What is the real draught of your boat.

Why so concerned about the minutiae of the draught? I would have thought most people have a comfort zone well outside the likely variation. I have ran aground a couple of times knowing that I was likely to do so and having calculated the risk as zero. The last time was arriving at Portpatrick in time for a meal at the pub, soft bottom, rising tide. I gently went aground, swung on the boom to get off, moored for 20mins (phoned pub to put in orders), tried again, ran aground again, swung again, ploughed on in giving the keel a polish and had a nice fish and chips with a pint of heavy. I have had 10cm under the keel on the way into Arisaig in similar conditions and for similar reasons.

In general my alarm goes off at 3m which is 2m under the keel. This lets me have time to think before ploughing on. Our waterline varies with the amount of crew and stuff but not so much that the 10cm increments on the depth gauge would notice.
 
pmagowan said:
Why so concerned about the minutiae of the draught? I would have thought most people have a comfort zone well outside the likely variation. I have ran aground a couple of times knowing that I was likely to do so and having calculated the risk as zero. The last time was arriving at Portpatrick in time for a meal at the pub, soft bottom, rising tide. I gently went aground, swung on the boom to get off, moored for 20mins (phoned pub to put in orders), tried again, ran aground again, swung again, ploughed on in giving the keel a polish and had a nice fish and chips with a pint of heavy. I have had 10cm under the keel on the way into Arisaig in similar conditions and for similar reasons.

In general my alarm goes off at 3m which is 2m under the keel. This lets me have time to think before ploughing on. Our waterline varies with the amount of crew and stuff but not so much that the 10cm increments on the depth gauge would notice.
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If you're an East Coast creek crawler, you soon learn to do your tidal calculations very carefully otherwise you never go any where.
 
maby said:
If you're an East Coast creek crawler, you soon learn to do your tidal calculations very carefully otherwise you never go any where.
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Yes, luckily I don't have that problem so much and I find the tide calcs done on my plotter and iphone seem to be pretty accurate enough for my purposes. The only real concern I have is the tidal gate at Strangford but that is not because of the bottom, just the fact that wrong timing leaves you either out when you want in, in when you want out, or with the donk on full pelt travelling at a sedate 0.3knots SOG. I have a long keel made of reasonably hard stuff attached to mahogany but I would not like to try and dry out standing upright in the mud as the ballancing act could be rather complicated!
 
Nostrodamus said:
A few questions about keeping off the bottom.

1) When a boat manufacturer gives the draught is this without fuel, water and all the other stuff like anchors, chains, and the collection of stuff including yourselves kept on a boat. If this is the case roughly how much would you expect to add to the draught when fully laden? As others have said, its a design daught that is always less then the draught when measured in real life. How much difference in draught the extra weigh makes is subject to the canoe hull shape. There should be somewhere in the Naval Architects calculations a 'so many cm's per tonne' indication. In real life on the average cruiser its not a lot and in real life its almost irrelevant as you can't change it. The draught increased on our previous boat by an inch or two per tonne of stores on board. The real question is 'What is my draught?' not, 'What is it supposed to be?'

2) is there much difference in draught between a boat in salt water and the same boat in fresh water? No - perhaps a (very) few cm's When we take our boat into fresh water, it sits about a very few inches or so lower.

3) In a swell or wind waves is the bottom of the wave a lot below the same sea level when when it is flat. How much of a difference can there be? In theory, the mean height of the sea is half way up the swell. In other words, the swell goes down as well as up. Watch out for grounding in big swells and remember the swell gets bigger as it starts to 'sniff the bottom'...

Thanks
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I have tried to précis into short sentences the responses you need. The real answer is to go gently aground in benign conditions and get a lead line out and MEASURE! Note what the e/s says but bear in mind many echo sounders get confused between soft surface of the bottom mud and the hard stuff underneath.

However:

If the bottom is soft mud and its a rising tide and the conditions are benign, who cares if you go aground or touch the bottom?

But if its a lee shore of rock or shingle/pebbles or hard sand and you are being blown on and its a falling tide and a rising wind, then you are in deep smelly stuff unless you act VERY quickly with a tow or the kedge etc.

There's no problem AT ALL with touching the bottom in the right conditions. Just make sure you know when the right conditions are would be my advice...
 
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Nostrodamus said:
Thank you so much for the replies which I found very interesting. Amazing some of the calculations involved.

