Brownie's Southport Divers loses divers

[LandonL]

thanks, not sure what the terminology of north/south current check means.

You drive the boat south at a fixed slow speed throttle position, take note of the average speed, turn around, drive north without adjusting the throttle, and take note of the average speed in the opposite direction. The current is the difference between the two values, and is headed in whichever direction you were going faster

 

[johndiver999]

You drive the boat south at a fixed slow speed throttle position, take note of the average speed, turn around, drive north without adjusting the throttle, and take note of the average speed in the opposite direction. The current is the difference between the two values, and is headed in whichever direction you were going faster

That might explain the high value. I see no reason why you would not let the boat sit passively and then look at the velocity data on the GPS. Speed and Course over ground are standard default settings for most any dive boat I have been on. Why drive all around?

Trying to drive at a uniform speed adds potential error(s) that seems to have no value?

I looked up the maximum speed of the Gulf Stream, and your method deliver a value above the maximum and it was only a few miles from shore.

 

[LandonL]

That might explain the high value. I see no reason why you would not let the boat sit passively and then look at the velocity data on the GPS. Speed and Course over ground are standard default settings for most any dive boat I have been on. Why drive all around?

Trying to drive at a uniform speed adds potential error(s) that seems to have no value?

I looked up the maximum speed of the Gulf Stream, and your method deliver a value above the maximum and it was only a few miles from shore.

Actually, it is the opposite of what you claim. Driving at a uniform speed and direction counterbalances most other forces applied here (wind, current, direction). We are near shore, not the Gulf Stream. You method results in extremely unreliable data some days. Giving the throttle a little push, then measuring the distance is how you derive a reliable value.

All the best,
Landon

 

[johndiver999]

If someone has done it a few hundred or thousands of times, it is not difficult to have the boat sit still and see what direction and at what speed the boat is moved by the current within about 90 seconds.

The resulting (raw) speed and direction, needs to be adjusted for the wind, which is not perfectly uniform and some boats "catch" more wind than others. In addition, at slow drift speeds, output readings of the gps units tend to bounce around a bit (plus or minus maybe 0.5 kts) every second or two. So the captain has to mentally average out the speed and do some vector math in their head to cancel out the drift associated with the wind and come up with a decent current estimate. Not that difficult in practice unless the wind is howling or the current is very weak (or both).

Regardless of these inherent weaknesses, I see zero benefit of driving up and down a current (with respect to optimization of time utilization and fuel consumption) nor improvement in the resultant estimate. The method you describe also introduces another (unknown) variable in that running a boat in different directions will subject the hull to a different wave regime (i.e., going into or with the seas), which brings in a concomitant change in hull efficiency (drag) that seems undesirable as well. Your described method is also limited by the captain's ability to steer precise headings and the boat engine(s) maintaining a uniform rpm when going with the wind (and seas) and against them.

Perhaps your methodology is one reason why your current estimate is faster than the gulf stream itself?

 

[LandonL]

Well then I guess all the South Florida captains have no idea what they are doing then. All the best.

 

[VsubT]

Regardless of these inherent weaknesses, I see zero benefit of driving up and down a current (with respect to optimization of time utilization and fuel consumption) nor improvement in the resultant estimate. The method you describe also introduces another (unknown) variable in that running a boat in different directions will subject the hull to a different wave regime (i.e., going into or with the seas), which brings in a concomitant change in hull efficiency (drag) that seems undesirable as well. Your described method is also limited by the captain's ability to steer precise headings and the boat engine(s) maintaining a uniform rpm when going with the wind (and seas) and against them.

Perhaps your methodology is one reason why your current estimate is faster than the gulf stream itself?

Had meant to post this earlier - there’s another good reason for running both directions to check current, aside from averaging out wind & wave differences. Occasionally the deeper current reverses, decreases, or increases from the surface current. With wrecks being fish magnets, modern instruments can show where & how the fish are grouped (generally on the down-current side of the wreck). The captain can use that information to make an informed estimate of what the deep current is doing. This additional data can make or break a hot drop.

