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OT thinking about the BJ 24


capt jake

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Boy have I been thinking about Tom's boat! :) There is a fellow in the Seattle area who has started!! He has the hull panels scarfed and cut out, but had a set back during stitching due to a broken panel. :(

Anyway, I have been scouring over the plans and pictures and I have the bug big time! If I could just get the shop addition done; then I could order plywood and start!!

I find myself waking up thinking about her. Not a very productive way to try and sleep. ;)

Just open rambling and dreaming.........

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Dana, I can't do that. It would take up way too much space. I am currently awaiting a few bids to see if I can have somebody else do it. ;)

I am getting caught up on the BJ24 plans and reshreshers on S&G. :) Actually doing some pricing. I already have the sink for it! :) :) Nothing else to do with this blasted head/chest cold. :(;)

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John,

Who is it that had a panel fracture?

It has happened to many people, including me, when twisting a sheet of plywood, especially if the plywood has any flaws. Lower quality plywood is much more apt to have this problem.

I need to tell him about a cure for that problem which I assume ocurred near the bow in the bottom panels.

A good fix is to screw a batten on the outside of the panel about 1 1/2" to 2'' below the chine. The batten should be about 1" thick by 1 1/2" wide by 6' to 8' long and the screws should be through plywood washers from the inside. This will prevent stress concentrations from causing failure at a weak point by spreading the load through the batten. The reason for having the batten away from the chine line is to allow wire ties and some epoxy reinforcement before removing the batten to finish the joint.

Ray, Jake is not waiting to ask questions. :lol:

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Tom, it was Hugh M. It was a fracture in the bow on the lower section, just as you describe. Is there any bearing on the type of plywood being used? IE 1088 vs 6566 (7 vs 9 ply in Meranti) or 1088 or 6566 Okume (both are 7 ply)? His failure does sound as though it was a concentrated amount of pressure as you stated. I am still tossing around which ply to buy. Meranti is cheaper (emphasise ), stiffer, heavier, more rot resistant, but is may not bend as well. Thoughts?

You are right, I have a list of questions, but I am having Hugh review them as it appears that he has already asked you about a few of them.

I am really getting excited! I am going to have to make a duplicate set of plans as I am sure to wear these out before i start. :)

A good fix is to screw a batten on the outside of the panel about 1 1/2" to 2'' below the chine. The batten should be about 1" thick by 1 1/2" wide by 6' to 8' long and the screws should be through plywood washers from the inside.

Wouldn't this flatten out the designed curve created when opening the bottom up? Or does it bend well enough to maintain the designed shape while still providing the needed temporary strength?

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I copied this from the Woodenboat forum.

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Since I designed the Bluejacket 24 I guess it is time that I got into this problem after just talking with Hugh.

The break was caused by stress concentration caused by the assembly sequence. The fact that the bottom panels were opened and took their design shape indicates that there was no undue stress up to that point. When the port topside panel was stitched in place and the fairing blocks screwed in, the hull undoubtedly warped a bit and put more stress on the starboard side bottom panel. Hugh then attempted to fit the starboard topside panel in place and screw the fairing blocks down BEFORE tightening the wire ties. this combination put a stress concentration on the already stressed bottom panel just in front of the block as seen in the photo and the panel failed.

The fact that Hugh is using 9 ply plywood may also have contributed since it is said to be stiffer than 7 or 5 ply.

The proper sequence to install a panel that needs bend and/or twist is to start at the end with the most severe bend or twist. This is almost always at the bow. All wire ties should be put in from this point and tightened snug. This will help prevent stress concentration by allowing some transfer of stress to adjacent areas as the panel is gradually bent into shape. The fairing blocks should not be put in until all the panels are in place and wires tightened. The purpose of these blocks is to force the panels to make a fair intersection at the chine near the bow, which they may not like to do.

In severe cases, installing a batten an inch or two below the chine and on the outside will be even more effective in preventing stress concentration. Another help is to squegee on a thin coat of epoxy on the outside (tension) side of the curvature which will delay local fiber failure by spreading the stress. Sometimes water and/or heat can help but on thick plywood, it may not work too well. There are limits to which plywood can be bent without fracture but using these techniques will enable you to reach that limit.

Some more stuff to be added to the plans I guess since not everone is familiar with these problems or solutions and they are not in any of the books I know of. At least that is fairly easy for me to do since I make my plans in a loose leaf format that is not too much work to add to or modify.

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Adding a batten to the edge of the plywood will not intoduce flat spots provided the batten is long enough to extend well past the area of severe bend. In fact the batten will make the curve more fair by eliminating the "edge effect" of the plywood. Try bending any sheet of material and notice that the center part does not take the same curve as the edges. This is caused by any point in the center being supported by other points on either side while the edge is supported on only one side. Some materials won't work because you gotta get at least near the elastic limit of the material in the bend. I don't necessarily understand this stuff since I'm an electrical engineer but I'm told it is so and it sounds reasonable since we have the same phenomena (nearly) in physics.

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Thanks Tom. It almost sounds like it might be better to use 6566 Okume, at least for the bottom.?? I am still wondering if 6566 Meranti will take teh bend.?

I appreciate you prompt answers here and at the WBF, Sounds like Hugh got his question answered. :) Also sounds like I can expect a new page for the plans eh? ;) ;)

thanks again,

John

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