Minimum depth under keel

[Kd9truck]

I draw 7.5 ft, my alarm is set at 8ft, I have not herd the alarm in 3+ years. My vessel is fairly deep as far as keels go, I know this, I plan accordingly, I do not look to “squeeze” or “squeak” into thin areas. That removes the options others have And those who have shallow draft vessels have made compromises, I had 2 shallow draft vessels they are fun (Hobie cat 18 she drew inches and a 1990 bene 285 she drew 4 ft), but very much protected water/ near shore small wave sailing in my humble opinion.
When I have to approach shallow places I go very slow if I can drop my anchor and Rib to get where I need to go I stop at 9ft then let chain out.
I have never sailed a vessel over 40 ft that drew less than 6ft, I can not speak to the performance or attributes, l love the way my vessel performs, it is up to me to keep it performing, plowing things are not part of my sailing experience.
Cheers

[grantmc]

+1 to what the earlier posters have said, but I'd just add that before entering a shallow area, having a recovery plan in your head in the event that you do ground fast is good insurance. Lots of options one might consider, but knowing if your monohull/keel can roll on to her side without damage to wait out a tide is always helpful.

[Tupaia]

I am always amazed by the number of owners who don't know whether their depth sounder is calibrated to read the water depth, the depth below the transducer or the depth below the keel.

[jen1722terry]

Ditto to what grantmc posted.

Most sailors don't know how their monohull boat will settle if it gets grounded in a neap tide. Some will be fine on their bilges until the tide returns, others may be in danger of swamping if there is any kind of wave action. At the very least, close all portlights and hatches, including cockpit lockers.

Cheers

[wingssail]

Approaching San Luis in Brazil we were dubious when the depth seemed less than the coming tidal drop but fellows on shore waved us onward and we docked with the keel touching in the mud, a first ever for us. But we were next to a Gulf Star so maybe it was going to be OK.

Then I checked more carefully! Not only was the tide going to drop more than our 8' draft but it wouldn't be as high again as our arrival for over month! OMG!

By that time we were already stranded. Our last chance was to leave early the next morning, plowing our way out, which we did.

[geoff3nebel]

I draw 6’ and get nervous in anything less than 8’ in a flat calm - I never know what’s lying on the bottom… I have a long fin keel and a balanced spade rudder

[clakiep]

It is never a question of how many inches....do not expect such precision be it from tide tables, charts or your depthsounder. Always atempt it with rising tide, so you can back off shortly after touching ground. Never atempt a critical entry or departure with waves. And the rest depends on your hull shape - long keel, fin keel, twin keel. The first of course can take much more punishment.


Capt. Claus - ocean tramp of the eighties

[acunningham]

Let's take La Rochelle for example. There's a spot in the main channel that's charted as 0.4m deep:

https://webapp.navionics.com/?lang=e...ey=y%7DsxGpqdF

Low tide this afternoon is predicted to be 1.77m at 14:59. High tide this evening is predicted to be 5.02m at 21:58:

https://www.tide-forecast.com/locati...e/tides/latest

Let's say your boat draws 2.15m, and you arrive off La Rochelle right at low tide. You've never been to La Rochelle before. At what time do you attempt to enter harbour?

Do those who say "one inch" attempt it immediately? After all, 0.4 + 1.77 - 2.15 = 0.02m to spare! Or do you wait to provide more of a safety margin?

[svfinlandia]

Originally Posted by Bobby Lex

Like Benz said, there are lots of variables. Too many to have one hard-and-fast rule. Add to his list: 1. wave action. You may have enough depth if the water is flat. But with 1' waves you may be bouncing off the bottom with the same depth. 2. bottom type. I'm much more willing to risk a soft grounding with a forgiving bottom (sand, silt, etc.) than with a hard bottom (coral, rock, etc.). 3. lastly, one the OP mentions, rising vs falling tide. If a soft grounding is quite possible due to little depth, I'm much more likely to risk it if I know the tide is rising and I can float off after a bit of a wait. OTOH, if the tide is falling I'm very wary of risking a grounding, knowing that I could get stuck there for hours.

Bob

+1 Bob
My sailboat draws 5.8 feet and I round it up to 6 feet I have my depth sounder set to 7 feet and if I know the bottom is mud or sand I have been known to run across it with the depth sounder alarm blaring.
I have a 75 HP diesel and a large prop so I don’t worry too much about it.
I have been known to cut a slot in the bottom on many occasions and have only had to call towboat US once.

