Thursday, April 24, 2014

Water Levels for Boatbuilding

In my recent post about building stocks for wooden boats I mentioned using a water level for getting things aligned on the building stocks but I didn't explain how it works.

A water level is the most basic of measuring instruments and is very easy to use. Properly used it is very accurate but there are some principles that need to be taken into account in order to achieve that accuracy. If you ignore them then you can work on blindly assuming accuracy that is not really there.

A water level works on the basic principle that water will always find its own level; it will flow from where the surface of the water is highest to where it is lowest, until the two are equal. It is a simple tube with water inside it, open at the ends so that atmospheric pressure can push down on the exposed surfaces. It can be done with a simple length of clear plastic tubing, or it can be an ordinary garden hose with a length of clear plastic tube attached at each end so that you can see the level of the water.You will need about 500mm (20") of empty tube above the water level, to prevent water from running out one end when moving the tube from one bulkhead to the next.

To get two equal-level points on any structure, place the tube against the structure both at your reference mark and where you want to place the new mark. Raise or lower the tube at your reference mark until the water surface inside the tube is exactly at the level of the reference point. At the other end of the tube, draw a mark on the structure exactly in line with the surface of the water inside the tube. Ideally it needs two people working together, with one at each end of the tube to make fine adjustments to get the reference level correct but it can be done by one person if the tube ends are secured against the structure.

You can use this when setting up a bulkhead so that it is level across the boat. First, the bulkhead must have the waterline drawn accurately on the face, which should be done on the workshop floor when drawing the bulkhead for cutting. Position the bulkhead on its legs with clamps, with the centre of the bulkhead at the centrepoint of the building stocks. Position your water level so that the water is at the level of the waterline at one end. Raise or lower the other end of the bulkhead until the marked waterline is exactly at the level of the water in the tube. Check the transverse alignment of the bulkhead on the stocks with a plumb line attached to the centreline string above your building stocks, as described in my previous post about building stocks for wooden boats. Similarly, use the water level to set up other bulkheads at the same level as the first one.

Simple isn't it? But, it has potentials for error that you must take into account, or your new mark will not be accurately placed.

  1. Don't use a tube that is smaller than about 12mm (1/2") diameter. The smaller the diameter the slower it will be to react to adjustments in level, due to friction of the water against the wall of the tube. The larger the tube, the more easily the water will flow and the faster it will react. The large tube also has more water surface exposed to atmospheric pressure, helping to speed the reaction. A tube of 16-20mm (5/8-3/4") diameter is ideal.
  2. Don't use a tube that is excessively long. Ideally it will be long enough to reach the most distant point to be marked, with a bit extra to allow the tube to lie on the ground. Any additional length increases the friction in the tube due to increased surface area. It also allows the tube to lie in loops that will slow the movement of the water and possibly trap air to form airlocks. An airlock can stop movement of the water almost completely.
  3. Purge all air bubbles out of the tube before you start. Air bubbles will affect the accuracy of your readings and will slow the movement of the water inside the tube. The smaller the tube, the more that it will be adversely affected by air bubbles.
  4. Try to do your work in fairly stable conditions, so that the tube and water are not heating up and affecting the readings. Warm water will flow more easily than cold water and a warm tube will expand in diameter, holding more water. You don't want the tube to be moving between sunshine and shade because the expansion/contraction of the tube and water are likely to affect accuracy. This is particularly important if you are using a dark-coloured garden hose with clear plastic ends because the hose and water will expand considerably in sunlight.
  5. Don't be in a rush to mark your new reference line. It takes a few moments for the water to find its level before you can make your mark. It will first surge up and down a few times before it stops. This is when you will see the importance of a tube that is not too small because it is possible for the friction inside a small tube to overcome the atmospheric pressure and give inaccuracy of 20mm (3/4") or more for awhile before it very slowly equalises.
  6. Flick the tube with your fingernails before assuming that it has found its final level. The shock waves running through the water will help it to stabilise at the correct level.
  7. Don't do this work in windy conditions. Wind blowing over either or both of the open tube ends will exert a venturi effect and change the readings. The air flowing across the mouth of the tube forms a vacuum inside the tube. That sucks the water up the tube, so the level will be higher at that end than the other. Even if the wind is blowing over both ends, it must be exactly the same speed over both ends at all times to have no effect, a very unlikely situation. If you must work in the wind, make sure that both ends of the tube are well sheltered from the wind. If the wind is strong then just don't do it because wind blowing over a bulkhead will cause high pressure on one side and low pressure on the other, so you will be unsure of the accuracy of your readings.
  8. Tie a length of string to each end of your tube. Between readings, tie the strings to something at about the same level as your reference line. This will prevent the tube from dropping down and emptying some of the water unexpectedly.
  9. When you have one bulkhead that is accurately set up and secured so that it cannot move in any direction, use that first bulkhead as your point of reference for the waterline when setting up all of the other bulkheads. Don't move the reference point to any other bulkhead because you could end up with a cumulative error, which is minimised by always working with the same origin as reference.
  10. Buy or make some tight-fitting  bungs to plug the ends of the tube. It does not matter whether they are rubber, plastic or wood, but they should be non-porous. Use them to keep the water inside the tube when not in use.
I have no images of water levels of my own but Roy McBride of CKD Boats kindly supplied the images below, with permission to use them. His company is my agent in South Africa and supplier of plywood kits for many of our designs.
Two water level tubes. The one at left is discoloured by age but still usable. A simple plastic tube and water.

