Centerboard slot...

[SparkG]

Building a strip-plank catboat, hull is done, f/g'd, not turned over yet. Making a few smaller parts during the winter.
I've been reading a lot here about white oak and epoxy so before I assemble this centerboard slot I thought I'd ask a few questions. Parts are cut out in this image - laying on its side, top is on the right , the inside is already e/f/g/10oz. the ply is 1/2" marine, will be e/f/g with 4oz f/g outside.

http://webpages.charter.net/sparkg/cbslotparts.jpg

Questions are:
What order would you assemble this and with what glue?
I'm thinking, sanding the white with 80 grit and acetone right before gluing with thickened epoxy. I'd use 1" silicon bronze screws from the inside of the slot into those two long top w/o braces.
Same for the white oak end logs but the silicon bronze screws would be put in from the outside.
Then I'd f/g with 4oz over the outside - over those filled screws.
Or what do you think of using 3M 5200?
Or using PL or Elmers or Gorilla polyurethane glue?

[Yeadon]

Of all the adhesives you mentioned, I'd consider the PL Premium for gluing the slabs to the posts. Fastened, too.

Edited to add - As far as hooking it all together, I'd get everything lined up, drilled, countersunk, and prepared for the fasteners. Then I'd glue it up, fasten/screw it snug and clamp a bit around any spot that needed a bit more squeeze-out.

[James McMullen]

Why are you using white oak for a boat that is being built with liquid joinery? White oak is fantastic stuff in the right place but it is also a wood that swells and shrinks in greater proportion to its size than a great many other woods. I would not recommend using white oak for any part of a strip-planked anything--I think you ought to use a wood that glues well with epoxy and will be more stable in comparison to the rest of your stripper, a wood like mahogany or perhaps D. fir. White oak is at its best in traditional types of construction, not modern, glue-intensive forms. Fiberglassing right over an inch thick chunk of white oak that sits at bilge level is not a really good idea.

If you must use the white oak, then your best bet for a lasting watertight seam would be the 5200 and fasteners. The CB case to hull joint will take an enormous amount of stress if you clobber a shallow rock with your board at speed and needs to be extra strong. Through bolts, not just screws, for your CB trunk logs to the keel is a good idea.

[Thorne]

I'd be tempted to use epoxy for every joint except the bedding of the CB case onto the hull -- for that 5200 or Vulkem116 or another flexible glue/sealant will be best.

Several makers have epoxies they have formulated for oily woods and WO, Smith & Co. is the only one I've tried. And I use MEK rather than acetone for pre-glue (and pre-paint/varnish) wiping -- per Steve Smith of the above company, it evaporates fully and much more quickly, won't leave any residue in the wood that might cause the glues to fail or weaken.

The ply doesn't look like marine ply, but either way I'd be sure to seal all the edges with epoxy, and take care with any holes you drill through the plies.

For the CB pivot pin hole, drill it oversize and fill with thickened epoxy, then drill the correct size hole through the hardened epoxy.

[Yeadon]

What about a bronze flange bearing for the cb pivot?

[James McMullen]

What about a bronze flange bearing for the cb pivot?


That's what I did for the CB pivot pin on Rowan--machined some bearings to fit the pin precisely. I think it's a good idea for a heavy centerboard, but Thorne's idea of drill oversize, fill and re-drill is probably plenty good for a light, unballasted board.

[Thorne]

Either way, you'll want to set the bearing or pin through epoxy, not ply - so drill the hole oversize, fill with thickened epoxy and let cure.

I set a SS bushing in epoxy in the centerboard, which is the only piece of ply on my dory skiff. So no worries about that bolt wearing through the epoxy filler during use...

My entire CB case is built from WO, glued with Smith's epoxy, painted with red lead and marine paint on the inside, bedded with Vulkem116. No problems so far, but the boat is mostly dry-sailed.

[SparkG]

Thanks all for the input...I think I will change the end logs over to doug fir. The longer w/o planks are on the top of the slot, not the bilge. I'd like them there for looks and durablility, getting banged into being at the top of the trunk.
That is marine ply - veneer core, no voids, been running a small piece it for a month now very hot water, hot steam drying 3 times a week in the dishwasher and I'm pretty impressed with how it holds up. Not sure of the outer layer wood type - doug fir or pine? -but it will be covered with 4oz f/g on the outside and painted to avoid checking.
A few other notes on this boat...it'll be dry sailed, fresh water, moderate climate, in StLMoUSA. Also, this will have a 1/4" steel centerboard, in a 3/4" slot - unless someone talks me into a 3/4" c/b with a 1" slot ;) But the steel called for in the plans.
Keel will be thru-bolted.
Keel and rudder are 1"thick, doug fir laminated with pl premium, with 3/8" coated steel drifts, maybe e/f/g:
http://webpages.charter.net/sparkg/otherparts.jpg

Here's a shot of the f/g'd hull thus far when making the template for the above keel:
http://webpages.charter.net/sparkg/hull.jpg

[esingleman]

I disagree with the objection to using white oak in this application. The pieces used are thin, and movement will be insignificant.

Also, the centerboard box on my boat (which looks identical to yours)was specified by Joel White to be adhered to the boat with bedding compound like 5200 and to be screwed through the hull not bolted. Of course the seats in my boat fully restrain the upper ends of the the centerboard box making it solid as a rock. I have had no leaks in eight years.

