Measuring Draft

[BobD17833]

With the boat on the hard and ready to launch I pay particular attention to anything I can not access once the boat is in the water. I'll be recalibrating my depth instrument after launch and while rechecking measurements I questioned how and where DRAFT is measured. When a manufacturer publishes 5'3" draft where is the measurement taken?Water line to keel base? Loaded - unloaded? Keel joint to keel base? Other?BobDRendezvous

 

[Steve]

Bob raises a good point

I am also on the hard at the moment -- splash date is 8 May. While I am out of the water, I think I will take some measurements. Since I know where the waterline is on the side of my boat (which represents my typical loaded line) I will take a level and measure from the bottom of my keel to the waterline. That should give me my typical draft. I can then do the same for the head of my depth sounder and get an accurate depth below the waterline. These two measurements will allow me to calculate my offset. For example, if my draft is 5'-9" and the sensor head is 2' below the waterline, then that means it is 3'-9" from the sensor to the bottom of my keel. In that case I would set a 4' offset (to be safe). I like having accurate measurements rather than using the manufacturers. I always wondered if my offset was right, thanks Bob.

 

[Dirk]

Run aground

I was told when we first got our boat to run it agound to see what the depth sounder says. Sounds kind of strange, but th logic was that running aground when you could pick the time, place and conditions isn't really that big of a deal. It also lets you really know what you're dealing with. I didn't follow this guy's advice but the first thing I did when I hit bottom in a shallow channel last summer was look at the sounder. Now I know that 4.9 feet is my limit.

 

[Ross]

If you measure from the transducer

to the ground you will get a better measure than you will from a grounding. If you ground in soft mud your sounder may show a much shallower depth than is true. you would need to ground on hard sand.

 

[Don12364]

Plumbing the Shallows!

The following may amuse. I wrote it in 2004 for Wavelines our Owners Association newsletter. Plumbing the ShallowsToday's echo sounders have some clever features, one of which is a means of offsetting the readings to take account of the depth of the transducer below the waterline. Some people now use this feature to set the calibration so that their sounder reads the actual depth of water below the bottom of their keel. They say it saves all the mental arithmetic when coming into shallow water.I do not do this, preferring my instrument to read the actual depth of water from surface to the bottom which I can then compare with the charted depth. The following may help to explain why:- My first digital echo sounder was bought in 1977 and was made by a firm called Aquatronic. At the time it was a 'state of the art' job but it did not have the offsetting feature described above.As a keen racing man, and when tide cheating, I regularly sailed the boat towards the shore until she just touched bottom and then threw her about onto the next tack. Of course we did this only when the bottom was sand or mud.Although Marksman's draught was 1.8 metres (5' 11"), when she grounded the sounder read 1.6 metres (5' 3") because the transducer was .2 metres below the surface of the water. To give the crew a warning of impending grounding and the inevitable crash tack I had written "1.6 METRES = AGROUND" in chinagraph pencil round the bezel of the digital display.All went well for several years until one day I was jilling around outside a French marina waiting for the large illuminated tide gauge to indicate sufficient depth over the sill. We were in company with several French yachts all of which were pressing to be first over the sill and so to secure the few vacant visitors berths. We watched the marina's depth gauge slowly counting up in 0.05 metre increments (2" in English speak) and, as soon as it reached 1.6 metres I was into gear and away. Immediately a great deal of shouting of "Monsieur, monsieur" and much gesticulating began amongst the French skippers. "Hah, those Frenchies don't know that I have a DIGITAL echo sounder" - I thought smugly.We had travelled only a few metres when there was an almighty BANG and Marksman stopped dead right on top of the sill wall. Much cheering ensued followed by ribald remarks - in French. I had just tried to take a 1.8 metre boat across a 1.6 metre concrete wall.My laboriously polished, super finished racing keel was bu**ered in an instant!Don Alexander

 

[ed]

If your gonna do this,

If you measure it on the hard, measure both sides. and make sure the ground is level too. if your gonna worry about inches a little bit of sloped ground or a heal on the hard will really mess your measurements up.

