Hi Stuart
I think those are the ones that Seafarer use in their 680.
I cannot speak for their ability to protect your back but what I did see about them when I looked at a few older boats to see how durable they were was that some didn't look too healthy.
I have a feeling that these were not maintained / freshwater washed etc after use.
I did L4 and L5 in my back hence my interest in these.
At the end of the day there seemed to me to be four options
1 try the units your looking at and service them well and hope
2 get a proper suspension seat (with the appropriate cabin to keep the sea water out) big bucks!
3 travel slower and
4 get a foam wedge for your existing seat so you use your feet more with your back at a safer angle
http://www.cbc.ca/health/story/2006/...t-posture.html
You can get high density foam wedges cut to the right angle to sit on from Foam World at Mermaid Junction (Q Shopping Centre)
I opted for a combination of 3 and 4 and touch wood its worked OK so far.\ ie 4 years later
Hope thats of some help
Cheers
Chimo
Hows the boat going? Looked great so far.