[James McMullen]

Good job, esingleman. Of course your solitary example of anecdotal evidence completely trumps standard, modern boatbuilding practice! My point is not that it absolutely won't work, but that there are other woods that are much more suitable that don't have the potential problems of white oak. Why would you knowingly build in a potential problem if you could avoid it?

And as far as screws vs bolts. . . .screws are cheaper and quicker to install, bolts are substantially stronger. Take your pick and make your choice.

[Thorne]

Now James -- We shouldn't brangle over this -- the boat will be dry sailed. Issues with WO and swelling/shrinking plus issues with WO and epoxy may not really apply in this case, whereas they might be much more important for a larger boat that lives in the water.

I used WO extensively in the restoration of my dory skiff, and have had no issues with it cracking or failing even when immersed for up to a week -- so you can add one more case of "anecdotal evidence" if you wish.

Now if he wanted to use red oak, or spruce, I'd say we'd need to jump in and recommend against it -- but WO has a long history of use in boatbuilding. Look at all the Scandanavian boats built entirely from oak...and the smaller ones get pulled out of the water for the winter, so they aren't all wet-sailed.

[James McMullen]

Amigos, I've got nothing against using white oak in boats--in the proper applications that is. I used more than 300 board feet of it last year in boatbuilding and boat repair projects last year, for steambent frames, floor timbers, breasthooks and knees, and buttblocks for planking and through-hole reinforcements. What I didn't do--and nor should you--is try to glue it with epoxy or "encapsulate" it.

I repeat, why build with a material that has known potential problems when there are other alternatives readily at hand that don't cost any more and do the job as well or better? You don't bake cupcakes with bread flour when there's pastry flour at hand.

[esingleman]

Our original poster showed picture of a box not yet glued together. Sure WO is not compatable with epoxy, but there are plenty of other glues available to to do the job 5200 being one of them, as well as the home depot P/L polyurethane glues, and good old recorcinol. Since the sides of the CB are going to be screwed together anyway it probaly doesn't matter too much which one of those glues is used, it doesn't have to provide mechanical bonding as much as be a seal.

As far as the screws versus bolts go, again I think in this application it is overkill. If it were a stinkpot being hammered over the chop of white caps I'd be concerned.

[Clinton B Chase]

As a practicing boatbuilder, I want to second James's point...it is a best practice thing. We did a boat with W. oak in an otherwise monocoque structure (frames in a dory) and had issues. Go with best practice...you'll never regret that move, but you COULD regret trying to get away with W. oak in this application.

[esingleman]

We did a boat with W. oak in an otherwise monocoque structure (frames in a dory) and had issues.

What type of issues did you have?

[SparkG]

Talking about the centerboard, slot and pivot (for the boat that started this thread ;) , this is how it's designed - pivot point is below the hull...
http://webpages.charter.net/sparkg/sf%20catboat.jpg
Won't have to worry about any leaks but this will present a whole other set of problems. The plywood c/b slot underneath will be well braced on either side and tapered into the stern skeg/keel and the forward keel. But once again, I'll have to give the assembly seqence some thought - there are no hints on the plans.

[TerryLL]

Not really sure I should poke my nose in here, because I don't have anything bad t say about white oak. But H. mahog has always been my preferred wood for bed logs and posts. No glue issues, very stable, plenty strong for this application, holds fastenings well, bores straight and true, holds paint well. What's there not to like?

[JimConlin]

Gougeon's G/Flex epoxy is alleged to work well with white oak.

I'd suggest coating the inside of the trunk with an epoxy-graphite mix.

[SparkG]

Thanks Jim, in the image at the top you can see I already coated the inside with 10oz f/g and epoxy.

BTW all, plans call for a 1" wide slot with a 1/4" steel c/b. I'm thinking that's a bit sloppy so I'm going to make it a 3/4" slot.

The only advantage I can see to a 1" wide is if I'd ever want to go with a 3/4" wood c/b (3 layers of 1/4")

Anybody think a 1/2" slot would be better?

[TerryLL]

A one-inch slot seems a bit wide for a 1/4-inch steel board. But I'd be wary of making changes without asking the designer about it. A warped board or some swelling in the case can be a real nightmare if the slot is too narrow. Another option, probably safer, is to pad out the sides of the plate with some slippery plastic like UHMW. Looking at the profile, it appears you could pad just that portion of the board that stays in the slot. The UHMW would reduce the wear on the interior faces of the case, and reduce the size of the opening, resulting in less turbulent flow past the slot.

[James McMullen]

Are you supposed to sandwich the steel plate with plywood cheeks so that you can fair it into a foil section? A 1/4 thick flat plate is a relatively low-performance centerboard shape compared to what you can do with a foil with a 1" chord.

[SparkG]

The plans didn't say anything about sandwiching the plate between anything...just 1/4" mild steel plate.
From what I've read I'm not so sure that a foil shaping would make that much difference with a 13' tub like this. Then again, you could argue that it'd need all the help it can get ;)
TerryLL, yes some uhmw would work well and if I end up making a 1" slot I will definitely use that.
But I'm leaning to a 3/4" or 1/2" slot...

For now, just thinking about keeping the total width if the c/b down so as to not make such a wide hole through the hog. But just looking at the way that board sits so low, there's going to be a lot of strain on the lower part of that cb trunk.