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Old 28-07-2008, 23:53   #1
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Challenge: How do you solve this?

I suggested in the “seamanship” thread that we could start a “How would you solve this?” thread where a poster describes a tricky situation and then the rest of us come up with the thought processes to solve the problem. (Hidden surprises are allowed and encouraged!)

Can be technical, nautical or whatever you like….No restrictions other than it relates to boating.

Once it is solved someone adds another challenge to this thread and so on…..


So I will start the ball rolling with a technical challenge from this scenario:

You are making a mid summer crossing from Japan to Seattle and during a storm an electrical fire has wiped out most of your Nav/Com. No radio, no GPS, no sounder…. only a magnetic compass and an old, but strong radar is still working.

Seasonal FOG has been with you the last 5 days and it is becoming thicker sometimes down to 50 ft visibility, but from your DR you know you are now closing in somewhere on the dangerous Washington/BC coast.

Your plan as you approach the coast is to compare Radar targets with your paper charts and by sizing the openings of the many inlets, determine your position and work your way to Juan de Fuca Straight by precise parallel indexing in this dense fog using radar alone.

The only problem is that the track ball and knob for your VRM feels “mushy” so you do not know if the readings are accurate.

You are getting good radar returns from passing ships but can not see them in the fog. You have no radar monitor for testing.

So how do you prove that your VRM is correct?
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Old 29-07-2008, 07:07   #2
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Hopefully I still have internet access.

Distance to an object can be calculated by only knowing the bearing to an object, as long as you know your own course and speed. You take a bearing, then change your own course 90 degrees, and then take another bearing to the object. Using a simple calculation(which eludes me at the moment, it's been a few years since I needed it) you can then accurately know the distance to the object. This is how submarines measure distance to targets using sonar. Pretty easy to plot out on paper, which I would need to do in this instance, to verify the radars range markings.

Of course, I would never be without a vhf in the first place, because I 'unthinkingly' always carry a handheld everytime I go out on any boat. Good seamanship.
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Old 29-07-2008, 13:09   #3
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And I'll also add, while I'm waiting for Pelagic to come back and tell me if I got it right, that for a fixed object you can make a 90 degree turn and then use the basic triangle a2 + b2 = c2 to measure the distance to the object. The basic equation that I've forgotten, is what we used to measure the other vessels speed across the horizon as well as distance, which we could figure just from two bearings to the target.

Without electronics or calculators, this is how the U-boats used to figure out the targets speed and distance in order to line up the torpedo shots. They didn't just "shoot from the hip" like in the old movies.
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Old 29-07-2008, 14:31   #4
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So how do you prove that your VRM is correct?
You do not bother with that. You hang out and lay low until that fog lifts.
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Old 29-07-2008, 14:54   #5
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Quote:
Originally Posted by Pelagic View Post
I suggested in the “seamanship” thread that we could start a “How would you solve this?” thread where a poster describes a tricky situation and then the rest of us come up with the thought processes to solve the problem. (Hidden surprises are allowed and encouraged!)

Can be technical, nautical or whatever you like….No restrictions other than it relates to boating.

Once it is solved someone adds another challenge to this thread and so on…..


So I will start the ball rolling with a technical challenge from this scenario:

You are making a mid summer crossing from Japan to Seattle and during a storm an electrical fire has wiped out most of your Nav/Com. No radio, no GPS, no sounder…. only a magnetic compass and an old, but strong radar is still working.

Seasonal FOG has been with you the last 5 days and it is becoming thicker sometimes down to 50 ft visibility, but from your DR you know you are now closing in somewhere on the dangerous Washington/BC coast.

Your plan as you approach the coast is to compare Radar targets with your paper charts and by sizing the openings of the many inlets, determine your position and work your way to Juan de Fuca Straight by precise parallel indexing in this dense fog using radar alone.

The only problem is that the track ball and knob for your VRM feels “mushy” so you do not know if the readings are accurate.

You are getting good radar returns from passing ships but can not see them in the fog. You have no radar monitor for testing.

So how do you prove that your VRM is correct?

You interpolate using the range rings already on your radar screen. You don't need your VRM

If you don't have range rings (what radar does not have range rings?) then you make a paper ruler depending on your range scale setting and subdivide it into miles. If your on 24 miles for example, then divide it in half, 12, divide it again, 6 and so on down to 3 miles. Then make some marks on your screen out of a grease pencil or a Dry Erase marker to make it even easier.

