Sailboat Bottom redesign.

[Stormfield]

Okay, so I've found the boat I'm going to build. It's an old William D. Jackson design from Svenson's free boat plan site. The 14' Zephyr.
Problem is, the plans call for a curved bottom panel, and to ease construction I'd like to make it flat. The boat isn't too different from Bolgers' June Bug (and as that's a flat bottomed boat I was hoping I could go ahead with the idea).
Any thoughts?
Thanks all,
Eddie Burkhalter

[Raggi_Thor]

I think that might work.
If you draw a straight line through each arc so that the area you lose below it is pprox tge same as that you gain above it, your boat will have the same volume and distribution.

It's always good to post teh picture.

[Raggi_Thor]

sorry for my spelling, laptop on my lap!

[Stormfield]

Thanks so much. I feel much better about it now. I know I'll loose a little in comfort, as the flat bottom might slap in the chop a bit more, but it sure will be easier than trying to force the plywood into a curve it deosn't want to make. I'm also going to loose the pivoting centerboard in favor of a drop-in, and use a kick-up rudder to ease launching. Can't wait to cut up some wood.

[Raggi_Thor]

If you divide the round bottom in three developable panels your are very close to the originale, but that is much more work. I would try the flat bottom.

[PAR]

You'll lose a fair bit of displacement by removing the arc from the bottom of this boat. You could plywood plank the bottom of this boat (as an arc bottom) with to pieces (one per side like a V bottom). This might require a slit or two, but it doesn't look like it'll need any, just the centerline. The arc bottom boat will perform better too. It may be possible to use a single piece, but I don't know enough about the curves. I am assuming they're very close if not completely developed, which Jackson would have done.

This would be a great candidate for a taped seam conversion. She'd be lighter, stronger and more water tight.

There are plenty of flat bottom designs in this size range to consider without butchering this one.

[gggGuest]

Its very probablw that the curvature makes a significant contribution to the stiffness of the completed structure. It shouldn't really be a big deal getting the curve into the plywood if that's how it was designed to be built...

[gonzo]

A flat bottom may need a thicker planking. As gggGuest says, it contributes to the stiffness

[TeddyDiver]

Looks quite tricky having round bottom and rocker.. Anyone done that with a single piece of ply?

[Raggi_Thor]

I thougt he could keep the same displacement by moving the flat line down on each frame to make the area he loses iin the middle approx the same as the area he gains on the sides.

Transverse planking over a couple of stringers should be easy here on the round bottom.

[apex1]

Just to add my mustard to the dish:
it looks like a developable design, and to "torture" the ply a bit adds noticeable to its strength, so, why "redesign" (which it in fact is´nt) a proven one? Do a tiny little model in 0,5 mm Birch ply, you will see it´s easy!

Regards
Richard

[PAR]

The curve is compound, if you take the bottom as a whole, but if divided down the centerline it's conically developed. The plans show 1/4" plywood for the bottom planks, without a centerline cut, so it's clear Jackson is torturing the plywood into an arc bottom shape. I can tell you from experience the arc bottom will be stronger, more comfortable and faster underway then a flat bottom version. This is an ideal candidate for taped seam conversion, which will probably remove 30% to 40% of the rough hull weight and most of it's interior framing too. Less to build, less to buy and more waterproof.

Jackson was one of the early designers to embrace developed plywood planking.

[apex1]

Thanks Paul,
as usual you found the better words to describe what I meant. At least you did not contradict me, you made my day!

[Stormfield]

I'd already planed on going for taped seams and glassing the bottom, and now I think I'll stick with the arc bottom. It'll handle better, and with enough temporary screws and clamps 1/4 ply should take the curve. It'll be interesting, but I bet this boat hasn't been built in several decades. I'd like to use a simpler rig, as in Michalak's ladybug (75 sq ft ballanced lugsail) whcih should be much easer to rig, though slower. Thanks to all for your invaluable help.

[ancient kayaker]

Sounds like you will end up with a skiff. I have read that skiff design is harder than it sounds so you may spoil the qualities of the hull by changing the bottom to a flat plank.

I developed a method for a severely tortured bottom using 2 pieces, tried it once and plan to use it sometime on a wee Lassie (classic 1880's Rushton canoe design). It worked but I didn't take pictures! I will try to explain it with these FreeShip pics to help, in case it is of use to you.

The developed planks are cut out of ply, a stringer is glued to the bottom edge of each piece. These stringers will form the keelson. The two planks are clamped midships with their outside faces together then twisted into shape. In this position, the stem(s) twist upwards, not inwards, so the two planks have their intended shape but are actually rotated 90 degrees outwards as shown. Then I plane a flat on the top edges to create 2 flat surfaces; when the planks are rolled back 90 deg and glued together a perfect gapless joint results. A keel can be sandwiched between them if required.

It's a lot easier to do than to explain, but if you are still with me I see no reason why it could not be adapted to a sailboat bottom on a glued lapstrake hull.