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Tim Marchetti
05-01-2012, 03:15 PM
How wide should the slot be for a 1" thick centerboard? and what is the best way to seal up the slot for strip composite construction

TerryLL
05-01-2012, 03:36 PM
The width of the slot somewhat depends on what the board is made of and how much you expect it to swell. The last thing you want is a the board firmly stuck in the slot. A metal plate CB can stand fairly close tolerances, but a solid wood board might need a fairly wide gap to allow for expansion. A fiberglassed plywood board will be fairly stable, up until the time you wear a hole in the glass and the board soaks up water. My boat plans specify a 1-1/8" slot for a 1" epoxy/ply/glassed board.

Tim Marchetti
05-01-2012, 03:47 PM
The width of the slot somewhat depends on what the board is made of and how much you expect it to swell. The last thing you want is a the board firmly stuck in the slot. A metal plate CB can stand fairly close tolerances, but a solid wood board might need a fairly wide gap to allow for expansion. A fiberglassed plywood board will be fairly stable, up until the time you wear a hole in the glass and the board soaks up water. My boat plans specify a 1-1/8" slot for a 1" epoxy/ply/glassed board.
My board is plywood and will probably end up 1 1/16 or so when it's glassed. I was thinking 1 1/4 for the slot. Is that too much slop?

wizbang 13
05-01-2012, 03:50 PM
strip composite ,what does that mean, strip plank with glass? Is that the boat or the board?
seal the slot, what does that mean, caulk the trunk/case?
remember that a 1 inch board is only going to be 1 inch in 1 place. Yer probably going to foil shape it, or at least taper it by eye?
If it is an ol' fashion board , it'll swell , like TLL says.

TerryLL
05-01-2012, 03:56 PM
My board is plywood and will probably end up 1 1/16 or so when it's glassed. I was thinking 1 1/4 for the slot. Is that too much slop?

A board that's a bit sloppy in the slot is a whole lot better that one that's stuck tight. You can always attach a couple external strips to the hull alongside the slot to take up some of the slop if necessary. UHMW works very good for that because it's completely stable and very slippery. But a 3/16" gap sound reasonable.

Tim Marchetti
05-01-2012, 03:59 PM
strip composite ,what does that mean, strip plank with glass? Is that the boat or the board?
seal the slot, what does that mean, caulk the trunk/case?
remember that a 1 inch board is only going to be 1 inch in 1 place. Yer probably going to foil shape it, or at least taper it by eye?
If it is an ol' fashion board , it'll swell , like TLL says.

The hull is strip plank and glassed in and out. When I cut the slot the wood core will be exposed. I'm thinking glass and goop, but is it better to seal it before the trunk is intalled or after?

wizbang 13
05-01-2012, 03:59 PM
huh, I was thinking the other way, buzz, or lube, the board a tad if it sticks.
then again, if it's too sloppy, add stuff to the board!

Tim Marchetti
05-01-2012, 04:07 PM
A board that's a bit sloppy in the slot is a whole lot better that one that's stuck tight. You can always attach a couple external strips to the hull alongside the slot to take up some of the slop if necessary. UHMW works very good for that because it's completely stable and very slippery. But a 3/16" gap sound reasonable.
Thanks TLL, That's my thinking too. Stuff seems to grow in the slot also.

JimConlin
05-01-2012, 04:11 PM
It depends on whether the board and the trunk will vary in width.
If the board is well sheathed and the 'thwartship dimension of the trunk is stable, the you can go pretty tight.
Damfino's daggerboard is more composite (carbon, foam and glass) than wood, and the trunk is quite well sheathed, so it's quite stable.
The slop is under 1/16" after painting.

Ian McColgin
05-01-2012, 04:27 PM
I like a structure that has no endgrain in the slot and minimizes the number of joints. This means that if the case is essentially framed plywood, it makes sense to make the plywood extend down through the slot in the keel or bottom plank or whatever. If it's build up planks, I have seen some nice old boats where the lowest centerboard trunk plank was wide enough that it went through the keel and came up about half way up the bedlog. Thus each seam is blocked.

But much depends on the boat's design. Did the designer provide any ideas and what sort of boat is it for - 1" being such a middling thickness.

