Thread: Twisting in a 9mm ply garboard plank.

What are you building? Post a photo showing the garboard you are talking about. How are you intending to fasten it?

I have done a hull in 9mm 5 ply, epoxy glued only and no metal fasteners. No problem at all with springback, just leave clamps on for a couple of days.

If the plywood is so tightly twisted that you are having trouble getting it into place, try hot water and towels and pull it into place gradually over a period of time rather than in one hit. Plywood is pretty tough stuff and you can torture it a long way before it will split if it is good quality.

Cheers, Cameron.

A common problem with garboards. Use a heat gun and you'll probably get it to go. Once the plywood has been forced into place, it will take a "set" meaning it will remember this twist, which will be seen if you remove it after a few days. You can also drape heavy towels over the offending areas and pour boiling water over them. The towels help keep the heat in, during the cussing part of the operation. Lastly, you could scarf on a section of 1/4" (6 mm) plywood and twist this in, which will be easier. After it's in, apply two layers of 12 ounce (400 gsm) biax fabric over the skinnier plywood (on the inside). This will make it just as stiff and strong as the 3/8" (9 mm) stock that's suppose to be there. Inventive cursing is helpful I've found, so be creative and let us know how it works out.

How far up the stem does the garboard climb? On a boat like yours, this can be challenging. Some clever clamping, ratchet straps, odd shaped blocks of wood screwed to the planking, etc. all will likely come to play. The rest of the planks will be relatively easy compared to the garboard on your little putt putt. The turn of the bilge planks might you off a little, but you'll have developed several new and effective curses by then, so it'll be a bit easier.

Thanks Par and Seafarer,
Left plank on for a couple of days and it has taken a set as you described. Did not need half as much shoving as first tried. I do have a steam box that will accommodate it if necessary. Or would it be preferable to heat it with a heat gun if it fails to relent.. Some pictures are here about,I hope.
Cheers Dan.

That doesn't look so bad, the forefoot has some fullness to it. I've done some sailboats that would make you cry, with hollow forefoots and wicked twists.

On a garboard like that, I use temporary clamps and predrill the plank for it's fasteners (even if temporary ones). I'd then use the fasteners to "suck" it down as you work forward. Personally, I don't hang them aft end first, but I start with the bow, where the twist is worst, so the length of the plank can be used to advantage. Again, I'd use the fasteners to suck it down to the rabbit and let gravity help with the midship and after portions.

Heat is the real ticket, though steam has some benefit. Plywood doesn't react as well as solid stock to steam, but it will to heat from a gun. The boiling water over towels trick also is effective on wayward plank ends. A Spanish windlass is a good tool for convincing planks to lay down where you want them. I usually place blocks or wedges under the straps to force the plank into position, drill my pilot and clearance holes, then screw the sucker down. I generally don't glue anything right away, but let the plank "live" with the set for a few days. It'll relax a bit, so the strain on the fasteners or glue line isn't as severe. This lets you make finial adjustments to it's position and get ready to hang her for good.

I've found if you're good, you will "present" the plank to the boat 3 times. The first for fit and she has to come off for some trimming and dressing, the next is the "dry" run to be sure of fastener holes, spacing, gains, lap lands, etc., then finally you get to really "hang" the puppy and the goo and fasteners go in for good. If you're not so good, you'll "present" that plank a few more times to the boat. They call it presenting the plank, because your boat has to accept your work and any misfits or other adjustments, is the boat's way of saying "screw that, try again buddy". Eventually, the boat and you will come to an agreement and she'll let you hang it properly. Of course when you do, you must click your heels three times, turn to the north and spit. Other wise, the next plank will require at least one more presentation, just from boat spite. They can be evil, vindictive wenches at times.

Thanks Par.
The reason for springing the board from amid ships to the stem is that it takes a radical curve in that direction from nearly horizontal to vertical. At midships the rabbet is virtually concave and accepts the whole edge of the garboard. However as you know the rabbet tends to open as you progress towards the stem. I found it easier to trap the plank into place this way. Thanks to you these planks are coming around to OUR way of thinking. I shall keep you posted. Just as an aside I am making a router jig in order to do the splices I have calculated that I will need to do approximately 108 of them to complete the planking. Shall post a picture of the jig once done.

