How Flat is Flat?

[Mandoliniment]

I'm building my construction jig for my (as yet theoretical) Penobscot 14. The jig is, basically, two 1X12's on edge; with the top edges defining the top of the jig.

So I have to get one edge straight. They vary from straight now on the order of 1/8" over their (12'3") length. I'm not convinced my #5 Jack plane will get them significantly straighter; we're talking about less than one part in 1000 here. Can my lumberyard do an acceptable job? what about the 5' jointer at my work (The idea of wrestlying a 12 foot board over a 5' jounter is not appealing). And more to the point - how close is close enough? That wood's going to move around as I build, regardless. What should I strive for? 1/16? 1/64"?

[bud]

Your plane can get them as straight as anything else, it's up to you. If you need to secure them on edge up for your form, sight down them & plane them straight. Your eyeball is actually a pretty good instrument for that. It will be important that they form a true plane across the two on them, so once you have two straight edges move along the top with a level & make sure it reads level between the 2 boards all the way along. If not shim up below them. Have fun, recite as you work, "my plane is my friend, my plane is my friend." ;)

[Ian McColgin]

I was pretty sure that I knew more than John Gardner and everyone else a quarter of a century ago but I've become progressivly more ignorant with age. From that perspective -

It'll probably float but errors have a way of compounding as the building goes along. Be anal here and it will be easy to stay right all the way along. If not, you may be like me and live with a rather wonderful boat (my dory Leeward) that has a visible woofle in the garboard to midstrake seam caused entirely by thinking that a small error would not matter.

Boat floats and she's given good service these last 25 years and with the dark paint it's mostly me that sees the flaw, but it galls me to have committed such a crime to such a fine craft.

[Boyd]

I am not speaking from a great deal of experience here, but I have a suggestion to offer. I hope that I have not missed the point.

A close friend built a 16' Whilly boat on a similar jig. He got the 1 by strongback as flat as possible and built station molds. Before attaching the station molds to the strongback he stacked them together on a flat surface and drilled a hole clear through all of them at once. When he put them on the strong back he shown a laser pointer through station one and two (two points to define a straight line). If the laser light shines straight through to the far wall, the station molds are lined up (in at least two directions). If the light does not make it the adjustments that need to be made are very clear. I thought that was a pretty clever solution.

[Don Maurer]

When I built the strongback for my boat, I placed the 2 x 10s edge to edge along the top edges to determine where they needed to be planed down. Once there was a tight joint between them, I reversed one of them and did it again. When the frame was assembled, I ran a string about an inch above the lowest point of the keel parallel with the 2x10s and on the centerline. I measured the station height from the string and used a plumb bob just touching the string to center the moulds.

[NormMessinger]

Just to be contrary, if not obstreperous: The important thing about your jig, strong back, is that it be rigid. It is not so important that it be flat because you are going to set your forms up on the jig and shim them into place. It is improtant that they not move and that they are lined up. My eye is good for 1/16" and I would try for less and not settle for more.

--Norm

[Mandoliniment]

Thanks, all. I did about like Don suggested, except that I clampep and screwed the boards together and planed them as a piece; in the end I got it straight to within 1/32".

Each one of these steps I keep getting psyched out by turns out to be easier than I ever expected. Onward...

Apologies to anyone who's secretly annoyed that I posted this topic in the wrong forum. Won't happen again.

[mmd]

Tut-tut, Mandolinament! :D If this isn't the place for such questions, where is? The target might have been construed as building rather than design, but we all read all categories religiously (don't we, guys 'n' girls?) except for when there is a particular rant going on in Misc. and we're disgusted for a few days. redface.gif

[imported_Steven Bauer]

I was out in my Thistle Minnow tonight watching my 9 year old daughter in her first sailing regatta and saw a really cute small sailboat heading back to the ramp. I peddaled over and talked to the skipper/builder. Yep, it was a Penobscot 14. Very nice boat. Good choice. Keep us posted. Pics, too.
Steven
P.S. she did great - out of ten boats Holly had a 7th, a 5th and a 2nd.

[kcarlon]

It looks like I'm more than a day late and a dollars short but here goes my two cents worth. I haven't been able to start any boat construction for the last couple of years because last year I built a guest cabin for my kids up at the lake. This year its been an addition on the house and a sauna at the lake. Next year it will probably be a new boat shop, finally, up at the lake. The only reason that I'm boring you with this is that I have been using good quality lumber, douglas fir etc. The bottom line is the lumber looks good going into the truck but by the time it gets used, as soon as possible, it doesn't seem to be none to straight anymore. The next time I build a jig I think that I'm going to try engineered lumber, in particular I joists. I suspect that once there planed straight they should stay straight. Good luck with your project.