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Old 08-08-2009, 00:33   #1
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USNA Short Course: Chapter One

Thought recycle/share some stuff I posted on in our Seamanship forum.

Most of you probably know this, but from some of the reactions of other skippers on the race course, apparently it's not common knowledge. So many times you know you're going to clear that stand-on vessel, but he freaks out anyway, alters course and protests. Those are the ones that don't know about bearing drift. And some of you may be self-taught and new to sailing. I've also talked with experienced sailors that think they just "know" if they're clear but can't express why. Their brain is looking at bearing drift without them knowing.

So, in short, you can use bearing drift to figure out if you're on a collision course with another vessel.
- Look past the boat in question and find a reference. The farther the better. Land is good, celestial bodies are best, a static cloud formation is okay. Pick something that stands out like a radio tower. A hand held compass is of course great, but isn't always handy and distracts you if you're at the helm.
- As you sail on a steady course, if the boat is drifting to the left, he will pass to the left. If the boat is steady on the reference, you will collide. In the Navy it's called "CBDR". Constant bearing Decreasing Range.

Notes: This is for "eyeballing" it. You have to inject some logic and take appropriate action. If the bearing drift is slow, it will be a close meeting. If the vessel you're looking at is a 100ft long, you could have left bearing drift on the bow and right drift on the stern... meaning you're on a collision course with the middle. So take the drift of the part of the boat you need to clear. Also consider where your eye is. If it's a close one and you're spotting form the bow, you probably want to be sure that the stern of you boat will clear as well. In racing you're usually checking if your stern will cross his bow without a problem. Lastly, if either boat changes course or speed, it's a whole new ball game.

That was an early seamanship lesson at the Naval Academy and I shared that to a few wide-eyed sailors. You could see the light bulb over their head when it registered. Of course the Skipper of High Flight is a pilot so it was old news. The pilot equivalent is putting yout finger on the canopy covering the contact. If he's still under your finger after a bit, you're on a collision course.

Frankly I wish everyone on land knew this because it works for driving and walking too! I hate having to dodge morons in a crowd because they can't figure that out. 8)
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Old 11-08-2009, 04:02   #2
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That's good.

You can also line the target up with a shroud or stanchion on your boat and achieve similar results If the target in question moves left of the vertical "sight" you will pass to its right. Non-moving targets are the concern.

At night you can do the same with lights. It is very important to know if the light you are sighting is a statinoary target - buoy or land or whether it is a moving target - ship.

Similar to cars are airplanes. Want to know if you are going to clear the mounain ridge ahead? Pick a dirt spot on the window. If the target drops below the sight then you will clear the ridge.
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Old 11-08-2009, 09:13   #3
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I should have mentioned, at night, stars and planets are best. But you should stay away from shipboard items in making your range. That changes with every wave and rudder wag, and it's so close that if you move your head the lineup changes. So unless you can keep your head perfectly fixed to the boat and the boat on an exact course, it's not much help. (Just thought of a good hazing for a newbie crewman.)
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Old 11-08-2009, 09:29   #4
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It also works with an intermediate object (like a shroud). If we're approaching another boat and their bearing doesn't change relative to our shrouds, we'll hit. Good to bring it up explicitly because after a while you do it without thinking. Ours doesn't wiggle much which is why it works.
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Old 11-08-2009, 11:30   #5
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Again... for safety sake, distance is the key. Anything on your boat should be considered useless. You need something farther than the target. Thinking you can keep your head in one place is folly. If we're talking miles of separation, no harm done. If we're talking crossing situation during a race, don't even think about it. Wind changes, the boat heels, you're up and down waves, you never keep a steady heading so the line from your eye to that shroud is constantly changing. It's just asking for trouble.
Conversely, if you pick a distant object on land or in the heavens, you can walk up to the bow and and it's still essentially the same line.
Just looking out for y'all.
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Old 11-08-2009, 12:02   #6
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And if no distant object presents itself?
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Old 11-08-2009, 12:17   #7
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Take a rough bearing off your compass and don't check it more often than once a minute, longer if the contact is far off. The longer you go, the more noticeable the change should be. Again, this backup is for the open sea, not close quarters, since getting a bearing from a compass on the binnacle is rarely easy or accurate. But at least you're eyeballing a fixed line in a consistent manner. I've done this on cloudy nights and accurately predicted the movement of the contact long before anyone else could pin it down.
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Old 11-08-2009, 12:30   #8
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From a practical standpoint, if you're not noticing a change in bearing (even averaging against your moving boat/head/whatever) odds are your course is too close for comfort. Your compass would swing too if you're slewing in the waves. I'd say fixed point land or celestial is best if available, but sighting off hardware is good for quick and dirty. When in doubt, turn.
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Old 11-08-2009, 13:54   #9
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A hand-bearing compass makes this process easy versus trying to find something to line up on or keeping your head in one place and sighting across a shroud. They aren't that expensive. When I'm sailing in an area with a lot of traffic, I keep my hockey puck style hand-bearing compass hanging around my neck.
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Old 11-08-2009, 15:42   #10
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Quote:
Originally Posted by Hud3 View Post
A hand-bearing compass makes this process easy versus trying to find something to line up on or keeping your head in one place and sighting across a shroud. They aren't that expensive. When I'm sailing in an area with a lot of traffic, I keep my hockey puck style hand-bearing compass hanging around my neck.
I've found those more useful for taking ranges to something on land. I haven't found the need for such great accuracy in collision avoidance.
That said, if you want to use a puck, that's great.
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Old 11-08-2009, 16:50   #11
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Nice explanation Down.