, S/V Finlandia

[svfinlandia]

Originally Posted by jen1722terry

Ditto to what grantmc posted.

Most sailors don't know how their monohull boat will settle if it gets grounded in a neap tide. Some will be fine on their bilges until the tide returns, others may be in danger of swamping if there is any kind of wave action. At the very least, close all portlights and hatches, including cockpit lockers.

Cheers

Also, keep in mind how close to the waterline your fuel tank vents are.
Don’t ask me how I know.

Al, S/V Finlandia

[billgewater]

Time and tides wait for no man. Chaucer.
Or, conversely, why procrastinate now when you can put it off until tomorrow?

Have touched the bottom three times. In each case, entirely my own fault, but ultra careful now. Line up all the stars. i.e. consider all the possibilities plus how to save the situation if things do go wrong.

[wingssail]

Originally Posted by acunningham

Let's take La Rochelle for example. There's a spot in the main channel that's charted as 0.4m deep:

https://webapp.navionics.com/?lang=e...ey=y%7DsxGpqdF

Low tide this afternoon is predicted to be 1.77m at 14:59. High tide this evening is predicted to be 5.02m at 21:58:

https://www.tide-forecast.com/locati...e/tides/latest

Let's say your boat draws 2.15m, and you arrive off La Rochelle right at low tide. You've never been to La Rochelle before. At what time do you attempt to enter harbour?

Do those who say "one inch" attempt it immediately? After all, 0.4 + 1.77 - 2.15 = 0.02m to spare! Or do you wait to provide more of a safety margin?

Isn't that an error? The stated tidal height of 1.77 would be over Mean Lower Low Water, not 1.77 over any and all obstructions. The depth of that spot would be .04, also at Mean Lower Low Water. So at low tide of 1.77, you'd have only 1.37m.

With a draft of 2.15 you need that much over the .4m spot, or 2.45m tide. I'd say you could go at about 16:00, about three hours after low water.

Do I have that right?

[David B]

Originally Posted by acunningham

When entering or leaving an unfamiliar tidal harbour that has inadequate depth for your boat at low tide, what's the minimum depth of water that you're willing to tolerate under your keel when planning your arrival or departure time? Does rising tide versus falling tide make a difference? How about onshore wind versus offshore wind, day versus night, etc?

Depends on the tidal range and wind - depends on the bottom. We have more than once pushed the keel through soft mud to get in or out of a place, and on one occasion with a firm bottom, just rested there until a bit more tide came in, so we could continue entering.

Other places with hard or rocky bottoms, 100mm is comfortable if there is no swell, but I want to be sure once we are in, that there is half a metre of so under us - after allowing for tidal and wind effects. A harbour that develops swell a day or so after you have berthed, needs room down there.

[kish]

Originally Posted by acunningham

When entering or leaving an unfamiliar tidal harbour that has inadequate depth for your boat at low tide, what's the minimum depth of water that you're willing to tolerate under your keel when planning your arrival or departure time? Does rising tide versus falling tide make a difference? How about onshore wind versus offshore wind, day versus night, etc?

When I learnt about navigating in an area of tidal ranges of 4 metres plus, the perceived wisdom on entry with a yacht of 1.8 metre draft [5.5 ft] was to be on a rising tide in flat seas, with an allowance [error] of a "pilots foot" [30cm] in depth calculation below keel in a harbour with well charted approaches.
Also if there is a high barometric pressure above 1020mb+ the tidal height could be less than predicted height and if there was a slight wave action,in either case to adjust the minimal safe depth to 1 metre [or 3 ft].
It is much less stressful to avoid having to navigate into [and out of] such depths by appropriate arrival times or by avoiding problematic areas.

[Lodesman]

Originally Posted by wingssail

Isn't that an error? The stated tidal height of 1.77 would be over Mean Lower Low Water, not 1.77 over any and all obstructions. The depth of that spot would be .04, also at Mean Lower Low Water. So at low tide of 1.77, you'd have only 1.37m.

With a draft of 2.15 you need that much over the .4m spot, or 2.45m tide. I'd say you could go at about 16:00, about three hours after low water.

Do I have that right?

No.

Spot depths are depth of water at chart datum - this is MLLW on US charts - not sure if that is the case with Navionics charts of French waters. Regardless, height of tide is added to that figure to give you supposed depth of water. I say "supposed" because there are a lot of variables, errors, changes with time, etc that make that affect the accuracy of that depth calculation. The OP had the calculation right.