 

[Dave]

Steve, a final step to take...

Steve, as a final step, when you are in the water drop a weighted line over the side and measure the actual water depth, which you can then compare to what your instrument shows. If you know how far underwater the transducer is, you can adjust the calibration to be very accurate.

 

[Stu Jackson]

Dirk's is the easiest and most reliable

and Donalex's complicates the matter by trying to compare to charts (and he gave us, quite frankly and honestly, the results of his attempts).The difference between measuring from your waterline or to the transducer is only a small percentage of the difference that will be between your keel and the bottom, so it makes little difference (unless you always sail in the Bahamas, Florida or other really thin water). I often sail in narrow channels with varying depths based on tides.What you are interested the MOST in is how much how much water is under your keel, not necessarily what the actual water depth may be, so that WHEN you are going to hit is based on how much water is below your keel, that simple.We have a 5'-7" draft. I care not where that's from (transducer location, waterline, loaded or unloaded), because ALL I know is that WHEN my depth sounder hits 4.4 I'm aground. I did this by going aground gently on a known mud shoal when we first got our boat.If the depth sounder is showing 8.4, I KNOW FOR CERTAIN that I have 4 feet of water below the keel, FOR SURE. I can then do all the chart comparisons I want to do. With a nominal 6 to 8 foot change in water depths here on SF Bay, and the charts at MLLW, I can do the math anytime. I've used the charts to follow depth contours as practice for navigation, and it works very simply.Keep it simple and it'll work for you.Stu

 

[Ross]

When we show about four feet on the depth

we bump a little. The ground is always a little lumpy. So just for peace of mind we try to stay outside of the six foot line. My sounder also shows bottom contours and slopes. if the bottom is quite flat then five or six is good but when it starts to slope up even in ten feet we get very cautious.

 

[Seattle Scott]

I set my sounder to water depth

Sounder was replaced when boat was in water. Measured depth of water with lead line and then just puched the offset button on the sounder until it read water depth. Specs on my boat say 5 feet 1 inch and since local beaches usually have 1 foot rocks scattered on them I just figure 6 ft on the sounder is limit if tide rising and 7 ft on sounder if tide falling.

 

[Alan]

Mine is set ...

....at the keel off-set. When it reads .1 I'm safe, unless some jerk in a power cruiser shoots by plowing a wake. I've tested it in zero wind and waves and approaching very slowly a gentle sloped shore. At zero I can feel the touchdown in soft mud. So as long as I have any reading other than zero, I'm safe. Best of all, anyone piloting my boat knows in an instant whether we're clear or not.

 

[BobD17833]

Academic question

There are some interesting posts here regardng how to calibrate instrument for low water depth. I've never considered actually soft grounding to verify setting. I too use Alan's zero math method, no reading-no water.But back to the original question - or is it too obvious; where is the measurement taken by the manufacturer for publishing a vessel's draft?If you want to be exact, and I'm not that anal, I expect there are two exact point between which you measure that define draft.Just a question seeking an answer.BobDRendezvous

 

[Warren Milberg]

KISS it....

What am I missing here? Keep it simple is right. With your boat in its slip, moored, at anchor or whatever, turn on the depth sounder. Then drop your lead line or any line with a weight on it over the side. When it hits bottom, pull it up and measure how much water you are sitting in. Then compare that to what your sounder is reading. Once you know the difference between the actual depth, and what the sounder is reading, you'll know forever how to use your sounder to keep from running aground. My boat draws 4 ft. There is about an 18 inch difference between what the sounder reads and actual depth. I know that I can go until the sounder reads 2.5 ft before I touch bottom. What could be simpler?

 

[Lou 14643]

My simple strategy

I keep my sounder calibrated to show the depth from surface to bottom (adding back the offset from sounder to waterline). In this way I never have to do any calculation. I simply know my draft and therefore know how little water I can safely sail (or anchor) in. As someone else said above, it also allows for navigating by the chart, as the sounder displays the same depth as the chart indicates - assuming the chart is accurate. No extrapolation necessary. In Chesapeake bay sailing/gunkholing, where the bottom usually comes up very gradually, I've spent many nights at anchor with a foot or less under the keel at low tide. It works for me.

