Sunday, May 1, 2011

The Battle of the Scanmar self steering, Whangarei NZ

Scanmar self-steering unit on the brackets Dave built to hold it outboard of the solar panel.

Another rainy day.
There is one small event to report: the rudder hinge piece for the Scanmar self-steering has returned after passing through the hands of yet another metals shop.
The little bit I've learned about self-steering: Self-steering options include electronic (course is sensed and directed by electronic gear, and course changes are made, in the Marquesa's case, by a hydraulically-assisted motor mounted down by the lower steering column) and mechanical. Mechanical self-steering systems are wind based, which work well offshore, where wind direction tends to be steady and sails can be trimmed and left for hours. Mechanical systems include trim tab, auxiliary rudder, and servo-pendulum- e.g. the Monitor.
In a trim-tab system a wind vane directs a tab on the trailing edge of the boat's rudder. While the windvane doesn't produce enough force to turn the entire rudder, it can turn the smaller trim tab on the end of the rudder.
Similarly, in an auxiliary rudder system, the relatively small force of a windvane directs a secondary rudder which is smaller than the main rudder.
Servo-pendulum systems were a real breakthrough in the cruising world. They use a pendulum, servo gear, and a small oar/rudder to hydraulically enhance wind force so that it is sufficient to turn the main rudder. The stronger the wind/faster the boat speed, the more force created. The system that produces the force is complicated and both the windvane and the rudder end up rotating around a horizontal axis as well as a vertical one. Motion is passed on to the wheel via gears and ropes.
Scanmar touts themselves as the 'world's largest manufacturer of self-steering equipment', and their Monitor model is described by Bluewater Sailing as 'the most popular windvane ever built'. Dave wishes he'd bought an Aries (tougher but more expensive).
The self-steering has been the bane of Dave's existence for some time now. His college degree is in metallurgy and he comes out with some pretty good diatribes when he encounters shoddy metal-working jobs. The unit hasn't seen a whole lot of use prior to this trip, but apparently it's poorly designed. It's broken 4 times so far. Near Tahiti the hinge needed repair due to stress fractures in the welds; the welds hadn't been stress-relieved and the metal is too thin to be durable. In Samoa the hinge pin broke and had to be re-ordered from the States. The servo rudder hinge broke twice partway through the crossing from Tonga- New Zealand, due to what appeared to be metal fatigue. We're lucky we didn't lose the bottom half of the self-steering.
Dave took the old pivot piece to a guy in Opua who cut metal for a new one, but couldn't weld it up. Then he took the pieces to Terry Symonds at Alloy Stainless and Marine Ltd in Whangarei. Terry not only didn't finish the welding, but did such a slap-dash job of it- not bothering to block it or pin the holes - that the thing is next to unusable now. It took half an hour of filing to get it to the point where it can be forced into place, and the bolt holes don't line up. The new piece cost a total of $500 NZ.
The last bit of welding was done by Northern Marine Machining in Whangarei. We've had a great experience with them - their work is generally of good quality and speedy. Dave took an order back once after they messed up a measurement on his masthead fitting, and they re-did the work for free. They're a great resource.