Jamie Orr
05-01-2012, 06:30 PM
The hull is strip plank and glassed in and out. When I cut the slot the wood core will be exposed. I'm thinking glass and goop, but is it better to seal it before the trunk is intalled or after?

I had a similar issue with plywood end-grain being exposed in the slot in my Chebacco - I did the sealing after installation. It is possible, with care, to lay a strip of glass around the (rounded) edges and into the case to seal everything up tightly. It's a PITA, though, so good luck with it.

Jamie

Thorne
05-01-2012, 06:53 PM
What Jamie said. CB case leaks are very common, so I'd recommend going for the belt, suspenders (braces to you Brits) and glue method. Make the slot large, nothing to lose and everything to gain. Imagine how awful it would be to not be able to beach or trailer the boat if the CB is stuck down after and earlier beaching on gravel or whatever.

Run the fiberglass well up into the CB case, sealing all end grain as recommended above. If you'll be sailing in rocky areas, consider using a flexible sealant / adhesive around the case to hull join, as otherwise impacts can eventually (or suddenly) crack just epoxy. If using ply for the CB, make sure it is top-quality marine ply, well-covered in glass, and put a brass or other metal strip on the leading edge and bottom -- this helps reduce the chance of cracking the glass and then having the ply CB swell and stick in the case. As we say here on the Forum, "Don't ask me how I know this..."

johnw
05-01-2012, 07:07 PM
How do you plan to bed the logs?

Tim Marchetti
05-02-2012, 08:22 AM
Thanks folks. The boat is a 15 foot sailing dinghy of my own design and this is my first sailboat build. I will fiberglass the inside face of the trunk before I assemble it. Does it make sense to glass the slot before I install the trunk? This way the glass would wrap around the opening and I could see what I'm doing. Then I could tape the seam where the trunk and the keelson come together??
http://i877.photobucket.com/albums/ab331/123mardog/baby%20dot/IMG_2607.jpg?t=1335063019 (javascript:void(0);)http://i877.photobucket.com/albums/ab331/123mardog/baby%20dot/IMG_2638.jpg?t=1335786170 (javascript:void(0);)

TerryLL
05-02-2012, 08:36 AM
How wide is the keelson? A photo of the outside bottom would be helpful.

Ian McColgin
05-02-2012, 09:22 AM
Very nice.

I'd make the trunk of plywood sides that are glassed before assembly, 1-1/8" square framing-spacing inside the trunk front, back and top (assuming pivoting board, enclosed top) and wider facing (perhaps 6" to 8" and 1/2 thick) top, front and back for comfortable seat and stability. The wider facings land on the inner keel. The rest of the trunk goes through and finished flush with the bottom of the hull. Bedlogs on either side, of course.

Finishing the outer rim of the CB slot is interesting. A radical solution I did to Leeward that worked nicely with this CB trunk construction was to glass the outside of the hull. I just laid glass right over the slot not worrying too much and cut it to the slot after cure since that was easy. I taped inside and bridged the slot with tape to keep epoxy from dribbling down into the trunk during glassing. Then I had a sharp joint with glass meeting but not overlapping, which I did not like. I relieved the corner to a modest quarter round and put in as much CPES as the wood could stand. I thought about taping the corner but didn't like the thought of the bulge both in the trunk and I realized that with the CPES/plywood matrix I had something pretty plastic anyway. So I just mixed some epoxy with colloidal silica, smoothed it on, and called that a day. Lasted to the end of the boat.

We sometimes forget that fiberglass is not really all that abrasion resistant and we get carried away worrying about having it cover places where it's not really needed.

Gerarddm
05-02-2012, 09:28 AM
Why not simply coat the inside of the CB case with epoxy,and forget the fiberglass?

JimD
05-02-2012, 09:52 AM
Why not simply coat the inside of the CB case with epoxy,and forget the fiberglass?It will wear off much, much faster without the cloth.

TerryLL
05-02-2012, 10:05 AM
Tim, I found a photo of the bottom on your other thread.

http://i877.photobucket.com/albums/ab331/123mardog/baby%20dot/IMG_2558.jpg?t=1333827292

I was curious if the construction was keelson-plus-keel or keelson only, and if the strips fully lapped the keelson.

