Plastimo Iris 50 Damping

[zvidoron]

It appears that the Plastimo Iris 50 hand bearing compass is nearly unusable in a rough sea due to continuous movement caused by the acceleration and slowing down of the boat as it goes up and down steep waves. The compass card just wanders around an arc of some 12-15 degrees, never settling.

Is that your experience as well? and if so, is it possible to replace the liquid inside the compass with something with higher viscosity to increase the damping of unwanted movement?

 

[Guest]

I use an Iris 50 regularly and "yes" it does "wander" - and so does every other compass.

Increasing the density of the fluid in a compass will NOT increase the accuracy of a bearing when the compass itself is moving. All damping does is to slow down the recovery rate between where the compass needle is pointing to where it should be pointing.

Increasing the density will make the "wander" less apparent but the inaccuracy will still be there and may even be increased.

Take the problem to an extreme - imagine if the bowl was filled with golden syrup!! Point the compass at anything and it will give a "steady" bearing. On the other hand, it would take maybe an hour or more for a 30 degree change of position to register as the needle dragged its way through the syrup.

The Iris 50 already has a fairly viscous liquid inside. To check this out slowly rotate the compass in your hand and you will see that it is possible to turn the compass all the way through 360 degrees without too much trouble. Stop turning and it will steady on the new bearing in less than 10 seconds.

From a practical point of view, when the compass is "wandering in an arc of 12 degrees" take the mid-point of the "wander" and use that as the bearing. With a three point bearing using the "mid-points" should still give a reasonably accurate position.

Best regards )

Ian D

 

[zvidoron]

Thank you Ian, I see your point about viscosity and I totally agree. In the mentioned case I was just practicing trying to get a position using Berry Head, Start Point and Prowle Point in force 5-6 and I did resort to averaging. I did not however plot the position there and then and compare that to the GPS as there were too many other hectic things to do but I will try that next time.