Just as another point.. When you are sailing and healed over does you depth sounder give a totally wrong reading or should I say is it out by a lot. I certainly remember my days in the solent taking on the 5 m contour but I wonder just what depth we were actually in?
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The quick answer is no, it won't.

The longer answer is that, because the echo sounder beam is not very directional, it measures the shortest distance from transducer to bottom within the beam of the transducer. I've just had a quick look, and it seems that typical beamwidths for Airmar DEPTH only are in the region of 11 degrees, so you'd have to be heeled by at least that amount before you saw a significant drop off. 11 degrees doesn't sound like a lot, but it's actually quite a big angle of heel.

Finally, the sensor's beam-width is actually a fuzzy thing; what is published is the "3 dB" point; that is the point at which the sensor's performance is down by (about) 50%. So, if you've got a good strong reflecting bottom, then it will still register the shortest distance to it even if you're heeled a bit further than the nominal beam-width.

Finally, what is measured is the shortest distance to the bottom within the "footprint" of the sensor - if the bottom is steeply sloping, that COULD be substantially less than the actual depth beneath your keel. But it will never be MORE than the depth beneath your keel.
 
pmagowan said:
Why so concerned about the minutiae of the draught? I would have thought most people have a comfort zone well outside the likely variation. I have ran aground a couple of times knowing that I was likely to do so and having calculated the risk as zero. The last time was arriving at Portpatrick in time for a meal at the pub, soft bottom, rising tide. I gently went aground, swung on the boom to get off, moored for 20mins (phoned pub to put in orders), tried again, ran aground again, swung again, ploughed on in giving the keel a polish and had a nice fish and chips with a pint of heavy. I have had 10cm under the keel on the way into Arisaig in similar conditions and for similar reasons.

In general my alarm goes off at 3m which is 2m under the keel. This lets me have time to think before ploughing on. Our waterline varies with the amount of crew and stuff but not so much that the 10cm increments on the depth gauge would notice.
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I agree, joking apart (and I am renown for pushing it with depth which is why I tend to run aground occasionally), the actual draught is unlikely to be more than 100mm different from the design, even with a well loaded cruising boat and if you are really basing your navigation and anchoring decisions based on so small a margin (the tide/chart calculations are not that accurate) then you are going to get very badly bitten at some point.

Oh and don't forget the Mediterranean does have a tide, been caught on that one!
 
maby said:
If you're an East Coast creek crawler, you soon learn to do your tidal calculations very carefully otherwise you never go any where.
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The tideguage on the cill at Woodbridge Tidemill Yacht Harbour provides a good indication of just how much water ones boat really needs to float:rolleyes:

Rather surprisingly the published draft for my boat appears to be spot on.
 
Perhaps over simplifying but, having used weighted line to check depths close to shore before, could you not hand SWMBO a line then jump in holding other end and snorkel down to your keel? Bring it back up and measure, simples :)
 
The other thing is that the height of the tide is affected by more than just the position of the sun and the moon. Weather has a lot to do with it, pressure and wind as well as local effects and sea state. When you can throw a biscuit on to the shore then you are too close and when you can jump overboard without getting your hair wet you are too shallow. I was once involved in a cruise which was written up in the ICC magazine as 'Hydrography by impact' where we tested the 1840's chart data by motoring through a gully in a headland. The current was supposed to be against us and there was supposed to be 4 fathoms on the way in and 5 on the way out. Unfortunately when we 'engaged' we discovered the eddy sucked us in and a couple of seals were sunning themselves curled like bananas right in the centre of the channel. The sudden realisation that we were in a bit of a pickle dawned on us at the same time as we came to a crunching halt. Unfortunately the boat had been playing up in every way previously, the engine required a firm hammering with a solid mallet to get going, the VHF had fried, the sail was starting to rip off and we later lost the dinghy. We could not get up enough steam to escape full astern and were in great danger of being dragged side on into the gulley where the crew and 80 year old diabetic skipper would have had a fun time trying to climb the cliffs and walk to civilisation. We put into forward gear while I rode the bow shouting rocks a starboard, rocks a port and we danced through with the boat wiggling it's hips like a skier on a slalam. Out the other side the bottle of rum was extricated from the bilges to celebrate the first passage of a yacht inside the headland.

I will stick in future to only running aground if absolutely neccessary to make pub opening times.
 