Cheers Dan.
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Unless you're finishing your planking bright, then you can skip the scarfing jig. A hand plane, a power plane and a belt sander are all it takes. Just get it close enough, so you can bond them together within reasonable tolerances. Thickened goo will fill any defects and you don't have to fuss with jigs. I lay both halves of the scarf on the table, aligned appropriately, then several quick passes with a power plane to rough in the taper. I use the veneers as a guide to keep them straight. Next some finer passes with a low angle block plane by hand and finally I dress them up with a belt sander. A 5" (127 mm) wide plank of 1/2" (13 mm) plywood will take about 5 minutes, maybe less (both sides). Nope, it's not perfict, but it doesn't have to be with some thickened, wood butcher's friend. I've made some real hack jobs with a 10" angle grinder and a 24 grit pad. Works really fast, not especially precise, but as long as you align the two pieces right, it doesn't matter much when the paint goes down.

Food for thought . . .

Thanks Par.
I made a jig anyway. Took me just over an hour to construct and I can do a scarfe in about 30 seconds and is very neat and accurate. The 20 mm, gains at the bow and transom I shall do by hand with handsaw and rebate plane. Plan to fix down the garboard with epoxy glue and silicon bronze boat nails. Would spacing of the nails at .075 mm (3 inches) be adequate? Thanks for your time.
Cheers Dan.PC090064.jpgPC090065.jpgPC090063.jpg

I have a similar scarfing jig I use, though the angle can be adjusted for different scarf slopes and planking thicknesses. By the time I set it up, get the bit depth right, the stock shimmed and clamped into place, I've could have it cut with conventional tools. I do use it when I have a lot of scarfs to cut or want bright finish precision.

3" fastener spacing could be a bit tight, if the keel or keel batten isn't very thick. If it's not very thick, stagger the fasteners up and down (every other one), to prevent splitting the keel inside the rabbit. With epoxy holding things, you could use a 4" spacing, though by nature, I'd still stagger every other one, so there'd be 8" between fasteners sharing the same grain line penetration. Often the wood selection for the keel can determine this. For example, a species like live oak (one of my favorites), which has an interlocking grain, I wouldn't worry about the stagger, because splitting and checking along a line of fasteners, isn't common. On the other hand, a species like cedar or Douglas fir, I'd surely stagger the fasteners, to prevent this potential issue.

In the forefoot, where the garboard rises up quickly on the stem, you'll want to decrease the fastener spacing a bit, so the garboard doesn't snatch it's fasteners out, because of pre-load from the plank twist. It's a judgment call, but you'll see what's happening as you screw in the garboard and can adjust accordingly. On some soft keels, I've drilled out the fastener holes and reinforced them with a glued in dowel. This serves two purposes - the first is to isolate any potential splits along the grain from closely spaced fasteners and second is to harden up the fastener holes themselves. A bit of a pain in the butt to do (more steps) but it can really increase the holding power and life of the fastener holes, as well as make replacement during repairs an easy thing, as the dowels can be drilled and replaced, with likely little damage from moisture in the holes (the keel lives unmolested).

Can't wait to see her planked up. I'm a sucker for lapstrake.

Once again,thanks Par for your reply.
The boat has a .100 by .020 (4 inch by 3/4 inch) keelson of flooded gum (rose wood), an Australian hardwood indigenous to this area. There is plenty of meat to nail to either side of the keel so staggering the fastenings will not be a problem. I am a bit concerned about the forefoot as you pointed out, and am wondering if screwing here, with stainless screws would be a better option. The boat whilst I shall have it will never be moored. Picture here of bright finished transom interior.
Cheers Dan.

I always assume stainless or bronze fasteners, as I can't justify galvanized any more with recent quality. Which ever you choice, you have to stick to it, don't mix metals.