"CBDR" ...The Navy seems to have an acronym for everything!

Here is a nice guide to Naval acronyms:
http://www.all-acronyms.com/tag/naval/9

I'm working with a group from the Navy right now. Nice people.
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Old 11-08-2009, 22:44   #12
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Quote:
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Again... for safety sake, distance is the key. Anything on your boat should be considered useless. <snip>

If we're talking crossing situation during a race, don't even think about it. Wind changes, the boat heels, you're up and down waves, you never keep a steady heading so the line from your eye to that shroud is constantly changing. It's just asking for trouble.
<snip>

In a crossing situation in a race you have to take a bearing off the other guy. Anything on shore is useless to look at because both boats are moving.

In a race the crossing situations are constant and everyone who is any good at it is constantly taking bearings off shrouds, stanchions etc.

The key is not to take one look but to keep reevaluating. Collectively it's called situational awareness and you have to develop it to win.

If you are in a ducking situation and leave 5 boat lengths you will consistently lose races.

It is hairy when two 45 foot boats pass within 5 feet at 8+ knots each but that's racing.

I think we are talking two scenarios anyway. Standing off a rock, marker, buoy or lighthouse vs. missing other boats.
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Old 11-08-2009, 23:55   #13
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The geometry doesn't change with the target. Hate to be a nag on the point but a line between your eye, a part of your boat, and a boat 100ft away... useless. It comes down to parallax. The greater the ratio of the distances and the greater the distances, the better. Photographers know this. The moon behind a distant building won't change unless you move very far.

Both boats are moving and believe me, if the boat is moving to the left in relation to the land behind it, you're crossing behind. Moving to the right, you're crossing ahead. If land is more than a mile away it's as good as fixed.* I jump to the bow for a ducking sit. and take bearing drift on his transom and call it out to the helm. We always cross well inside the other skippers comfort zone. (Never touched, but could have shaken hands)

Like I said, some people "have it" and call it situational awareness, but your brain is doing exactly this. Try it out next time you race and then tell me it's wrong. You guys know what you're doing, but I just want to keep the newbs away from the shrouds and lifelines for safety sake.

*Good as fixed = land has negligible bearing drift to throw off your tracking of the target.
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Old 12-08-2009, 06:02   #14
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Is it just me that thinks there's something wrong with this? ("Never touched, but could have shaken hands"). Why would you do that? There's no way I would ever purposely come that close to another vessel. It's not necessary, it's dangerous and it's damn rude. I keep well clear. I take action early so the other guy knows what I'm doing. I don't play macho games on the water with right of way.

Maybe there's some point to this that I'm missing but this thing has been tweaking me since I started reading it.
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Old 12-08-2009, 06:06   #15
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That was in a race of course. Outside of racing I do quite the opposite. Then the goal is to see bearing drift to ensure you get a large CPA.
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