What to use to draw curved lines

[ZendoDeb]

Got my plans yesterday - Two Paw 8.  Think I have a local source for real marine grade ply - will save shipping costs!  (The other "marine ply" I found turned out to be fir.  Don't want fir.)

So it says when transferring curved lines to the sheets, to use a long, flexible piece of wood.  What works?

Have everything else I need except a reliable jig saw.  (The old Craftsman got caught in a basement flood.)  Hope to start this weekend.

[John Burritt]

I'd add to what Ray said is that the piece of wood be straight - not bowed, crooked, curled, or any of the other things that relatively small pieces of wood do to themselves.  The wood should be stiff enough to pull around the curve fairly, but not be wimpy.  That size depends on the wood and the severity of the bend.

[Howard]

My battens are of the type John and Ray describe. But a long, clear wood batten that takes a "fair" curve is hard to find. Although I've not tried it, I've heard of folks going to Lowes or Home Depot and buying long pieces of plastic or fake wood trim. As they are consistent, folks say they bend into a fair curve. Not that expensive to try as a test.

[PAR]

I have a dozen or so battens, most are wood, because they're long (24'+), but many of the short ones are plastic or aluminum extrusions. I have a favorite piece of 1/8" square aluminum stock that I use for small stuff. You have to be very careful with it or you'll kink or bend it, but it fits some fairly tight radius.

[Howard]

I have a dozen or so battens, most are wood, because they're long (24'+)

Are these family heirlooms or can you still find that kind of wood in Florida?  ;D

I thought I was doing well to find a nice clear piece at 12 feet.

[PAR]

I've had some of them a number of years, but the long ones are scarfed from 16' stock. Resorcinol is the adhesive. I can get 20' Douglas fir, and much longer oak and SYP.

[ZendoDeb]

Thanks for all the replies.

The suggestions that call for using a table saw imply I have a table saw I trust to cut a 3/8 batten (or thereabouts.)

I guess I will try the plastic-wood trim idea.

(I am trying to build a catspaw dinghy - not the Queen Mary.)

[Hirilonde]

Yeah, if all you need is one 10 foot batten to use for building one dinghy, then a piece of 1/2" quarter round or what ever from the lumber yard will work.

[PAR]

Always use a batten that is considerable longer then the curve you need, at least 25% longer or the resulting curve (from too short a batten) will have flattened ends.

[Ken_Potts]

I've gotten fair curves from a batten shorter than the final curve.  It works well as long as you don't draw the line to the end of the batten.  If you have a 10 foot batten and need to draw a 15 foot curve just make sure the middle portion of the batten lays nicely along the curve and don't draw the line all the way to the ends of the batten.  Draw 4 feet then reposition the batten so it lays nicely along the line you just drew and some more of the remaining points in the curve.

I've gotten very good results just drawing a few feet of the curve at a time.

Don't worry too much about finding the perfect material, just find a piece of anything that bends in a consistent arc and draw a little at a time (an inconsistent piece of wood is not as good as a good piece of plastic).  If you lay the whole batten out along the intended line and only draw a segment that is about a third the length of the batten you'll do well.  Just work your way along the curve - Start the batten at the middle of the curve and work your way outward.  Many beautiful curves (boats, airplanes, etc) have been drawn using a french curve to draw a long sweeping line in short connected segments and that's even less accurate than using a batten.

[Tom Lathrop]

Always use a batten that is considerable longer then the curve you need, at least 25% longer or the resulting curve (from too short a batten) will have flattened ends.

This is often overlooked but is important in getting a fair curve near the ends.  In order to get this fair curve, the batten must also be fixed along an extension of the arc beyond the ends.

[Designer]

It is rare when when Tom is wrong, but... While technically correct, we have to build to the real world. What Tom is referring to is called 'end moment', applying a force beyond the end of the curve. When bending the bottom ply around that curve, the builder does not have any plywood beyond the bow to apply a similar end moment and it wants to be flatter than the rest of the curve, therefore it is better not to force the batten to a shape that is hard if not impossible to build. What usually results is a gap which the builder fills with epoxy but if we are theorizing we need to understand the principles.

The easiest way for the first time builder without a lot of resources to get a good batten is to go the lumber yard and buy a piece of trim molding. They are usually very clear with straight grain and it comes in small sections.

My preferred batten is one that is stiff enough to just bend around the curve. If too limber, you end up spending more time fairing the batten where a stiff batten wants to be fair.

[Craig]

I like to use the trim moulding also.  I usually buy 4 at a time and store two.  Easy to replace if broken or thrown javilin like into the yard.  I also have 36 and 12" steel rules that work great for smaller arcs.  I don't throw or step on them.  8) 8) 8)

[Tom Lathrop]

Some might say that I am already 'round the bend. ;D

Actually, I have not noted a significant problem with fairing to the true arc in planking. If you are using a batten that is a bit too stiff for the arc, it could make a significant difference near the end.  My best large batten is a piece of straight as an arrow fir offcut from a Windmill mast I made over 40 years ago.  Shorter battens are whatever I find that will do the job.  A friend recently gave me a box of different cross sectioned plastic splines that work great with ducks for arcs up to 3 feet.

[lbrewer]

...if we are theorizing we need to understand the principles.

and THEN fill up the gap with epoxy

[wayfarer3130]

I used a piece of balsa 1/2" x 1/4" - edge on for flatter sections, and the other way for more curved sections.  I nailed finishing nails into the curve points, and drilled a hole through the wood, attaching to the nail with a tie at each end, then held the middle to match the previous curve 1/2 way to the first nail, and then 1/2 way to the next nail.  It worked pretty well - at least the curves look fair to me.  I just didn't want to have to drive to the nearest lumberyard and I had the balsa on hand. 

Bill

[Maria CS20 104]

I have used, quite successfully by the way, fake plastic quarter round moulding in 16 foot lengths, overlapping the ends of the plywood I was marking by about every  18 four inches,  I also use very small 1 inch finish nails to tack in place using pre-drilled hole about 18 inches apart.  Have had to reduce the 18 inch span to a 12 or 15 on span at times and all seems to come out well.  Tried strips of a variety of wood but the curve does NOT seem to be as consistent, and found out to my detriment that I had to rethink approach, AFTER I cut an $80 dollar hunk of ply, pissed me off because I knew it was wrong and did it anyway when I was in a hurry to get the side panels cut on our CS 20.  Stupid is as stupid does.  Lessons learned by others are often better than one we learn ourselves.

Jim