Do I need to tape the seams of plywood bottom panels?

[AndrewT]

Based on advice in another thread, I've decided to forgo the cross-planked bottom on my wooden skiff and instead install 3/4" plywood panels, sheathed in fiberglass. Couple more questions before I start that project.

1. Do I need additional framing members on the bottom of the boat? All the plans call for is an inner keel - no cross-pieces (due to the cross-planked bottom called for in original plans). I was planning to install an athwartships cleat under the bench seat and land the plywood seam there, but I'm wondering if I need to add any additional members.

2. Do I need to do anything special to the plywood prior to installation? Obviously I will coat all the mating surfaces liberally with unthickened epoxy prior to gluing and screwing, but I'm wondering if there's any other preliminary prep to be done.

3. After installation, do I need to "dish" the seams with a grinder/sander and then fill with epoxy/fiberglass tape prior to fiberglassing the bottom?

4. Any other tips/warnings/etc. before I start cutting stuff up?

[Oldad]

What dimension cross planking are you replacing? If it is 3/4 and you are using good quality ply you should be OK. Coat the edges of the ply liberally. Dish the seams? Depends on the level of finish. Usually you can fair the seams with some thickened epoxy.
Oldad

[AndrewT]

Plans called for 1"-thick planks, not more than 5" wide, from chine-to-chine, glued & screwed to chines and inner keel, caulked in-between, no epoxy or fiberglass cloth. I'm not terribly worried about finish level on the bottom; it'll rarely be seen, unless you're laying under the trailer or the boat somehow floats upside down, and with an 18-25 HP motor this thing ain't going to set any speed records anyway. I just don't want it to leak/flex too much/fall apart during use.

I remember seeing details about dishing & filling & fairing, but I might've been looking at a stitch & glue book at the time - can't remember. My panels will be glued to solid lumber on all four edges, and screwed with #10 or #12 screws every few inches.

[jackster]

AndrewT,

Standard practice is not to butt the ply, but to scarph it to adequate length.
And, depending on the thickness and span, maybe a stringer.

[AndrewT]

The bottom is 60" wide at max beam, with a 4" wide inner keel on the centerline - so max span would be 30" side-to-side. I really don't want to clutter the interior with cleats running across the bottom, so my plan is to place the panels so one seam falls under the bench seat and the other falls under the forward deck/platform. So, the aft panels will be roughly 30" wide x 60" long, the mid panels will be roughly 30" wide x 72" long, and the forward panels will be taper to the stem in width and be roughly 48" long.

Would prefer not to scarph if I don't have to, but I can if necessary. (It's another jig to build, more epoxy to mess with, more time to build/glue/clean up, and will take two people to install instead of being able to do it alone.)

[Lewisboater]

If you don't want to butt with wood you will need to support the seam somehow so the Payson fiberglass butt or Scarfing are pretty much your other options. You don't really have to dish out the ply but you will get a little hump if you don't, and I recommend it for the outside to leave a smoother bottom. You will need to go about 6" wide on across the seams on both sides of the ply. You could also use a hybrid method using a 12" wide strip of 1/4" ply on the inside and the glass joint on the outside... feather the edges of the ply and it would be barely noticeable.

[wizbang 13]

Or leave an eighth inch gap between the ply (filled with epoxy) and fit ply butt blocks on the inside.
this way, you will not need to pre apply the epoxy.
having a fair ( no bumps)outside is worth it, even for a clunky boat.

[AndrewT]

Or leave an eighth inch gap between the ply (filled with epoxy) and fit ply butt blocks on the inside.

Wouldn't the cleat across the bottom serve as a full-length butt block? I'm thinking something like a 1x4 piece from chine-to-chine, let in to both chines and the inner keel.

[AndrewT]

Another question: would scarphing eliminate the need for athwartship cleats? It dawns on me that cutting limber holes in the cleats sort of defeats the purpose.

[TerryLL]

The proper way to do this is to use 12:1 scarfs, no butt blocks, no seam taping, no athwartships cleats. You'll end up with joints that are just as strong (or stronger) than the surrounding panel, and a perfectly smooth inner and outer bottom. The scarfs should lay on the hull like the scales of a fish, with the overlap pointing aft. After cutting those monster gains in your planks, a couple simple scarfs will be no big deal.

But here's a question. With a bottom that is 60" wide, do you plan on laying the ply sheet crosswise on the bottom, or doing a seam down the middle at the keelson? If you plan a seam at the keelson you'll likely need a much beefier keelson than the one specified for the cross-planked bottom.

[wizbang 13]

Yes, I think "cleat" and "butt block" may be the same thing in different lingo.
Yes, scarphing will make a smoother inside. If you can scarph quick n simple, no prob, do it!

[AndrewT]

After cutting those monster gains in your planks, a couple simple scarfs will be no big deal.

You haven't seen how my gains turned out...

I was planning to seam them down the middle, but now I'm not sure. I guess I could lay them across the hull in a single pass; I'd have to scarph them on the long edge instead of the short edge, which is going to present some problems.

The bottom of this boat is turning out to be much more of a problem than I anticipated.

[jackster]

AndrewT,

The weight of one (of two) bottom section will be 80-90% of a 4'x8' sheet.
Not the lightest, and an extra pair of hands would make things go more then twice as fast, but, IMHO, doable by one person.

