The Ever-Settling Waterline (Page 2)

This page was last updated on 7 October 2002.

Setup and Striking a Level Waterline

One of the things I determined during the summer was that the top boottop scribe mark was not level and parallel to the water's surface, but in fact incorporated a fairly significant sheer.  This is appropriate, attractive and proper for the upper edge of a boottop, and I will provide some sheer for my new boottop when the time comes, but I feel that the bottom edge of the boottop--which in this case corresponds with the top of the antifouling paint--should be dead straight and, most importantly, parallel with the surface of the water. 

First, though, here's a basic summary of the status of the various lines on the boat, incase it becomes less clear later.

  • The BOTTOM SCRIBE mark (what one might term the original Designed WaterLine, or DWL) is straight and planar.

  • The UPPER SCRIBE mark (the upper edge of the original boottop, and the point to which I raised the antifouling paint when I repainted the hull) is NOT straight and planar; rather, it incorporates a curve, or sheer, which is proper for an upper boottop edge, but not for the waterline.  This upper scribe mark is also the bottom edge of my current boottop; the fact that it is not planar leads in large part to the strange waterline we had when the boat was fully loaded this season.

  • The TOP OF MY CURRENT BOOTTOP is also curved, or sheered.

  • Finally, what I am trying to accomplish is to have the new waterline (edge of the bottom paint) parallel to the water, and straight and planar--and more reflective of where the boat actually floats.  This will give the new boottop a straight lower edge also, and I will curve, or sheer, the top edge for an attractive look.

My first step towards a new, straighter, and higher waterline and boottop was to confirm my on-water observations of the unlevelness of the current paint job.  To do this, as well as strike a new level line higher up, I needed to establish a level line that simulates the surface of the water.  There are a few ways that this could be done, one of which is with a transit and/or laser level.  These are effective methods of going about this, but require that the boat be leveled not only side to side, but also end for end for best accuracy.  Plus, you need to have a transit on hand--I don't.

Additionally, I wanted a true visual representation of what was going on, so I really wanted to reproduce the ocean surface in my backyard to confirm my thoughts and ensure that the new striping would end up the way I wanted.  Therefore, I chose a different method.  This method, adapted from one described in Details of Classic Boat Construction:  The Hull, by Larry Pardey, is simple and effective, provides a true visual planar reference to help with the process, and I could easily accomplish it with materials on hand.

My first step was to level the boat side to side.  It was pretty close, but I had to adjust the stands a bit to get it nearly perfect.  Fore and aft leveling is not important for this method, as you will see in the description as we continue.

levelbowsupport.JPG (172055 bytes)

levelsternsupport.JPG (155556 bytes)

Using some old 2x4s left over from my construction shed, I erected a structure at each end of the boat, consisting of two vertical supports (one on each side), bracing to hold the verticals in position, and a horizontal beam spanning between the two vertical members.  To secure the various pieces to the ground, I drove stakes into the dirt, after cutting points on the end of the stakes, and screwed the various supports to the stakes.

leveltobowmark.JPG (151911 bytes)

Next, I leveled the horizontal beam side to side, and positioned it so that the top edge was at the exact same height as the current waterline, which on my boat is located at the top scribe mark of the original boottop from the factory.  Because the beams are located a short distance away from the hull at each end, I used a short level and a metal ruler extension to transfer the level of the beam back to the hull at each end.  I began with the original waterline because I wanted to confirm my impressions about the waterline sheer before I began the process of striking a higher line.

portstringorigwl.JPG (168084 bytes) stbspringorigwl.JPG (154603 bytes)

Once I had the beams leveled at the right height, I ran strings fore and aft on each side, hanging weight (in my case, paint cans) from each end to hold the strings in place and taut between the forward and after beams.  Then, I adjusted each string inward until it touched the hull at its widest point, and at the same point on each side of the boat.

stringtightamidshpsstb.JPG (156131 bytes) portstringorigwl.JPG (168084 bytes) portscribemarks.JPG (164617 bytes)

These photos show the position of the string amidships on each side of the boat at the original waterline height, bow and stern.  Note in the righthand photo the notations showing the levels of the two scribe marks and their locations relative to the string, which represents the actual loaded waterline.  (Yes, the boat is heavy!)  Remember, the string is right at the top scribe mark at the bow and stern, which shows the amount of sheer in the top scribe mark.

tightmidshipsstb.JPG (153274 bytes)

With the string touching the hull at its widest point on each side, I began the process of transferring the straight, level line to the hull from the string.  This is a test run only, as I will be raising the level by an inch or two in the final stage, but I wanted to go through the process.  To transfer the line from the string to the hull, I started by securing the string to the hull where it touched with some tape, then marked on the hull with a sharpie.  Then, I moved each end of the string inboard (by sliding the weighted string along the horizontal beams at each end of the boat) until it touched the hull again 4" - 6" further along from the previous point, then taped the string again and marked the line.  The taping (or otherwise securing) the string to the hull is required to prevent the string from slipping up or down the curvature of the hull as it tightens.  This ensures that the line remains level and straight as it is moved in.  I repeated this process towards the bow and stern until I had the whole line marked on the hull.  In the extreme aft portions beneath the counter, where the hull is highly curved and hollow, I transferred the string level to the hull with a level held between the string and the hull.

tighttobowstb.JPG (146140 bytes)

You can see how the string begins at the intersection of the boottop and antifouling paint at the stem and quickly extends into the white boottop.  This is a straight line running to the same intersection at the centerline in the stern.

wlmarksstb.JPG (184731 bytes) With the string tight along the hull the entire length, I now had marks indicating the level line between the bow and stern, so I could remove the string and eye my line for fairness and accuracy.  The black pen marks on the boottop are the result of this exercise; I didn't worry about putting the marks on the hull since this area will be sanded and repainted when all is said and done anyway.  Remember:  those black marks indicate a perfectly straight line between the stem and stern at the centerline.  Although it looks like my new line curves upwards amidships, this is simply an optical illusion caused by the fact that the old boottop and bottom actually dip down, or sag, amidships, the result of the sheer, or curve, of the top scribe mark.     

The process as described above, including second guessing, remeasuring, and double checking, took me between three and four hours on a beautiful fall afternoon to complete.  I proceeded very methodically to ensure that I didn't make a mistake.

With my trial run complete, confirming the existing waterline, I moved on towards raising the line by 1" - 2"...I haven't decided yet.  Since this is the loaded waterline, I might only raise it one inch so that it doesn't appear to float way too high when the boat is less heavily loaded.  This is still under evaluation.  Before I quit for the day, I did raise the horizontal beams at each end of the boat, first to 2" above the existing line, which seemed ridiculously high, so I lowered it to 1" above the existing line, which seemed to be about right.

Please click here to continue.  


Glissando, Pearson  Triton #381

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