Fabrication of Chain and Rope (Dyneema) yacht steering

[coopec]

I'm installing a chain/rope steering in the yacht I am building but need help on a couple of aspects: advice from people "in the know" would be appreciated.

I assume the dyneema goes around the quadrant and is tensioned by the ? that I have hanging out of a hole in the quadrant. I will use a "thimble" and splice to terminate the rope

Both ropes would leave the quadrant aft of the rudder stock when the rudder is centered. (There would be a pulley either side of the quadrant to ensure this)

How do I attach the rope to the chain? I presume I use a thimble and splice to terminate the rope under the Whitlock pedestal.

Would it be wrong to consider two turnbuckles somewhere along each rope to enable further tensioning?

I have attached a photo of the quadrant, rope, tensioner and thimbles (but I have 10mm thimbles on order)



Hopefully you can see two stainless steel strips I intend epoxying to the hull. Tomorrow I intend giving about one square meter of the hull a coarse sand and then epoxy glass cloth to the hull. Then I will epoxy the SS strips to the hull using about six layers of glass under, then back over the strips. I will then finish it off with about four layers over the top.
Once I work out exactly where the pulleys/sheave have to go I will make up a frame to suit.
How am I going? (Thanks in advance)

 

[sarabande]

If that pulley is roughly in the position where you intend to install it, you may have to look carefully at the angles the dyneema leading from the quadrant makes at full deflection. It doesn't look at all right to me.

The pulleys in a wheel system originally installed on my boat were at least twice the diameter, and much stronger, though of course it is difficult to gauge accurately from the pic.

Do you intend the eye bolt to remain that far out of the quadrant ? The leverage is going to be very large ....

 

[coopec]

Thanks for your response.
No that pulley is certainly not in place: it is just sitting there for information purposes. When I mount them I imagine they will be just outside the arc of the quadrant.

You say they are small but they are 100mm (4 inch) which would be too small for SS as the strands would break down on a tight radius. If I was using SS I think I would have to use 7 in. sheaves and they are not cheap.

No the eye bolt was stuck in there just so you could see it. I plan to bolt a piece of SS angle through the hole where the eye bolt is stuck in and then insert the eye bolt through a hole in the ss angle and tension up the dyneema that way.

Thanks for you comments.

 

[Ian_Edwards]

I can't comment on the specific installation, there's not really enough info' in your post.

But some general points you may or maynot be aware of:

Look for info' on the d/D ratio (rope diameter to sheave diameter) for the dyneema you are using and see if you can find the minimum recommended by the manufacture or at least how much you should detrate the dyneema for the d/D you are using.

All rope goes "oval" as it goes round a sheave, it flattens as the fibres on the outside of the bend see more tension. This happens more with dyneema rope than SS wire. This means that the shape of the grove in the pulley is important, a pulley designed for wire rope will not be optimum for dyneema.

The recommended length of a dyneema splice is around 50 times the diameter of the rope, so the splice will probably be going over a sheave or quadrant will be thicker, and with relatively short lengths of dyneema you may find it hard to get the the required length of splice in the space available.

There's also a significant initial stretch in a dyneema splice when it is first tensioned, as the splice pulls in, which makes the initial set-up tedious. I use a dyneema backstay and I had to shorten the bridle several times when I first installed it. It's a single backstay to a low friction ring, then a bridle to the stern quarters, with a hydraulic tensioner on the port side.

In normal dynamic use, dyneema doesn't stretch very much, but it will creep, i.e get longer in an irreversible way, if the static tension is to high, as a rule of thumb, you need to keep the static tension less than 10% of the breaking strain, less than 5% is even better. So don't worry to much about the load you put on it when steering the boat, but be careful how you tension the system when you set it up.

I hope this helps a little.

 

[coopec]

Thanks Ian

That certainly does help particularly the length of splicing as it will certainly go over the quadrant. That means I will have to use a smaller diameter Dyneema as the groove on the quadrant and sheaves is 10mm.

You've made some other very interesting points and I will address those later on this evening.

I like to think I have just about finished the yacht. It is not perfect but I am pleased with it. You can see it here:



https://www.flickr.com/photos/44071060@N06/albums/72157631756633356

Clive