The major issue here is how to seal the raw edge of the strips once the slot is cut. Fiberglass really doesn't like making a sharp 90-degree bend, so wrapping the slot in glass will be a challenge unless the edge has a decent radius. If you do fiberglass the slot, it'll be easier to do before the trunk is installed. I have always glassed the inside surfaces of the case before assembly.

If you plan to never ever ever remove the case, then you can epoxy the case right down onto the keelson. But the CB case is often a trouble area, so I'd advise a non-adhesive bedding compound and screws.

wizbang 13
05-02-2012, 10:10 AM
Here is a pic or two from my catboat build a few tears ago. similar size to your boat,http://farm7.staticflickr.com/6045/5898532736_48d960f036_z_d.jpg CB is on the left, diving weights epoxied in. CB case halves on the right, with the dry dynel. dynel is much tougher than glass for this rubbing thing. http://farm6.staticflickr.com/5272/5891080830_6401b51b38_z_d.jpgI just cut a hole and spot glued it in place, then put a big ol filet of 403 around it. no wood log, no soft caulking. bam.

wizbang 13
05-02-2012, 10:15 AM
What is" never ever remove the case"? I could cut mine out and stick it back in in an hour.Faster cleaning up "non adhesive" goo and unscrew a buncha screws, not to mention putting it all back together. You have solid wood planking, not ply, so sealing the planking exposed is no a starter. just slap some resin on it. If it gets dinged by a jammed rock or whatnot, one inch plus of trunk width is plenty of room to stick your fingers down there an inch or two to repair it.
http://farm7.staticflickr.com/6135/5935220559_772cffcb6e_z_d.jpg

SHClark
05-02-2012, 12:00 PM
You hope to never see the inside of your centerboard trunk ever again after you have built it.
So if ever there was a place to be anal, this is it.
How much extra width depends on whether you are going to have slot gaskets to clean up the flow when the board id lowered. The trunk needs to be wide enough so that if the gaskets get swallowed into the slot as the board id raised, the a whole thing doesn't jam up. There are lots of options for slot gaskets, but the trunk needs to be wide enough for the board and two thicknesses of slot gasket.
I glass the inside face with 10 oz glass. before I assemble the pieces. I sand the glass and flow coat until there are no pin holes and I can sand the surface to 120 with no print.
I To keep thing neat after that, it usually pays to mask the inside face for the rest of the assembly because epoxy drools down your centerboard trunk are a pure pain in the ass.
I usually assemble the trunk around a blank the inside shape of the trunk so I know it is straight, and often tab the fore and aft ens with glass as well. This can be done with a bit of plastic and a piece of scrap wood cut to fit. Everything to assure that the inside face is waterproof and capable of enduring a lot of wear, chafe and abrasion.
With an ample keelson, I would cut the slot large enough to allow the trunk to stick all the way through, such that when trimmed you would see the edge grain of the plywood at the bottom. I would glass around the corner. Once again a shaped block is your friend as it will compact the glass to the sides of the trunk. With some cleverness you can push the glass down on the hull as well.

Your centerboard should not swell up. A good one has enough epoxy and glass on it that it stays the same shape. Nevertheless, the bearing surface wants to be tighter in the trunk than the foil surface. The less slop the better. The best way to do this is to use plastic to shim the head of the board out to the thickness of the trunk. build the foil shape, glass it and then build the head out to thickness as a separate process. A bit of plastic works well in this environment. I buy ski bottom P-Tex because I also build my own skis. (That's another really fun project!) There is a web site skibuilders.com that sells the HDPE that has been prepared to be bonded with epoxy. This stuff rocks generally and makes all sorts of things that slide by each other cooler. You should be able to get it so that your board goes up and down and doesn't wiggle around in the trunk much at all.

On a similar subject, I don't like the lead in the board option to make the board go down and stay down. A nice bit of line that pulls the head back and which can be cleated off to prevent the thing from sweeping aft when you get going quick is my choice. If you are concerned about hitting bottom, a bungee in the system works well.
SHC

Tim Marchetti
05-02-2012, 12:41 PM
Thanks for all the input everyone. I think I can now formulate a plan I will get the trunk sides glassed and go from there.
SHC: some great suggestions, I like the P-tex idea. I helped my son build a pair of tele skis using materials and info from skibuilders.com
http://i877.photobucket.com/albums/ab331/123mardog/IMG_0696.jpg?t=1335979970

Gib Etheridge
05-02-2012, 12:59 PM
Bias weave tape is much easier to apply over a radiused corner.

