Dyneema pull pull steering system

[Jagarwal]

Hi

We are considering replacing our pull/pull conduit steering system in our catamaran. Currently we have hydraulic steering in the starboard hull, from there the quadrants are linked via two runs of conduit to achieve the pull/pull.

As our preparations are for long range remote cruising; the current condition of the conduit is not acceptable.

We are considering either replacing the conduit, or setting up sheaves and a tension based pull pull system. We are leaning towards the sheave setup, as we can make it ourselves onsite and be more independent for future maintenance. However we would like to hear from anyone who has experience in these kinds of steering systems.

Can we expect a similar amount of friction between a 8 meter conduit run vs a 6 sheave pulley run? Are the loads on the pulley cable vs the conduit cable going to be different? How do we go about estimating the loads within the system?

The current cable is 6mm SS. If we went the sheave route we would like to use Dyneema. Any recommendations on sizing the Dyneema as not to introduce stretch into the system? Any recommendations on sizing the sheaves? If dyneema we had imagined 8x rope diameter.

Is plain bearing sheaves going to be enough or are ball bearing sheaves going to be necessary?

What are we not considering? Any general advice is welcome

Jack & Sol

[SailingHarmonie]

First, a properly designed sheave and line system should have significantly LESS friction than the conduit system, although if your prime driver is a hydraulic system at the helm, I doubt you'll notice.

Traditional wire systems use oiled bronze sleeve bearings. Ball bearings are not needed. Making these yourself will require quite a bit of good engineering. Loads can be VERY high.

Especially when using dyneema, you need to pay very close attention to sheave alignment. Everything has to be perfect to prevent chafe. How you will get the sheaves perfectly aligned over the distances involved will be a challange in the design process.

The dyneema should be sized not based on breaking strength, but rather on the amount of stretch. It is true that a 1/4" diameter dyneema will have a breaking strength similar to a steel cable of the same size, but it will stretch a lot more. You need to upsize the dyneema quite a bit to be sure that the stretch is acceptable. You can not afford to have slack on the unloaded side to the point where the line can EVER fall off the sheaves.

While I appreciate the coolness factor of dyneema, be sure that you are using dyneema instead of stainless wire rope for good reasons, and not just because it is cool.

[catsketcher]

I used to have a system that had 11 sheaves - it was pretty terrible. Friction and stretch.

I replaced it with a tiller bar and a system that uses dyneema and 4 sheaves - it's great.

You don't need to use thin bronze steering pulleys, I just use normal roller cheek blocks of 75mm diameter for two of the pulleys (it is rope after all) and two composite blocks I made myself for the other two. Making your own cheeks is really easy. Find a piece of wood the same thickness as your sheave. Rout over the both sides of one edge. Wrap in plastic and glass over the edge and sides.

Then cut out sections to make sheave holders and pop the sheave in and reave the line. Then glue onto boat. Then glass on with db 440 and epoxy. This has worked well for me for about 18 years. My sheaves are very wide compared to the 6mm dyneema so a little misalignment is not an issue.

[wholybee]

I replaced my wire with Dyneema. A basic 2 Sheeve system. Getting the sheeves aligned my case was trivial. Dyneema is a far better material for this application. It should last forever.

[SailingHarmonie]

Originally Posted by wholybee

I replaced my wire with Dyneema. A basic 2 Sheeve system. Getting the sheeves aligned my case was trivial. Dyneema is a far better material for this application. It should last forever.

But properly installed, so does wire rope...

What are the properties of dyneema that make it better for this application?

Just so you know where I am coming from, I rigged a cruising ketch with dyneema standing rigging, and it was a great performance improvement over wire rigging, but was also more maintenance. So I do understand there are places where it has advantages, but there are also always tradeoffs. Wire rope in a properly installed and maintained steering system is essentially a permanent installation.

I know that dyneema is capable of working in this system. But why--exactly--is it better? I am not saying it is or is not, it's just I don't see the compelling case. Make it for me...

I don't have a dog in this fight, my steering system is not a cable system, so it's not personal.

[catsketcher]

I reckon one reason the dyneema is better is flexibility. To turn a rope around a pulley requires that you bend the rope - the amount of energy required to do this with rope is negligible, but with wire it takes some energy.

This energy has to be fed into the system by the mug on the wheel - so it makes the wheel slightly harder to turn. Also wire needs to be really tight or it falls off the thin little sheaves Edson make. But if you use normal deck pulleys - my 75mm blocks have sheaves about 12mm wide, then tension is not an issue - the blocks are designed for tension to be on or off the blocks so no problems with the rope going over the side of the sheave.

On top of that the alignment issue is not an issue - because the sheaves are so wide you don't have to "aim" them so well - at least I didn't and it has been fine for years. My steering rope alters its angle as it leads to the tiller arm - so the ability for the rope to alter angle to the sheave is a must.

Then there is the hassle of wire - it needs swaging tools to replace and gets meat hooks on the cables when older. Dyneema never hurts and you end it with a knot, splice or loop of some sort. You couldn't pay me to put steering wire on my cat - modern rope is the best.