Both ends plugged with bungs to keep the water in.
This illustrates the principle. The tops of the lower two pieces of tape are level with the top of the water at opposite ends of the tube, so they are level with each other. Here they are close together but they could be 10m apart without any loss of accuracy.
Close-up of the tube and water surface.
 Thank you Roy and  CKD Boats for allowing the use of your photos.
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Monday, April 7, 2014

Building Stocks for Wooden Boats

There are a few ways to construct building stocks for building a wooden boat but most are variations on the basic method that I will show here. These photos have been sent to me by Fred Grimminck, who has previously built our Didi Mini and is now building the bigger sister, the Didi 950. This is a boat that is designed to fit into the Classe 950 Rule. It is a radius chine plywood boat with hard chine in the topsides.

Fred's building stocks, or beds, are on short posts that are fastened to the concrete slab in his workshop. If he were building off earth then the posts would be extended down into the ground about 300-400mm and set in concrete to make them stable. Other than that, the rest of the configuration would be the same.

Click on photos to enlarge, to see detail.
Didi 950 building stocks looking aft.
Didi 950 building stocks looking forward.
Rails and transverse braces are bolted to the posts. It is good to set the rails accurately so that they can be used as an accurate reference at any stage during construction. That means setting them so that the tops of rails and transverse braces are level, with the centrepoints of the transverse braces on centreline and the rails identically positioned each side of centreline. If any bulkheads will be attached to transverse braces then those transverses need to be set up at 90 degrees to the centreline, which will happen automatically if the two sides of the beds are identical. The centreline string on the beds can be seen in the photos above, providing a good reference for checking accuracy.
Transom doubler being set up on beds.
The photo above is of the transom doubler being set up on legs that are bolted to the aft transverse brace. If the transom will not have doublers on the inside then the transom itself would be in this position. This attachment is done in the same way as a bulkhead except that the bulkhead legs are generally bolted to the rails instead of a transverse brace. If you zoom in on the photo you can see the small piece of steel angle that Fred has used to fasten the post at bottom right to the concrete slab. The angle is bolted to the leg and to the slab. This must be done for all of the posts.

A secure and accurate set of building stocks is the foundation for an accurate build. Another essential to get this right is for the waterline and centreline to be accurately drawn onto every bulkhead. These lines will be used in setting up the bulkheads themselves and also many times during construction to set up joinery, engine alignment etc.
Didi 950 bulkheads being set up on beds.
 Fred is working forward from the transom in setting up the bulkheads but you can work from either end. I normally set up the forward bulkhead first, then work aft. This is because the forward bulkhead may extend below the top of the stocks and need to be either clear of the front transverse brace or attached directly to that transverse.

In Fred's photo above, the bulkhead closest to the camera is at the front of the cockpit, so forms the aft end of the cabin and serves as a good example. Each bulkhead must be set up accurately in all directions for the hull to be built to the shape intended by the designer. It must be at the correct location fore/aft, laterally and vertically. It must also be square to the centreline and it must be vertical both fore/aft and laterally.

Correct position fore/aft is done by measurement from the first bulkhead that is set up. Check it also against the distance to the previous bulkhead because this will highlight previous errors that may have been missed.

Correct position laterally is done by means of a plumb bob hung from a line above the stocks that is exactly over the centreline of the stocks. In a building this line can be attached to the ends of the building. If outside it will need posts to be set up beyond the ends of the hull and the line strung between them. The plumb bob is visible in the photos above.

With the plumb bob hanging from this line you have three checks that can be made on each bulkhead. They are lateral position of the bulkhead, vertical alignment of the bulkhead face against the plumb bob string and vertical alignment of the centreline drawn on the face of the bulkhead. This last one is a basic check that one end of the bulkhead is not lower than the other end but is a check only, not the place to set it up accurately.

Correct position vertically can be done with a waterlevel, which is a whole subject of its own and best dealt with in a separate post. An alternative and quicker method is to use a land surveyor's theodolite or a laser level that can be set up beyond one end and to one side of the beds, with a clear view of the whole length of the beds. Use this to horizontally align the waterline marked on each bulkhead so that both ends are at the same level and all bulkheads are aligned to that same level.

When each bulkhead has been correctly set up, bolt it to the support legs and brace the legs to prevent movement. Once all bulkheads are aligned and secured, you will be ready to start adding longitudinal structure.

A few things about this process should be noted.
  1. Wood is a living material, it changes dimensions with variations in humidity and temperature. If you set up a bulkhead on a dry day and a week later you set up the next bulkhead on a humid day, you will find that the dimensions may have changed by a millimetre or two. Work as accurately as you can but don't sweat over these minor changes. If you do, you will chase yourself around correcting and recorrecting whenever the weather changes. Just accept the changes and move on. Constructing the stocks from steel will stabilise them from changes due to humidity but not temperature. If you are like me, you like to work with wood and really don't like working with steel, so you will stick with wood for the stocks as well.
  2. Don't attach the legs to the stocks or to the bulkheads with wood screws, through-bolt them or use lag screws. Each bulkhead will not weigh a lot when you set it up but by the time that you have added backbone, stringers, sheer clamps, hull skin and possibly even built much of the interior, your wood screws may be bending or even sheering off under the load.
  3. A solid foundation to build your hull is the principle to aim for. Use diagonal bracing to keep everything at correct relative positions and angles.
  4. You need to be able to get through between the structures of the building stocks and the hull as it takes shape. You need access at many positions along the hull, not just aft under the transom. That needs the longitudinal rails of the stocks to either be hard against the slab or lifted enough to slide under. I recommend lifting them enough to slide under, which is easy by grabbing the rail with both hands and swinging your body through. If the rails are on the ground you need to hang onto the sheer clamp or some other structure to swing through and you will likely suffer numerous bruises to your back from impact with the rail during the course of the hull construction.
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