IF you scarph by yourself, rough cut the three pieces (per side), allowing for the 12-1 bevel once at each scarph, scribe the joint at the keel to a final fit, make sure both scarphs fit well and leave the chine fit a little fat and perfect after the glue-up.
All the fitting can be done one piece at a time, dry fit until satisfied, and glued up on the boat, also one piece at a time.
And as Terry says, work forward for the transom.

[TerryLL]

Andrew, another question:

I'm curious how you plan to finish off the exposed ply endgrain at the chines? In a ply boat this joint is typically glassed. But with solid wood lapped planks that's not a good option.

And yet another question:

Why did you decide against the cross-planked bottom in solid wood? For a boat of this type and construction, that is the right type of bottom.

[AndrewT]

Plan was to radius the plywood edges and wrap the glass cloth partway up the garboard plank.

Decided against the cross-planked bottom because I didn't want to mess with caulking between adjoining planks and between the planks and chines, which seems like a maintenance issue waiting to happen. But the more I get into this, the more I'm considering just doing it that way and dealing with it. Would save me the hassle & expense of fiberglassing, too. At this point I'm completely clueless as to how to proceed. (I guess in that instance, following the instructions is probably a good idea.)

[TerryLL]

Glassing the ply bottom to the solid wood garboard plank will likely be a bigger maintenance issue than maintaining the cross-planked bottom.

[P-man]

adding splines to the cross planked bottom might reduce some of your concerns about cross planking.



this picture is from JMadison's "Building the Maid" thread.

[AndrewT]

Not sure what the point of that (splines) is? Does it replace the caulk? Reinforce the caulk? Does epoxy between the splines/planks not cause the same issue as fiberglass over solid wood? Or are the splines caulked in place?

[P-man]

Andrew, i'm not an expert, just relaying something i had seen in another thread. http://forum.woodenboat.com/showthre...he-Maid/page12 the spline is assembled dry without adhesive or caulk. bedding compound is used on the chines. the idea, i believe, is to allow the wood to swell and contract and still be watertight while eliminating caulking and epoxy.

[AndrewT]

Huh. My plans call for caulk between the plank edges, and caulk & cotton at the plank end/chine joints. None of which I'm wild about - but maybe that's because after ten years as a (home) builder, "caulk" is synonymous with "quick way to cover a mistake made by a lazy trim carpenter." Bedding compound, though - doesn't that stuff stay "tacky" forever? Seems like it would give as the planking moved. Might be worth looking into.

[Breakaway]

Build it cross-planked. I built a small skiff screw and glue. Scarfed the ply, glassed the bottom and over the chine, feathered and faired the edges of the glass. All of it was pretty easy. But about half way through I realized that if I had built with lumber instead of plywood:3.

1. I would have been done already
2. It would'nt have been as messy
3. I wouldn't have to work stop and go fashion waiting for glue to dry.
4. I wouldn't have to worry about ambient temperature.

Two-cents from a one-boat builder.

kevin

[TerryLL]

Having built four epoxy-ply boats to date, and one riveted lapstrake in solid wood, I have to say, building in solid wood without all that slimy glue mess is a pure joy.

[Gib Etheridge]

I would install the inner keel and a cross frame below the thwart then glue and screw on one layer of 3/8 fore and aft on each side of the keel with the long seam 1" to one side of the centerline, then another layer of 3/8 in thickened epoxy spread with a notched trowel and running the same way but with the long seam 1" to the other side of the centerline. I would stagger the butt joints on the cross frame the same way. I would scarf the butt jounts but only about 2 to 1. I see the cross frame as 1 1/2" X 2 1/2" on the flat. The 1 1/2" provides enough thickness notch over the batten keel leaving a limber hole on each side of the keel and notch over the chine the same way, also with limber holes.. The 2 1/2" allows for staggering the butt joints. The chines would be laid on the flat, just as structurally useful but thin enough to notch over and the upper edge won't be inclined to hold water and catch dirt.

I usually do things the hard way, which I know is not everybody's cup of tea.

If I already had the 3/4 I would do a 12 to 1 scarf on the shop floor then dry fit as long pieces on either side of the centerline, screws and all, then remove and replace in thickened epoxy. I'd cover the centerline joint with a rub strip also bedded in epoxy.

[AndrewT]

Okay, after sleeping on it, I think I've about decided to just go with the cross-planked bottom as shown on the plans (I know, I know - what a concept). These are 1" x 4-5" solid eastern white pine (although I will likely use Cypress, since that's what's available), nailed/screwed to chines and inner keel and caulked with something called Life Caulk and cotton (at the chine).

Questions about this method: 1. Should I still coat everything in epoxy afterwards, prior to prime & paint? 2. Is "Life Caulk" the thing to use, or, as someone mentioned above, should I look into bedding compound, or 3M 5200, or something else? Or are they all six on one hand, half a dozen on the other?

[TerryLL]

I'd stay away from an aggressive adhesive like 5200. I haven't used Life Caulk, but it sounds like the kind of stuff you want. My go-to bedding compound is Dolfinite.

No epoxy, no glass. Prime and paint.

http://www.jamestowndistributors.com/userportal/show_product.do?pid=4400&engine=adwords!6456&keywo rd=dolfinite

[AndrewT]

No epoxy, no glass. Prime and paint.

What am I going to do with all the time and money I save by not using epoxy and fiberglass?

[TerryLL]

Can't help you with the spare time, but I'll PM my address so you can send all that extra money you don't know what to do with.