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DrB 07-21-2007 08:38 PM

How Much Water?
Like many of you, my boat is on a mooring. Probably not like a lot of you, my boat is moored in the Northeast US where tides are 7 to 9 ft.

My boat is a fixed keel and draws a 5' 11" draft.

How Much Water Depth Is Recommend Based on A Boats Draft? Draft + 3, 4, 5, X feet? What's the consensus?

The Story:

I recently obtained a mooring in a location where the depth at MLLW is about 8 ft. I beleive that this is the case because based on the my chart of the area I am in, the depth is noted as 8 to 12 ft. I also monitored my depth today, using my depth meter, at low tide and the meter read 7.9 ft. Since this is a new boat, I don't know if the meter is calibrated to include the depth from the waterline to give the overall drepth from the waterline or just measures the depth below the transducer. The depth meter transducer is in the bilge and measures through hull. I estimate that the bilge is 1.5 ft below the waterline. So if it is measuring from the bottom of the boat, the 7.9 would now be 9.4.

When I applied for my mooring permit with both the harbormaster and mooring company, I clearly stated that my boat draws 6' of water. When the Mooring company that installed and placed the mooring told me it was ready, I ask what the depth, at low tide, was the boat in and they replied about 10 ft. 10 ft would be fine, but I am concerned if it is truly 8 ft.

I guess I'll wait until low tide again and measure with a long stick to get an idea of the true depth.


US25 07-21-2007 08:51 PM

I don't think even 10ft is enough water. What happens in rough weather? Or historic low tides?

sailingdog 07-21-2007 09:17 PM

I think that 10' MLLW is cutting it a bit close, unless the mooring is fairly well protected from surge and has limited fetch in most directions, you may have a problem in a storm.

SchoonerMISTRESS 07-22-2007 06:34 AM

Ya gotta love a lead line. Simple, effective, and does not require batteries. We have the regular depthsounder, we have the hand held, and the old Nantucket Sounder which more often than not is used to verify what the others are trying to tell us.
We draw six and a half and are in thirteen feet at low tide. Around here, there is about two feet of tide.

bestfriend 07-22-2007 12:27 PM

I draw 6'3". At a super low tide my sounder reads 5'4" and I am still floating, but obviously I must be very close to grounding. Also, I am tied to the dock, so the swell is not a big factor. Next time you haul you need to measure from the sounder to the bottom of the keel with a plumb line to get the real number for using your sounder. Or, I don't know off hand, but, maybe it can be callibrated for the difference. Kathleen's advice to get a line is a solid and time tested idea.

Valiente 07-22-2007 05:22 PM

I have an even cheaper suggestion: Borrow a whisker pole or even the longer sort of extendable boat hook. Tape two extended boat hooks together if you don't even feel like leaning over the lines. Go to the point on your boat where the deepest part of the keel is; most of the time this will be slightly aft of the chainplates on a fin keeler. Tie a piece of crappy white bread or a cheap hamburger bun to the end of the hook, the spongier, the better.

Dip. Measure. Examine bun end.

Now you know what are sailing ancestors knew by swinging a lead line with a lump of tallow: the exact depth and the nature of the bottom.

This matters in a marginal mooring depth-wise because if it's a foot of loose silt over sand, I would worry somewhat less than if it was boulders or a rock shelf. I would still worry, mind you, but not that the first grounding would tear me a new seacock.

SchoonerMISTRESS 07-22-2007 05:47 PM

A Nantucker Sounder has a small impression hole in the bottom of it. When it hits the bottom, it will bring back to you some of the bottom so you know what you are dealing with. Obviously, if it hits a rock you get nothing.

jrd22 07-22-2007 06:39 PM

An adjustable wrench with a small line tied to the hole in the handle works in a pinch. Lower it slowly until it just hit's then pull it up and measure the wet line. Then compare to what your depthsounder is reading. The difference is your offset.

DrB 07-22-2007 09:20 PM

How Much water- Part Deux

Went to the boat today. At today's low tide, I did the plumb bob string measure technique in several areas around the boat. My measurements were around 11' give or take a few inches. The sounder was reading 9.4 feet. So my original guess of the transducer being about 1.5 below the waterline was about right assuming it is accurate for it's hull location

Today's low tide was theoretically 1.5 feet above MLLW, so I guessing at MLLW I am in about 10 feet of water. According to the harbor chart, the extreme LLW is -3.5 feet, so if that is the case, that puts me in 6.5' of water for a 5' 11" draft. So I have 7" of draft at the extreme low tide assuming no waves. The area that I am has a sandy/muddy bottom, so that is a plus. I am also pretty far into the harbor, so exposure should be lessend. The harbor is surrounded by high land on three sides. The fetch distance to open water is about a 1400 yards, whereas the nearest shore is 300 yards and the other shore is 900 yards. The closed end has a depth of 4' or less at MLLW for most of the area (1500 X 1200 yrds). Distance from my position to the area where the depth is 4' or less is less than 300 yards.

I looked at the tide charts for the next three months, (the boat is pulled by then) and the lowest tide is -2.0 ft, so that is around 8 ft or 2 ft of clearance.

I think I am probably pretty safe, but tomorrow I am going to put a call into the harbormaster to see if I can get in a little deeper water. If not this year, hopefully next. I will ask for at least 12 ft at MLLW, which should be more than plenty even in extreme conditions.


sailingdog 07-22-2007 10:33 PM


Sounds like a good plan.

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