Mooring depth? deep enough? - SailNet Community

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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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Old 09-28-2013
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Mooring depth? deep enough?

Hi All

I think I know the answer to this but I would like a second opinion after all a saving of $800 a month marina fees is at stake!
So here's the facts the place is Brisbane river Port Office the time and date my son depth tested the spot was 11:AM 28th August 2013 and measured 2 meters on a rising tide which on the tide chart was 0.8 meters on low at 10:02 AM AEST (Australian Eastern standard time) the boat draws 1.8 meters the spot is reported to be less than the chart which says 2meters. So I will be interested in your response if u have a good idea of understanding of enclosed waters navigation. A second or more opinion would be most appreciated as I am a loooong way from 'the spot'.

Thnx in advance

JC
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Old 09-28-2013
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Re: Mooring depth? deep enough?

Confusing post.

Are you saying that you draw 1.8 meters, by low tide is 0.8 meters? It doesn't seem like your question could have such an obvious answer, unless you are asking whether burying the keel is a good idea. Some do it, but its a very bad idea.
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Old 09-28-2013
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Re: Mooring depth? deep enough?

You may save $800 a month but the price may be your boat.
If the boat draws more than the bottom, or really close to the bottom, you are definetely in trouble. If more...it is pretty obvious. If close, you are one big wave from bouncing on the bottom.
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Old 09-28-2013
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No it's tide low is .8 mts today the depth on the chart is 2 mts so today at low it should be 2.8 mts but I don't trust the chart depth that's y I measured it
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Old 09-28-2013
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Re: Mooring depth? deep enough?

Quote:
Originally Posted by JohnnyChristo View Post
Hi All

I think I know the answer to this but I would like a second opinion after all a saving of $800 a month marina fees is at stake!
So here's the facts the place is Brisbane river Port Office the time and date my son depth tested the spot was 11:AM 28th August 2013 and measured 2 meters on a rising tide which on the tide chart was 0.8 meters on low at 10:02 AM AEST (Australian Eastern standard time) the boat draws 1.8 meters the spot is reported to be less than the chart which says 2meters. So I will be interested in your response if u have a good idea of understanding of enclosed waters navigation. A second or more opinion would be most appreciated as I am a loooong way from 'the spot'.

Thnx in advance

JC
JC--

What you need to know is the datum for the tide measurements and the extent of Spring Tides at the particular location. US Chart Depths are based upon MLLW or "mean lower low water". The key word in that phrase is "mean". One needs know the variation about the "mean" to know what the lowest low tide might be. Moreover, you need to know the extend to which the mooring site is subject to wave action, whether natural waves or those created by passing vessel traffic. Lastly, you need to know the extent to which the site is subject to offshore winds and the effect an extended offshore wind will have driving water out of the anchorage. One can easily loose a foot (-.3 meters) of depth to prolonged offshore winds in some locations. Couple that loss with a Spring Tide that gives a negative low water (e.g. -.25 meters) and perhaps waves of .25+/- meters and you could find your 2 meter mooring with an effective depth of only slightly more than 1.2 meters at the bottom of a wave. The coincidence of the foregoing conditions might be rare but really only has to happen once (to your misfortune) and as the old saying goes, if something can happen, it will eventually.

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Last edited by svHyLyte; 09-28-2013 at 04:07 PM. Reason: correct typo
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Old 09-28-2013
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Re: Mooring depth? deep enough?

Johnny, boats don't sit still. In any kind of winds the water and the boat bounce around, so even if that spot has 2m of water at dead low tide, as long as your boat draws 1.8m it is guaranteed to hit the bottom and take damage in any kind of bad weather.

I'm also confused by your posts, in the US as HyLyte said charts show a datum and are normally at mllw, so they are the least depth you'd ordinarily find. But again, that's not the shallowest that they will ever be, it is just a mean depth.

If your boat can take any damage from pounding the keel or rudder on the bottom, then you'd want at least a meter under it at all times, I would think. A half meter if you're a bit less conservative with a small boat and a new rudder won't cost you a lot.
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Old 09-28-2013
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Re: Mooring depth? deep enough?

a half meter isn't much
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Old 09-30-2013
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Simple Answer...Find Another Spot

Boat draws 1. 8 m (~ 6 ft), chart says 2 m, assuming that is a MLW or MLLW reading, so you have only 0.2 m (8 inches) of water under your keel in ideal conditions. If low tide actually lower (frequently), than you're in the mud/rock or if you have any sizable waves even at normal low tide hieght , keel could be hammered constantly into the sea floor on a frequent basis.

Here in the Massachusetts, US North Shore area, the average tide is 9 ft. But unusually high tide range, it could be almost 13, or +/- 2 feet on each side of the tide. At Mean Low Low Water, chart values, if 8 ft is represented on the chart and is accurate for that area, than during the large tides, the the height in that chart area could be as low as 6 ft.

My boats draws 6 ft. My mooring at MLLW on is about 9 ft. During the Spring and Full tides, the low tide depth at my mooring can be as low as 7 ft. A little dicey for my liking, but the bottom is soft mud, which isn't as bad as rocks.

My recommendation, if you can is to get a mooring spot where the low water height is at least 50% more than your draft. So for your 1.8 m draft, 2.7 m as a recommended minimum, 2.4 m may be OK if in a very calm (area with no waves or strong winds). However, that still may not be enough if your area has a huge tidal height range.

DrB
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Old 10-24-2013
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Re: Mooring depth? deep enough?

Did you check the depth at low water spring tide?

The Brisbane River bottom is mostly mud, but it is also littered with debris from old moorings and previous floods. Everything up to and including old motor vehicles could be down there. All waiting to punch a hole in your hull. You really need to wait for a very low tide, then get in the dinghy and do a thorough survey of the area that your boat will swing around in all conditions.

The tide rips through that river as a strong current - 6 knots is not uncommon - so you won't want your boat stuck in the mud when that happens. Personally, even if the swinging area is totally clear of obstructions, I would not accept a mooring spot with less than 1.5 metres clearance under the keel at LWS. Also you will need to thief-proof your boat.
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