Originally Posted by Jim Cate
I've long wondered about the "foam under bulkhead" practice. Seems to me that such a structure puts a lot of shear loading on the bond between the vertical part of the tab and the bulkhead and increases flexibility of the joint where rigidity is desirable.
Rod's suggestion of direct contact but with the filet spreading the load over a large area sounds practical and effective. After all, the same end effect as using the foam is reached, but without the increased shear loading. Seems a win to me.
Always great fun (though sometimes a bit frustrating) to respond to these---myths? Or should we call them 'arguments from authority'?
Because you say so, right? As noted, the 'experts' recommend cabosil or milled fibers or some such combination of the two. The point is that the true strength of the joint comes from the glass fiber, not the filler. This is indicated and supported by the well documented failures of methyl methyl acrylate glues in mass production boat applications...surprised that as a professional (oh soooo hard to avoid using the dreaded quotation marks there) you continue to disregard this. As asked many moons ago, please explain how does using a glue, putty or filler stronger and harder than the material being glued, puttied or filled not result in the (also much dreaded, apparently) hard spot?
Originally Posted by Terra Nova
Very poor tech. Microballoons have extremely poor strength and should only be used for final fairing before primer application, NEVER under structural fiberglass
Originally Posted by sailortype
and again, as mentioned above, the bulkhead (or hard filler underneath it) should not be in direct contact with the hull. It creates hard spots and the constant flexing over the hard spot will eventually fatigue the structure of the fiberglass
. A 1/4" foam insert with 45 degree angles cut into it makes a good filet to smooth out the bend of the tabbing cloth.
On my previous boat, this was not done and when
it was hauled you could clearly see the hard ribs all along the hull
Um...I'm having a little trouble understanding your point here. What type foam are we talking about? Chair cushion foam (haha) or ultrahard 25 lb/ft3 (about the same density as plywood) PVC or urethane (and before anyone brings it up, I'm aware that 'density' does not necessarily equate to 'hardness', but I think we're safe in using one as a proxy for the other in this case...)?
That you "clearly see the hard ribs all along the hull" likely has little to do with the way the bulkheads are tabbed (though the lack of a radiusing filet or too few layers of tabbing could be a contributing factor or even the cause) , and has probably more to do with any number of other things; the actual layup schedule of the hull skin itself, the placement of the 'hard ribs' in the hull, the design of the hull and placement of the 'ribs' in relation to it's accumulated induced stresses...
Here I'll break my own rule and respond to yet another example of your non understanding.
Originally Posted by NevisDog
I see some poor advice has been offered here, including that Sail video.
1. Best practice is never place the ply bulkhead in contact with the hull, always place it on top of a foam section to avoid hard spots on hull.
2. Best practice is drill holes or slots along edge of ply and feed glass tape thru to mechanically anchor
Before commencing rebuild
work, I recommend you read the chapter in 'This Old Boat' by Don Casey. Better to get it right first time.
1. Why? How does this 'avoid hard spots on hull' without weakening the joint?
2. Please elaborate, because I have no idea what you're talking about and certainly don't understand...
To which chapter are you referring? How do you or Mr. Casey know what is 'right' or 'best case' in any given situation?
As usual, the right question and the correct, real world answer.
Originally Posted by ramblinrod
Actually, I disagree with the foam between bulkhead and hull. Instead, I recommend solid FRP between bulkhead and hull, but with substantial fillet and
Radius to the tabbing.
The more are the load is distributed over the better, and load is not ditiributed by foam.
Sorry about typos, trying
This with POS iPhone
And, though I'm not very meticulous about keeping visual records, here're a few, as well as a line drawing showing the design flaw inherent in using a comparatively soft foam 'shim, spacer or load spreader' (apparently, judging from the 'advice' given) unattached or glued to either the bulkhead or hull; as well as a possibly more 'proper' way to tab plywood to hulls, especially if there are shortcomings in the hull layup;
1. An MCA (micronized copper azole) plywood transom in a Wellcraft skiboat converted into a 55 mph+ Lake Ponchartrain fishing
boat 11 years ago. (had it out last week to test the carb I rebuilt for her, solid as the day she was launched). Treated plywood must be allowed to thoroughly dry before it is glassed in. The plywood is bedded against the fiberglass in iso poly resin mixed with glass beads, mixed fairly wet, with raw catalyzed resin painted on the plywood with a fresh layer of mat laid on the fiberglass. The plywood is clamped to the fiberglass skin with screws and fender
washer into holes drilled through the skin, which are later removed and filled.
Note the filet, unglassed and glassed, in the right side of the picture as well as the cylindrical 20 lb pvc foam inserts to receive the bolts for the swim platform and isolate them from exposure to water
2. Another view of the same boat at a different level of completion. Note the different color of the transom and the stringers, the stringers are CCA treated plywood, which in my opinion is much more resistant to rot
. It too has to be thoroughly dry prior to glassing. Also note the deck
, which is 1/2 coosaboard, I guess making this a composite composite construction...
3. From another angle, all fileted and ready for glassing. The gray rectangle is a coosaboard insert, to isolate the wood in the transom from the cutout for the outdrive.
4. What the final product should look like. Since might be unclear in the picture, the red lines are the transom, the blue the bottom of the boat, the black the stringer, and the yellow the deck
5. Two bulkheads prior to glassing, made from CCA treated, 5/4 deckboards salvaged from the owners dock
after Katrina (couldn't resist the 'poetic justice'). Again there are foam inserts (for limber holes) to isolate the wood from water
6. A duplication mistake actually, but I'm too lazy to take it out. Dry fitting the stringers.
7 & 8. Just doubling down on my sin of using glass beads where they're inappropriate, this time in a 80+ mph bass boat. Guarantee this lunatic runs the (overpowered) boat like a maniac. After three years, no cracks anywhere...
9. More of the same, though this boat has gone 24 years since this 'modification'. Again 'run hard and put up wet', the only places we've had any problems at all are where Mr. New Land suggests is the only appropriate place for glass beads, that is as filler under primer. Yeah, the paint
is cracking after 21 years (we ran it for a while before painting it).
Colored line drawing;
1. The suggested 'right' way. Brown is the bulkhead, blue is the existing hull, green is fiberglass, gray is foam, purple (4) is putty.
2. Since the foam is comparatively softer than either the bulkhead (plywood) or the hull (fiberglass)(or what would be the point?), when the
hull deforms, as exaggerated by the inclined blue line, it crushes the soft foam on one side putting the fibers on that side in compression
(not good), and in tension on the other (at least this is the better way to use glass's properties...)
3. The end result; as the joint is put through these reversing cyclic loadings, is a void between the 'foam spacer' and the bulkhead, leaving the bulkhead supported by 'legs' of glass on each side, which are themselves stressed and subject to delamination
, in a kind of detrimental positive feedback loop of deterioration...
4. If one has problems with 'ribs showing through the hull', this might be a better solution. Several layers of glass under the bulkhead, then bedding and generously fileting the bulkhead in position (if one feels they must use some super strong and hard putty-making material in this application, its no skin off my back) followed by multiple, staggered layers of glass for tabbing.
And finally, a disclaimer. By all means do what the experts tell you. The things I've shown and said here have worked and continue to work for me, and may not work for anyone else...