Keeping the boat straight

In this post I will suggest you a boatyard procedure to level your boat and keep her leveled for the whole building process.

Why am I writing this post ?

Because in complex boat construction (apart from kayak and small dinghies) your only reference lines on board to place correctly all the structures and furniture are the DWL (design waterline, that is to say the horizontal floating plane) and the centerline of the boat (CL); if you build the boat not perfectly leveled there is a high chance first to make a crooked boat, and second to place all the structural elements and the furniture in the wrong position

These internal structures, stringers, bulkhead, floors, engine beds , cabin separation panels, galley and berth plans, and so on, have to be properly placed in a hull which is a complex curved shape with no reference points, no straight lines and no square angles.

In all this geometrical mess, we can use three measures inside the boat to “fix” an exact position:

  • distance from bow (or more frequently from the first bulkhead starting from the bow) or from transom face plane
  • distance from the centerline (CL)
  • distance form the Waterline plane (DWL, design waterline), or any other plane parallel to this and at a given distance; you can easily trace on board the DWL plus one meter, or plus 1,5 meters, and so on

Each internal structure has to be cut, adjusted on board, glued and strengthened with glass lamination, and they all have to be positioned on board with a precision that has to be reasonably high; this operation has a capital importance in realizing what you really have on building plans ; precision can’t be underrated or dismissed as a designer’s wish ; in my boatyard experience 1cm of error in up/down direction and in distance from the transom face exceeds the maximum tolerable mistake in a 10 meters boat; mistakes bigger than this may create troubles or slow down the construction since you have to measure and heavily adjust every single panel to keep aligned what you have on the plans with the reality.

Now let’s face the main question:

How can we manage the measuring/aligning process during the construction ?

There are two main phases where this kind of measure are super important:

  • setting up the bulkheads/frames at the beginning of the building process
  • after the hull rotation when have to reposition the boat on the cradles, straight and leveled until launch

  • It all start with the frames/bulkheads placement on the assembly scaffold: it is mandatory that these elements are correctly aligned to build a straight hull ; each frame/bulkhead has the DWL and the centerline marked on plans, and we’ll use a laser level to properly align all the frames/bulkheads during the first stages of building
  • take your time to place each frame/bulkhead in the perfect position on the scaffold, keep the laser level in the same exact place for the whole process
  • make a “safe” DWL/CL mark : laser levels have the nasty tendency to attract people wandering around the yard , and being kicked and thrown on the ground; so, place a “safety mark” of the DWL and of the centerline, in a part of the yard wall or on the floor , or anywhere else it will not disappear after the level has been kicked , which sooner or later will invariably happen; this marks will be really helpful to reposition the level in the correct way ; we will mark them again after hull rotation
  • transferring the DWL and the centerline from frames/bulkheads to hull: this is an often underestimated operation that is as important as the previous one: we have to mark the centerline and the DWL in the internal face of the hull and in the outer face too (we’ll understand later why it is important to mark DWL even outside) ; simply place the laser level within the boat , and mark the DWL using the frames/bulkhead marks as a reference

  • using the laser level: using the laser level is a skill quite easy to achieve but a couple of trick may help you to avoid painful mistakes:
    • get a decent tripod: a quite heavy, stable, sturdy tripod with a good precision in the head mechanism will be our best ally ; you don’t need 200 bucks professional photoshooting tripod, just a sturdy decent one, you can find it for 25 bucks
    • standard pendulum lasers levels: these are the most common laser levels used in this kind of operation; they work well, they are quite cheap , are self-aligning , we just have to pay attention to place them leveled enough to be in the range of self leveling with the internal pendulum, but it is quite straightforward to check this. Laser beam varies in power, they are not so strong and they are quite battery hungry, so keep a set of spare batteries on the yard.
    • Marking the lines: I personally prefer not to trace a continuous line but pieces of line 5 cm long (2 inches), spaced 30-40 cm (one feet) , it is important to use a quite thin marker, 1 mm of line thickness is fine, if you use a super big marker making a 5 mm or more thick line you are basically wasting your time.
    • Keep the light low on the yard: it will help to better see the laser beam, mostly when you are quite far from the level ; sometimes a black cardboard maybe useful to evidence where the laser is on the hull and follow it
    • Marking the DWL on the outer face: we can we start marking the floating plane on the inner face of the hull using the building frames as a reference, before the hull rotation we will need to mark the DWL plane on the outer face of the hull too; someone drill a small hole in the hull on the DWL trying to keep it perfectly horizontal to “transfer” the DWL mark on the outer face of the hull; I don’t think it is the best practice , mostly if you have thick planking ; I prefer to use the transom, place vertically a stiff long batten with clamps on the outer face of the transom , mark a DWL plus one meter (or similar) on the internal hull topsides, it has to be a mark high enough to allow me to be in the lower part of the batten and out of the transom panel, mark the same DWL+100 on the batten with the laser level, measure exactly one meter on the batten, this is your DWL on the extrtanl face of ther hull; now you can take your laser level and mark DWL all around the boat
    • rotating the boat : after the boat rotation it is mandatory to place the boat on cradles or any other system perfectly straight and aligned
    • how to lift the boat to adjust position after rotation : two or three hydraulic bottle jacks and a two-three extra trestles and wooden wedges and chunks is all that we need to do the small lift operations that we will do to set the boat perfectly leveled once she is rotated, even for quite big hulls
    • now we are sure that the boat is leveled and we can be confident that any measure that we will take from now will have a decent degree of accuracy
    • rechecking all the lines 1: when the loading conditions change , for example when you load on board big and heavy stuffs, engines, gensets, big tanks, check again that the boat is leveled and make small corrections if required
    • rechecking all the lines 2: every time you do an operation that will erase the referring lines, sanding, laminating, painting, spend the few minutes required to mark them again both on the inner part of the boat and on the outer surfaces