Determining Air And Water Draft

[Jd1]

Originally Posted by Seaworthy Lass

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 .

Awesome graphics !!! Yes, this should get me within a few inches anyway. The error that would creep in would be because of a non-level floor and any boat tilt error would even out if the side measurements are added and divided by two. I will give that a try and see what I get.
Thanks !

[Seaworthy Lass]

Originally Posted by Jd1

Awesome graphics !!! Yes, this should get me within a few inches anyway. The error that would creep in would be because of a non-level floor and any boat tilt error would even out if the side measurements are added and divided by two. I will give that a try and see what I get.
Thanks !

Fine artwork was never my forte LOL.
Yes, as long as the ground is level, the results should be within a few cm inches. If you want even greater accuracy for some reason you could average a few measurements. It is a simple method that wouldn't take long .

[Paul Elliott]

Another way to extend a level measurement across obstacles or a distance is to use a hose filled with water. They may still sell clear extensions that couple onto garden hose, or you can use clear tubing. Put on end at the cabin top (or whatever reference point you have), and the other at the boat rail. Adjust the hose to set the water level at your reference point, then drop a tape measure to the ground from the water level at the rail. To compensate for not-level ground, repeat port/starboard and average.

True, a laser level is probably cheaper and easier. The last time I used a hose-level was when I was shingling the side of a house, and they hadn't yet invented laser-levels.

[Andrew Troup]

I would hang a plumb-line off to one side, say from a building or another boat, and hold a square (a square piece of cardboard would be fine) against the line. Sight across the top and see whether you're below or above the mast collar or whatever.

Walk up the ladder till your eyeline is level with the point you're trying to measure and put a piece of tape on the ladder, the building, or hitch a loop in the string.

Do the same to establish the waterline, then measure between your marks.

For the degree of accuracy you need, you could even dispense with the stringline and just sight across the water surface in a wide glass tumbler to position your marks.

[sestina]

When taking the draft measurement, measure from the base of the keel to an assumed point on the topsides on both sides. Best done at about the maximum beam. Then when afloat, measure down from the point to the water. Much easier than trying to get the waterline spot on.

Measure the mast whilst its on the ground to a suitable point. With the aid of a bricklayer's spirit level (or a short one and a straight length of timber) project the point out over the side and and measure down. Do this from the dinghy so heel doesn't affect things.

I've done this loads of times measuring boats for IRC ratings. To get the overhangs a plumb bob and a old wooden yardstick (floats) is the way to go.

Simples!