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post #105 of Old 02-24-2018
colemj
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Re: Multihull Popularity and Interesting Designs

Quote:
Originally Posted by Arcb View Post
If you say so, most catamarans i am familiar with have two rudders, two steering linkages leaving one to steer with in the event of a failure.

Yes, I have used high end autopilots in big cats offshore.

The boat in the video was overpowered. The bows were stuffing. A few more knots and the bows could have buried, there could have been a rapid deceleration forward, the breaking waves at the stern would have had a 52 foot lever to push the stern around the bow broadside to the waves at 27 knots. I dont beleive an autopilot could have steered out of that. Autopilots are reactive by nature. A helmsman can see a breaking wave or a gust before it hits.

They were sailing that boat on the edge, having blind faith in your technology to get you out of problems in conditions like that is unwise in my opinion.
Depends on the steering system design and which part breaks. If the chain or associated sprocket or cable holders breaks, you lose all wheel control. If a cable breaks, you also lose wheel control. If a crossbar breaks, you may or may not have control of one rudder depending on if the system uses a single quadrant or splits between two. Ours is a pull-pull system, so a cable issue anywhere disconnects the wheel from the rudders. If a hydraulic system using independent drives on each rudder, then you have control of one rudder if one drive or cross link fails. If they are all integral, you lose it all if there is a hydraulic failure.

I would say at least 50% of catamarans will lose steerage from the wheel if any link breaks in the system. The number is probably higher than 50%. All single helm ones will lose it if the chain/sprocket system breaks.

I agree with being overpowered. However, I don't see a human getting out of a broach or stuff either. Particularly with that boat, which isn't the most responsive or light helm.

Modern autopilots are not purely reactive. Ours predicts. You can see this in action. 20-100 times per second it is getting pitch, roll, heave, heading, wind, SOW, and SOG data. A helmsman can see a breaking wave or wind gust on the water before an AP, but I'm not sure how much realistic space is there to do anything predictive in those conditions. You certainly aren't going to make large alterations in steering to prevent or adjust to that. Almost everyone in these conditions will have slowed the boat down so that breaking seas and wind gusts do not cause steering issues.

How good is a human in these conditions at night? Worse than a good AP IMO.

Those ocean racers are on AP constantly and they only have one rudder/steering system, where a break anywhere takes them down.

You may have experience with modern AP's in big catamarans offshore, but the only actual reference I have seen you give has been large commercial power vessels. These are apples and oranges to small recreational sail cats in both boat type and AP gear. Can you be more specific as to which recreational sailing catamarans and which AP's you have experience with?

Mark

Manta 40 Catamaran "Reach"

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