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Old 12-04-2010, 15:34   #1
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MOB, Charplotters and Autopilots

Some chart plotters do have an MOB feature. Has anyone tried this MOB feature and how satisfied were you with the result? I would assume that all systems are not equal and some have serious shortcomings. To know these shortcomings could be of some help to anyone reading this forum.
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Old 12-04-2010, 15:57   #2
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We have had several in depth?? (get it) discussions on the first thing to do in a MOB situation. I have argued that hitting the MOB is number 1, 2, and 3 priority. The main reason is there is no cost, just instantly hit the button. That part works great and now you have a proximity reference marked.

The reason I'm so hard over on this is I have spent many years diving and looking for wrecks. I have experienced how far a boat can move in an amazing short period of time if there is wind and waves.

One mistake during the panic and confusion of a MOB situation and you may never get back to even an approximate reasonable search location.

With the MOB position marked remainder can be sorted out by searching down wind and/or down current of reference position.
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Old 12-04-2010, 16:07   #3
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We have had several in depth?? (get it) discussions on the first thing to do in a MOB situation. I have argued that hitting the MOB is number 1, 2, and 3 priority. The main reason is there is no cost, just instantly hit the button. That part works great and now you have a proximity reference marked.

The reason I'm so hard over on this is I have spent many years diving and looking for wrecks. I have experienced how far a boat can move in an amazing short period of time if there is wind and waves.

One mistake during the panic and confusion of a MOB situation and you may never get back to even an approximate reasonable search location.

With the MOB position marked remainder can be sorted out by searching down wind and/or down current of reference position.

And how does your system perform once you have initiated an MOB? If this feature is available to you, do you use POSITION or DR to mark your's MOB?
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Old 12-04-2010, 16:19   #4
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And how does your system perform once you have initiated an MOB? If this feature is available to you, do you use POSITION or DR to mark your's MOB?

On each of my Furuno, RayMarine, and Garmin systems there is a button labeled MOB. Once the MOB button is pressed (in any mode I think) a waypoint is created.

On my Furuno NavNet the waypoint is given a number. The automatic waypoints are sequential so the largest numbered waypoint is the MOB position (unless you have been labeling waypoints with numbers.)

Once you have the lat/lon you navigate back to it using your GPS using any one of a number of modes.

In the chartplotter mode on the Furuno I can monitor my position relative to the waypoint automatically plotted on the chart. All I have to do is search relative to the MOB waypoint on the chart.
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Old 12-04-2010, 16:28   #5
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Some instrument systems support a MOB based on DR, while others are based on GPS waypoint being set. My NX/2 instruments support the DR MOB and the Furuno chartplotter supports GPS MOB. One less obvious advantage to using the DR instead of the waypoint is that the DR tends to compensate for drift. So the DR MOB shows the relative position to participant in the water relative to the boat. This assumes the boat and person are affected by the current about the same. The waypoint will show you the GPS location when the button was pushed, but does not compensation for the drift of the in water participant.

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Old 12-04-2010, 17:18   #6
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One less obvious advantage to using the DR instead of the waypoint is that the DR tends to compensate for drift. So the DR MOB shows the relative position to participant in the water relative to the boat. This assumes the boat and person are affected by the current about the same.
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Yes some brand of chart plotters will state that DR is more accurate. For DR to work, the system requires the necessary data (current and wind) to be able to calculate set and drift.
The danger of DR is the set and drift can be toward a reef, a sand bar or any other obstructions and to sail to the DR could be in fact running into danger. For this reason should an autopilot be able to track to the DR or should any form of tracking be prevented?

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Once the MOB button is pressed (in any mode I think) a waypoint is created.
An electronic MOB should be tested. The way the system will behave may be quiet surprising.
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Old 12-04-2010, 17:34   #7
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Yes some brand of chart plotters will state that DR is more accurate. For DR to work, the system requires the necessary data (current and wind) to be able to calculate set and drift.
The danger of DR is the set and drift can be toward a reef, a sand bar or any other obstructions and to sail to the DR could be in fact running into danger. For this reason should an autopilot be able to track to the DR or should any form of tracking be prevented?

An electronic MOB should be tested. The way the system will behave may be quiet surprising.
You don't need set & drift to do the DR MOB. My NX/2 instruments use compass course and speed to do a DR. Just like you would on paper. The assumption is that the boat and the MOB are drifting at close to the same speed, therefor the DR is drifting with the person in the water and the direction-to and distance-from it are displayed accurately from the current position of the boat. I have played with this in light air but with current running, and it was pretty accurate.