I get what you’re saying about a static drift, but the powered method eliminates most non-current variables when done correctly. If I’m spending most of a day and a bunch of money to get dropped on a 150’+ wreck, I want the captain to have as much accurate information as possible to set up the drop distance and direction with a solid chance of making it onto the wreck. It’s a lot like figuring out winds at different altitudes when you’re spotting in skydiving, if you’re familiar with that.

Lance

 

[johndiver999]

Had meant to post this earlier - there’s another good reason for running both directions to check current, aside from averaging out wind & wave differences. Occasionally the deeper current reverses, decreases, or increases from the surface current. With wrecks being fish magnets, modern instruments can show where & how the fish are grouped (generally on the down-current side of the wreck). The captain can use that information to make an informed estimate of what the deep current is doing. This additional data can make or break a hot drop.

I get what you’re saying about a static drift, but the powered method eliminates most non-current variables when done correctly. If I’m spending most of a day and a bunch of money to get dropped on a 150’+ wreck, I want the captain to have as much accurate information as possible to set up the drop distance and direction with a solid chance of making it onto the wreck. It’s a lot like figuring out winds at different altitudes when you’re spotting in skydiving, if you’re familiar with that.

Lance

Nope know nothing about sky diving. Done a lot of drift dives and I'm familiar with local currents and have dove with quite a few professional divers. Never have I seen someone doing what was described in order to determine the surface current and it makes no sense to me since it serves to add additional uncontrolled variables (as I specifically described in earlier post).

Also, I think you are quite incorrect about where the fish schools will be with respect to a wreck and current.
Hint: they are not down current. I've dropped divers on wrecks that are well past recreational depths several hundreds of times and I've done the dives myself a lot too.

 

[VsubT]

Nope know nothing about sky diving. Done a lot of drift dives and I'm familiar with local currents and have dove with quite a few professional divers. Never have I seen someone doing what was described in order to determine the surface current and it makes no sense to me since it serves to add additional uncontrolled variables (as I specifically described in earlier post).

Also, I think you are quite incorrect about where the fish schools will be with respect to a wreck and current.
Hint: they are not down current. I've dropped divers on wrecks that are well past recreational depths several hundreds of times and I've done the dives myself a lot too.

Cool, if it works for you. Interested to hear how the vector math that you mentioned works for factoring out wind & wave effects, can you give a quick example?

Lance

 

[johndiver999]

Sure you put the boat in neutral, you determine the speed of the surface drift and also your course. Then you look around at the wind, determine the speed and direction of the wind. Come up with an estimate of the relative effect the wind is having relative to the actual drift and subtract out the wind effect.

For example if the drift is straight north and at a low speed and the wind is whipping straight out of the south, then you can assume that there is little actual current at the surface.

If the drift is 2.5 mph north, and the wind is whipping out of the north, then you know for sure that the current is screaming north.

If the drift is mostly north and a little toward the west and you have a moderate southeast wind, then you can assume the actual current is north and the speed is probably a little less than the speed of the boat.

Your gps provides a measurement of the net effects of the wind and current as you sit in neutral and you have to use your brain to break the net velocity vector down into the two vector quantities consisting of wind and current and then subtract out the affects of the wind. The waves direction is probably not too relevant unless it is terribly rough and if that were the case, the wind would be a big factor.

It takes a little practice to be able to do the estimates, but it doesn't involve driving up and down the presumed current.

You are trying to estimate the surface current. It does not have to be super accurate with respect to speed, but it is pretty important to get the direction as accurately as possible. You can always give the divers a little more lead up current, but if they get blow in or out, then it is much more likely they miss the target.

Plus the gps is usually not that great at measuring the speed at a slow speed. You will see the reading bouncing around, plus or minus maybe 0.5 knt or something, so you also have to mentally average out the non constant speed data and make an estimate of the actual average speed. Generally the course data has very little variability in it, once you have let the boat drift for a minute or so.

 

[VsubT]

Ah, got it. You’re doing rough wind effect estimates, adjusting to get surface current from GPS speed / direction numbers based on experience, and essentially calling it crude vector math. Thanks for the explanation.

Just for grins, maybe try what Landon described above sometime. I think you’ll find it interesting to compare both methods, especially for an unfamiliar boat, rough seas, etc. Always good to have another tool in the toolbox, no?

At worst, just keep doing the static method if you’re happy with it.

Lance