Strongback

There are two methods of building boats under 50' (15m), Right side up and Upside Down.* When building Right side up the frames/stringers are erected with the help of a cradle, upside down uses a strongback to rest the boat on. The accuracy and fairness of the entire project depends on the strongback being level and straight so it is ill advised to take any shortcuts at this stage. The strongback should be of the material with the most similar properties as the boat, a metal boat should have a metal strongback, while a wooden boat can have a wooden strongback.** Keep in mind that for every mm (fraction of an inch) error in the strongback the same error will be reflected in your hull. Be very careful when building on "soil", try to use concrete foundations and remember that frost has the potential to dislodge even the strongest foundations.

Some designers will include strongback drawings but others will just send you "standard drawings" on which to base your own design. Most of the time this will include a recommendation for the thickness/type/size of material to be used for constructing the strongback. When designing a strongback the ability to use CAD*** is a huge benefit, what follows applies equally to manual/cad design. The first decision is how high the hull should rest on the strongback (distance from DWL***** to the top of the strongback), this depends on several factors. Firstly calculate how low the hull can sit without the stern hitting the floor, second calculate how high above the floor this places the keelplate. If you plan to build indoors or in a shelter this height+30cm (1') is the lowest possible ceiling height.**** For those who build outside the question becomes one of comfort, pick the lowest height from where you can work on the sheer and are able to get into/under the hull. Don't be afraid to experiment with this, remember the golden rule of work positions, if you can write a letter comfortably in the position it is good enough to weld/grind.

Once the height of the hull from ground is determined the next step is to figure out how tall the strongback stands, this depends on how high into the hull it protrudes. Since the strongback should be paralel to the DWL the next step is to calculate the distance from the DWL to the lowest point of the sheer line, to fully support all the frames this is the maximum distance the strongback can be placed from the DWL. It is preferred to place the strongback as far away from the DWL as possible because this enables you to build a wider strongback which will be closer to the sheer-line resulting in more strenght and less flexing/distortion. This is where things get a bit tricky and the CAD programs really get handy. Now that you have the distance from the top of the strongback to the DWL you have to go to your table of offsets, we need to figure out the distance from the centerline***** to the hull plating at the height you specified for your strongback. Those of you building a round hull or radius chine might be getting a bit scared at the prospect of having to use a lot of math you labeled as "I'll never need to use this". No need to worry, you can for the most parts ignore the rounded sections since the strongback will only intersect these at the stern where they don't cause any problems. The distance from the centerline to the intersection point of the hull skin is the value of interest. Once you have done this for all the stations/frames make a 2d drawing from a top view using the intersection points, connect the points to make an approximation of the hull skin.

Now it is time to calculate the clearance required for frames and stringers, add up the width of the stringers to the frames******, subtract the depth of slotting, add the thickness of the hull skin and you have the maximum distance of the frames from the intersection points. Mark these on the drawing, and draw a "virtual" inside skin. Now you can experiment with different forms and see how close to the "virtual skin you can fit them for every frame, remember that the only thing that matters is how close you are to the skin at the frames themselves. Usually a compromise must be made and a design with 3 or 4 joints is chosen. Again using some geometry and trigonometry and the table of offsets you can calculate the length of each part and the angle of intersections.

Once this is done and the number of legs has been decided (each joint should have a leg) the length of the legs can be calculated.

When setting up the strongback and welding I highly recomend getting a lateral laser level, with these you can project the entire plane of the strongback.

If a laser level is not available the next best approach is to use a string between two poles. Strictly speaking the DWL and Centerlines do not have to be 100% accurate since all the measurements will be made from those and teh external environment is never used as a reference point.

It is a good idea to make the legs adjustable, this way each part can be welded on a straight welding table/floor before final assembly, never count floors/walls to be level. The degree of accuracy required for boat builing far exceeds that of house building.

*Over 50' (15m) boats are almost always built right side up.

** Wood is in my oppinion to unstable for metal boat work.

*** We recomend Rhino 3D for beginners www.rhino3d.com

**** Remember that you also need to be able to get the boat out without demolishing a wall!

***** DWL stands for Developed Water Line, CL stands for Centerline, these are two of the axis from which every measurement in your boat is made.

****** This applies equally to temporary frames