Spira International Boat Plans header graphic

Boatbuilding Tips and Tricks

Friday, June 8, 2012

"Hey Jeff, You Forgot This on Your Plans..."

I occasionally get emails from people thinking I "forgot" to include things like the side angles on the drawings or the shapes of the planking. Trust me, I've thought these things through and try to ensure a first time builder does not fall into obvious traps that more experienced builders have learned by the hard (and expensive) way. I have developed the building technique in the manuals to make sure your boat comes out right, and you make the fewest mistakes.

I design boats with the understanding that people aren't precise in their construction. Let's take the side angles for instance. My miter box is only accurate down to about 2 degrees. So that means the width of the frame that's 24" high can vary by (pardon my trigonometry) the tangent of 2 degrees times 24 inches, or 0.035 x 24 = 0.84 inches, so nearly one inch. Remember that is on each side, so one frame could be nearly 2 inches narrower than the next. Imagine what your sheer line would look like if the frames varied by two inches in width from their design nominal when you sight down it: wood boat plans

When you build the frames using my method described in the book, most builders can get them within +/- 1/8th inch of their theoretical position and so the the sides will look fair, have that pleasant sweep without lumps and dips. Still think you should build the frames using angles? Can your miter box or chop saw resolve and accurately cut down to the needed 1/4 of a degree?

The keelson is another example. If you cut the strongback within 1/2", cut the forward end angle within 2 degrees and set the height of the stem up from the strongback within 1/4" then made the transom and aft end angle and height as accurate as the bow, your keelson length could vary by as much as 1-1/2". How pissed would you be if you spent an hour's time and big bucks and picking out on a nice piece of clear lumber for the keelson, then cut it to my recommended length and it turned out to be 1-1/2" too short? To make sure you don't launch your hammer through the garage door in frustration, I recommend you attach the keelson to the transom and let it hang over the stem, glue and screw it in place, then trim it later, so it comes out the perfect length.

From the manual:

wooden boat plans

On some boats like the Alamitos, where the stem is pre-attached to the keelson, the strongback jig does not hold the stem in position. It settles wherever your building tolerances allow it to.

The same goes for why I dont give the exact sizes to cut out the plywood for framed hulls. Well, its not only me but no designer of framed hulls can ever, nor should they ever, tell you exactly what size to cut out the plywood. If you build each frame within plus or minus 1/8 and locate them precisely within plus and minus 1/8 the plywood planking can vary by as much as 2! Its probably more likely that most of these dimensions will vary by a good deal more than that. If a builder gets within 1/2 on some of those dimensions, hes probably doing a pretty good job on his boat. This means that the plywood planking shape can vary by as much as 6 from nominal. It just doesnt make any sense to try to cut this out beforehand.

The correct way to cut out the plywood is to first frame the hull. Put all elements in-place, glued and fastened so that the precise hull size and shape is fully established. The boats framing is then used as the only reliable pattern to cut out the plywood covering. This is done by clamping the plywood in place where you wish to locate it on the framing as shown here:

free boat plansYou then trace around the framing onto the plywood with a pencil being sure to draw it a little oversized to ensure full coverage of the area. Then remove the plywood and cut it out along the lines you drew. Then attach it and trim later.

Hope this sheds a bit of light on why my plans are different than most and why I lay claim to my boats being Easy-to-Build. I'd like to think it's why I get many, many emails from people saying that this was their first boat and they're surprised at how well it came out.

- Jeff


Blog Contents

Archives