Obviously you could make this more complicated and take a GPS waypoint, then feed in drift and set, predict where the MOB is in lat/long, but this isn't how the simpler instruments work.

I'm not convinced that these automated MOB buttons are going to do you much good. If you have a full race crew and someone goes over, then there is a good chance of recovery. There's someone to keep their eye on the victim and lots of crew to manage sail and get back to the pick up zone. In the typical Mom & Pop cruising scenario the odds of a successful pickup are very small. The effort and cost expenditures need to be focused on staying on the boat, not on recovery.


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Old 12-04-2010, 19:41   #8
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You don't need set & drift to do the DR MOB. My NX/2 instruments use compass course and speed to do a DR.
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I am not convince that every system use a compass course and speed to do a DR.

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The assumption is that the boat and the MOB are drifting at close to the same speed, therefor the DR is drifting with the person in the water and the direction-to and distance-from it are displayed accurately from the current position of the boat. I have played with this in light air but with current running, and it was pretty accurate.
Paul
It is an assumption. In practice it is interesting to see how quickly Position and DR separate.
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Old 12-04-2010, 21:37   #9
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I am not convince that every system use a compass course and speed to do a DR.
What do they use then?

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It is an assumption. In practice it is interesting to see how quickly Position and DR separate.
I think you are missing the point. A DR position gets off because of a number of factors, like drift. In the case of MOB the error induced by the drift is canceled out, in theory, because it affects the boat and the MOB the same. Something that does not occur if you use a GPS waypoint.

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Old 13-04-2010, 08:37   #10
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We have several Garmin units aboard our boat--a 162 at the helm, a 2006C at the nav table and a hand held 76 that we use in the dinghy. All have the same MOB function. Punch the MOB button twice and then the Enter Key and the yacht's position at the time the MOB button was first pushed is marked as the MOB point and the unit displays the bearing and distance to that position. We, fortunately, have never had to use the function for a MOB situation but we do use it to mark the point where our anchor hits bottom while anchoring the yacht (and later rename the resulting MOB waypoint as "Anchor") which has proven very useful. For what it's worth one needs frequent reminders as to the proper sequence of buttons for the MOB function to work properly but we go over that frequently aboard our boat as a matter of routine.
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Old 13-04-2010, 17:01   #11
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What do they use then? Paul L
This is the question. May be some manufacturer agents working under cover in this Forum may be able to enlighten us in such a way that members of this forum can assert if the MOB features on their instruments is a reliable tool or just another gadget.

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A DR position gets off because of a number of factors, like drift. Paul L
Agreed. Taking into consideration that at the occurrence of a MOB the vessel is likely to travel at is best speed and the Mob will stop traveling someone will require a fairly good navigator to plot a reliable DR in the shortest time possible. Compass deviation, tidal data, current data, effect of the wind on the vessel etc, will have to be properly assessed.
Given a dark night even a large racing crew may have difficulty to establish the position of the Mob. Now electronic MOB plotting suppose to give you an accurate position instantly, based on the onboard instruments and relevant onboard electronic cartography.

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I think you are missing the point.
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The point of the thread was to evaluate electronic MOB. Thank you for your contribution.
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Old 13-04-2010, 19:39   #12
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we do use it to mark the point where our anchor hits bottom while anchoring the yacht (and later rename the resulting MOB waypoint as "Anchor") which has proven very useful.
This is interesting. We have 2 C-series, repeaters and pilot and a hand held Garmin. With the C-series, once the MOB is activated it is not possible to place a waypoint at the MOB position or rename a MOB icon because most of the functions on the MFD become unselect-able. Furthermore if the DATA BAR is not ON, the MFD will only display a Mob icon on the cartography but no data about the position. It is not possible to switch back ON the DATA BAR because access to it is denied. The only alternative is to cancel the MOB, reactivate the data bar, and press the MOB again, which of course is not anymore the exact position of the original MOB. Quiet odd. It is to be noted that in the first instance POSITION and DR are still transmitted to other MFDs and repeaters. In our system the repeaters show the POSITION only because they are not able to show DR, and the MFD will show the DR. This could be quiet confusing to someone unaware, but handy if someone wants to know both positions.

The question remains, would it be desirable for an autopilot to be able to TRACK to DR and/ or to a POSITION, knowing the danger involved in tracking toward a person or possible dangerous ground? In our system the manufacturer decided not to allow tracking.

Thank you for your contribution.
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