If you thicken the last couple of coats of epoxy with Teflon Powder it will be very slippery and therefore easier to clean the inside of the slot with Scotchbrite on a stick.

Teflon powder available here:

http://www.epoxyproducts.com/

johnw
05-02-2012, 01:29 PM
If you plan to never ever ever remove the case, then you can epoxy the case right down onto the keelson. But the CB case is often a trouble area, so I'd advise a non-adhesive bedding compound and screws.

I always figure the case is going to move, it's just in the nature of the stresses on it, and if the way it's fixed to the hull is too rigid it will crack. I'd use a non-adhesive bedding compound with the logs. Seton Gras, who advised me on the first centerboard case I built, told me that if I was delighted with the fit, I should use Dolphinite, if I wasn't so sure, I should use something like Boatlife. So far, on both cases I've used Boatlife, and never had a leak. There are some here who have built a lot more cases than me, though.

TerryLL
05-02-2012, 01:47 PM
I've always used Dolfinite where bedding is called for. But I've heard such good things about Boatlife that I'll just have to give it a try.

Tim Marchetti
05-02-2012, 04:27 PM
Well,here's the first step. I got the trunk sides glassed. I will document the rest on the Building Baby Dot thread. Thank you everyone for your input.
http://i877.photobucket.com/albums/ab331/123mardog/baby%20dot/IMG_2652.jpg?t=1335993730 (http://javascript<strong></strong>:void(0);)

kbowen
05-03-2012, 11:13 PM
I will fiberglass the inside face of the trunk before I assemble it. Does it make sense to glass the slot before I install the trunk? ??

The Gougeon book recommends as follows: plan the CB trunk to go all the way to the outside of the keel, cut the slot in the keel for that, then widen the top of the slot (inside the boat) so it is tapered. Install the CB trunk (Inside already glassed) with the hull right-side up and tack a strip on the outside so goop doesn't leak through. Pour epoxy with a heavy mix of structural filler in the gap between the board and keel. The advantage is that you don't have hidden and inaccessible trunk to bedlog to keel joints. When this hardens, you turn the boat over and seal the end-grain of the trunk and wrap glass over it and down inside the trunk. I haven't tried this, but it sounds reasonable. If your hull is relying on the bedlogs for strength that the "T" section trunk can't provide, then..... nevermind.....

JimConlin
05-03-2012, 11:33 PM
The Gougeon book recommends as follows: plan the CB trunk to go all the way to the outside of the keel, cut the slot in the keel for that, then widen the top of the slot (inside the boat) so it is tapered. Install the CB trunk (Inside already glassed) with the hull right-side up and tack a strip on the outside so goop doesn't leak through. Pour epoxy with a heavy mix of structural filler in the gap between the board and keel. The advantage is that you don't have hidden and inaccessible trunk to bedlog to keel joints. When this hardens, you turn the boat over and seal the end-grain of the trunk and wrap glass over it and down inside the trunk. I haven't tried this, but it sounds reasonable. If your hull is relying on the bedlogs for strength that the "T" section trunk can't provide, then..... nevermind.....

A very good sequence. I'd add one wrinkle. Before glassing the side panels of the trunk, taper them a bit toward the bottom so that when the trunk is eventually glassed to the bottom at the mouth, it'll all end up (more) flush on the inside.

Ian McColgin
05-04-2012, 05:13 AM
Yep, the approach I favor is not far from the GoogeBros except I don't see the need to make a taper and puddle of epoxy between the trunk and hull slot since the fit that loose enough to comfortably fit the whole CB trunk assemply will have room to add thickend epoxy to any areas you're not happy with the squeeze out. After slathering the inside of the slot through the keel with unthickened, I put a thin layer on thickend, a bit of thickend on the outside bottom part of the sides, front and back of the box, and start sliding it in. I add epoxy to the box as I ease it in so there should be a nice roll at the top when it's home. I make it about as thick as peanutbutter. I have the opening slot in the trunk taped for protection and tape over the bottom of the slot through the boat for no really good reason.

