dyneema steering arangement

this is rough schematic of possible steering arrangement for light-displacement 10.5m catamaran.
the steering pulley is 12.7 cm diameter
2 turns lock to lock for +/- 40 degree rudder movement
total rope length ~3 m per side
the green tiller bar is 4.4 m long and lies outboard of the aft cross beam.
protected but still open to the elements.

would 8mm dyneema sk-75 be suitable? too stiff?

 

Just a note, you are not showing the distance from the tiebar end to the pintle axis. The tiebar moves in an arc so it will pull on the dyneema cord at an angle. The angle would be significant on the short side and you have no provision for the added length it would need -so yes, anything stiff would be too stiff for that geometry.

 

oh oh

thanks for the reply
seems that I'm asking the dyneema to do the impossible
be really stiff to give good helm feel but be soft enough to take up the length changes.
The sheeves on the steering wheel are simple UHMW screw or convolute. Could they be machined to provide the length differences?
Have to nut out exactly how much difference is involved.

 

One problem with line like Dyneema is that even though the material stretch is very low, when stretched it does not spring back. Also, you must deal with construction stretch, which becomes more difficult to remove as the thickness of the line increases (as it requires higher pre–tensioning to reduce) and also does not spring back while in use. The result is that the line may become loose very quickly, which may seem counter intuitive.

You will need a mechanism for maintaining the tension in the system that is easily adjusted as the Dyneema stretches over time. Or use a cord with more elasticity (in the common sense of the term) that will spring back when stretched.

Just my $0.02 worth. ;-)

 

for a rope length of 3+ meters and the loads involved the stretch (hopefully) will be insignificant.
pre-tensioning 8 mm dyneema will take some force, but doable with a come-along.
Maintaining tension also impacts rudder centering so the vertical pulleys under the steering wheel sheaves will have be to individually adjustable. Problem is likely to be all the slop in the dyneema fittings/terminations.
$0.02 more than welcome

 

one possibility is to split the sheaves allowing one to be rotated to take up slackness and then pinned against the other which is fixed to the shaft

 

accounting for geometry

this irregular sheave will handle the skew in dimensions as the tie bar moves aft at full lock (40 deg). Definitely a job for CNC. This means the 2 sheaves must be pinned to the shaft and slack/alignment handled by shifting the vertical pulleys separately. Rope now 4mm dia.