| Author | Message | ||
     Ted Lavino Senior Member Username: Tlavino Post Number: 257 Registered: 01-2004 |
For those interested, lots of info relating to tides and currents is available at NOAA's web site: http://co-ops.nos.noaa.gov/index.html Tide Predictions at: http://co-ops.nos.noaa.gov/tide_pred.html Real time tide info at: http://tidesonline.nos.noaa.gov/ Current predictions at: http://co-ops.nos.noaa.gov/currents04/currpred.html Table 3 (Height of Tide at Any Time): http://co-ops.nos.noaa.gov/publications/table3.pdf | ||
| Mark Howe Moderator Username: Unclemark Post Number: 163 Registered: 08-2003 |
Two ways you can figure the maximum tidal range: One is to look at the range scale on B dock, the other is to look at a tide table and see what the ranges are in Jan and July. We had some extremes this year; I previously called the high at 7 ft. but there were some a bit in excess of that this year. A am confused about what Susie and Ted are discussing; Susie first asked about the "greatest" range but then the discussion went on to address "mean" readings. Understand that means are like averages and greatest is like maximums. A "spring" is a maximum range for that lunar month and of course any of these data can be averaged. Once again, the devil is in the details and definitions. | ||
| Ted Lavino Senior Member Username: Tlavino Post Number: 255 Registered: 01-2004 |
In the same discussion with Susie was anchoring and using depth sounders to judge the amount of rode needed. We were discussing the accuracy that should be expected of these types of instruments, which got us into my favorite topic (as you know  of preparation. One of the items on my pre-cruise, particularly for an untested boat such as you might find in a charter fleet, is to guage the accuracy of its instruments, i.e. depth sounder and compass with a Lead Line and also to verify to what offset the depth sounder was set, i.e. waterline, water under the transducer, or water under the keel. Great diagram of this is in the Raymarine ST-60 Depth Sounder manual, page 27, which can be accessed at: http://www.raymarine.com/raymarine/SubmittedFiles/Handbooks/Instruments/ST60/810 38_4www.pdf T. | ||
| Mark Howe Moderator Username: Unclemark Post Number: 162 Registered: 08-2003 |
The tidal range is about 9+ ft. From MLLW it goes up 7+ and down 2 | ||
| David Sheriff Board Administrator Username: David Post Number: 156 Registered: 10-2002 |
I know a "lead line" is kinda old-fashioned, but given the amount of uncertainty regarding shallow depths reflected in these last couple of posts, perhaps it would be good device and skill to revive. Pilgrimage has (or once had) a lead line with an actual piece of lead designed for the purpose. It has a semicircular depression on the bottom of the piece of lead which can be filled with beeswax or something else sticky to give you an idea of what's on the bottom. (bonus) I really like traveling with a softball sized piece of sticky wax in a ziplock bag, so that would be a ready source of wax for me. The "sticky wax" is just several wax toilet sealing rings smashed together. in the event water starts coming through the hull, I think it's a lot more versatile than a set of wooden plugs. One size fits all holes or cracks. I credit Bucky Bailey for giving the idea to me. I made myself a lead line last summer when I helped a client without a depthsounder move his boat from Wilmington to Santa Barbara. it's just a weight and maybe 40 ft. of light gauge yellow polypropylene line knotted every fathom. its one knot at the first fathom, two knots at the second fathom and so forth. With a line only six fathoms long the knots don't seem to get out of hand. knots also beat thread or tags in the tactile simplicity of the thing. I tied the bitter end of the line to a piece of wood so it was less likely to get away from me. the wood also gives me something to wind up the line on. | ||
| Mark Howe Moderator Username: Unclemark Post Number: 160 Registered: 08-2003 |
If you take the depth reading as the water depth you usually have a cushion as Susie says. To be sure, you would have to know if the reading was from the transducer, the keel or the surface, but the depth would never be shallower than the meter said unless somebody really screwed up. I would hesitate to add the draft unless I was sure the meter was calibrated to the keel bottom. Usually nobody gets around to calibration at all so its only the depth from the transducer. Sure, you could add 2 ft., but be very careful with what you do with that extra room. [I like cushion] First class trip to Little Harbor we got outside the reef and the propeller shaft got loose from the coupling and slipped out to the point where it jammed the rudder so's we could only go in circles. We got an anchor down in time and were able to eventually get the coupling bolted back together but learned to install a hose clamp on the shaft so the shaft couldn't slip far enough back to jam the rudder. That lesson may have been lost to the cobwebs of time. Anybody heard of that? Another time we monitored calls for help from a large [40-50 ft.] sailboat in the harbor at high tide. On a high tide the swells come over the top of the reef and can cause havoc if there is a sea running. We were snug at anchor at Sta. Barbara Is. but could relay info to San Diego Coast Guard as they did a helicopter rescue of some 6 kids aboard. Must have been quite a thrill for the kids and the parent that went with them. After the tide went down what had seemed an emergency became no big deal, so that reef is pretty important. The CG had to rely on our communications because they couldn't hear anything from behind the island; not even their own helicopter. Lessons learned: never hesitate to relay info for a vessel if their com isn't getting thru. And, figure on a rough ride in Little Harbor at high tide. The famous class was when we were set aground in Little Harbor on a ~40' vessel with a wing keel. You think just wait for the tide to come in and float you free, but no. That's a story for class. Low tide in summer is about 4 am; know your tides! That said, I recall as a kid running aground about 4 am. every time we spent the night in Little Harbor. No deal; just make an adjustment and go back to sleep. But not with a wing keel. You learn a lot more when things go wrong than when everything goes right. More later. | ||
