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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #131  
Old 03-29-2008
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SURV69 is on a distinguished road
I Expect To Run Aground

If I religiously follow the charts of Lake Erie.

The charts are based on some obscure, probably made-up low water datum with the average depth of water being 4 feet above datum and high water marks almost a foot above that.

MY new(old), boat draws 4 feet.

ACCORDING to the charts, since the depths are based on "low water datum", shouldn't I be able to take my boat ANYWHERE THAT THE CHARTS SHOWS WATER existing?

The idea of using low water datum is that a 4' depth would for all intensive purposes, be some number of feet deeper(like a "safety" margin). In this case, 4 feet deeper. ANYPLACE on the chart that shows 1 foot of depth, should mean I can transit that route safely, which is plain and simple, just not true. In fact I know of many places which is shown as 1 or even 2 feet depth, where the smallest of boats(planing hulls) get stuck on the bottom all the time.

REALISTICALLY . . . shouldn't any place which has a depth of less than 3-4 feet(real-time actual depth), actually be shown as uncoverings or dry-land on the chart . . . or at the very least, a color-change exist at the lake bottom(average depth) where the low-water datum would meet the air?

THIS seems to be especially critical to me since my draft is the same as the difference in real-time depth and low-water datum.
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  #132  
Old 03-29-2008
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We have a saying that the depthsounder is only of historical interest. Assuming that the swinger is 2 m. before the keel, then sailing at the speed of 4 knots, it will take 0,5 seconds from the moment one have the information from the instrument to the moment of truth.

I have only one explanation for my groundings, sloppy chartreading.
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  #133  
Old 03-29-2008
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sanctuarysam will become famous soon enough
as i've said before...

much like 'bikers', there are two types of bikers, those who have dropped their bikes, and those who haven't dropped it yet. and when 650+ lbs of hot motorcycle falls...can be painful. yes..i have dropped my bike.
i've been sailing all my life..and have been aground soft and many a hard grounding (actually sailed into a hole at the mouth of the piankatank in '78), near gwynns island..early january..imagine cold and confounding. old farts cg aux had to pull me off (ugh..and there were young women on board too)
as said above, the sounder is a retrospective device..by the time you see that it is too shallow, you are probably already bumping..fast spin around, if possible ("GYBE HO!!!) and hope i don't spill my bass ale
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Sam
Sanctuary, Sabre 30 mkIII
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