beengone]

Have four six inch bronze sheaves with pins and straps. $50 each. San Diego.

[jkishel]

Originally Posted by Rowglide

I have read the article and gone over the Edson chart... what am I missing?

It appears to me that the loads referred to in the article are at the rudder post, a twisting action. When you move out from the center of the rudderpost by however many inches of radius on the quadrant, you are reducing the load.

Also, if you look at an 11 tooth Edson sprocket, the radius of the chain center is a hair over 2".

No?

per the article, 3751 lbs on the rudder acts 2 inches away from the centerline of the rudder post, resulting in a moment of 3751 x 2 = 7502 inch-pounds on the rudder post. The article's quadrant has a radius of 16 inches, so the force on the edge of the quadrant (the wire rope) is 7502/16 = 469 pounds, that is the wire rope and chain tension. The chain is driven by a 3 inch sprocket, so the moment on the sprocket is 469 x 1.5 = 703.3 inch-pounds. The helm has a radius of 15 inches, so the force on the edge of the wheel is 703.3/15 = 46.9 pounds.

More simply, the quadrant provides a mechanical advantage of 8 and the wheel adds a mechanical advantage of 10 in this example, for a total mechanical advantage of 80. 3751 pounds at the rudder divided by 80 = 46.9 pounds at the wheel. The wire rope sees 469 pounds. The breaking strength of 1/4 inch wire rope is about 5000 pounds when a straight length of new rope is pulled apart in a lab. Applying an appropriate safety factor and providing for the loss of strength in bends (thimble) and wire rope clamps gets you to 1/4 inch or maybe 5/16 inch pretty quickly. As mentioned above, the length of the rope may be a factor as well. Edson has a lot of expertise in this area, ignore their guidance at your peril.

[steamgoat]

Zephyrworks makes custom sheaves,I think he is in Washington state.

[coopec43]

This formula came from an excellent article on the elasticity of SS cable which I have bookmarked for my future reference.

According to my calculations (can someone check me?) a 100kg load on a 4 meter of 6 mm cable would stretch it 116mm. If that is right that would play havoc with the steering.

Going from 6 mm to 8 mm cable would nearly halve the stretch (28 to 50)
Going from 6 mm to 10 mm cable would reduce it by 2/3 (28 to 78)

Elastic stretch can be calculated by the following formula:

Elastic Stretch = (W x L) / (E x A)

W = Applied Load ( kN )
L = Cable length ( mm )
E = Strand Modulus ( kN/mm²)
A = Area of Cable = (D2 x pi) / 4 (where D= Dia of cable mm)
Typical values for E are:

1x19 = 107.5 kN/mm²
7x7 = 57.3 kN/mm²
7x19 = 47.5 kN/mm²
Dyform = 133.7 kN/mm²


EXAMPLE
Load 100 kg (1kn = 100 kg?))
4 M length steering cable.

1kN X 4000 mm / ((47.5 kg/13 mm2 ) X (6 mm x 2 X 3.1459)/4 =
4000 / (3.65 X 9.42) =
4000/ 34.44 = 116 mm

https://www.s3i.co.uk/wire-rope-technical.php

The mm2 of a cable can be calculated here
http://www.sengpielaudio.com/calcula...ss-section.htm


Clive

[Snowpetrel]

Originally Posted by coopec43

1kN X 4000 mm / ((47.5 kg/13 mm2 ) X (6 mm x 2 X 3.1459)/4 =
4000 / (3.65 X 9.42) =
4000/ 34.44 = 116 mm

https://www.s3i.co.uk/wire-rope-technical.php

The mm2 of a cable can be calculated here
http://www.sengpielaudio.com/calcula...ss-section.htm


Clive

Pi x D^2/4 not pi x dx2/4

6 mm x 2 X 3.1459/4 should read

6 mm^ 2 X 3.1459/4

[coopec43]

Originally Posted by Snowpetrel

Pi x D^2/4 not pi x dx2/4

6 mm x 2 X 3.1459/4 should read

6 mm^ 2 X 3.1459/4

I'm not familiar with your ^ symbol. But could you solve the equation to find a result? Please.

Just Now The penny dropped! The "2" is supposed to be a power? ie "squared".

I just had a look at the original article and even that shows it as "D2". It should read (D²) then?

How did you know that! (I bet you are an engineer or something)

That would make a difference then. I'll re-work it as soon as I get a chance.

Thanks



Clive

[Snowpetrel]

^^ that's the one!

For what it's worth I think 6mm 7x19 should be fine unless it is an exceptionally large unbalanced rudder with a high gearing ratio.

Be aware that a lot of books recommend a sheave diameter of 18 times the wire diameter. So for 10mm wire that would suggest a 180mm sheave which is getting very large.

[coopec43]

Originally Posted by Snowpetrel

^^ that's the one!

For what it's worth I think 6mm 7x19 should be fine unless it is an exceptionally large unbalanced rudder with a high gearing ratio.

Be aware that a lot of books recommend a sheave diameter of 18 times the wire diameter. So for 10mm wire that would suggest a 180mm sheave which is getting very large.

My yacht is a center cockpit so the steering cable is quite long and the yacht has a large unbalanced rudder.

I have decided to use Dyneema instead of SS cable so sheave diameter is not so critical.

I think you'll see the rudder in this photo.

Clive

[thunderhoof]

I think you should seriously consider 10 mm Dyneema. It has a rated breaking strength of 20,000 lbs. The recommended sheave diameter is 8X to 10X the rope diameter, or 80 to100 mm (3 1/2 to 4 inches). I know that Harken and others recommend a different pulley profile, one with a flatish bottom, to improve rope performance.

Rope like Dyneema has many advantages over wire rope:
1. Lower stretch under load. This makes it easier to "tune" your steering rig to maintain the correct tension under a range of sea conditions.
2. Better fatigue life. Rope will take more load cycles, at higher average loads, compared with wire rope -before failure.
3. Easier to install. Leading wire rope around your aft compartment will be not easy. Splicing eyes into the rope ends is easy. No sharp ends or "fishooks" to contend with.
4. Lower weight - maybe not important to a cruiser.
5. Better in the marine environment. No corrosion.

I have Dyneema steering lines in my boat. A Beneteau 40.7 it has a good sized rudder and the boat is sailed fairly hard. I have replaced the lines twice at 6 year intervals. I inspect them yearly. No wear has been observed.

[coopec43]

Thank you thunderhoof.


Your post is very reassuring coming from someone with first-hand knowledge of the matter: been their, done that.

3. Easier to install. Leading wire rope around
your aft compartment will be not easy.

You got it one! My mind is working like a computer evaluating different options. I've got all my engine components (like filters, fuel lines, strainers etc) tacked in with "tech screws" because it is highly likely I will have to move them to make way for the steering cables

Cheers

Clive

[tomtriad]

Schaefer Marine has a large selection of blocks and sheaves for wire, rope, etc.
https://hardware.schaefermarine.com/
They are very helpful and can assist you with specifications.

I would not use synthetic line as I've had it fail on a steering system because it couldn't take the shock load when the boat backed down on the rudder while reefing. I was using 3/16" Vectran in lieu of 1/4 ss 7x19 wire. Nevermore!

[Jimmyhenry]

We broke a SS steering cable half way BoraBora to Niue. 12 mth old Benetau 61 so the cable was not old. It fractured at the sheave.
Analysis. The cable was badly deteriorated, little bits everywhere around the sheave. It looked like the sheave diameter was too little. The cable was about 5mm. Way too thin. We fixed it with a short length we added and used those U bolts fasteners (essential back up stuff)
I think if the cable had been dyna of same size it would have not happened even given the small sheave size.