Shallow Depth Alarm Settings

[andrewwoods]

I have a Seawych, Bilge Keels with a draft of 0.72m according to <A target="_blank" HREF=http://www.seawych.org/Specifications.htm>http://www.seawych.org/Specifications.htm</A>

I am currently only sailing in Poole Harbour, while we gain experience and confidence - I don't have a huge amount of sailing experience, and my wife and children have next to none. I have done the DaySkipper theory, but not yet the practical.

With a draft of less than a metre, it seems to me that much of the northern half of Poole Harbour should be available to us. If there is at least a metre of tide (as there is except at LWS) then I can sail anywhere Blue on my Admiralty chart. Yes, I have an up to date chart, and yes, I have a full set of tide tables (tide planner software, actually).

BUT, I'm a cautious chap, at least when family are on board, and I have the depth sounder shallow water alarm set to 1.5m, to give me a chance to turn before I actually run aground. However, the alarm is going off very frequently at 1.3m of water or so.

So, my question is this, should I believe the chart and tide data (blue + 1m of tide = OK) and adjust the depth of the alarm to 1m, or carry on as I am? What do you have shallow water alarms set to, in similar situations where you're in shallow waters in a boat without much draft?

I feel that I'm setting myself up for some flames along the lines of 'get some experience, boy' but I don't want to be forced outside the harbour into Poole Bay before I'm ready just to get some water under my keels, when in fact there's plenty of water there already.

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[snowleopard]

it's a bit of a dilemma. if you set it to give a couple of feet under the keel as you and i both have, it will keep going off when you are in shallow water. if you set it to 1m you may go aground without warning. i put up with it, especially when getting on and off my drying mooring. if its a raymarine you can silence the beeps for a while by pressing reset.

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[Benbow]

You need to know what the offset on your sounder is. ie does 1m mean 1m under the keel or 1m total depth ? I have mine set to a negative number to read from the surface, others prefer a positive number to give depth below keel. Obviously critical you know which. When set, I would suggest you check it with an old-fashioned lead line.

The other general piece of advice is on a rising tide, take all the risks you like, but on a falling tide be extremely careful!




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[bedouin]

Lots of variables here.

Firstly you need to know the offset of your depth sounder. That could be set to read depth of water, depth under keel or even depth from transducer. The most useful thing to know is the reading of the sounder when your keel touches bottom - this is easy to find out by practical experiment as I have demonstrated on several occasions /forums/images/icons/smile.gif

Once you've discovered that depth chose a shallow alarm setting relative to that

Also note that tidal predictions are only predictions, and the true height of tide can be influenced by a number of issues such as the atmospheric pressure (high = lower tide), the wind and the amount of rain.

Charts too get out of date - places like Poole Harbour have constantly moving channels and depths and even if your chart itself is up-to-date it may be based on a survey that is years old.

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[jimbouy]

Andy

Seeing as we are almost in the same boat, which ever way you look at it.

I have set the offset so I have depth beneath keels showing and added a bit for luck. about another foot.

My view is to "take risks" ( not really the right turn of phrase) on the rising tide and learn from this experience, I already have much improved knowledge of Ryde sands!!

Regarding the shallow water alarm... i haven't set it. I felt i would probably become too reliant on it. Better to train myself and crew to keep an eye on it when needed.

Are you going to do the Dayskipper Practical?? I've done the first of three weekends with the Hamble School.. Not cheap but very enjoyable and IMO worthwhile.

I mentioned your name to Gordon, I think he remembered you, he must so a lot of faces.

Cheers

Jim

<hr width=100% size=1> "It is a pleasure to give advice, humiliating to need it, normal to ignore it"

 

[andrewwoods]

I don't have a manual for the echo sounder (see <A target="_blank" HREF=http://www.ybw.com/cgi-bin/forums/showthreaded.pl?Cat=&Board=pbo&Number=347410&Search=true&Forum=All_Forums&Words=andrewwoods&Match=Username&Searchpage=0&Limit=25&Old=allposts&Main=347410> this posting</A> ).
The offset is set to 0.5m, which I was assuming was to cater for the sounder device being 0.5m below the surface, but it might mean depth below keels.



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[AndrewB]

In a place like Poole harbour that is shallow and with a fairly flat bottom, at least in the shallow places, it is true you can afford to set your alarm much shallower than in places like the west country, which is often 'steep-to' and the bumps come suddenly.

