What is the real draught of your boat.

Nostrodamus

Nostrodamus

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www.cygnus3.com
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?

2) is there much difference in draught between a boat in salt water and the same boat in fresh water?

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?

Thanks
 
1. I'd expect draught to be to the design waterline, which should include a moderate allowance for stuff on board. But many boats (especially cruisers like yourselves) will be overweight.

2. In theory a boat should float deeper in fresh water. I've no idea by how much.

3. Assuming waves that aren't breaking, the "flat" water level is halfway up the wave. Otherwise you'd have to import (or dispose of) extra water from somewhere. How much of a difference - how big are the waves? Getting bounced on the seabed in the trough of a wave is certainly a risk in shallow water.

Pete
 
Draught is usually measured to the design waterline. There is an ISO standard for the calculation. It is approximately based on basic boat plus fuel and water, but not cruising clobber, which can add upwards of a ton to displacement. How much this increases draft depends on the size of the boat and the underwater shape. More effect on small narrow boats than on large flat bottomed boats.

Boats are more buoyant in saltwater than fresh and again the difference in draft depends on the size and shape of the boat.
 
I had a big tidy up recently (32ft motorsailer) and by taking home all the galley kit,paints,some tools,clothing,dinghy and general junk she is now about 1'' higher in the water.
 
I measured my transducer to the bottom so all I know is when the number reads 1m we are aground! We sit very low in the stern when cruising as we seem to pack everything in here. As the cruise progresses and the drink is drunk we rise up.
 
My rule of thumb is that there is always at least 10% less water than expected. My draught is a nominal 1.5m but I have measured it in the boatyard at 1.6.

The mean level of the water in waves would be at half their height if they were sine waves but often the waves are more peaked than that and I tend to think the mean level is not much above the base level quite often.
 
I know of a boat that anchored in sandy bay for lunch, thought he had plenty of depth until a high speed ferry passed 2 miles out. When the waves from the wake reached the boat his fin keel smacked off the bottom. When he inspected the boat he found cracks around the ladder frame that supports the keel inside the boat. One to watch out for. Oh and he hasn't done anything to repair the cracks.
 
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?

2) is there much difference in draught between a boat in salt water and the same boat in fresh water?
Click to expand...

Neither iof these two questions can be answered without details of the shape of the boat itself. A big fat modern boat and an old fashioned narrow shaped boat, both of the same weight , will both displace the same amount of water but the modern boat being much wider will sink less far down than the older design. Salt water is about 3% heavier than fresh so your boat will push 3 % less by volume when its in fresh water.

Nostrodamus said:
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?

Thanks
Click to expand...

Waves are approximately sine waves - the trough goes down below average sea level as far as the peak goes above it. So if the depth above the bar is say 2m and there is a 1m swell running then the depth of water over the bar will vary from 2.5 to 1.5

|P.S. You have me playing with my calculator. I work out that my boat sinks down 1cm for every 220kg . Probably a bit more than 220kg because the shape model I have used is crude. Coincidentally, it would also sink by one cm if it went into fresh water
 
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As well as your UKC (Under Keel Clearance) you need to take into account your Squat calulations when under way. As you move through the water, normally under power howeevr to a less degree under sail as well, the stern of the vessel will always 'sink' by the stern in proportion to the speed of the vessel. In Shallow water this will double! (Always remmeber to slow down when in shallow water, MAster of the QE2 didnt once off Canada and ran aground!).

As a rule of thumb when passage planning I use either the max beam of the vessel + 15% or Draft of vessel + 15%, which ever is the greater and then use as my UKC.
 
prv said:
My stern has to dip down quite a long way before it ends up lower than the keel :)

Pete
Click to expand...

Good point Pete, I learnt to sail on a Devon Yawl, with a lifting keel, sailing accross Pool Harbour with the keel up could often make the rudder leave bigger tracks in the mud!
 
dharl said:
Good point Pete, I learnt to sail on a Devon Yawl, with a lifting keel, sailing accross Pool Harbour with the keel up could often make the rudder leave bigger tracks in the mud!
Click to expand...

Indeed, my last boat had a traditional shape like a baby pilot cutter. The lowest point was the shoe joining the lower end of the rudder to the after edge of the keel, so on all but the steepest of banks that's the part that ran aground first. I could often raise it out of the mud and escape by running out onto the end of the bowsprit with the engine running hard astern :)

Pete
 
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?
 
One day I am going to learn to sail properly but the more I learn the more I have to worry about. Staying in ignorant bliss makes for a far more enjoyable sail !!!
 
My Rival will dig in by about 2' at the stern when motoring at a good chat. Some of that will translate into a deeper draught at the aft end of the keel. If I was that worried about depth I wouldn't be going that fast anyway. Regarding design draft, the Rival 41C is 1.8m but I measured 1.9m from keel bottom to water line when she was laid up. As the design is from the early 1970s perhaps there was less accuracy back then. I have compared the waterline with other Rivals and it is the same, so perhaps the design specification and 'as built' facts are within the tolerance expected with early 70's calculations. She is not that heavily loaded. From my very basic understanding of yacht design (from just a simple book) the calculation of water line position is a bit iterative anyway when calculated manually. Of course today, it can be calculated accurately, I assume. Maybe I am wrong on this and a 10 cm difference is significant or my eye was squint.
 
There was a very sorry case of a brand new yacht being wrecked off the north Kent coast a couple of winters ago because the skipper hadn't factored in the wave height in his depth clearance calculations. I remember him remarking at the time that while it should have been obvious, nobody had ever mentioned it and I reflected then that I had passed Dayskipper Theory and Practical and had completed the Coastal Yachmaster theory syllabus (without having yet sat the exam) and had seen no mention of taking wave height into account.

If you take into account that a forty foot yacht can easily weigh ten tons, that's a lot of inertia. If you are in a 1 meter swell, I bet it sits down several centimetres more as it bottoms out of each wave.
 
Agree, I use keel offset so '0.1m' is the last 4 inches for doing something...alarm at 1m under the keel about right for tacking on mud slopes..or not!

Nostro, many boats are built 'heavy' unless in a highly factory controlled process...Oyster, hmmm. Unlikely to be underweight?
I weigh a theoretical 7.5 t and reckon a normal 9t with enough clobber to go a few months.
The first thing I did on buying this boat ( which has been cruised already) was to add another 50mm to the waterline. Which was already usefully raised 50mm.
Going aground is fine , just watch ( or think about) dropping in a swell on hard bottom eg sand etc. and the vulnerability of the rudder if it is deep?
The mast can rattle when you bump which can be a bit disconcerting!
When you cruise over to South America ( which I am sure you guys will in your own sweet way and time), a high boot topping is pretty useful for keeping the goosebarnacles off the topsides when immersed on the same tack for 3 weeks..and oily harbours at bay
I would say weight can often be a bonus, its just where you stow it securely to balance the boat and motion..Obviously not high up nor in the ends of the boat..
Btw the difference between density of seawater and fresh is 1025 over 1000 so very little, but enough for example on my heavily loaded Corribee to put the cockpit drains awash in fresh water canals..
 
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