Hi Folks, We're sailing to Victoria, BC for the Canada Day/Fourth of July weekend. In checking over the tide tables I noted that we're going to experience some extreme low tides (up to negative 3.5 feet) middays over the weekend. This has me thinking. I'm familiar with datum soundings and mean lower low water. But does a minus 3.5 feet in the tide table mean the water depth at low tide will be 3.5 feet less than soundings published on the charts? I think so and plan to steer well clear of anything close to our draft. But I'm curious. Any thoughts? Gary Wyngarden Shibumi (335) Orcas Island, Washington
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Phil, are you sure?
I thought chart soundings are MLW, Mean Low Water, meaning the average of the low tide readings. At times, the low tide could be lower, since MLW is an average of low tides, but it shouldn't go down by a full 3 1/2 feet from the chart, but will be lower than the recorded depths. The exact deviation at times like this weekend isn't readily available, but it shouldn't be 3 1/2 feet. (As a sidelight, I thought that bridge clearances are normally Mean High Water. Not as sure about this one!) This is "shooting from the hip", my Dunhill's Navigation is on the boat, so I'm willing to be corrected
I thought chart soundings are MLW, Mean Low Water, meaning the average of the low tide readings. At times, the low tide could be lower, since MLW is an average of low tides, but it shouldn't go down by a full 3 1/2 feet from the chart, but will be lower than the recorded depths. The exact deviation at times like this weekend isn't readily available, but it shouldn't be 3 1/2 feet. (As a sidelight, I thought that bridge clearances are normally Mean High Water. Not as sure about this one!) This is "shooting from the hip", my Dunhill's Navigation is on the boat, so I'm willing to be corrected
Minus tide
The minus 3.5 foot tide does means that the water level will be 3.5 feet below chart datum. I think this might be the lowest tide of the year up in the Pacific Northwest this year. PNW has a pretty wide tide variation unlike many other locations around the US. Great weekend to hunt for dungeness and rock crab from the beach - there will be lots of beach to hunt from...
The minus 3.5 foot tide does means that the water level will be 3.5 feet below chart datum. I think this might be the lowest tide of the year up in the Pacific Northwest this year. PNW has a pretty wide tide variation unlike many other locations around the US. Great weekend to hunt for dungeness and rock crab from the beach - there will be lots of beach to hunt from...
Chart Datums in Canada
Welcome to Canada Gary The Chart Datum on Canadian Charts ( Salt Water) is referenced to 'Lowest Normal Tide' which is synonymous with 'lower low water, large tide', a low water datum taken from the average of the lowest low waters, one from each year for the last 19 years. The difference to US charts being that the datum used in the US is Mean Lower Low Water. This figure tends to be a higher sea level than the Canadian. Bear in mind that on both US and Cdn charts bordering the boundary line both datums are used on the same chart depending which side of the border you are on. This is by agreement. So there is a difference in depth depending on which side of the border you are on. And no it has nothing to do with us measuring alcoholic content of beer by volume vs weight or is it the other way around? Good crusining and enjoy the 1st and the 4th
Welcome to Canada Gary The Chart Datum on Canadian Charts ( Salt Water) is referenced to 'Lowest Normal Tide' which is synonymous with 'lower low water, large tide', a low water datum taken from the average of the lowest low waters, one from each year for the last 19 years. The difference to US charts being that the datum used in the US is Mean Lower Low Water. This figure tends to be a higher sea level than the Canadian. Bear in mind that on both US and Cdn charts bordering the boundary line both datums are used on the same chart depending which side of the border you are on. This is by agreement. So there is a difference in depth depending on which side of the border you are on. And no it has nothing to do with us measuring alcoholic content of beer by volume vs weight or is it the other way around? Good crusining and enjoy the 1st and the 4th
Clear as Mud (Thank you Ray!)
Ray, Thanks for your welcome to Canada. Your answer is clear as mud...perhaps stirred up by my Hunter running aground at mean lower, low water. Actually your response makes sense to me (that's scary). What you're saying is that if a minus 3.5 foot (1.07metres) tide happened to be the average of the lowest tides for the last 19 years, then water depth at low tide would be equal to the sounding on the chart. At least in Canada. But I might still run aground in the US. I think I'll play it safe and stay in deep water and drink some of that Canadian beer you mentioned. Happy Canada Day!!
Ray, Thanks for your welcome to Canada. Your answer is clear as mud...perhaps stirred up by my Hunter running aground at mean lower, low water. Actually your response makes sense to me (that's scary). What you're saying is that if a minus 3.5 foot (1.07metres) tide happened to be the average of the lowest tides for the last 19 years, then water depth at low tide would be equal to the sounding on the chart. At least in Canada. But I might still run aground in the US. I think I'll play it safe and stay in deep water and drink some of that Canadian beer you mentioned. Happy Canada Day!!