| Susie Campbell Senior Member Username: Susie Post Number: 124 Registered: 10-2002 |
Thanks for the rule of 12ths lesson! I've heard of it before but never had it explained so well! When you get that fathom reading don't foget you can add the draft of your vessel (from water line to bottom of hull) to the depth reading. That will give me about an extra 2 ft. to play with. When we got to Little harbor I kayaked out to the reef that extends across the entrance about 1/3 from the shore. I read that the reef is complety submerged at high tide! YIKES! We used the big black rock that sits on shore in Shark Harbor as a range reference when coming in. I lined my self up with that rock when I was in the kayak could see that you would have plenty of room to come in, if you did this. When we hiked up to the top of the hill and looked down at our boats, we could see that the drop off from shore was pretty deep. A reading of 10ft. seems awfully shallow when the chart datum tells us it's 18ft. at MLLW. | ||
| Marc Hughston Moderator Username: Hughston Post Number: 266 Registered: 03-2003 |
From my perspective, this was another very good trip with new situations, and new lessons. We had more motoring than sailing this trip, and that lead to more emphasis on how to fix the engine problems with Half Moon. Many thanks to David Sheriff of Electric Marine for getting Half Moon going again much more rapidly than we could have done for ourselves – avoiding a long-distance tow from Cat Harbor. In addition to those engine issues, many other interesting things occurred. These are my observations, and I invite your comments. Anchoring at Willow With wind coming over the top of the hill out of the southwest, Campbell’s Sloop was in first with Pilgrimage next. Seems like every boat dropped their stern anchor first, then motored out to drop the bow anchor. We ended up with four boats closer together than I can remember, and rafting together was a breeze. How we would lie together for the night was a concern of mine, but we worked out an arrangement that had us staggered so that each boat was either forward or astern of the other. It turned out that Half Moon was abeam of Pilgrimage and maybe 40 feet away, and both were forward of La Mouette, who was forward of Campbell’s Sloop. Concerns about Anchoring at Willow I was concerned that if weather conditions did not hold to the forecast, we were too close to one another. I was also concerned that Pilgrimage was abeam of Half Moon, but even when the wind died we did not get very close. Lessons re Anchoring at Willow 1. We all had plenty of stern rode out and that was our primary anchor considering the direction of the down-slope wind during the night – I think this was a key factor in my feeling for our security. 2. A more conservative strategy would have us separated further apart for a worry-free night. 3. Picking up anchor on departure: Because Pilgrimage had to pick up her bow anchor first, and because of the close proximity to the other boats, we arranged to drop our stern rode on a float made of rafted PFDs. Kevin on Half Moon told me he decided to do the same, but I’m not sure if he actually needed to. It turns out that because every other boat was in motion, we had room and we turned around, picking up the stern rode from our bow. (A drawing on the whiteboard would make this much more clear). Anchoring at Little Harbor This was a snap, and only La Mouette decided to keep sailing during the couple of hours we spent here, with winds blowing straight across the reef. Lessons re Anchoring at Little Harbor The Rule of Twelfths Pilgrimage settled to bow and stern anchors in 13 feet of water at 1330. I calculated, using the Rule of Twelfths that we would lose approximately another 2.5 feet of depth due to the dropping tide, and so I was comfortable that we would not go around. The Rule of Twelfths is a rough but fairly close guide, and here’s how it works: 1. Assume that the average time between tides is 6 hours – so in our case on 2/27/05, the high was at about 1030, and the low was at about 1630 (the actual times vary by a few minutes). 2. Assume that in the first hour, 1/12 of the total tidal range goes down (or comes up as the case may be). In the second hour it’s another 2/12s, in the third hour it’s another 3/12s, (so in 3 hours 6/12s or ½ of the total tidal rage has gone down), fourth hour is another 3/12s, fifth hour is another 2/12s, and the sixth hour is another 1/12. In tabular form it looks like this: Hour-----Twelfths 1----------1 2----------2 3----------3 4----------3 5----------2 6----------1 Since we anchored at 1330, and since low tide was at 1630, I knew we had six more twelfths of tide, or another ½ of the total drop before the low. Since the total tidal range that day was 4.6 feet, six twelfths, or half of that, was 2.3 feet and I rounded up to 2.5. This is a neat way to very quickly get a handle on how much the tide will rise or drop when you are in any anchorage, at any time of day. 3. Assume there will be local variation to this “Rule.” By 1505, the time we left, we were reading depth of 10 feet, and there was still about another hour and a half until low tide. Our draft was six feet and so we had plenty of room in the calm conditions. If we had been playing a game of inches with our draft though, we may well have gone aground. There is more about radio communications and the engine situation on Half Moon, but it’ll have to wait for the next post. Marc |
