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Old 24-04-2013, 21:17   #1
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Determining Air And Water Draft

The boat is on the hard, the mast is off and it is an ideal time to get some accurate numbers. Alas, short of major contortions, how do you get the figures accurate ? Is there an obvious trick I am missing ? Seems that whatever I come up with involves setting up a perfect horizontal board on either the mast or the keel to get around the boat hull. I can do that but it seems a lot of pain ......
So, let's say the mast is down, I have a reference point on the mast near the coach roof and I can accurately measure from the top of the VHF antenna to that reference point. I now need to add the distance between the reference point and the water line. Yes. I can rig up a horizontal board from the mast past the safety lines and then measure between the board and to the water line but is there a less complicated way ?
Same question about the keel .... I can set up a perfectly level horizontal board just below the keel and then measure up to the water line stripe but is there an easier way ?
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Old 24-04-2013, 21:27   #2
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Re: determinimg air and water draft

Relax: you don't need to know the measurements down to his 16th of an inch take the measurement from your rail. to the dirt then just subtract the measurement from the dirt to the bottom of your keel. that should be close enough for the numbers you need. even in the thinnest waters and the worst of times I always try to keep a least a foot under the bottom of my keel
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Old 24-04-2013, 21:34   #3
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Re: determinimg air and water draft

I agree, you don't need accuracy down to an inch because chart soundings and bridge clearances are not accurate to an inch either. Just get out the tape measure and eyeball it where you have to. That will get you within a foot.
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Old 24-04-2013, 21:56   #4
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Re: determinimg air and water draft

No sh^t, within a foot LOL

How about a laser level and a pole?
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Old 24-04-2013, 22:11   #5
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Re: determinimg air and water draft

No sh-t, are you talking some damn theory?
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Old 24-04-2013, 22:19   #6
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Re: determinimg air and water draft

So I guess the answer is that there is no easier way
I would like to start out with an accurate measure and then add a safety margin, For me that is a lot more reassuring then just eyeballing it and thinking I got enough room. Call it 'peace of mind' ! Things can get very expensive very fast when eyeballing things !
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Old 24-04-2013, 22:28   #7
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Re: determinimg air and water draft

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No sh-t, are you talking some damn theory?
theory? no.

Take a pole high enough to measure highest point which is where the mast enters deck I guess. Now take the laser-level and pin the laser on the bottom of the keel while holding it level along the pole. Mark position. Now pin the laster on the waterline and mark position. Next is the base of the mast and mark position. Tape measure the two distances marked on the pole.

Next is tape-measure the tip of the VHF to point on mast that is where it enters the deck.

Draft was measured directly from the pole. Air draft is all three measurements added.

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Old 24-04-2013, 22:34   #8
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Use a level on a string.
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Old 24-04-2013, 22:37   #9
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Re: determinimg air and water draft

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Use a level on a string.
At the home depot you can get a laser level for $15 so I thought let's not do the string this time
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Old 24-04-2013, 22:38   #10
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Re: determinimg air and water draft

Oh, I thought you were going to measure the third leg of a triangle using your brain or a shadow etc.
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Old 24-04-2013, 22:43   #11
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If they are that cheap, then yes. Laser level. Everyone should have one to keep that sinking waterline true.
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Old 24-04-2013, 22:53   #12
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Re: determinimg air and water draft

Keep in mind how abstract a waterline is in the first place. Change the salinity of the water in which you sail, and you change the draft. Change the weight of your cargo, and again the draft fluctuates.

Isn't it funny that the only time you can be obscenely accurate about a boat's waterline is when it's on the hard?
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Old 24-04-2013, 22:54   #13
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Remember in high school when they wanted you to find the height of the flagpole by its shadow? .... Neither do I
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Old 25-04-2013, 00:06   #14
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Re: determinimg air and water draft

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Originally Posted by Bash View Post
Keep in mind how abstract a waterline is in the first place. Change the salinity of the water in which you sail, and you change the draft. Change the weight of your cargo, and again the draft fluctuates.

Isn't it funny that the only time you can be obscenely accurate about a boat's waterline is when it's on the hard?
True but you can measure accurately given your normal configuration and add a foot for safety to cover all those eventualities. You will have hard limits instead of waving the arms in the air and saying 'I should have another extra two feet down there before I h.....damn, I guess not'
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Old 25-04-2013, 01:04   #15
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Re: Determining Air And Water Draft

An easy method would be to use two bits of string with weights (eg shackles) attached as plumb bobs (two are needed for the mast measurement). You also need a metal tape measure, a chair/stool and a helping hand.

This method assumes the ground is roughly level.

To measure the draft from your nominated waterline, stand on the stool and drop the plumb bob adjacent to the centre of the keel on the waterline. mark the string where the shackle hits the ground.
Repeat on the other side.
Halve the distance between the two marks and measure this length, remembering to include the shackle.
Measure the distance from the bottom of the keel to the ground.
Subtract this off the plumb bob measurement and you have your water draft.

For the mast height extend a metal tape measure on its side (it will not sag significantly this way) over the 'reference' point, so that it overhangs the boat on either side (easier than trying to find a long pole or board).
This is where you need two people.
Drop a plumb bob either side just skimming the side of the boat and mark the string when the weight hits the ground (simultaneously on both sides so that the tape measure is not changing position).
Average the measurements of the two bits of string.
Subtract the waterline plumb bob measurement off this.
Add your mast length to your ref point and you have your air draft .
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