I think you'll find that after a few sails you get a pretty good feel for where the shallow bits are and do not need the alarm so much. Until then, like everyone else in Poole, you'll sail with one eye on the depth-sounder all the time, whether or not the alarm is set.

I totally agree with Benbow about checking the accuracy of your depth sounder against a lead-line (piece of weight on a string), up to about 5m, so that you know exactly how much clearance you have from the depth-sounder reading. You don't necessarily need to adjust the depth sounder - but you MUST have confidence in it.

But I wouldn't be too worried about going aground, particularly at speeds under 4 knots. Nine times out of ten, where the bottom is soft reversing under engine will pull you off. Otherwise kedging - pulling yourself off with the anchor - works well in Poole. And if the the bottom is firm, you can even jump out and push, but beware getting left behind. If the worst comes to the worst, on a flat surface all you need do with a bilge keeler is sit and wait for the tide.

 

[Benbow]

an offset of plus 0.5m is likely to mean that zero is half a meter below the transducer, probably the bottom of your keel.

Check it with a weight on a bit of string !

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[andrewwoods]

Yes, I suspect you're right.

Before I go to Poole again I will prepare a (substitute) leadline and check it against the depth sounder while still on the mooring.


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[Robatbradwell]

I got grounded on the south side of brownsea island last wednesday. I was trying to keep to the withies but instead i should have followed what my gps was telling me. With keel up i can draw only 450mm but still grounded just before low tide. When i got out to walk to the island i found that 4m away from me i couldnt stand and had to swim! I have a Garmin with bluechart and now think it is extremely acurate for the area.

By the way, sometimes it can be calmer out in the bay than in the harbour so give it a try. It can be very choppy when going past the chain ferry near low tide but often fine once past.

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[gjgm]

fraid i m power, not sail, but everyone but me on the YM course was sail, so I ll pass on their thoughts. Pretty much as here. Be confident you know from what your sounder is reading.. bottom of keel, or water line. Otherwise, going aground in sand/mud on a rising tide might not be more than an embarassment;going aground on rocks on a falling tide could be very serious. Guess one of the things i learned on the course, is that being on the water isnt mathematical..its more about assessing the risk and making a judgement. Some times you will want a greater safety margin than others ! So for example, you might take a greater "risk" with a hardend crew, but much less with aboat full of kids. Same amount of water underneath you though....

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[ex-Gladys]

Having been about small boats and sailed most of my life, Amber is my first foray into cruiser ownership and sailing. She is lift keel, and only draws 2' 1" keel up, I'm still being very conservative though. On the Blackwater with the keel down (6' draft) I set my alarm first to 12', then to about 9'. I do start to get nervous around the 15' mark under sail, as there are some fairly "steep to" bits around, and so don't gain the max I would have in a dinghy or cat. I'm really just getting the "feel" of the water around me. Didn't stop us hitting bottom weekend afore last returning to the mooring on a rising tide, but on analysis that was a "prat on the stick" mistake. I should have been on the buoy side of a moored boat not the stern...

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[northener]

As stated you need to calibrate the sounder on the boat in the water.
Its up to you how you do this - but my own preference is to set to read actual water depth then you can always relate it to chart and tide tables.
Also remember in a bilge keeler your draft will increase as you heel over.

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[andrewwoods]

Thanks for all your advice.

I will check the actual depth on my next visit, and then calibrate the depth sounder to show from the surface - which is what I was expecting it to do now, but I may be wrong.

Andrew

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[IanPoole2]

Christ - your alarm must be going off all the time. The only place I know on the Blackwater where you get shed loads of water is the hole off of Bradwell - where all the fishing boats are.

Where did you hit the mud - Tollesbury?

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[JimMcMillan]

stick a 5' pole over the side

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[ex-Gladys]

Nope, entering Mersea Flete at low tide. Didn't hit mud either, we had the keel most of the way up and came to a full stand with a crunch. Fortunately the engine was at idle...

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[surekandoo]

I too sail in the Blackwater & surrounding areas & have a swing keel. One trick I use if I'm approaching shallow water under engine, is to have the keel partially raised. Then if I run aground, a few quick turns on the winch will reduce mydraught and enable me to either continue or reverse course.

Incidentally, most sailors touch bottom off the east coast sometime, or so I'm told! As long as you don't have a deep fin keel, all that usually happens, is a boring wait for the tide to return, unless of course you've been really silly and got yourself neaped.

Happy sailing, I'll look our for you next time I'm on the water

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