 

[Dennis Thomas]

The less math the better

Draft is from the waterline to the base of the deepest part of your boat. That deepest part is almost always the base of the keel. It is possible - but rare - that the rudder is deeper. As Ed noted, check the measurement from both sides of the boat. Hang a plumb line from the waterline to the ground at a point on the side of the boat closest to the base of the keel. Record that measurement. Measure the distance from the ground up to the base of the keel if it is setting on block and subtract. Do this on both sides and you should have the draft of your boat.I favor setting your depth sounder to actual water depth for all the reasons already mentioned. I also like to communicate water depths with friends on other boats when gunk holing. I don’t want to have to calculate the true water depth before I report it to someone else and I don’t want to rely on them making the right calculations before reporting their depth to me!Stu, I have often admired your posts but I think you missed the point of Donalex’s story. He had been using the same procedure you suggest and it led him to hit the sill because he forgot to do the math. Had he been able to adjust his sounder to read ACTUAL depth he wouldn’t have made the error. If you adjust your depth sounder to read actual depth then WHEN it reads 5.6 you’re aground. If the depth sounder is showing 9.6, you KNOW FOR CERTAIN that you have 4 feet of water below the keel, FOR SURE. Then you can look at the charts etc. and NEVER have to do the math anytime.Have a good sailing summer!S/V Anodyne36’ Catalina #34

 

[Warren Milberg]

For Dennis and Lou

First, there is no math involved, even for the "math challenged," in my simple system. All it takes is remembering one number, in my case 2.5 feet to know I'm in trouble. In your case, the number is "0". Not difficult to remember either way. Secondly, as to the gradual nature of the changing depth in the Chesapeaks, Lou is only somewhat right: take a look at the charts for Bloody Point Light and you'll see an amazing convergence of fathom lines. Depth goes from over 100 ft to less than 10 ft in a VERY short distance. Lots of other examples in the Bay. And, as to anchoring in a place where you know you are going to be in water of less than a foot under your keel, better buckle up cause you are going to be in for a bumpy ride some night when you find yourself aground at 3 am.... If you know anything about the Bay, you know it can produce some very abnormal high and low tides now and then.

 

[Ross]

If you have programmed an offset

on your depth sounder, and it shows 1 foot, can you step over the side and only have the water come up to your knees? When you decide to anchor how do you decide what a proper scope is?

 

[Stu Jackson]

Comment from Dennis

Thanks, and I agree with you, it just seems to me that what he did may have simply been having the "settings" backwards.I just like how mine works, and each of us has their own preferences.Your boat, your choice.Happy gunkholing to all.

 

[Lou 14643]

Pete, I think you missed my point

I certainly don't go blythly around with less than a foot of clearance under my keel. What I said was intended to refer to gunkholing. I enjoy spending the night on the hook up some of the lovely small creeks off the bay and have been able to get, thanks to my shallow draft (2' 11") pretty far up some quite places that others may have to explore by dinghy or binoculars. I watch tide tables carefully and generally don't cut things that close unless I just have to get around that last bend. In spring of '04 I spent six weeks cruising the bay and only touched bottom once when I was leaving an anchorage behind Dobbin Island in the Magothy (the party was getting a little too wild for me!) and I cut a little too close around the bar on the east end of the island.I don't think our disagreement is a major one, just that for you, knowing the depth under your keel works, and for me, knowing the total draft works. Neither is wrong, just different strategies. We chartered a few weeks ago in Antigua and as we were getting the boat briefing the base manager asked how I'd like the sounder set up, to show under the keel or total depth. I opted for total depth and spent the trip watching sounder, chart (and water color) and knowing into how shoal it was safe for me to travel. He commented that everyone has their preference and there is no "right" strategy. By the way, we were there as Roz Savage completed her transatlantic rowing expedition. Now that's a challenge!