Scantlings for stitch and glue spritsail skiff, 20'6"x 5'4"

[nss]

Hi,

Just found this forum - looks great.

I've got a question for experienced stitch and glue builders out there, regarding scantlings.

I'm looking to build a 20' v-bottom spritsail skiff, using stitch and glue. Pretty much straight from the NC Maritime Museum plan #5, but I'm converting the plans to S&G:

http://us.st11.yimg.com/us.st.yimg.com/I/yhst-45081188800216_1952_13462941

This is a hard chined design, with bottom and side panels only (2 each side, what with the v-bottom). The warping required is minimal except for the forward-most part of the bottom panel at the base of the bow, where I plan to cut the bottom panels short and carve/saw out a solid block to insert and make the final 16" or so transition to the stem (they have to do this in traditional construction of these boats as well, as those thick juniper planks can't make this severe transition either).

It will be an open boat - structure will include short deck at the bow (a foot or two long), with a bulkhead at the aft end of it; a thwart at the mast, centerboard trunk, thwart at aft end of trunk, and thwart in rear, in front of removable sternsheets. No side decking, just a strong rail/gunwale. I will probably make gussets on either side of each thwart, extending from rail to chine, and box in under the thwarts (flotation filled). As per the traditional skiff, there will be a relatively heavy deadwood starting at the aft end of the centerboard trunk and extending to the rudder. No keelson forward, probably a minimal one aft to bolt the deadwood through to.

I will epoxy/glass the exterior, and possibly glass the interior too (haven't decided yet). Ply will be Okoume BS1088.

I'd like to make this boat as light as possible for maximum speed; it will be basically a toy for tooling around the Beaufort NC area, Bogue Sound, etc., with a goal being to see if we can outrun the faster of the traditionally built skiffs.

I'd like it to be as stiff as possible for maximum performance, moreso than for great seaworthiness. Note the rig will be a bit taller than that shown in the plans; the norm for the museum built boats seems to be 15' or so mast height, vs. the 12' measured for the workboat in the plans.

So my question is, what thickness plywood should I use? The original boats were planked with 7/8" juniper, with quite a few frames holding the planks in place. With S&G I will have no frames, other than the aforementioned structure.

I was leaning toward 3/8" for bottom and side panels, but am wondering if I should make the bottom panel 1/2" just to stiffen things up, and for walking around on (hoping to get away without any floorboards). I think I can do the minimal warping necessary with either 3/8" or 1/2" ply on the bottom panel, so that shouldn't be an issue.

Any opinions?

Thanks in advance,

NSS

[BWD]

Here's my best guess:

From just the sailplan posted doesn't look like much warp to the bottom. Hard to say without seeing full lines.
But 1/2" ply is not very bendable. And serious overkill on strength and weight. and $$$.

I would guesstimate 3/8 all around or 3/8 bottom and 1/4 sides with 6-12oz glass in and out, plenty of tape on seams. If you do 1/4 sides you may want a few stringers (1x2) and partial frames (3/8 ply) between thwarts/seats, but that is what I would do for the purpose stated.

Only an amateur's guess though....
Best luck!

[nss]

Thanks. I think I'm leaning back towards 3/8" all around. I want it as clean and uncluttered as possible, so would like to avoid stringers/frames.

NSS

[PAR]

A light weight version of this boat would ask for 3/8" bottom planking and 1/4" topsides. A heavier duty version increase the planking dimension by 1/8".

For a true taped seam conversion, which will make it the lightest, eliminate all the stringers, except a centerline plank keel. No chine logs and a minimum sheer clamp/shelf arrangement of 1 by stock, glued on the flat. Your athwart stiffeners forward are good, make them plywood ring frames, rather then solid bulkheads. The thwarts can rest on a cleat, glued to the side planks at the correct height (and boxed as you mentioned) and try to tie the centerboard case to the side planks too. A small aft deck or thwart will help "racking" of the hull there. You also don't need transom framing. These pieces are all replaced with reinforced epoxy fillets covered with 'glass tape in many cases. A small skeg will help tracking, but don't get carried away, you don't want much down there to hold her back. Of course, tape all exteior seams, if not a full sheathing of fabric.