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Setting Up the Strongback and Bulkheads


Below is an example of a strongback with the bulkhead stations in place. The station markers are 2x4's screwed to the strongback and a string is snapped down the center which will be the location mark for the centerline of the bulkheads.


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The strongback is 6' wide made up of 2x6's. I have pre-made sections of various lengths that I screw together to get whatever overall length I need. Once the strongback is laid down I use a laser level and go around the perimeter to find the highest point. This will be the mark that the strongback is leveled to. I go around the strongback and shim with wedges as needed to get the strongback level. If the strongback is warped it may require some weights in spots to keep it on the floor. When the strongback is level and all on the same plane I use a can of spray foam and anchor it about every 4' making sure to spray around any wedged and shims to keep them in place. The blue line in the picture below represents the center as determined by the string I snapped.


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Below is a typical setup with the bulkheads and longitudinals in place.


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The constant dimension in lofting all the bulkheads is the waterline, it is always the same dimension above the "baseline" which is the top of the strongback. The bulkhead blanks can be made big enough to go all the way to the baseline, but that results in a lot of waste, so I make the blanks large enough to go from the keel to at least 6" beyond the sheer. Spacers are made to add the needed length to get the proper dimension from the waterline to the baseline. Plans may specify a distance but it is completely arbitrary as long as the sheer is above the baseline. I sometimes add more to the waterline to baseline dimension to make it easier to get under the sheer inside the hull to work.

I drill a 1' hole at the intersection of the waterline and centerline in all the bulkheads. When all the bulkheads are in place I will sight through the holes to see if any of the bulkheads are out of place and adjust as needed. I then run a string through the holes and can really fine tune the alignment. Any side to side adjustment is a no brainer, but if there is a height difference I check the centerline plumb and the discrepancy may be because one of the sides is to low or high. It may now get to a point of spliting the differences. It is important to measure an re-measure when lofting because these are the lines that determine the placement and orientation of the bulkheads to the strongback. The triangular fall-down pieces are set aside when the bulkheads are cut out, they will become the temporary braces for setting up and securing the bulkheads to the strongback.


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In the image below a bulkhead is going in place. The spacers for this bulkhead are only a couple of inches tall, at some of the stations they will be a couple of feet tall. I fastened the spacer to the 2x4 and then fastened a piece of 1/4" plywood to it keeping it inside the outer edge of the strongback so it does not get in the way of the bulkhead brace. When the bulkhead is set in place and centered, the 1/4" piece of plywood helps to keep the bulkhead from shifting from side to side. To help with any minor adjustment to the bulkheads later, I set a couple of tongue depressors on top of the spacers. If later I find a need to drop one of the sides of a bulkhead I can remove a depressor or two instead of removing, trimming and re-setting the spacer.


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Below is an example of a bulkhead in place. To set the bulkhead brace I will put a 2x4 next to the strongback keep the brace off the floor and then run screws from one side of the bulkhead through to the brace. The 2x4 is removed and that side of the bulkhead can be plumbed and the brace screwed to the strongback.


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The first three bulkheads in place...


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If any of the bulkheads are off more than 1/8" you will see it when you sight through the 1" holes. I put a piece of 1/4" plywood on the first and last bulkheads so the string is centered. When making bulkhead adjustments don't assume the first and last bulkheads or correct. If for example the second bulkhead is off to one side 3/8" and the third bulkhead is off to the same side 1/4" and the fourth bulkhead is off 1/8", it is a good indication the only the first bulkhead needs to be adjusted and not 2,3 and 4.


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When the bulkheads are all in place they will most likely be bowed side to side in the center and or top to bottom. Even if they are not, they probably will once the hull panels are placed. I generally run 2x6's on edge at the sheer and another about half way to the keel. This boat will get longitudinals in three of the sections so higher 2x6 is placed so the longitudinal will rest on it. I also installed temporary vertical cleats to screw the longitudinals to. The longitudinals are made out of the 1" bulkhead stock. I will cut them to the proper length and about 4" taller than they need to be. When they are in place I will run a batten over the bulkheads and scribe the shape to the longitudinal. I like to cut the centers of the longitudinals a bit shorter, it will be much easier to shim the hull panels out than to trim the longitudinals if needed.

I will use the longitudinals to help keep the bulkheads plumb, but were there are no longitudinals I will run 2x6's from top to bottom. I try to keep all the 2x6 stiffeners about one foot short of all the edges of the bulkhead so they do not get in the way of tabbing or glassing.


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The image below is not of the Kokanee 38 but it shows the longitudinals in place. In the lower left it shows two longitudinals in place but were cut out with the bottom edge left a bit long. After all of them are installed like this a batten is laid across the bulkheads for and aft so the longitudinals can be scribed and cut. I like to cut them a little low in the center and use shims to fair the bottom panels as needed.


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