found this in my search for info,
Hull Blueprinting 101 by Jeff Bennett-
Hull blueprinting means different things for different hull designs. I will limit this discussion to vee bottom jet boat hull blueprinting. It is assumed that the reader has a working knowledge of body work and fiberglass repair. It is best to talk with or get assistance from someone who understands this skill. Also, there are many ways to blueprint a hull, this is the way I go about it.
Basically, vee bottom hull blueprinting means to return the bottom of the hull to the shape that the manufacture originally intended. Over time, hulls can change shape or warp due to improper trailer fit, or hulls being removed from the mold too early. This warping often results in a 'hook", which is when the bottom of the boat at the transom is lower than the adjacent hull forward. This hook resembles a trim tab or cavitation plate in the down position. The result of this hook is to generate more lift at the rear of the boat, which forces the nose of the boat downward. The faster the hull speed, the more this occurs. Because of this tendency to push the nose down, the boat is prevented from getting loose (free from the water) and top speed is reduced. Usually the prime motivation of hull blueprinting is to increase top end speed. There are many steps to hull blueprinting.
First, the amount of hook must be measured to determine whether or not the effort is worthwhile. The easiest way to do this is to use a 5-foot straight edge and place it lengthwise from the transom forward. Examine the bottom at several locations; down the centerline, at each lifting strake and at least one location between each lifting strake. You should concentrate on the lower part of the hull, usually at least 2 or 3 lifting strakes outboard from the centerline. At every location measure the gap(s) between the straight edge and the hull. The gap(s) is the amount of hook. If the hook is any more than 1/16" then a hull blueprint may be worthwhile. If you decide to go ahead and blueprint the hull, there are a series of steps required to accomplish the task.
1) The boat needs to be stripped (remove the motor, battery, fuel, steering wheel, possibly the upholstery and anything that will fall out or be in the way when the boat is turned upside down.
2) Now it is time to turn the boat upside down. Used car tires with blankets over them work well for this. First lay out 6 to 8 tires on the ground behind the boat and with the help of 6 or 8 of your friends (more if the boat is over 20 feet) and slide the boat off the trailer and on to the tires. Next, line up 3 or 4 tires in a location that when the boat is lifted up on one side, they will be between the ground and the hull on the opposite side. Have every one except 1 person gradually lift up on one side of the boat and the other person make sure that the boat does not slide off the tires. When the boat is at 90 degrees and resting on its side have each person one by one go around to the opposite side and then gradually let the boat down. One person needs to move the now vacant tires to the other side to be a resting-place for the boat when it is upside down. As you are letting the boat down, make sure nothing drags on the ground and that the tires are completely supporting the boat. Should you have a hoist, there are many ways to aid in this process, but I am not going to go into those here.
3) Using the straight edge, make a detailed contour map of the bottom by measuring and marking how deep the gap is at various locations. A grease pencil is a good choice for this map. The blueprint area should be 5' long and out to the 3rd strake outboard from the centerline (2nd if you have a 2 strake per side boat).
4) The amount of the step will determine how the 'hook' should be filled in. Never, by the way grind out a hook; many manufactures have made this mistake and ruined otherwise good hulls. Anything less than a 1/16" gap should be filled with body filler. Any body filler will work as long as primer and or paint seal it. The best filler is 3m Premium grade filler, which is waterproof. Between 1/16" and 1/8" gap can be filled with either Duraglass or fiberglass mat and resin. Anything over 1/8" should be filled with fiberglass mat and resin.
5) Next, you need to grind the areas that will be filled. Use a 40-grit disk on a body grinder. The gel coat does not have to be completely removed, but all the shinny spots need to be eliminated and the surface must be rough.
6) If you need to fill any areas with fiberglass, use 3 oz. mat and resin and do them first. A single lay up of 3 oz. mat is roughly 1/16' thick when soaked with resin. If you are not going to add fiberglass, go on to Step 7. Use a roller and remove any air bubbles. Try not to work in the sun. You can use fiberglass cloth over the mat, but you really do not need to add any strength and in this case mat alone will work fine.
7) Once the fiberglass is dried, grind it with the body grinder and prep the surface for the body filler by washing that portion of the hull with acetone on a rag.
Fill next with the Duraglass filler. Mix up a small batch of Duraglass and spread it on the areas you wish to fill. Using a straightedge and standing on the side of the boat, quickly place the straightedge lengthwise (relative to the hull) and slowly pull the straightedge towards you as you scrape the excess material off the bottom. Start the filling from the centerline and work outboard. When the Duraglass dries, use the body grinder again and smooth out the high spots. Repeat this process over the blueprinting area. Apply another application(s) until the hull is within 1/16" of being true.
9) Now do the final filling using the regular body filler. This procedure is basically the same as Step 8, but instead of using the body grinder, use a long board with 40-grit sandpaper, sanding lengthwise. It will take three or four applications before the hull is straight.
9b) The hull to intake interface needs to be blended. Just make sure that there is no part of the intake hanging below the boat, no forward facing steps and that you fill and smooth any gaps between the intake and hull. The finished blend should be a gradual entry and the intake should look as though it is part of the hull.
10) Once the boat is straight, again use the long board with a 100-grit sandpaper and detail out the bottom. Some areas, especially around the intake will require hand sanding without the long board.
11) Use glazing putty to fill the pinholes. Again sand with the 100-grit sandpaper.
12) Mask the blueprinted area off and prepare the surface for primer by cleaning the surface with a thinner or acetone. If you use acetone, be careful to not remove the glazing putty, which will be attacked by the acetone.
13) Primer the blueprinted area using a polyester primer. Put on a thick coat because it will help fill any remaining pinholes and seal the body filler. Dust coat the primer with a black lacquer to be used for the wet sanding step.
14) Wet sand the primer with 220-grit wet/dry sandpaper. Follow that with 400-grit wet/dry.
15) Now you are ready for the finish coat. Since polyester primer has been used, you have a few options on type of paint or Gel coat. I usually use catalyzed acrylic enamel or a lacquer based graphite finish (AKA speed coat), which is available from Rex Marine or Eddie Marine. Gel coat is the strongest finish, but it requires the most work. As I mentioned at the beginning, it is assumed that you have a working knowledge of bodywork. If you do not feel comfortable with the final paint step, either find someone who can do it for you or use the graphite paint because it is almost impossible to screw up.
16) Let the final finish dry thoroughly (24 hours) and be careful not to scratch it when you put the boat back on the trailer. Now you are ready for the lake. Good luck!
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