It's nice to have dry fitted the bedlogs with their studs coming up through the hull or keelson so that they can be plopped on while the epoxy squeeze-out from the trunk is workable so that the bedlogs can help keep the trunk perfectly verticle and alligned.

In a small boat an epoxy structure like this will not crack or lead - at least Leeward's didn't in her working live of about thirty years.

G'luck

kbowen
05-04-2012, 08:57 AM
I have the opening slot in the trunk taped for protection and tape over the bottom of the slot through the boat for no really good reason.
G'luck

The Gougeon book shows one additional wrinkle but doesn't treat it in text: They show a shallow rabbet around the slot to put one of those vinyl slot-covers that are intended (I assume) to keep the slot from gurgling too much.

Ian McColgin
05-04-2012, 09:58 AM
Absolutely a slot cover is good. I did not put one on the dory Leeward because I didn't think it was needed in such a slow boat with a CB filling most of the length anyway. Wish I had. Every beaching led to sand jamming in the trunk. I had a hole fo the f&#k-stick but had to also add a hardend flat on the CB itself to keep from driving it through the trailing edge. Slot covers do more than smooth out water flow. They would have kept the crud out.

kbowen
05-04-2012, 10:46 AM
Where does one get the material for a slot cover? The one on my little ODay feathers from the attachment edge toward the board, and seems to have held up despite being 40-ish years old.

SHClark
05-04-2012, 12:47 PM
APS in Annapolis sells specialist stuff that is Mylar and Dacron laminate.
It is pretty nice because you can glue it onto the hull and thus avoid the runners and all the pesky screws.
It's not very thick.
For small boats:
The "best" and most durable traditional gaskets were made from 5 oz Dacron sail cloth with one edge tabled.
You need the runners to do this.
You get a bit more than twice the length of you slot.
You lay it so that the tabled edges are overlapped about 1/4"... you don't cut it yet, just loop it back forward, you have to throw a twist in so the folds are both the same way.
Screw the front in place and then rig a Spanish windlass between the back end and something like the lower pintal. Pull quite hard... not quite hard enough to through the front screws.
Make sure the tension on both strips is the same
Screw the runners down and then cut the aft end at something like a 45.
Refinements are a little piece that spans the entire slot in front of the leading edge of the centerboard.
Our standard slot gasket strips were 1/8" x 1/2" aluminum with #8 FH screws every 4" or so.
These sat in a gasket recess that was about 5/8" wider than the trunk itself and about 1.5" longer at the front and back. Screws were doubled at the ends.
More traditional types will favor bronze or stainless half oval. If you chose to use half oval, you shouldn't bother with the recess. The half oval also provides a bit of protection for the gasket.
Nice thing about the sailcloth or specialist stuff from APS is that it is thin, so you don't have to cut recesseses if you don't feel like it.
These aren't forever parts of the boat. They were something you paid attention to as you moved the boat around and something you renewed every year or two ( depending on use and abuse.)
McMaster Carr sells urethane flat belting belting which is a pretty good choice for larger vessels.
SHC

SHClark
05-04-2012, 01:03 PM
One more thing: I don't mind in the very bottom of the trunk is a wee bit narrower than the trunk above, You actually want to the side load from the board transferred into the hull skin and not carried by the inside wall of the trunk. So the thickness of a pass of glass tape (~.0150) is in no way a bad thing. The slickest way to do that detail is to cut a piece of wood that fits over the cornerand extends maybe an inch up the slot and an inch outside the slot. Cover the thing with packing tape so it won't stick to anything.
layup your glass on the piece and let it get gummy. Trim with a razor blade and before it goes off all the way.
At the same time paint the open grain of your trunk with neat epoxy and let it soak in ( some heat helps!)
When everything is still a bit gummy, paint the glass with thicker goo and press it into place. Use wedges, threaded rod, shores and divine intervention to push everything HARD into place. When everything cures, you should have something that inspires pride.
Clean up is bit of a war but did you remember that we have done a very good job of keeping the masking on the trunk... Maybe even leaving the peel ply in place? So that the drools can be chipped away cleanly with a chisel?
Another slick trick is to stuff a piece of Styrofoam inside the trunk to make the dorools puddle up in such a way that they are easier to smack off.
SHC