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post #1 of Old 03-04-2004 Thread Starter
Michelle Potter
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Deep Thinking in the Bahamas

Don't be deceived by the  differing colors. The lighter shade of blue means a sandy bottom, while the darker shade means grass, but the depth is about the same in both places.
When we arrived in the Abacos, Bahamas, we were stunned to poke our noses over the bow and see dolphins swimming alongside our boat in beautiful, clear water. We saw shades of blue, green, and aqua that we didn't know existed. From a navigational standpoint, the lighter colors usually indicated shallower areas, while the deeper colors let us know that we had more depth.

That was the easy part.

The clear water that made the Bahamas so alluring and beautiful, however, was also a source of frustration. We never knew how deceptive it can be to read water 'by sight.' For example, when we were sailing our first narrow passage from West End to Great Sale Cay, I excitedly leaned over the side and yelled, "Conch!"  "Starfish!"  "Bottom, I can see the bottom!" only to have my husband gently point my attention to the fish finder. While I was convinced we were down to five feet of water, the fish finder indicated seven feet.

"OK, you try it," I said to my husband. He looked over the side and shook his head. Even though we stayed in the Abacos for two months, neither of us could ever tell the visual difference between five feet and eight feet of water. This didn't make a world of difference to us, living aboard a boat that only draws only three feet, but if we'd had a boat that drew six feet, that difference would have been critical.

Shifting sands and tidal states can change charted depths. And sea grasses, along with sun angle and changing light can each alter how deep the water appears to be.
When we tried to read the water, we noticed that something as insignificant as a cloudy sky could throw our entire perception of "light" and "dark" water out of whack. On the rare choppy day, the wave motion stirred up the bottom and gave us nothing but murky water leading to unreliable guesses.

As for trying to follow the charts, well, that could get a sailor into a bunch of trouble, too. The charts were reliable, but the navigable courses around some of these little islands seemed to change on a daily basis. With a constantly shifting bottom surface, we found that whoever was at the helm had to be alert and ready to change course at a moment's notice.

Throw in the dramatic effect of the tides and you can see why so many boat carcasses litter the reef-strewn areas around these picturesque islands. Two harbor entrances, in particular, stand out in my mind as downright tricky: Little Harbor and Black Sound, Green Turtle Cay. I admit that I'm the conservative captain, and I've been known to fudge the danger factor once in awhile, but on these two entrances I was right to be cautious. If we'd read the tides wrong, like the catamaran that entered Little Harbor four hours after we did, we, too, would have grounded. Granted, it was a bit of a luxury to witness a grounded catamaran (how many times does a captain see that in her life?), but the catamaran drew the same three feet that our boat does, so if we'd been a little off in our navigation or tide tables, we could have easily grounded, too.

This view of the entrance to Hopetown Harbor, taken from the top of the Hopetown Lighthouse, illustrates sandbars (above the powerboat) and the deeper channel.
In the end, it's best to navigate an area like the Abacos with your eyes wide open and with a little technology by your side. Yes, I know, there are plenty of old salts who throw out a lead-weighted line with a dab of peanut butter on the end and claim that this system works perfectly. The line tells them the depth and the sticky peanut butter tells them if the bottom is sand, clay, rock, or grass. For those of us who don't want to spend the day yelling, "Mark Twain!," there's always technology. A depthsounder is for all you number-crunchers out there and, for those of us who would rather have pictures, there are fish finders.

"On our first night out, we re-anchored three times because I was convinced that the fish finder was indicating a steeply-banked shallow bottom directly in front of the boat."
I have to admit that we went a little overboard when we bought our fish finder. We were so excited that we could buy an Interphase probe with a forward-looking sonar that we didn't really think about the realities of using this instrument while sailing. On our first night of anchoring (in the middle of a storm, of course), we re-anchored three times because I was convinced that the fish finder was indicating a steeply-banked shallow bottom directly in front of the boat. What if the wind shifted and we grounded during the night? After we anchored the third time, we discovered that the fish finder was either telling us where our mass of anchor chain was (directly in front of us) or the side lobe interference was causing it to read incorrectly. Exhausted, we finally fell into berth and slept soundly until morning. Now we know that we should have turned off the forward-looking sonar feature that night, but we thought it was so cool that we let it confuse us for another three months before we turned it off for good. We did enjoy having a fish finder rather than a depth sounder, though, because the picture really was worth a thousand words. If the bottom was getting shallower, its contour came up dramatically on the fish finder, which was great for reinforcing the information at hand.

Bringing a new meaning to 'hard aground,' the skipper of this vessel strayed a bit too close to the wrong colored water.
Another feature of our fish finder was that it worked as a GPS repeater. Since our GPS unit was down in the cabin, the repeater came in handy. We could access our latitude, longitude, direction to a waypoint, and our track (course-over-ground) to that waypoint without leaving the cockpit. Thus, we could keep our main GPS unit snug and dry at the nav station, while captain and crew were left to brave the soft breezes and sunny days of the Bahamas.

The old saying not to rely on one means of navigation holds true, especially when negotiating shoal waters. What the eyes see isn't the only story, and is likely to present a ball-park figure when it comes to determining depth. In the same way, what the depth sounder reads can also be subject to various distorting factors. A cool head and cross refrencing both with what you see and what your instruments read is necessary to keep your boat in safe water and your cruise from winding up on the beach.

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