Low tide
Gary You'd think that knowing all that I could keep my Hunter 30 off the putty, but no it happens to us all.
Gary You'd think that knowing all that I could keep my Hunter 30 off the putty, but no it happens to us all.
MLW
Just a little correction. Mean Low Water is not the average of the low tides, but rather the point in which half the tides are higher and half are lower. That means this number could be substantially different that the average low water, possibly in either directions. How's that for high school algebra over thirty years ago? In either case stay clear with a -3.5 tide Bryce S/V Spellbinder H410
Just a little correction. Mean Low Water is not the average of the low tides, but rather the point in which half the tides are higher and half are lower. That means this number could be substantially different that the average low water, possibly in either directions. How's that for high school algebra over thirty years ago? In either case stay clear with a -3.5 tide Bryce S/V Spellbinder H410
Smaller Correction to MLW
The number that you're describing (half are lower and half are higher) is the median, not the mean. MLW is mean low water, as correctly defined previously. Isn't it a shame how our memories fade with time?
The number that you're describing (half are lower and half are higher) is the median, not the mean. MLW is mean low water, as correctly defined previously. Isn't it a shame how our memories fade with time?
Everyone is Right!
Okay, I found my "Dutton's Navigation and Piloting", 14th edition. On page 128, "Mean Low Water (MLW), the average of all the low tides. This plane was used for many years on NOS charts of the Atlantic coast...but is now in the process of being discontinued." "Mean lower low water (MLLW), the average of only the lower of the two daily low tides. This plane has long been used on charts of the Pacific Coast of the United States, the Hawaiin Islands... It has now been adopted for the Atlantic and Gulf of Mexico coast...but the change has no practical significance for navigators." My further reading reminds me that tide tables can have values with a minus sign. That indicated that you have to subtract that amoung from MMLW or MLW in order to estimate depth. So, normally, the charts will show the approximate low tide levels. At certain times of the year, in some locales, you have to subtract more! (Gee, everyone was right!)
Okay, I found my "Dutton's Navigation and Piloting", 14th edition. On page 128, "Mean Low Water (MLW), the average of all the low tides. This plane was used for many years on NOS charts of the Atlantic coast...but is now in the process of being discontinued." "Mean lower low water (MLLW), the average of only the lower of the two daily low tides. This plane has long been used on charts of the Pacific Coast of the United States, the Hawaiin Islands... It has now been adopted for the Atlantic and Gulf of Mexico coast...but the change has no practical significance for navigators." My further reading reminds me that tide tables can have values with a minus sign. That indicated that you have to subtract that amoung from MMLW or MLW in order to estimate depth. So, normally, the charts will show the approximate low tide levels. At certain times of the year, in some locales, you have to subtract more! (Gee, everyone was right!)
In addition...
...to the great definitions: Remember that mean lower low water is just as the definition: the average of the lower of DAILY tides. This does not mean (pun..gotta love it) that is the lowest possible tide. Lower low water is often used in place of what it is intended to identify: Lowest Low Water which occurs twice per year and varies greatly with latitude. To make a long story short, if you read the little paragraphs on the corner of your chart it will tell you what the chart datum is and what the chart's definition of the datum is. (Fine print.) ...at least on NOAA U.S. charts after 1991. So, HOW do you get -3.5 on a tide table? Datum on chart is mean lower low water (average of DAILY lower low water) and tide on that day is probably lowest low water. Might be neat to pull up to the rock (unless it's a huge beach) and write your name in an environmentally friendly, permanent paint/ink. Maybe a brick on concrete block engraved "Kilroy was here." ...or something like that. I would suggest that your boat name is not a good idea as they can look you up and fine you for littering...no?
...to the great definitions: Remember that mean lower low water is just as the definition: the average of the lower of DAILY tides. This does not mean (pun..gotta love it) that is the lowest possible tide. Lower low water is often used in place of what it is intended to identify: Lowest Low Water which occurs twice per year and varies greatly with latitude. To make a long story short, if you read the little paragraphs on the corner of your chart it will tell you what the chart datum is and what the chart's definition of the datum is. (Fine print.) ...at least on NOAA U.S. charts after 1991. So, HOW do you get -3.5 on a tide table? Datum on chart is mean lower low water (average of DAILY lower low water) and tide on that day is probably lowest low water. Might be neat to pull up to the rock (unless it's a huge beach) and write your name in an environmentally friendly, permanent paint/ink. Maybe a brick on concrete block engraved "Kilroy was here." ...or something like that. I would suggest that your boat name is not a good idea as they can look you up and fine you for littering...no?
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