Autopilot

[NahanniV]

Thanks,

I'll keep that in mind if I ever get going that fast !

Cheers,
JM.

[colemj]

Originally Posted by NahanniV

Thanks,

I'll keep that in mind if I ever get going that fast !

Cheers,
JM.

Actually, it isn't really about going fast - it is more about the boat changing speed downwind. Waves play a big role here. Apparent wind angle changes quite a lot as boats change speed on waves, while true wind angle doesn't. Upwind, this doesn't happen as dramatically. It isn't a catamaran thing, although as Paul points out, catamarans may be better suited for taking advantage of it because their mastheads are more stable.

Mark

[Paul L]

Originally Posted by Dockhead

You're right, but very fast boats, especially ones which surf, can change the apparent wind so much that you steer an unnecessarily erratic course, if using apparent. Then you use true and trim for the apparent wind at the normal speed.

Or use a compass course, which in practice is basically a long term averaged true wind angle if you have someone on watch.

On a related issue, on a Cat when you get a dangerous wind shear or sudden wind speed increase, are you safer with the AP on compass, AWA or TWA? As I understand it, most of the Cat boat flips that were due to wind occured while under Autopilot.

[colemj]

Originally Posted by Paul L

On a related issue, on a Cat when you get a dangerous wind shear or sudden wind speed increase, are you safer with the AP on compass, AWA or TWA? As I understand it, most of the Cat boat flips that were due to wind occured while under Autopilot.

Well, first, you might want to put the flipping thing into some perspective. We aren't worried at all regardless, and very few production cruising cats need to be. You also may be confusing being on autopilot with not being at the helm to dump sails for those that have flipped.

I don't think it really makes any more difference than on a mono as to what the autopilot is set to steer by when an unexpected large and high velocity wind sheer/shift hits. The AP will not be responding fast enough to make a difference on any boat.

Mark

[colemj]

Originally Posted by Paul L

Or use a compass course, which in practice is basically a long term averaged true wind angle if you have someone on watch.

Assuming the true wind direction is not changing, and one is not running so deep as to risk jibing when "averaging" by compass.

Mark

[Paul L]

Originally Posted by colemj

Well, first, you might want to put the flipping thing into some perspective. We aren't worried at all regardless, and very few production cruising cats need to be. You also may be confusing being on autopilot with not being at the helm to dump sails for those that have flipped.

I don't think it really makes any more difference than on a mono as to what the autopilot is set to steer by when an unexpected large and high velocity wind sheer/shift hits. The AP will not be responding fast enough to make a difference on any boat.

Mark

OK, in perspective it is a very rare occurrence. In the analysis of the Atlantic Chris White cat that went over, he makes a point that all of the wind induced flipped cruising cats (the few) happened on autopilot. That's what got me thinking about what the AP might have been steering to and what was the safest in those possible conditions. So is dumping sail the only action in this case, or is there an appropriate steering action also, i.e turn up hard or fall off hard?

[colemj]

Originally Posted by Paul L

OK, in perspective it is a very rare occurrence. In the analysis of the Atlantic Chris White cat that went over, he makes a point that all of the wind induced flipped cruising cats (the few) happened on autopilot. That's what got me thinking about what the AP might have been steering to and what was the safest in those possible conditions. So is dumping sail the only action in this case, or is there an appropriate steering action also, i.e turn up hard or fall off hard?

In pretty much all production cruising catamarans, heading up like one would do in a mono is the correct instinct. Unless the sudden increase of wind is behind you - but the point is the instincts here are the same as in a monohull.

For higher performance and lighter catamarans, heading up could pass you through the death zone and be dicey, and which way to steer is dependent on wind angle and sea state. If near a beam reach or below, steering down to decrease apparent wind can be better.

For all out racing boats, where the apparent wind is always ahead of the beam, steering toward increasing wind is usually the right instinct again.

All of this assumes the sails are being let out to dump air.

It is very difficult to flip a production cruising catamaran by wind alone. It is very difficult to flip them regardless. For giggles, consider that in cruising trim, a Lagoon 450 weighs 40,000lbs, and a Lagoon 620 weighs over 80,000lbs, and almost all of that weight is near the water.

Mark

[Paul L]

Interesting. Thanks

[goboatingnow]

I must say , as my experience is all monohulls, that steering deep downwind, using a true wind calculation is a brave thing indeed, given the typical variations I see in true wind calculations. I suspect all your sensors would need to be top notch, and in particular the wind vane.

personally, I find I want to sail to a destination, unless I fancy a masochistic experience, I will ensure I have a free wind , wherever possible. IN that regard I find using the AP in either GPS mode, or Compass mode more then adequate, Ive crossed oceans with no more then that.

I certainly mistrust wind angle steering, personally I prefer to trim and adjust rathe then the AP trying to outthink the wind !.

Downwind in any seaway, I can't see any point and some disadvantage in using anything other then compass/GPS mode. primarily because aspect to the wave train is often more important that pure response to true wind. ( and I would have preventers etc )


Dave

[monte]

We've done many miles downwind in wind vane mode. I'm not sure if our Raymarine pilot is configurable to TW but we set it on 170 degrees and it works well under spinnaker or wing in wing. Never had an accidental gybe. The difference between AWA and TWA wouldn't be much even if speeds range from 7-15K, maybe 3 degrees

[colemj]

Originally Posted by monte

The difference between AWA and TWA wouldn't be much even if speeds range from 7-15K, maybe 3 degrees

Not DDW, but they do vary considerably when at higher downwind angles.

Mark

[Paul L]

We almost never sail that deep, 170 apparent - with or without an AP.. That is pretty much DDW. I don't trust an AP to handle this with seas behind us. Even a hands on driver will have issues sailing super deep after awhile.

[colemj]

Originally Posted by Paul L

We almost never sail that deep, 170 apparent - with or without an AP.. That is pretty much DDW. I don't trust an AP to handle this with seas behind us. Even a hands on driver will have issues sailing super deep after awhile.

On the other hand, we sail DDW all the time, for days even, in all types of seastate with the AP handling it just fine. It is one of our favorite points of sail on our boat.

This will depend solely on boat and autopilot, I think.

Mark

[jensbusk1]

HI. I look for bey, for a Cetrek autopilot 701, aminbox for cockpit, second hand.
I have look over all, but have not find it! If I can find one hu Work, can I chare to Invest 2000 Enlish paund! PLEACE HELP, MAYBE YOU CAN GIVE ME A IDE TO FIND IT!
Kindly Jens Busk S/Y Thor-Christ of Denmark. (NOW the boat is in Turkey Greek!)

[skipmac]

Originally Posted by jensbusk1

HI. I look for bey, for a Cetrek autopilot 701, aminbox for cockpit, second hand.
I have look over all, but have not find it! If I can find one hu Work, can I chare to Invest 2000 Enlish paund! PLEACE HELP, MAYBE YOU CAN GIVE ME A IDE TO FIND IT!
Kindly Jens Busk S/Y Thor-Christ of Denmark. (NOW the boat is in Turkey Greek!)

Hi Jens

Cannot give you any ideas or help in Europe but can tell you what I would try in the USA.

First, have you tried eBay? I have found many old, out of date products on eBay. If you have an account on eBay you can put an automatic search in your account that will send you an email when the item you want is listed.

Also, in the US there are a lot of shops that sell marine gear that is old, used or from wrecked boats. There must be 10-15 that I have heard of. Try google to search for "marine equipment used" or "marine consignment" or similar search words. Some of these stores list their inventory on the internet but for many you would have to email or call to find out if they have a particular item.

Good luck.