Depth Offset

[arran]

Has anyone got any suggestions on what offset to have on our digital depth gauge? I'm part of a syndicate owning a fin keel yacht and there seems to be varying views from zero? below keel to 2 meteres!

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[gunnarsilins]

I use to...

...offset the actual depth of the transducer, so the reading is relative water surface

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[montezuma]

Though it's nearly a religous discussion, I've always prefered to calibrate it to read the depth from below the keel. Ulitimately, this is the number you're interested in. In moments of tension, a quick glance at the depth guage is all it takes. It you want to compare measured depth with charted depth, for example, this tends to be in less stressful circumstances where a quick burst of mental arithmetic is less prone to error.

A little note somewhere around the chart table just to remind everyone does no harm either.

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[charles_reed]

There are a range of opinions on this subject.

Popularity in descending order appear to be:

Offset to read from keel depth
Offset transducer to give actual depth of water
Do nothing
Offset from a "safe" depth.

I personally believe the second to be best and set my audible warning for keel + clearance of 400mm


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[chriscallender]

Its down to personal preferance, do you want it to tell you how close you are to the bottom (to avoid running aground) or how deep the water is (eg to work out how much anchor chain to use)? Whichever you go for, you'll need to do some rough mental arithmetic to work out the other one. My personal preference is for depth of water but I have a lifting keel so depth under keel would depend on whether it was up or down...! Anyway I feel its a bit easier to know that I draw x metres, so if depth sounder starts to get close to x then start to worry. But as I say, just personal preference, as long as you know what it is and agree it amongst the syndicate then no problem either way!

What you don't want to do I believe is leave the keel offest at zero, since the transducer won't be on the waterline of the boat, and you'll then need to do some mental arithmetic in either case to work out both depth of water or depth under keel from the arbitrary level of the position of the transducer...

Chris

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[stephenh]

yes - it is almost a religous issue...

no - don't leave " a note on the chart table "

Have a permanent (engraved plastic? embossed sticky tape thingy?) label glued nearby reading :-
"This boat runs aground when this guage reads 2.00 metres" or whatever.

From this it doesn't matter which offset is used as long as it is known.
and you must agree amongst yourselves that to alter the offset will be a keelhauling offence .

The above is taken from OYC boats with changing crews and works very well.

Happy groundings..

Stephen

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[jleaworthy]

As you are a syndicate with, presumably, different skippers acting independently it would be wise to forget about notes whether written on paper or on engraved plaques and set the sounder to register the least depth of water ie the depth below the keel. That way, if any skippers get it wrong they will have more water than they think and will only have a problem if deliberately trying to dry out - but no catastrophe.

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[quaelgeist2]

Re: I use to...

...giving you an additional idea on tidal movement and current depth as an additional datum for navigation.

Together with the earlier plate "Grounding at x.xxm" you are safe in the stressiest moments, aren't you?

But then again - it's all a matter of personal preference!

chris

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[davehu]

What a lot of trouble modern electronics cause. The old depth sounders had no offset so it was automatically the depth of water below the transducer, in most boats that meant it showed a little less water than you actually have. Why not set it to the actual water depth then there is no doubt it reads the depth of water and your skipper can then make the judgement on a safe depth dependant on the draught, sea state and type of bootom. Lets keep sailing simple

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[richardandtracy]

Simple Depth Sounder

Electronic depth sounders "Simple"? How's this for simple...
Use a lump of lead (brick?) on a string. No offset. No arguements. No confusion. No money.

Improves seamanship because you avoid areas that may cause groundings like the plague. Just like your insurers want you to. Just like your wallet needs you to.

Regards

Richard.

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[FlyingSpud]

Re: Simple Depth Sounder

You don’t sail on the east coast, that’s for sure, on your basis you would never let go of the side /forums/images/icons/smile.gif

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[Reap]

Re: Simple Depth Sounder

Yes and try using a lead line when beating up a narrow river, you won't have time to swing the lead, you 'll be aground long before that!
However I am a believer in being able to use the leadline, when piloting up a muddy creek it can be indispensible.
And lets not forget its not always undesirable to run aground!

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[anabel]

What you want to know is: 'how much water is under the keel'?. So I would suggest you set it to your draft. Make a note of it in the log so that anyone sailing the boat knows exactly what it is set to.

anabel


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[alant]

If your part of a syndicate, ie possibly different skippers each trip, I suggest using a leadline each time you get on the boat, just to confirm where the depth sounder it is reading from. A common problem on charter yachts is people 'fiddling' with the offset, hence check it each time you sail. Easily done even if no leadline, just a lump of metal (anything) + a piece of string.
I personally prefer to know how much water I have under the keel. Although depth read from the waterline is useful if your keen on checking your tidal heights/secondary port info etc.

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[Trevor_swfyc]

Another problem to add to the list of cons about being in a syndicate which was discussed some time back but think we missed this problem.
The reason you have a choice on the instrument is for the very reason the vote on how it is set is going to be about 50:50. Lets hope you can have a democratic vote within the syndicate and most important all agree to stick to it. The alarm must be set so that you can take avoiding action before you hit the bottom mine on the east coast is set to 2 metre water depth which works for me. I have the gauge set to actual water depth then it as has been said it then relates easily to the chart datum for navigational calculations. At least you have info to sway the vote or are you just doing the ground work!

All the best.
Trevor



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[l'escargot]

If other people are going to use the boat, at some time someone is going to play with the settings.
If you have set it at depth from transducer and it is changed, any other setting will actually give you more water than the reading suggests (unless someone can put in a negative offset value). So leave it at depth from transducer and shouldn't be caught out. If you set it at anything else and it is changed without you knowing, you may have less water than you think.

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[Piers]

Calibrate it to show depth of water under the lowest part of the boat (keel or prop depending upon yacht or motor), or calibrate it to the water level.

The former is helpful if you are in a sudden 'ouch' situation. The latter is easier when doing chart work and calculations.

All other settings will, at some time, involve mental gymnastics when in a tight spot.

Piers du Pré
www.playdeau.com

 

[vyv_cox]

Charles,

I'm amazed to read that you set your alarm as low as 0.4 metres. You must either have lightning reactions or only sail in very deep water (which I guess is now the case). Sailing out of Menai Straits we always had the depth alarm set on 5 metres below the keel, occasionally reduced to 3 metres if crossing some of the shallow routes, e.g. the swatchway to Conwy. Inland in the very shallow waters of Holland the alarm is always set at 2 metres and we have run aground a couple of times. At 0.4 metres I suspect that our grounding rate would have increased enormously.

Vyv

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[richardandtracy]

Re: Simple Depth Sounder

Just the River Mudway (sorry, Medway) and surrounds.

Regards

Richard


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