-OR-

Ok..I'm going to bend the rules. If all your hard wired electronics were wiped out from a fire then your hand held VHF probably survived because it was not anywhere near the fire...probably near the steering station. Call the Coasties and have them get a VHF fix on you or call VTS in that area and have them get a radar fix on you. Or call a nearby vessel and ask if they will reduce speed so you can follow them in....they should not be speeding through the fog in the first place.
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Old 29-07-2008, 18:09   #6
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Sorry Fishspearit you’re in the FOG,….. you can’t see diddly !
Oh and the drawer that held your 2 handheld vhf’s and the backup Garmins was burned in the fire

Morgan Paul, logical thought but FOG in the Pacific North West has been known to hang off the coast for weeks. You have already been at sea 29 days, because of the storm and fire, water and provisions are very low and you are hoping you have enough fuel left to make it to first port Neah Bay ….where you can clear customs and immigration and get your boat back in order. Sorry….can‘t wait…..oh and to make matters worse, one crew is showing symptoms of appendicitis!

You are on the right track about the fixed rings David M and what you suggest would work, but difficult to do when you start Radar plotting in confined waters, increasing traffic and frequent changes in Display Range (Still Zero Visibility).

The technical solution to verify your VRM has been explained in books about Radar operation, since the mechanical knobs that expand the VRM or rotate the EBL have been known to get out of alignment after some heavy handed use.

Answer in 24hrs if no one comes up with a better solution than David’s which is still technically correct.
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Old 29-07-2008, 23:08   #7
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Throw your MOB pole overboard, with your radar reflector on it. Take a wild guess as to your boat speed, and sail away from it (you are just trying to get close here), and follow it on the radar. If your boat speed is 5kts, sail for 12 minutes. That's a mile. Mark it on your radar. Turn around, and repeat. Use an average between them.
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Old 30-07-2008, 07:05   #8
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...You are getting good radar returns from passing ships but can not see them in the fog....
To heck with the VRM. I always carry a pad of "maneuvering board" sheets onboard. You can plot the relative positions of each of the ships that appear on your radar, and derive their courses and speeds. Then follow the courses that lead into port.
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Old 30-07-2008, 07:48   #9
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Sorry Fishspearit you’re in the FOG,….. you can’t see diddly !
Why can't you use the radar to get the bearings to a fixed object?

Isn't that the basis for getting a position fix with a radar? Two bearing lines to two known objects?
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Old 30-07-2008, 09:32   #10
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Sorry Bstreep even though you are in FOG there is a sloppy swell and current making it impossible to track your pole with any certainty. This will not prove your VRM which you will need to accurately measure distances between strong radar targets and try to match this to your charts to determine where you are.

The problem Hud is that you are making a landfall after now 30 days at sea in an area called the “Graveyard of the Pacific.” You don’t know if Juan de Fuca entrance is north or south of you. You are steering 090 T hoping to make a landfall and determine where you are. Any ships you have seen lately seem to be steering either NW or SE which could be Alaska or San Fran. (You just don’t know)

Fishspearit you need to be able to identify the object you are taking bearings from for it to mean anything in terms of a fix. That is the immediate problem when you start to see radar returns from this rugged coastline without knowing where the hell you are in this Fog.

Focus on proving the VRM and it will get you out of trouble
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Old 30-07-2008, 10:14   #11
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Fishspearit is correct. You start by sailing a line to a mark. Take a bearing. Then by turning 90 degrees to that line, your new course is along the baseline of a right triangle. After traveling some convenient distance along the baseline, say one mile, you take a bearing to the mark. The angle enclosed by your right triangle is the difference between the original bearing and the new bearing minus 90 degrees. ( Subtract an additional180 if the difference is greater that 180. ) Your present distance to the mark is calculated by dividing your distance run along the baseline by the cosine of the the angle.

There are convenient angles for which the calculations are easier. For instance if you sail until you are 30 degrees off your original bearing then the distance from the mark is twice your distance sailed along the baseline.

I certainly hope you at least have your Chapman's.
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Old 30-07-2008, 10:32   #12
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Look, you can't keep changing the "rules" on this problem, just 'cause folks come up with a solution that doesn't match YOURS. You have several very plausible solutions given to you. I'm done with this...
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Old 30-07-2008, 10:45   #13
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If I may borrow a phrase from another thread....
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I think the danger when you equate seamanship with rules is that you become blinded to the exceptions that exist in this fluid state when fate conspires to make you think outside the box.
There may be more than one way to skin this kitty.
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Old 30-07-2008, 10:58   #14
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Are you close enough to the passing ships to get a "double-echo"?
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Old 30-07-2008, 10:58   #15
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To be a little more specific, you plot and label on your radar screen ahead of time the distances (CPA's) you want to be off specific waypoints, like channel markers or points of land, so there is no rapid radar plotting necessary while hand steering in the fog, unless you find it necessary to plot the RML of another vessel. This takes reduces the stress and the greater potential for mistakes and makes it relatively easy.

Also, if your EBL fails you can always use a straight edge.
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