Thank you for all your answers. I may not be the most technically gifted of people, in fact in my last job I did my bit in the water shortage by closing off two lanes in a swimming pool.
Artic.. thanks for you clear answer which i did not know. Just glad there are a few wizards out there to make my life easier.
On my own boat it was one of the first things I did. Drop the lead line and set the depth (with an added bit). I know it is pretty accurate from the couple of times I have skimmed the bottom.
I know people do it differently but mine is set to the bottom of the keel which works for me.
I agree that on most occasions you don't need to be so accurate as there are also other factors that influence the charted depth. It is still a bum squeezing time when you go into the many marinas of Northern France that have sills or the entrance dries out.
I just thought it was an interesting subject. Usually when you speak to a seasoned sailor and you ask their draught they are never accurate. Usually they just say.. about "xxxx"
Once again thank you for the replies.
 
The load that will sink the boat by 1cm is .01 x (Area of waterplane). Of course that supposes the sides are vertical, which is not the case, but for small increments it is a useful guide.

Area of waterplane can be measured accurately, if you do not have the lines plan then for a good approximation take between around 70% of LWL x BWL (Beam Water Line).

Example LWL = 10m LWL, BWL = 3m, AWP about 21 m^2 ---> 210 kg to sink the boat 1 cm.

Narrower or fuller bodied hulls might change that from 200 to 220kg, which in practical terms is irrelevant.

Knowing AWP you can also compute the increase in draft from salt to fresh water.


addition
not that that changes a lot, but more correctly multiply AWP for 1.025 for salt water :)
 
There's an interesting variation on the theme - just to keep the thread running - how is your depth gauge calibrated? Depth of water, or depth below keel? I've always preferred depth of water since it makes it easier to compare the reading with the charted depth plus predicted tide height - but with a 2m draught, it's very easy to calculate depth under keel.
 
Water under the keel. I don't think I've ever formally reduced a measured depth to datum and used it for navigational purposes, so setting the sounder to remove one step of that process (while still requiring other arithmetic) seems daft. (That's not to say I don't check that the depth is roughly plausible.)

It's also fail-safe when other people are involved.

Pete
 
prv said:
I don't think I've ever formally reduced a measured depth to datum and used it for navigational purposes
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quite surprised to hear that, in the Bristol Channel we have so many 'changeable depths' that I do it as a matter of routine if passing over a covered mudbank. That said, I also have my sounder calibrated to the keel, though that is because the previous owner put it that way and if I ever read the manual I will change it :)

Strongly agree about the safety aspect wrt other people reading the log.

Cheers
 
bitbaltic said:
quite surprised to hear that, in the Bristol Channel we have so many 'changeable depths' that I do it as a matter of routine if passing over a covered mudbank.
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We must be meaning slightly different things, then. I was referring to using depth information to help pinpoint where you are. I've never had to do that, because although I sailed a little before GPS was common on yachts, I was too young to be involved in the navigation. By the time I was in charge, seemingly every boat had a Garmin GPS128 at the chart table.

Obviously the above process doesn't work if the charted depth is "changeable" - so are you effectively doing a mini survey for when you come back that way on a lower tide?

Pete
 
prv said:
We must be meaning slightly different things, then. I was referring to using depth information to help pinpoint where you are. I've never had to do that, because although I sailed a little before GPS was common on yachts, I was too young to be involved in the navigation. By the time I was in charge, seemingly every boat had a Garmin GPS128 at the chart table.

Obviously the above process doesn't work if the charted depth is "changeable" - so are you effectively doing a mini survey for when you come back that way on a lower tide?

Pete
Click to expand...

I see what you mean. I have found that, besides the parts marked changeable, the mudbanks which are not so designated are not consistent enough for confidence. So yes, I am reducing the deph to datum to check the chart and do a mini survey, rather than using a contour as a position line.

Cheers

Ps, just fitted a second 128 this winter, you don't mean to suggest there's newer technology out there? ;)
 
bitbaltic said:
Ps, just fitted a second 128 this winter, you don't mean to suggest there's newer technology out there? ;)
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I almost wish there wasn't :). I feel most comfortable navigating with a stack of up to date charts, a Yeoman plotter, and a GPS128. I've gone to some effort to obtain such a system for Ariam (not actually installed yet) even though she came with a Raymarine plotter :)

Pete
 
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